In a Belgian horses, coat coloration can either be chesnut brown or roan (brown with white mixed in). The coloration pattern in these horses is governed by a single gene. The roan allele is completely dominant, so heterozygous horses are roan-colored. When you mate several sets of heterozygous roan horses over a number of years, you get 99 roan-colored offspring and 53 chesnut-brown offspring. You begin to suspect that one of the alleles for the coloration gene may be lethal. Why does this data seem to indicate that a lethal allele is involved? Which allele appears to be lethal when two copies are inherited?

Question

In a Belgian horses, coat coloration can either be chesnut brown or roan (brown with white mixed in).  The coloration pattern in these horses is governed by a single gene.  The roan allele is completely dominant, so heterozygous horses are roan-colored.  When you mate several sets of heterozygous roan horses over a number of years, you get 99 roan-colored offspring and 53 chesnut-brown offspring.  You begin to suspect that one of the alleles for the coloration gene may be lethal.  Why does this data seem to indicate that a lethal allele is involved?  Which allele appears to be lethal when two copies are inherited?

blues · Answer

The total number of horses is 99 + 53 = 152. Of these 34.8% are chestnut and 65.1% are roan. You note that these numbers are ~ 1/3 and ~ 2/3 respectively.

Whenever you see ratios in thirds, you should suspect that either one or the other homozygous state is lethal. In the case when the recessive homozygous state is lethal and the other allele is completely dominant, you see only the dominant phenotype. In this case, you do see some of the recessive phenotype meaning that the dominant homozygous state is lethal.

Clear?

You can (and should) do a Punnett square to illustrate this for yourself.