Question

You have inherited a racehorse and decide to put him out to stud. In looking at the studbook, however, you discover that the horse's grandfather exhibited a rare disorder.The disorder if hereditary and results from homozygosity for a recessive allele. If your horse is heterozygous for the allele, it will not be possible to use him because the genetic defect may be passed on to it's offspring. You happen to have a neighbor that has 3 horses. 1 horse is homozygous dominant, another is heterozygous, and the last is homozygous recessive.How would you determine whether your horse carries the allele

Answer 1

You would perform what is known as a "test cross", this means you would cross it with the homozygous recessive horse.

From this cross you'd expect 1/2 to show the dominant phenotype and 1/2 to show the recessive phenotype. This would be impractical though because horses typically give birth to one calf at a time, so you'd have to do this over a few years. Even so, you're not guaranteed that you will see the recessive phenotype show up because there is 1/2 chance of getting either every time.

The only other scenario you could get would be if all offspring show the dominant phenotype, then your horse is homozygous dominant. Your horse could not be homozygous recessive because it would show that phenotype.

Answer 2

Thank you so much. I have a hard time understanding the difference between homozygous recessive and homozygous dominant

Answer 3

no problem!

um let me see if i can clear that up for you:

homo = same (and hetero =different) (these are from latin)
zygous = alleles (a certain type of gene)

dominant overpowers recessive, this is in terms of proteins (because genes code for proteins). You can think of it in terms of efficiency, the protein from the dominant allele is better at it's job, so you wont see the effect of the recessive (weak/inefficient) protein.