Xa is an allele of an X-linked gene that denotes the presence of an antigen on r

ID: 188369 • Letter: X

Question

Xa is an allele of an X-linked gene that denotes the presence of an antigen

on red blood cells. It is dominant over an allele for the absence of the

antigen (X-). The inheritance of these alleles was studied in children with

chromosome abnormalities to determine where nondisjunction of sex

chromosomes occurred. For each mating below, indicate whether

nondisjunction took place in the mother or father and, if possible, whether

it occurred in meiosis I or meiosis II.

A) X-X- x XaY -> Xa (Turner syndrome)

B) XaX- x XaY -> X- (Turner syndrome)

C) X-X- x XaY -> XaX-Y (Klinefelter syndrome)

D) XaX- x XaY -> X-X-Y (Klinefelter syndrome)

2) What is the probability of obtaining progeny with the genotype aaBBCcXmY

from the following cross: AaBbCcX+Xm x AaBbCcX+Y, where a, b, and c

represent autosomal genes and X+and Xm represent X-linked alleles in an

organism with XX-XY sex determination?

Answer :

1. Non disjunction takes place in the father for answer choices B and C.

2. For gene a : Aa * Aa

For gene b : Bb * BB

For gene c : Cc * cc

For the sex linked gene r: X+Xr * X+Y

aa Bb Cc X+X+------>1/4*1/2*1/2*1/4= 1/64

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