Is it possible for a sex chromosome to be homologous?
How can a male fly be pure-breeding?


A fly or any organism can only be pure-breeding for a certain trait, not all traits at once. If the trait-determinant allele is found on an autosomal chromosome, this is not a problem. If it's on a sex-chromosome (chr1 in flies), it is sex-linked, and therefore cannot be pure-breeding because it may depend on the sex of the offspring whether the trait is expressed. For instance, you may have a male carrying an allele on its X chromosome; its female offspring will inherit the X, but male offspring will not (those inherit the Y chromosome), i.e. it is not pure-breeding. Male flies cannot be pure-breeding for traits determined by loci on their sex chromosomes. Similarly, you may have a heterozygous female carrying an allele on one of its X chromosomes (A/a); its female offspring will only inherit one allele, either A or a, so it is not pure-breeding either. A female fly must be homozygous for an allele on its sex chromosomes to be pure-breeding for that trait.

Male flies can be pure-breeds for traits determined by alleles on autosomes.

Also, cool and useful funfact, Drosophila sp. male flies do not experience homologous recombination during meiosis. This is called achiasmy! This is useful to know when designing complicated crosses when there is a difficulty in balancing a genotype.


  Does it make sense to speak of homologous sex chromosomes?


The X chromosomes in females (X + X) are homologous, but the male Y chromosome is not homologous to the X. They are vastly different, just like the X and Y sex chromosomes in humans.


  Pure-breeding implies a homozygote gene


Correct. Pure-breeds are allele homozygous so that all offspring, regardless of recombination, inherit the allele.