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How many allele for one gene are found inside our cell?

How many allele for one gene are found inside our cell?



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I know it depends on phase. So i am talking of -

1) in human (diploid organism) with cells before prophase (before replication, when 23 pair of homologous chromosomes are present ). I think answer is 2 if we not consider male XY sex chromosome because they are not homologous in which case the answer should be 1. But then i get confuse why genotype is shown like Tt (doesnt this means two alleles per chromosome).

2) in gamete cell. I think the answer should be 1.

Please tell if i'm correct.


In short. Yes.

Each diploid cell has 23 pairs of chromosomes (22 of which autosomal)

In relation to the genotype part of the question. The "Tt" refers to each allele on the chromosome and wether it is dominant or recessive. Take for instance, hair colour. On one chromosome, there would be an allele from one parent and on the other an allele from the other parent. This is why two letters are needed, to show the allele given by each parent.

You can think of dominant and recessive in terms of how powerful the copy of the gene(allele) is. An allele that is able to overpower and mask traits is called dominant, one that is not strong and cannot mask traits is called recessive. This is why you need two recessive alleles for recessive disorders, otherwise the dominant allele would cover it up.

As this is a homework question I have tried to explain this in simple terms which you should understand. Hope this helps.


What Is an Allele?

Cristina Mutchler is an award-winning journalist with more than a decade of experience in national media, specializing in health and wellness content. A multilingual Latina, Cristina's work has appeared on CNN and its platforms, local news affiliates across the country, and in the promotion of medical journal articles and public health messaging.

Lauren Schlanger, MD, is a board-certified primary care physician with a focus on women's and transgender health.

Mark Garlick / Science Photo Library / Getty Images

Alleles are copies of genes that influence hereditary characteristics. Each person inherits at least two alleles for a particular gene—one allele from each parent. They are also called allelomorphs.

A good example of how alleles are expressed is eye color whether we have blue or brown eyes depends on the alleles that are passed down from our parents.   Because they help determine what our bodies look like and how they're structured, alleles are considered an important part of the blueprint for all living organisms.


What’s a gene?

In order to answer the question of how many genes we have, we must first agree on what we mean by the word “gene”. The definition has evolved ever since Mendel, but the focus as the HGP got under way was primarily on protein-coding genes i.e., regions of the genome that are transcribed into RNA and then translated to create proteins. However, many genes are noncoding: the HGP’s original paper, in 2001, acknowledged that “thousands of human genes produce noncoding RNAs as their ultimate product,” although the paper itself reported just 706 noncoding RNA genes [2]. For this discussion, then, let us use the following definition of a gene:

Gene: any interval along the chromosomal DNA that is transcribed into a functional RNA molecule or that is transcribed into RNA and then translated into a functional protein.

This definition includes both noncoding RNA genes and protein-coding genes, and it also groups all the alternative splice variants at a single locus together, counting them as variants on the same gene. It is meant to exclude pseudogenes, which are non-functional remnants of true genes. Admittedly, though, this definition raises the question of what is meant by functional, and a truly comprehensive definition of the term gene would likely take many pages to describe.

Using this definition, though, do we have agreement on the number of protein-coding genes? The short answer is no. The human genome began with the assumption that our genome contains 100,000 protein-coding genes, and estimates published in the 1990s revised this number slightly downward, usually reporting values between 50,000 and 100,000. The two initial human genome papers reported 31,000 [2] and 26,588 protein-coding genes [3], and when the more complete draft of the genome appeared in 2004 [4], the authors estimated that a complete catalog would contain 24,000 protein-coding genes. The Ensembl human gene catalog described in that paper (version 34d) had 22,287 protein-coding genes and 34,214 transcripts.


How our cells use mother's and father's genes

Credit: CC0 Public Domain

Researchers at Karolinska Institutet and Ludwig Institute for Cancer Research have characterized how and to what degree our cells utilize the gene copies inherited from our mother and father differently. At a basic level this helps to explain why identical twins can appear rather different, even though they share identical genetic makeup. With this knowledge we will better understand the variation in outcomes of genetic disorders.

Humans have two copies of all autosomal genes, one inherited from the mother and one from the father, and often the two copies are not perfectly identical due to small differences in their DNA sequence. Therefore, variation in the utilization of the two copies in cells has functional consequences, but the nature and patterns of their gene copy utilization has remained largely unknown. Now, the researchers have provided answers to this longstanding question in molecular genetics. They used allele-sensitive gene expression analyses, so called "single-cell RNA-sequencing", on the newly divided cells to characterize the dynamics of gene copy expression in mouse and human cells in remarkable detail.

"Our experiments allowed us to determine which genes get locked into expressing only one gene copy and which genes that dynamically switches between the two gene copies over time", says Björn Reinius, at the Department of Cell and Molecular Biology one of the lead authors of the study published in the journal Nature Genetics.

In genetics, the two gene copies are referred to as the two "alleles" of each gene. Indeed, DNA differences in the mother's and father's genomes explain why siblings can appear physiologically rather different from each other, as the siblings inherit different sets of alleles and thereby have non-identical genetic makeup. However, even between so-called identical twins, which carry precisely the same set of alleles, there are still differences in manifestation of some genetic traits. For example, sometimes only one of two twins suffers the effects of a genetic disorder – even though both twins carry the same disease allele. Historically this was mainly explained by variations in external environment and life history. However, since the last few decades we know that purely random molecular events taking place inside the cells can actually affect how and when the set of alleles are expressed. The mother's allele of a certain gene may be expressed in some of the individual's cells while the father's allele is expressed in other cells of the very same individual. Whether this "choice" of expressed allele tends to be forwarded through cell division or whether the allelic choice takes place independently in each cell again and again over time has remained unknown.

These competing scenarios would result in crucially different physiological outcomes since the first results in patches of cells in the body having the same set of expressed alleles, while the other scenario results in allelic expression patterns that "flicker" between the mother's and father's gene copies over the coarse time. The results of the present study, demonstrate that most autosomal genes dynamically fluctuate in expression of the two alleles, while only as little as 0.5–1% of genes are fixed into expressing only one.

"The knowledge gained from this detailed study on the nature of gene transcription will help researchers and medical doctors to better understand and model the mechanisms underlying variable outcomes in genetic disease", says Rickard Sandberg, at the Department of Cell and Molecular Biology at Karolinska Institutet and the Ludwig Centre for Cancer Research, who supervised the project.


Explainer: What are genes?

We inherit genes from our parents. Genes are like a recipe that’s passed along from one generation to the next. But the number of genes doesn’t explain why we are more complex than simple animals or bacteria.

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February 8, 2019 at 6:30 am

Genes are the blueprints for building the chemical machinery that keeps cells alive. That’s true for humans and all other forms of life. But did you know that with 20,000 genes, people have almost 11,000 fewer genes than water fleas? If the number of genes doesn’t predict complexity, what does?

The answer is that our genetic material contains much more than the units we call genes. Just as important are the switches that turn a gene on and off. And how cells read and interpret genetic instructions is far more complex in people than in those water fleas.

Genes and the switches that control them are made of DNA. That’s a long molecule resembling a spiral ladder. Its shape is known as a double helix. A total of three billion rungs connect the two outer strands — the upright supports — of this ladder. We call the rungs base pairs for the two chemicals (pair) from which they are made. Scientists refer to each chemical by its initial: A (adenine), C (cytosine), G (guanine) and T (thymine). A always pairs with T C always pairs with G.

In human cells, the double-stranded DNA doesn’t exist as one gigantic molecule. It’s split into smaller chunks called chromosomes (KROH-moh-soams). These are packaged into 23 pairs per cell. That makes 46 chromosomes in total. Together, the 20,000 genes on our 46 chromosomes are referred to as the human genome.

The role of DNA is similar to the role of the alphabet. It has the potential to carry information, but only if the letters are combined in ways that make meaningful words. Stringing words together makes instructions, as in a recipe. So genes are instructions for the cell. Like instructions, genes have a “start.” Their string of base pairs must follow in a specific order until they reach some defined “end.”

Explainer: What’s on your genes

If genes are like a basic recipe, alleles (Ah-LEE-uhls) are versions of that recipe. For instance, the alleles of the “eye color” gene give directions for making eyes blue, green, brown and so on. We inherit one allele, or gene version, from each of our parents. That means most of our cells contain two alleles, one per chromosome.

But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.

Scientists Say: Chromosome

By combining two sets of 23 chromosomes — one set from the egg, one set from the sperm cell — that new person ends up with the usual two alleles and 46 chromosomes. And her unique combination of alleles will never arise in the exact same way again. It’s what makes each of us unique.But we aren’t exact copies of our parents (or siblings). The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene (instead of two), packaged into 23 chromosomes. Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person.

A fertilized cell needs to multiply to make all of a baby’s organs and body parts. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell. Then the process repeats itself many times as one cell copies to become two. And two copy to become four. And so on.

To make organs and tissues, the cells use the instructions on their DNA to build tiny machines. They control reactions between chemicals in the cell that eventually produce organs and tissues. The tiny machines are proteins. When a cell reads a gene’s instructions, we call it gene expression. A fertilized cell needs to multiply to make all of a baby’s organs and body parts. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell. Then the process repeats itself many times as one cell copies to become two. And two copy to become four. And so on.

How does gene expression work?

Gene expression relies on helper molecules. These interpret a gene’s instructions to make the right types of proteins. One important group of those helpers is known as RNA. It’s chemically similar to DNA. One type of RNA is messenger RNA (mRNA). It’s a single-stranded copy of the double-stranded DNA.

Making mRNA from DNA is the first step in gene expression. That process is known as transcription and happens inside a cell’s core, or nucleus. The second step, called translation, takes place outside of the nucleus. It turns the mRNA message into a protein by assembling the appropriate chemical building blocks, known as amino (Ah-MEE-no) acids.

All human proteins are chains with different combinations of 20 amino acids. Some proteins control chemical reactions. Some carry messages. Still others function as building materials. All organisms need proteins so that their cells can live and grow.

To build a protein, molecules of another type of RNA — transfer RNA (tRNA) — line up along the mRNA strand. Each tRNA carries a three-letter sequence on one end and an amino acid on the other. For example, the sequence GCG always carries the amino acid alanine (AL-uh-neen). The tRNAs match up their sequence with the mRNA sequence, three letters at a time. Then, another helper molecule, known as a ribosome (RY-boh-soam), joins the amino acids on the other end to make the protein.

One gene, several proteins

Scientists first thought that each gene held the code to make one protein only. They were wrong. Using the RNA machinery and its helpers, our cells can make way more than 20,000 proteins from their 20,000 genes. Scientists don’t know exactly how many more. It could be a few hundred thousand — perhaps a million!

Explainer: What are proteins?

How can one gene make more than one type of protein? Only some stretches of a gene, known as exons, code for amino acids. The regions in between them are introns. Before the mRNA leaves a cell’s nucleus, helper molecules remove its introns and stitch together its exons. Scientists refer to this as mRNA splicing.

The same mRNA may be spliced in different ways. This often happens in different tissues (perhaps skin, the brain or the liver). It’s like the readers “speak” different languages and interpret the same DNA message in multiple ways. That’s one way the body can have more proteins than genes.

Scientists Say: DNA sequencing

Here’s another way. Most genes have multiple switches. The switches determine where an mRNA begins to read a DNA sequence, and where it stops. Different start or end sites create different proteins, some longer and some shorter. Sometimes, transcription doesn’t start until several chemicals attach themselves to the DNA sequence. These DNA binding sites may be far away from the gene, but still influence when and how the cell reads its message.

Splicing variations and gene switches result in different mRNAs. And these are translated into different proteins. Proteins also may change after their building blocks have been assembled into a chain. For example, the cell may add chemicals to give a protein some new function.

DNA holds more than building instructions

Making proteins is far from DNA’s only role. In fact, only one percent of human DNA contains the exons that the cell translates into protein sequences. Estimates for the share of DNA that controls gene expression range from 25 to 80 percent. Scientists do not yet know the exact number because it’s harder to find these regulatory DNA regions. Some are gene switches. Others make RNA molecules that aren’t involved in building proteins.

Controlling gene expression is almost as complex as conducting a large symphony orchestra. Just consider what it takes for a single fertilized egg cell to develop into a baby within nine months.

So does it matter that water fleas have more protein-coding genes than people? Not really. Much of our complexity hides in the regulatory regions of our DNA. And decoding that part of our genome will keep scientists busy for many, many years.

Power Words

allele One of two or more alternative versions of a gene.

amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic building blocks of proteins.

base pairs (in genetics) Sets of nucleotides that match up with each other on DNA or RNA. For DNA, adenine (A) matches up with thymine (T), and cytosine (C) matches up with guanine (G).

biology The study of living things. The scientists who study them are known as biologists.

cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells.

chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

chromosome A single threadlike piece of coiled DNA found in a cell’s nucleus. A chromosome is generally X-shaped in animals and plants. Some segments of DNA in a chromosome are genes. Other segments of DNA in a chromosome are landing pads for proteins. The function of other segments of DNA in chromosomes is still not fully understood by scientists.

coding (in genetics) The instructions contained in DNA (or its genes) that allow a cells to know what proteins to make and when to make them. (in computing) A slang term for developing computer programming — or software — that performs a particular, desired computational task.

decoding Figuring out a message hidden in some code.

development (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape. (v. develop)

DNA (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

egg The unfertilized reproductive cell made by females.

exon Part of a DNA or RNA molecule that holds the directions for creating part of a protein or peptide.

expression (in genetics) The process by which a cell uses the information coded in a gene to direct a cell to make a particular protein.

gene A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

genome The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.

guanine One of four substances that organisms need to produce DNA.

helix An object with a three-dimensional shape like that of a wire wound uniformly in a single layer around a cylinder or cone, as in a corkscrew or spiral staircase.

intron A section of DNA or RNA that does not carry the blueprints for making some protein.

messenger RNA (or mRNA) A type of genetic material that is copied from DNA. It carries the instructions for building a cell’s proteins.

molecule An electrically neutral group of atoms that represents the smallest possibleamount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nucleus Plural is nuclei. (in biology) A dense structure present in many cells. Typically a single rounded structure encased within a membrane, the nucleus contains the genetic information.

organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

protein A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.

RNA A molecule that helps “read” the genetic information contained in DNA. A cell’s molecular machinery reads DNA to create RNA, and then reads RNA to create proteins.

sequence (in genetics) n. The precise order of the nucleotides within a gene. (v.)
To figure out the precise order of the nucleotides making up a gene.

sibling An offspring that shares the same parents (with its brother or sister).

sperm The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

splice Originally meant to weave the ends of two pieces of rope together so that it becomes one longer piece of rope. It can now also mean to take two long things (movie film, pieces of lumber or pieces of DNA, for instance) and make them a single longer one.

stitch A length of thread that binds two or more fabrics together.

tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

translation (in genetics) The process of turning the mRNA message into a protein. A cell does this by assembling the appropriate chemical building blocks, known as amino (Ah-MEE-no) acids. Translation occurs outside of a cell’s inner core, or nucleus.

transcription (v. transcribe) To copy something down, word for word. (in genetics) The first step in gene expression. It's where an enzyme copies a selected piece of DNA into RNA (especially messenger RNA). Both DNA and RNA are made up of base pairs of nucleotides.

transfer RNA (tRNA) A type of RNA (ribonucleic acid) molecule that a cell uses to read a section of messenger RNA. This takes place during the production of a cellular protein.

unique Something that is unlike anything else the only one of its kind.

About Silke Schmidt

Silke Schmidt is a freelance science writer with degrees in biostatistics and journalism. She enjoys covering the environment, engineering and medicine. She has two kids and two places she calls home, Wisconsin and Germany.

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Allele

An allele is an essential term of genetics. Gene is the structural unit of the chromosome, which carries heredity from one generation to another. The alleles are the pair of genes, which is located in a specific area of the chromosome. On this page, we are going to define allele and discuss what is the meaning of allele.

Allele Definition - There are different variants of genes present in a chromosome. An allele is a variant of the gene, which locates in a chromosome's specific location as a pair of genes.

In the human chromosome, alleles are present in pairs and maintain the same trait. Therefore, humans are diploid organisms. Two similar alleles are present in each genetic locus, where one allele is inherited from each parent. Also, an allele can be two or more variants of a gene at one genetic locus. But all the alleles maintain the same trait at a genetic locus of the chromosome.

Alleles Meaning In Biology

Now, we will discuss alleles meaning. The word allele comes from the Greek word 'allos'. An allele is the modern formation of that word. The word 'allos' means other. In biology, an allele means different varieties of a gene. The alleles present in a particular genetic locus maintain the same trait. Though alleles are present in a locus as a pair, they can also be found in more than two numbers. In the human chromosome, alleles are present in a pair only to carry the heredity.

Genotype of Allele

Alleles are located in a particular location of the chromosome. The chromosome is the central unit of an organism. All the alleles present in an organism build its genotype. Genotype can be of two types depending on the similarity of the alleles.

When a pair of alleles are the same, they build homozygous genotypes. When the pair of alleles in a location are not similar, they build heterozygous genotypes. In the case of homozygous genotype, the allele is not dominant or recessive. But the heterozygous genotype includes one dominant allele. The dominant allele overrules the features of the recessive allele.

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Example of Allele

Now, we will be discussing some examples of alleles. Here, we are taking the example of a pea plant. The alleles for the colour of the flower build heterozygous genotype, where the purple allele is dominant, and white is recessive. For the height of the plant, tall is the dominant allele, and short is recessive. For the pea colour, the dominant allele is yellow, and the recessive allele is green. In these three cases, the dominant alleles overrule the recessive alleles' feature in the case of heterozygous genotype. Also, the eye colour and hair colour of human organisms can be observed as the example of alleles.

Difference Between a Gene and an Allele

There are some fundamental differences between gene and allele. The differences between a gene and allele are given below in a tabular form.

It is a hereditary information unit made up of DNA and consists of genetic information to transmit characteristics.

An allele is a variation of the gene.

They are located at a specific genetic locus, consisting of two copies, each of the parents.

The two copies at the specific genetic locus are called alleles.

A gene may contain different alleles.

An allele is present inside the gene upon which the character of a person is dependent.

Solved Examples

1. Give an Example of an Allele.

Solution: The genetic locus of each gene that consists of two alleles for different characteristics can be seen in the pea plant. In an experiment, it was seen that, for colour, the plants are purple due to the dominant allele and white due to the recessive allele. In height, they are tall due to the dominant allele and short due to the recessive allele. Like these, the other traits are governed by the dominant alleles in the specific genetic locus.

Conclusion

Biology is a vital subject of the Class 10 curriculum. The students should read all the chapters of biology sincerely. In Class 10, the biology syllabus contains introductory chapters of some important conceptual topics. The students should read all the chapters for their future convenience. These chapters will help them in higher studies. Genetics is a vital part of biology. The students can easily learn the primary concept of genetics in Class 10 from this page.


Polygenic Inheritance

Up until this point we’ve been talking about traits that are controlled by alleles from one gene and fit neatly into our Punnett square. But, some traits are controlled by many genes. Scientists estimate that your height is controlled by more than 400 different genes, for example!

The reason human height is controlled by so many different genes is because height isn’t a simple on/off, yes/no-type trait. We’re actually pretty complex critters for some types of traits!

There’s a lot of things that have to happen to make people tall—blood vessels, muscles, nerves, and bones have to grow and elongate more blood has to be produced to accommodate the extra tissue the brain needs to send out hormones to coordinate everything, etc. It’s a big job and it’s no wonder there are a lot of genes that come into play!

Many other human traits are controlled through polygenic inheritance, such as IQ, skin color, eye color, etc. Can you think of some of the things that might need to happen to produce these traits?


Alleles, genotype & phenotype

Alleles and genotypes are important foundations of genetics. An allele is a particular form of a gene and they are passed from parents to their offspring. A genotype is the combination of two alleles, one received from each parent.

The physical expression of a genotype is called the phenotype. The specific combination of the two alleles (the genotype) influences the physical expression (the phenotype) of the physical trait that the alleles carry information for. The phenotype can also be influenced by the environment

Alleles

An allele is a particular form of one specific gene. When Gregor Mendel completed his experiments on peas he was crossing different traits of one characteristic, such as flower color.

Genetically, the variation in traits, e.g. purple flowers or white flowers, is caused by different alleles. In most cases in the plant and animal world, individuals have two alleles for each gene one allele is inherited from their father and the second from their mother.

Depending on which alleles an individual has received will determine how their genes are expressed. For example, if two parents have blue eyes and pass the blue-eyed alleles onto their children, their children will also possess the alleles for blue eyes.

Certain alleles have the ability to dominate the expression of a particular gene. For example, if a child has received a blue-eye allele from their father and a brown-eye allele from their mother, the child will have brown eyes because the brown-eye allele is dominant over the blue eye allele. In this case, the brown-eye allele is known as the ‘dominant’ allele and the blue-eye allele is known as the ‘recessive’ allele.

Genotype

The genotype is the genetic combination of two alleles. If, for example, a child has received one brown-eye allele – represented by ‘B’ – and one blue-eye allele – represented by ‘b’ – then their genotype would be ‘Bb’. If, however, the child received two brown-eye alleles their genotype would be ‘BB’, and a child with two blue-eye alleles ‘bb’.

As previously mentioned, the brown-eye allele is dominant over the blue-eye allele so a child with the genotype ‘Bb’ would, in theory, have brown eyes, rather than blue or a mix between the two. Genotypes with two alleles that are the same, i.e. ‘BB’ and ‘bb’, are known as homozygous genotypes and genotypes with two different alleles are known as heterozygous genotypes.

Phenotype

The physical appearance of the genotype is called the phenotype. For example, children with the genotypes ‘BB’ and ‘Bb’ have brown-eye phenotypes, whereas a child with two blue-eye alleles and the genotype ‘bb’ has blue eyes and a blue-eye phenotype. The phenotype can also be influenced by the environment and sometimes certain alleles will be expressed in some environments but not in others. Therefore two individuals with the same genotype can sometimes have different phenotypes in they live in different environments.

Definitions:

  • Gene – a section of DNA that contains the genetic material for one characteristic
  • Allele – a particular form of a gene. One allele is received from each parent
  • Genotype – the combination of the two alleles that are received from an individual’s parents
  • Phenotype – the physical expression of the gene which is determined by both the genotype and the environment
  • Heterozygous – a genotype with two different alleles
  • Homozygous – a genotype with two of the same alleles

Contents

Genes are pieces of DNA that contain information for the synthesis of ribonucleic acids (RNAs) or polypeptides. Genes are inherited as units, with two parents dividing out copies of their genes to their offspring. Humans have two copies of each of their genes, but each egg or sperm cell only gets one of those copies for each gene. An egg and sperm join to form a complete set of genes. The resulting offspring has the same number of genes as their parents, but for any gene, one of their two copies comes from their father, and one from their mother. [1]

The effects of this mixing depend on the types (the alleles) of the gene. If the father has two copies of an allele for red hair, and the mother has two copies for brown hair, all their children get the two alleles that give different instructions, one for red hair and one for brown. The hair color of these children depends on how these alleles work together. If one allele dominates the instructions from another, it is called the dominant allele, and the allele that is overridden is called the recessive allele. In the case of a daughter with alleles for both red and brown hair, brown is dominant and she ends up with brown hair. [2]

Although the red color allele is still there in this brown-haired girl, it doesn't show. This is a difference between what you see on the surface (the traits of an organism, called its phenotype) and the genes within the organism (its genotype). In this example you can call the allele for brown "B" and the allele for red "b". (It is normal to write dominant alleles with capital letters and recessive ones with lower-case letters.) The brown hair daughter has the "brown hair phenotype" but her genotype is Bb, with one copy of the B allele, and one of the b allele.

Now imagine that this woman grows up and has children with a brown-haired man who also has a Bb genotype. Her eggs will be a mixture of two types, one sort containing the B allele, and one sort the b allele. Similarly, her partner will produce a mix of two types of sperm containing one or the other of these two alleles. When the transmitted genes are joined up in their offspring, these children have a chance of getting either brown or red hair, since they could get a genotype of BB = brown hair, Bb = brown hair or bb = red hair. In this generation, there is, therefore, a chance of the recessive allele showing itself in the phenotype of the children—some of them may have red hair like their grandfather. [2]

Many traits are inherited in a more complicated way than the example above. This can happen when there are several genes involved, each contributing a small part to the end result. Tall people tend to have tall children because their children get a package of many alleles that each contribute a bit to how much they grow. However, there are not clear groups of "short people" and "tall people", like there are groups of people with brown or red hair. This is because of the large number of genes involved this makes the trait very variable and people are of many different heights. [3] Despite a common misconception, the green/blue eye traits are also inherited in this complex inheritance model. [4] Inheritance can also be complicated when the trait depends on the interaction between genetics and environment. For example, malnutrition does not change traits like eye color, but can stunt growth. [5]

Genes make proteins Edit

The function of genes is to provide the information needed to make molecules called proteins in cells. [1] Cells are the smallest independent parts of organisms: the human body contains about 100 trillion cells, while very small organisms like bacteria are just a single cell. A cell is like a miniature and very complex factory that can make all the parts needed to produce a copy of itself, which happens when cells divide. There is a simple division of labor in cells—genes give instructions and proteins carry out these instructions, tasks like building a new copy of a cell, or repairing the damage. [6] Each type of protein is a specialist that only does one job, so if a cell needs to do something new, it must make a new protein to do this job. Similarly, if a cell needs to do something faster or slower than before, it makes more or less of the protein responsible. Genes tell cells what to do by telling them which proteins to make and in what amounts.

Proteins are made of a chain of 20 different types of amino acid molecules. This chain folds up into a compact shape, rather like an untidy ball of string. The shape of the protein is determined by the sequence of amino acids along its chain and it is this shape that, in turn, determines what the protein does. [6] For example, some proteins have parts of their surface that perfectly match the shape of another molecule, allowing the protein to bind to this molecule very tightly. Other proteins are enzymes, which are like tiny machines that alter other molecules. [7]

The information in DNA is held in the sequence of the repeating units along the DNA chain. [8] These units are four types of nucleotides (A,T,G and C) and the sequence of nucleotides stores information in an alphabet called the genetic code. When a gene is read by a cell the DNA sequence is copied into a very similar molecule called RNA (this process is called transcription). Transcription is controlled by other DNA sequences (such as promoters), which show a cell where genes are, and control how often they are copied. The RNA copy made from a gene is then fed through a structure called a ribosome, which translates the sequence of nucleotides in the RNA into the correct sequence of amino acids and joins these amino acids together to make a complete protein chain. The new protein then folds up into its active form. The process of moving information from the language of RNA into the language of amino acids is called translation. [9]

If the sequence of the nucleotides in a gene changes, the sequence of the amino acids in the protein it produces may also change—if part of a gene is deleted, the protein produced is shorter and may not work anymore. [6] This is the reason why different alleles of a gene can have different effects on an organism. As an example, hair color depends on how much of a dark substance called melanin is put into the hair as it grows. If a person has a normal set of the genes involved in making melanin, they make all the proteins needed and they grow dark hair. However, if the alleles for a particular protein have different sequences and produce proteins that can't do their jobs, no melanin is produced and the person has white skin and hair (albinism). [10]

Genes are copied Edit

Genes are copied each time a cell divides into two new cells. The process that copies DNA is called DNA replication. [8] It is through a similar process that a child inherits genes from its parents when a copy from the mother is mixed with a copy from the father.

DNA can be copied very easily and accurately because each piece of DNA can direct the assembly of a new copy of its information. This is because DNA is made of two strands that pair together like the two sides of a zipper. The nucleotides are in the center, like the teeth in the zipper, and pair up to hold the two strands together. Importantly, the four different sorts of nucleotides are different shapes, so for the strands to close up properly, an A nucleotide must go opposite a T nucleotide, and a G opposite a C. This exact pairing is called base pairing. [8]

When DNA is copied, the two strands of the old DNA are pulled apart by enzymes then they pair up with new nucleotides and then close. This produces two new pieces of DNA, each containing one strand from the old DNA and one newly made strand. This process is not predictably perfect as proteins attach to a nucleotide while they are building and cause a change in the sequence of that gene. These changes in the DNA sequence are called mutations. [11] Mutations produce new alleles of genes. Sometimes these changes stop the functioning of that gene or make it serve another advantageous function, such as the melanin genes discussed above. These mutations and their effects on the traits of organisms are one of the causes of evolution. [12]

A population of organisms evolves when an inherited trait becomes more common or less common over time. [12] For instance, all the mice living on an island would be a single population of mice: some with white fur, some gray. If over generations, white mice became more frequent and gray mice less frequent, then the color of the fur in this population of mice would be evolving. In terms of genetics, this is called an increase in allele frequency.

Alleles become more or less common either by chance in a process called genetic drift or by natural selection. [13] In natural selection, if an allele makes it more likely for an organism to survive and reproduce, then over time this allele becomes more common. But if an allele is harmful, natural selection makes it less common. In the above example, if the island were getting colder each year and snow became present for much of the time, then the allele for white fur would favor survival since predators would be less likely to see them against the snow, and more likely to see the gray mice. Over time white mice would become more and more frequent, while gray mice less and less.

Mutations create new alleles. These alleles have new DNA sequences and can produce proteins with new properties. [14] So if an island was populated entirely by black mice, mutations could happen creating alleles for white fur. The combination of mutations creating new alleles at random, and natural selection picking out those that are useful, causes an adaptation. This is when organisms change in ways that help them to survive and reproduce. Many such changes, studied in evolutionary developmental biology, affect the way the embryo develops into an adult body.

Some diseases are hereditary and run in families others, such as infectious diseases, are caused by the environment. Other diseases come from a combination of genes and the environment. [15] Genetic disorders are diseases that are caused by a single allele of a gene and are inherited in families. These include Huntington's disease, Cystic fibrosis or Duchenne muscular dystrophy. Cystic fibrosis, for example, is caused by mutations in a single gene called CFTR and is inherited as a recessive trait. [16]

Other diseases are influenced by genetics, but the genes a person gets from their parents only change their risk of getting a disease. Most of these diseases are inherited in a complex way, with either multiple genes involved, or coming from both genes and the environment. As an example, the risk of breast cancer is 50 times higher in the families most at risk, compared to the families least at risk. This variation is probably due to a large number of alleles, each changing the risk a little bit. [17] Several of the genes have been identified, such as BRCA1 and BRCA2, but not all of them. However, although some of the risks are genetic, the risk of this cancer is also increased by being overweight, heavy alcohol consumption and not exercising. [18] A woman's risk of breast cancer, therefore, comes from a large number of alleles interacting with her environment, so it is very hard to predict.

Since traits come from the genes in a cell, putting a new piece of DNA into a cell can produce a new trait. This is how genetic engineering works. For example, rice can be given genes from a maize and a soil bacteria so the rice produces beta-carotene, which the body converts to Vitamin A. [19] This can help children suffering from Vitamin A deficiency. Another gene being put into some crops comes from the bacterium Bacillus thuringiensis the gene makes a protein that is an insecticide. The insecticide kills insects that eat the plants but is harmless to people. [20] In these plants, the new genes are put into the plant before it is grown, so the genes are in every part of the plant, including its seeds. [21] The plant's offspring inherit the new genes, which has led to concern about the spread of new traits into wild plants. [22]

The kind of technology used in genetic engineering is also being developed to treat people with genetic disorders in an experimental medical technique called gene therapy. [23] However, here the new, properly working gene is put in targeted cells, not altering the chance of future children inheriting the disease causing alleles.


Allele

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Allele, also called allelomorph, any one of two or more genes that may occur alternatively at a given site (locus) on a chromosome. Alleles may occur in pairs, or there may be multiple alleles affecting the expression ( phenotype) of a particular trait. The combination of alleles that an organism carries constitutes its genotype. If the paired alleles are the same, the organism’s genotype is said to be homozygous for that trait if they are different, the organism’s genotype is heterozygous. A dominant allele will override the traits of a recessive allele in a heterozygous pairing. In some traits, however, alleles may be codominant—i.e., neither acts as dominant or recessive. An example is the human ABO blood group system persons with type AB blood have one allele for A and one for B. (Persons with neither are type O.)

Most traits are determined by more than two alleles. Multiple forms of the allele may exist, though only two will attach to the designated gene site during meiosis. Also, some traits are controlled by two or more gene sites. Both possibilities multiply the number of alleles involved. All genetic traits are the result of the interactions of alleles. Mutation, crossing over, and environmental conditions selectively change the frequency of phenotypes (and thus their alleles) within a population. For example, alleles that are carried by individuals with high fitness (meaning they successfully reproduce and pass their genes to their offspring) have a higher likelihood of persisting in a population than alleles carried by less-fit individuals, which are gradually lost from the population over time.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers, Senior Editor.