Mutation, Incomplete Dominance, Sex Determination

Partial (Incomplete) Dominance Partial, or incomplete, dominance occurs when the heterozygous organism exhibits a trait in-between the dominant trait and the recessive trait

Partially dominant alleles for color are seen in several species of flowers and in mice. Ex. Homozygous mice are black (BB) or white (bb) and the heterozygous mice will be grey (Bb).

Sheep exhibit incomplete dominance in the trait for eye color. When a pure, brown-eyed sheep is crossed with a pure, green-eyed sheep, blue-eyed offspring are produced.

Codominance Codominance occurs when a heterozygote offspring exhibits traits found in both associated homozygous individuals. An example of codominance is the feather color of chickens

If a homozygous black rooster is mated to a homozygous white hen, the heterozygous offspring would have both black feathers and white feathers.

Roan is a coat color in horses (sometimes dogs and cattle) that is a mixture of base coat colored hairs (ex. black, chestnut) and white hairs. Neither the base coat color or the white hairs are dominant nor do they blend to create an intermediate color.

Roan

Under these circumstances, neither allele is dominant and neither is recessive. Therefore, each allele is denoted by a capital letter (RW)

If the genotype is WW the animal will be white If the genotype is RR the animal will be red If the genotype is RW the animal will be a roan

Overdominance Exists when the heterozygous animal is phenotypically better than either of the homozygous parents Posses greater vigor, are more productive, and more fertile Usually effect performance traits, not structural traits Also called heterosis or hybrid vigor

Mutations Genes have the capability of duplicating themselves, but sometimes a mistake is made in the duplication process resulting in a mutation.

The new gene created by this mutation will cause a change in the code sent by the gene to the protein formation process. Some mutations cause defects in animals, while others may be beneficial.

Mutations are responsible for variations in coat color, size, shape, behavior, and other traits in several species of animals. The beneficial mutations are helpful to breeders trying to improve domestic animals

Changes that can occur in chromosomes during meiosis include: Nondisjunction – chromosome number, Translocation or deletion – chromosome breakage, and Inversion – the rearrangement of genes on a chromosome

Changes in chromosomes are reflected in the phenotypes of animals. Some chromosomal changes will result in abnormalities, while others are lethal and result in the death of an animal shortly after fertilization, during prenatal development, or even after birth

An example of a trait that is believed to have been caused by mutation is the polled Hereford A cross between horned Herefords resulted in a polled Hereford. But the polled gene is dominant. The only way to explain this is mutation

Sex Linked Traits Sex-linked traits involve genes that are carried only on the X or Y chromosomes, which are involved in determining the sex of animals. The female genotype is XX, while the male genotype is XY.

The X chromosome is larger and longer than the Y chromosome, which means a portion of the X chromosome does not pair with genes on the Y chromosome.

Additionally, a certain portion of the Y chromosome does not link with the X chromosome. The traits on this portion of the Y chromosome are transmitted only from fathers to sons. Sex-linked traits are often recessive and are covered up in the female mammal by dominant genes. Called Sex-Influenced genes.

Answer these questions Explain what is meant by Codominance. Give an example. What is mutation? Give an example. What are sex-linked traits? When some traits are expressed as dominant in one sex but recessive in the other, what is this called? What are the changes that can occur in chromosomes during meiosis?