Question

Three coat-color patterns that occur in some breeds of horses are termed cremello (beige), chestnut (brown), and palomino (golden with light mane and tail). If two palomino horses are mated, they produce about \(1 / 4\) cremello, \(1 / 4\) chestnut, and \(1 / 2\) palomino offspring. In contrast, cremello horses and chestnut horses breed true. (In other words, two cremello horses will produce only cremello offspring, and two chestnut horses will produce only chestnut offspring.) Explain this pattern of inheritance.

Short answer

The coat-color pattern of inheritance in these horse breeds can be explained by simple Mendelian genetics. It appears that the color genes are two allele - dominant (C) for chestnut color and recessive (c) for cremello color. Chestnut and Cremello horses are homozygous (CC and cc respectively), while palomino horses are heterozygous (Cc). Thus, when two palomino horses are mated, the following combinations of offspring are possible: CC (chestnut), Cc (palomino), cC (palomino), and cc (cremello). This explains the observed ratio of 1/4 chestnut, 1/2 palomino, and 1/4 cremello offspring.

Step-by-step solution

Identify Cremello and Chestnut As Homozygous

Given that cremello and chestnut horses breed true, meaning when mated together they only produce offspring of the same phenotype, they must be homozygous. This means that cremello horses have two 'cremello' alleles, and chestnut horses have two 'chestnut' alleles. Let's denote 'C' for the dominant allele (chestnut color) and 'c' for the recessive allele (cremello color). So cremello can be represented as 'cc' and chestnut as 'CC'.

Determine the Genetic Makeup of Palomino Horses

The fact that two palomino horses produce about 1/4 cremello, 1/4 chestnut, and 1/2 palomino offspring indicates that palomino horses should be heterozygous, having one 'C' allele and one 'c' allele each. Thus, they are 'Cc'.

Explain the Ratio of the Offspring

When two palomino horses (Cc) are mated, the following combinations of offspring are possible (using Punnett Square method): CC (chestnut), Cc (palomino), cC (palomino), and cc (cremello). This explains the ratio of 1/4 chestnut, 1/2 palomino, and 1/4 cremello offspring that is observed.