While working as a medical geneticist, you encounter an unusual patient: a 47,XX

Question

While working as a medical geneticist, you encounter an unusual patient: a 47,XXY girl. You prepare DNA samples from the girl and from her parents. You confirm that the stated father is in fact the biological father by testing the family for a large number of autosomal SSRs. You also test the family for a series of SSRs distributed along the X chromosome. A diagram of the X chromosome is shown below, with the centromere indicated by an oval.

A. During the development of which parent’s gametes did nondisjunction occur? (Mom or Dad)

B. In which division of meiosis did nondisjunction occur?

C. Draw the following steps in the meiosis that created the gamete that led to the production of the XXY child shown in the pedigree. Please label each SSR allele and the centromere on each homolog of the X chromosome. Assume that SSR alleles 1A, 2B, 3B, and 4C are on a single chromosome in the mother’s somatic cells. Draw these steps only:

i.the cell in metaphase I with its chromosomes lined up showing any crossover events occurring

ii.the two cells in metaphase II with their chromosomes lined up

iii.the four final products of the meiosis (Please indicate the gamete that led to the creation of the XXY child with a star.)

D. What might account for this girl having developed as a female despite the presence of a Y chromosome? Explain how you would test your hypothesis.

E. How would you account for the presence in the XXY girl of a paternal allele for SSR1?

F. Later you have the opportunity to study a boy with one X chromosome and two Y chromosomes. You realize that you do not even need to use SSRs or other genetic markers to figure out the meiotic division in which nondisjunction occurred. In which arent and at which meiotic division did nondisjunction occur?

Explanation / Answer

A. During the development of which parent’s gametes did nondisjunction occur? (Mom or Dad)

Nondisjunction occurred in the mother.

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B. In which division of meiosis did nondisjunction occur?

Since the daughter is heterozygous at the SSRs that are closely linked to the centromere,

this indicateds taht --nondisjunction occurred in meiosis I.

===================================================================

C. Draw the following steps in the meiosis that created the gamete that led to the production of the XXY child shown in the pedigree. Please label each SSR allele and the centromere on each homolog of the X chromosome. Assume that SSR alleles 1A, 2B, 3B, and 4C are on a single chromosome in the mother’s somatic cells. Draw these steps only:

A---- A---- A ----------------------B

A----- A--*- A ---------------------B

                                   x

C----- B----- B---------------- C

C ------B ---*--B--------------- C

====================================

Meiosis I nondisjunction-----à

First polar body No X chromosome

And

A----- A--*- A ---------------------B

                                   x

C----- B----- B---------------- C

C ------B ---*--B--------------- C

Meiosis II

Gives

Second polar body

And

Egg

A A A C

C B B C

i.the cell in metaphase I with its chromosomes lined up showing any crossover events occurring

ii.the two cells in metaphase II with their chromosomes lined up

iii.the four final products of the meiosis (Please indicate the gamete that led to the creation of the XXY child with a star.)

===========================================

D. What might account for this girl having developed as a female despite the presence of a Y chromosome? Explain how you would test your hypothesis.

If the daughter got a Y chromosome from her father that lacked SRY, then she could develop as a female.

==========================================================================

E. How would you account for the presence in the XXY girl of a paternal allele for SSR1?

The presence of SSR1 from the father suggests that there might have been crossing over between the X and the Y chromosomes in the father. Consequently, the Y lost SRY and gained the SSR1 from the X chromosom

=========================================================================

F. Later you have the opportunity to study a boy with one X chromosome and two Y chromosomes. You realize that you do not even need to use SSRs or other genetic markers to figure out the meiotic division in which nondisjunction occurred. In which arent and at which meiotic division did nondisjunction occur?

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