In Drosophila melanogaster, the recessive allele for the sepia locus (se) causes flies to have very dark colored eyes. The recessive allele at the apterous (ap) locus causes the fly to lack wings.

A male from a true breeding line of sepia eyed, apterous flies (line A) is crossed to a female from a true breeding line of red eyed, winged flies (the "wild type" line B) to create an Fl:

Next, a female from the Fl is crossed to a male from line A.

The phenotypes of 1000 progeny from this test cross were scored as follows:

Eyes Body Number Observed

Red Normal 247

Sepia Normal 253

Red Apterous 251

Sepia Apterous 249 5.

What was the genotype of the F1?

A) SE se EB eb

B) se AP

c) se ap

D) SE se AP ap

E) Can't tell because of linkage

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.