How can you identify if a person is homozygous for a certain allele?

How can you identify if a person is homozygous for a certain allele?

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I've been thinking about starting a small private research project. In this project I need to find out whether a person is homozygous for a certain allele. The reason for this is that I'm really trying to find out whether has a mutation that makes him/her not produce this chemokine. Thus I need to identify in an easy way which people are "normal", which people have the mutation in one chromosome, and which people have the mutation in both. About 1% of the population have the mutation in both chromosomes.

A primer would AFAIK not be able to differ between the heterozygous and the "normal" people.

Do anyone know how to approach this?

You could use a 3-primers strategy, where you have a common primer, one that only amplifies the mutant allele and one that only amplifies the wt. This may or may not be feasable depending on the type of mutation. DNA from het people will amplify both product, while homozygous will only amplify one (wt or mutant).

Of course then, linking the mutation of a gene to the lack of a chemokine is not necessarily straightforward.

To be honest, however, I would use a different approach: take people who don't produce the chemokine, and people who do (simply by dosing the chemokine in the blood). Now sequence the gene in the two groups and look for mutations.

What is Homozygous? (with pictures)

In genetics, homozygous is a term which is used to refer to an organism which has inherited two identical copies of a gene. For example, if a plant is homozygous for red flowers, it means that it has inherited two copies of the gene which tell it to produce red flowers. For people who breed animals or who raise plants, being able to recognize a homozygous animal is important, as it will allow breeders to bring out desirable traits with careful breeding.

Organisms become homozygous for a trait when they are diploid, meaning that they inherit two sets of chromosomes, one from each parent. Organisms which reproduce sexually, such as humans, are diploid. When the two sets of chromosomes come together, each chromosome in each set has a complementary chromosome in the other set. A pair of chromosomes are said to be homologous, and each homologous pair contains corresponding genetic information, although the content of each chromosome is different because it comes from a different parent.

Each genetic trait can be located at a particular site or locus on a chromosome, and in a pair of homologous chromosomes, there will be two copies of the alleles which code for a particular trait. In homozygous animals, these alleles are identical. Heterozygous animals have two different alleles, in which case only one of the alleles will be expressed. Hemizygous animals have inherited only one copy of an allele, with genetic information missing on the corresponding chromosome.

Genetic inheritance is complicated, and it is rarely as simple as two alleles in a pair of homologous chromosomes, although beginning genetics students are usually introduced to the concept this way. When an animal is homozygous for a trait, the trait can be expressed in different ways as a result of other alleles which have an influence on development.

When an animal is homologous recessive, it means that it has inherited two copies of a recessive gene. People with blue eyes are an example of a homologous recessive. A person with blue eyes will pass the gene down, but his or her child will not necessarily have blue eyes unless the child inherits the blue-eyed gene from the other parent as well. On the other hand, someone who is homozygous dominant for a trait such as brown eyes will have brown-eyed children because only one copy of a dominant trait is needed for the trait to be expressed.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a InfoBloom researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Understanding Pedigrees: Grade 9 Genetics for IGCSE Biology 3.24

Sometimes genetics problems are based around a pedigree diagram. These diagrams show the phenotypes of individuals over several generations and allow deductions to be made about certain individuals phenotypes. Often pedigrees are used to show the inheritance of a particular disease in a family.

You can see that circles in the pedigree represent females, squares represent males. If the symbol is filled in, then the person suffers from the disease. Empty symbols represent people who do not have the disease.

Have a look at the pedigree above? What does this tell you about the disease?

Well the first and most obvious thing is that this disease is caused by a recessive allele, h.

If you see two people who don’t have the disease producing one or more children who do, then this must be a genetic disease caused by a recessive allele. In the top generation, parents 1 and 2 do not have the disease, but they have three children 2,3,4 one of whom has the disease.

What does this tell us about the genotype of parents 1 and 2 in generation I? Well if neither have the disease and they have a child who does, both 1 and 2 in the top generation must be heterozygous – Hh

Anyone with the disease must be homozygous recessive hh.

Have a look at generation II in the diagram above?

The man, number 2, who is a sufferer and so genotype hh marries woman 1 who does not have the disease. They produce 4 children, three with the disease and one without. What must the genotype of the woman 1 be? Well she must be heterozygous Hh. How do we know? What children would she produce if she were a homozygous HH woman?

A pedigree caused by a dominant allele would look very different. Every sufferer would have at least one parent who also suffers from the disease. Two sufferers producing some children who do not have the disease is indicative of a disease caused by a dominant allele. If we use the symbol P for the dominant allele that causes the disease, and p for the recessive allele that is “normal”, can you work out the genotypes of all 12 people on the diagram above?

The Punnett Square Approach for a Monohybrid Cross

When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids. Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F1 and F2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely.

To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively. A Punnett square , devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining. Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds ([Figure 1]).

Figure 1: In the P generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype. This cross produces F1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F2 generation.

A self-cross of one of the Yy heterozygous offspring can be represented in a 2 × 2 Punnett square because each parent can donate one of two different alleles. Therefore, the offspring can potentially have one of four allele combinations: YY, Yy, yY, or yy ([Figure 1]). Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm. Both of these possibilities must be counted. Recall that Mendel’s pea-plant characteristics behaved in the same way in reciprocal crosses. Therefore, the two possible heterozygous combinations produce offspring that are genotypically and phenotypically identical despite their dominant and recessive alleles deriving from different parents. They are grouped together. Because fertilization is a random event, we expect each combination to be equally likely and for the offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1 ([Figure 1]). Furthermore, because the YY and Yy offspring have yellow seeds and are phenotypically identical, applying the sum rule of probability, we expect the offspring to exhibit a phenotypic ratio of 3 yellow:1 green. Indeed, working with large sample sizes, Mendel observed approximately this ratio in every F2 generation resulting from crosses for individual traits.

Mendel validated these results by performing an F3 cross in which he self-crossed the dominant- and recessive-expressing F2 plants. When he self-crossed the plants expressing green seeds, all of the offspring had green seeds, confirming that all green seeds had homozygous genotypes of yy. When he self-crossed the F2 plants expressing yellow seeds, he found that one-third of the plants bred true, and two-thirds of the plants segregated at a 3:1 ratio of yellow:green seeds. In this case, the true-breeding plants had homozygous (YY) genotypes, whereas the segregating plants corresponded to the heterozygous (Yy) genotype. When these plants self-fertilized, the outcome was just like the F1 self-fertilizing cross.

Law of Segregation

Observing that true-breeding pea plants with contrasting traits gave rise to F1 generations that all expressed the dominant trait and F2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation. This law states that paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F2 generation of a monohybrid cross, the following three possible combinations of genotypes result: homozygous dominant, heterozygous, or homozygous recessive. Because heterozygotes could arise from two different pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and homozygous dominant individuals are phenotypically identical, the law supports Mendel’s observed 3:1 phenotypic ratio. The equal segregation of alleles is the reason we can apply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel’s law of segregation is the first division of meiosis in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei. This process was not understood by the scientific community during Mendel’s lifetime (Figure 8.7).

Figure 8.7 The first division in meiosis is shown.

Homozygous genotype contains the same type of genes responsible for a particular phenotype whereas heterozygous genotype contains one dominant gene with one recessive gene in the diploid genetic setup. So, this is the key difference between homozygous and heterozygous. Furthermore, there are two types of homozygous genotypes as dominant homozygous and recessive homozygous. On the other hand, heterozygous genotype has only one type. Hence, this is also a difference between homozygous and heterozygous. In homozygous genotypes, there are two types of phenotypes expressed while only one type is expressed in heterozygous genotypes.

The following infographic presents more information on the difference between homozygous and heterozygous.

Can someone clear up this confusion with alleles, genes, and proteins?

So I understand that alleles are essentially different versions of a gene. What I want to know is how are those differences in the genes shown in the code, for example, if they have a different nucleotide sequence or something, and are the alleles of the same gene in separate locations of the DNA. Also, are both alleles expressed and if so what causes one protein to be dominant over the others to cause the phenotype to show.

Alleles are variations of a gene. A gene is a sequence of DNA that codes for a protein. Variations in genes are due to different nucleotide sequence for that gene. You inherit two copies of a gene, one from dad and one from mom, and those copies are at the same location on homologous chromosomes. For example, if you have a gene on your chromosome 3 from dad, you find that gene at the same location on your chromosome 3 from mom (they just might have different nucleotide sequences). Both alleles get expressed. There are many factors that make a certain allele dominant or recessive, such as how active the protein is or if the protein is functional or not.

Law of Independent Segregation

Mendelian genetics is based on three laws that dictate how certain traits are transferred from parents to offspring. These three laws are: the Law of Dominance, Law of Independent Segregation, and Law of Independent Assortment. These three laws were proposed by Mendel in 1865 in his paper &lsquoExperiments on Plant Hybridization&rsquo, which he submitted to the National Science Society in Brno (now in the Czech Republic). In this article, we&rsquore going to focus on the Law of Independent Segregation.

What is a homozygous dominant genotype?

In a monohybrid (single trait) cross, there are three possible genotypes.

The Genotype is the possible pair of traits from the parents represented by letters called alleles

The Phenotype is the possible trait displayed by the genotype.

Using the Alleles
T = Tall
t = short

The three possible genotypes (pairs of alleles) are:

TT = Homozygous Dominant for Tall

Tt = Heterozygous Dominant for Tall

tt= Homozygous Recessive for Short

Homo = same zygous refers to zygote

Hetero = different zygous refers to zygote

A homozygous dominant genotype is one in which there are two dominant alleles.


In Mendelian genetics, a trait is governed by one gene with two possible alleles one dominant and one recessive. Genotypes are usually represented as letters, with a capital letter representing the dominant allele and the lowercase letter representing the recessive allele.

A homozygous genotype is one in which both alleles are the same, and an organism with a homozygous genotype is said to be true-breeding or purebred. A homozygous dominant genotype is one in which both alleles are dominant.

For example, in pea plants, height is governed by a single gene with two alleles, in which the tall allele #("T")# is dominant and the short allele #("t")# is recessive. A true-breeding tall pea plant would have the genotype with two dominant alleles, usually represented as #"TT"# .