Q5. (a) A protein found on red blood cells, called antigen G, is coded for by a

Question

Q5. (a) A protein found on red blood cells, called antigen G, is coded for by a dominant allele of a gene found on the X chromosome. There is no corresponding gene on the Y chromosome. The members of one family were tested for the presence of antigen G in the blood. The antigen was found in the daughter, her father and her father's mother, as shown in the genetic diagram below. No other members had the antigen. Grandmother Grandfather Grandmother Grandfath (has antigen G) Genotypes or........... Gamete or........... genotype Father Mother (has antigen G) Genotypes Gamete genotypes Daughter (has antigen G) Genotype genetic diagram, using the symbol Xe to show the presence of the allele for antigen G on the X chromosome, and X for its absence. (i) Complete the rest of the diagram. (iii) The mother and father have a son. What is the probability of this son inheriting antigen G? Explain your answer. Probability

Explanation / Answer

Answer 5a) Taking the facts into consideration, it can be stated that the genotype responsible for inheriting antigen G is X-linked dominant inheritance, that is only one copy of a diseased allele on X- Chromosome is required to be susceptible to the inheritance. In the table below,

                              XGXG refers to homozygous Dominant in female

                               XGXg refers to heterozygous Dominant female

                               XgXg refers to absence of antigen in female

                                XGY refers to presence of antigen in male

                                XgY refers to absence of antigen in male

i) The gene for G Antigen is dominant in X chromosome (X-linked dominant inheritance), but whether the grandmother is homozygous dominant for the antigen or heterozygous dominant for the antigen is not mentioned. But in both cases, the antigen is expressed since it is a dominant trait in the gene. So, one of the grandmother (Father's mother) has two possible genotypes, one for homozygous dominance and the other for heterozygous dominance of the G Antigen, XGXG for homozygous dominance and XGXg for heterozygous dominance.

Since, it is also mentioned that no other members possess the antigen, so the genotypes of other members are Xg containing in all cases.

Answer ii) The complete table:

Grandmother

(Carries Antigen)

iii) The mother has genotype XgXg and the father has the genotype XgY.

Since the son will inherit the X chromosome from the mother and Y chromosome from the father so the genotype of the son will be XgY. Hence the probability of the son inheriting antigen G is 0 (nil).

iii) Bivalent is the association of a pair of homologous chromosomes held together by atleast one DNA crossover. X and Y chromosomes differ from one another in their morphology and genetic content. They are of different sizes, X chromosomes carry almost 1000 genes while Y chromosomes carry 78 genes.

X chromosomes are quite larger and hence the chromatids are unable to line up.

Also, most of the length of both of the chromosomes is not homologous owing to the huge difference in their genetic constitution (1000 genes of X as compared to 78 genes of Y). Only short pairing regions are present which is not sufficient for bivalent formation.

So, typical bivalent formation is not observed during the pairing of X and Y chromosomes.

Grandmother

(Carries Antigen)

Grandfather Grandmother Grandfather Genotype XGXG or, XGXg XgY XgXg XgY Gamete Genotype XGXG or, XGXg XgY XgXg XgY

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