This means for every given pair of chromosome, all the genes are same and their location is also same. That is why we call them homologous chromosomes. This means we have two copies of every chromosome and thus we have two copies of every gene figure 2.
In one of our previous post we had seen that copies of the same gene are called alleles. In short, alleles are different form of the same gene which might differ slightly in sequence giving us slightly different phenotype such as gene for eye pigmentation can give brown, green, blue etc. Figure 2. Chromosome pair Homozygous for eye and hair color trait.
Figure 2 shows gene for eye color. If both the chromosome 1, maternal as well as paternal codes for the same color eye pigment blue that means they have the same allele.
In this case they are called homozygous for eye color trait. It is also possible that a given chromosome pair has two different alleles for the same gene, right! In that case they are called heterozygous for a given trait.
Remember, we use the term homozygous or heterozygous for individual gene. We cannot use it for the complete chromosome because a chromosome would have many different genes present and for each of them they might be either homozygous or heterozygous. Whereas homologous is the term we use with reference to chromosome in a pair.
To understand more details, watch a video on this topic here. For example, you could possess two identical or similar alleles for a particular gene that causes hazel brown eyes. Some alleles are dominant that is expressed more strongly while some alleles are recessive which get masked by the dominant alleles. However, in a homozygous genotype, the interaction between a dominant and a recessive allele does not occur as the individual holds either two dominant alleles known as homozygous dominant RR ot two recessive alleles known as homozygous recessive rr.
We know that each gene inherits two alleles and the only difference is the versions of the alleles. This results in two different alleles present on the homologous chromosomes. An individual who possesses a Heterozygous genotype will have two versions of alleles Rr.
For example, in terms of hair colour, a Heterozygous individual can possess one allele for brown hair and one allele for red hair. The relationship between the two alleles affects the traits expressed in an individual which also determines the characteristics which a carrier possesses. If a diploid organism carries two same or identical copies of the same allele of each gene, where the alleles may be coding for a particular trait, and this is called the homozygous genotype.
If a diploid organism carries two different copies of alleles of each gene, either dominant or regressive, where the alleles may be coding for a particular trait, and this is called the heterozygous genotype. Homozygous genotype carries a similar but, only one type of allele- either dominant or recessive for a particular trait. Example- either RR or rr. Heterozygous genotype carries different or dissimilar alleles for a specific trait.
It comprises both dominant and recessive alleles. Example- Rr. A Homozygous individual can either be homozygous dominant or homozygous recessive. The Heterozygous alleles in an individual can show either complete dominance and co-dominance or incomplete dominance behaviour. Self-fertilisation in a homologous allele results in the repetition of the identical or similar traits over generations.
Self-fertilisation in a heterozygous allele results in different combination of traits over the next generations. For a homozygous allele, only one type of gamete is produced.
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