Question

How does a sex-linked gene behave differently in genetic crosses from a gene on an autosome? How were prokaryotic gene transfer and recombination discovered? What are the differences between recombination after conjugation in prokaryotes and recombination in meiosis in eukaryotes?

Answer 1

10). In sex-linked or X-linked diseases, the defective allele is present in X-chromosome. In this case both the parents are said to be normal. Means the father is not having the defective allele. X- linked or sex-linked diseases could be developed in females only when they inherit two X-linked recessive alleles, one from the mother and another from the father. Whereas in males, only one defective allele will result in expression of disease phenotype.

Y-linked: Traits are passed only between males through the Y-chromosome.

Eg:

Phenotypes in colorblindness:

X-linked inheritance:

Colorblindness (red-green) is inherited due to mutations on the X-chromosome; The genes responsible for the production of retinal photopigments predominantly carried on phenotypic X chromosome; mutation or missing of these sex linked genes, result in the males with higher probability of color blindness the reason behind this is that the “males possess only one X chromosome which is a characteristic recessive allele that carries a gene for colorblindness”.

Whereas females possess two X chromosomes in which we can notice a functional gene on only one of X chromosome that enables them to produce essential photopigments.

1.      Females –chromosomes 46, XX, color blind------> both X chromosomes are defective with a similar deficiency.

2.      Males chromosomes 46, XY are color blind -----> single X chromosome is defective

The following diagram represents the characteristic phenotypes with recessive allele that carries

Parent without the gene Carrier mother who does not express the trait XY X = other trait x = recessive trait Childrern Daughter who is a carrier but does not express the trait-XX Daughter and son who do not carry the gene nor express the trait-XXL Son who has the gene and expresses the trait or conditionxY IXY