Sex-Linked Genetics

Sex-linked traits are traits that occur from the passage of genes on the sex chromosomes. A sex-linked trait’s allele is typically, unless otherwise noted, found on the X-chromosome.

Example sex-linked cross:

Notice the female gives each son an

X-chromosome, not the male. The male

gives his Y-chromosome. So having a

father with the disease does not

harm the boys. Carriers means they

are heterozygous.

When learning about heredity and the passage of genetic information between generations (either autosomal or sex-linked), an individual who is heterozygous for a trait (has a recessive allele but does not show the trait) is called a CARRIER.

Perform the crosses below:

1. XAXA x XaY

Ratio of males to females ______

Proportion of recessive men ______

Proportion of recessive females ______

2. XtXt x XTY

Proportion of recessive men ______

Proportion of carrier females ______

3. Since most sex-linked traits or diseases are recessive, explain which sex (male or female) is more likely to show the disease phenotype.

4. In a cross between a white-eyed female fruit fly and red-eyed male, what percent of the female offspring will have white eyes? (White eyes are X-linked, recessive)

% white-eyed females ______

5. Hemophilia in humans is due to an X-chromosome mutation. What will be the results of mating between a normal (non-carrier) female and a hemophiliac male?

6. A human female carrierfor the recessive, sex-linked trait causing red-green color blindness marries a normal male.

What proportion of their male children will have red-green color blindness? ______

What proportion of their femalechildren will have red-green color blindness?______

7. If ½ of the male offspring from a cross are colorblind, what were the genotypes of the 2 parents? (i.e. what was the cross?)

mother genotype _____

father genotype _____

8. Explain why, when referring to a sex-linked trait/disorder, men are never called carriers(see intro if you don’t know what a carrier is).

9. Women have sex chromosomes XX, and men have sex chromosomes XY.

Which of a man's grandparents could not be the source of any of the genes on his Y-chromosome? Be sure to show your work.

10. Women have sex chromosomes of XX, and men have sex chromosomes of XY.

Which of a women's grandparents could not be the source of any of the genes on either of her X-chromosomes? Be sure to show your work.

11. Explain why a diseased male (sex-linked disorder like hemophilia) will never pass the disease to his son.

12. It is commonly thought that the best way for a male to check if they will be bald in the future is to look at their mother’s father. Explain why this might be true, considering baldness is a sex-linked disorder.