Question

1) If nondisjunction occurs during meiosis II, what type of gametes would be expected?

2) Sex determination in Ewoks is similar to humans - female Ewoks have two X chromosomes and males have one X and one Y chromosome. A gene on the X chromosome determines eye color, where the dominant allele X^B gives produces black eyes and the recessive allele X^b produces hazel eyes. Two black eyed parents have a hazel eyed son. If they have two additional children, what is the chance they will have a black eyed daughter and a black eyed son?  Write your answer as a fraction, such as 1/2, 1/4, 1/8, 3/16 or .

Answer 1

1. Non-disjunction = Improper separation of chromosomes between daughter cells during anaphase
If non-disjunction occurs during meiosis-II, the daughter cell would be
i. n+1
ii. n+1
iii. n
iv. n

i.e. only two of the daugher cells would be affected while the other two would be normal (Assuming nondisjunction occurred only in one cell out of 2 cells that produced by meiosis-I)

2. XB = Black
Xb = Hazel

Parental cross: XBXb X XBY
Progeny:
XBXB = Black female
XBY = Black male
XBXb = Black female (Heterozygous)
XbY = Hazel male
The frequency for each progeny type is 1/4 (Not assuming binomial distribution for subsequent progeny)