Genetics

The gene involved in eye color (white-eyed mutant) is located exclusively on the X chromosome, with no corresponding allele present on the Y chromosome. So punnett sqaures doesn't apply like you would in Mendelian genetics. For a male, a single copy ofthe mutant allele would confer white eyes; since a male has only one Xchromosome, there can be no wild-type allele (w+) present to offset the recessive allele. On the other hand, a female could have white eyes only if both her X chromosomes carried the recessive mutant allele (w).

In drosophila, males only have one X chromosome as a sex chromosome, no Y chromosome is present. Females on the other hand have 2 X chromosomes.

The male produces two types of gametes, one with the sex chromosome, and the other without it. All gametes produced by females contain a sex chromosome.

So if a gamete containing a sex chromosome from a male participated in fertilization of a female gamete, the offspring would have 2 sex chromosomes and therefore will be a female. But if a gamete having no sex chromosome from a male fertilized a female gamete, the offspring would have only 1 sex chromosome and so would be a male.

Eye color is sex linked, meaning that the gene coding for eye color is located on the X chromosome.
Red is dominant over white, females can be homozygous or heterozygous, but males can only be hemizygous (due to the presence of only one sex chromosome).

Now concerning the problem.

W: allele coding for red eye color which is dominant.
w: allele coding for white eye color which is recessive.
O: designates the absence of a sex chromosome.

The male has red eyes. It doesn't really matter what his father had, because the sex chromosome of this male was that of his mother.
The genotype of this male would be : WO.
Gametes produced by this male: W or no sex chromosome in the gamete.

The female has red eyes. She has one X chromosome from her father (who definitely gave her the "w" allele), and the other X chromosome from her mother who must have given her then the "W" allele.
The genotype of this female would be: Ww.
Gametes produced by this female: W or w.


         Male                   W                     O
         Female

              W                  WW                 WO
        
               w                  Ww                  wO

Each genotype has a 1/4 chance of being produced.
Probability of phenotypes: 1/2 red-eyed females, 1/4 white-eyed males, 1/4 red-eyed males, 0 white-eyed females.

          Xw+    Xw
Xw+ Xw+Xw+ Xw+Xw
Y     Xw+Y     Xw Y

a.) red-eyed males?

25% because Xw+Y

b.) red-eyed females?

50% because Xw+Xw+ Xw+Xw

c.) white-eyed males?

25% because Xw Y

d.) white-eyed females?

0 because you would need: XwXw

thx karim...

np 

Sounds like both parents are heterozygous, just make the cross and determine your genotypes and phenotypes.