Meiosis, Predicting proportions

Question

Hi, I am stuck on this question about meiosis; I'm wondering if anyone could help me?

I was provided with two karyotypes, a male with a normal chromosome 18 load and a female with trisomy 18.

The question is:

Assuming successful reproduction of this male and female predict the proportion of progeny with monosomy 18. trisomy 18 and normal chromosome load.

I appreciate the help, thank you

Answer 1

I would approach this by creating a Punnett square. You could use any letter to represent the eighteenth chromosome, similar to using a letter to represent an allele on a chromosome.

I used A to represent chromosome 18. The female has AAA for that chromosome, while the male has AA. I think it is fair to assume the female could create either normal gametes (with A) or gametes with 2n+1 chromosomes (represented by AA).

T T

TT TTT TTT

T TT TT

By this logic, there is no reason to imagine the man and the woman would produce a child with monosomy 18 since the man has the normal amount of chromosome 18 and the woman has an extra copy. Based on a Punnett square crossing TTT with TT, it would appear the man and the woman have a 50% chance of having offspring with trisomy-18 and a 50% chance of having offspring with the normal chromosome load.