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Polled Animals

Courtesy of the American Simmental Association

The polled trait depends on just one gene, expressed by the symbol “P”. The opposite condition, the presence of horns, is expressed as the “p” gene.

The polled gene (P) is dominant to the horned gene (p). When an animal inherits the dominant P gene from one parent, it is the dominant P that shows up in the individual’s appearance as the polled trait. The only time the recessive horn gene (p) can express itself is when the dominant P gene is not present.

There are three possible gene combinations involving the polled trait. They are PP, Pp, and pp. Half of each combination is inherited form each parent.

The PP individual is polled and said to be homozygous because it possesses two identical genes (“homo” means “the same”). It will have all polled offspring regardless of whether the other parent is horned or polled, because it has only the dominant P gene to pass on to its progeny. PP bulls are sometimes referred to as 100% dehorners.

The Pp individual is also polled, but is heterozygous (“hetero” means “not the same”). The Pp individual passes two different genes, so it won’t breed true for the polled trait. Fifty percent of the time, the Pp individual will pass on the horn gene, p, to its progeny.

The pp individual is horned, and is also homozygous because it has two identical genes. The pp individual will always pass on the p (horned) gene to its progeny, because that’s all it possesses.

Keep in mind that each parent passes one-half of its genetic makeup to its offspring.

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American Dexter Cattle Association

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