Warm Up

• In pea plants round seeds (R) are dominant to wrinkled seeds (r) and yellow seed color (Y) is dominant to green seed color (y).

If a plant heterozygous for both is crossed with a plant that has wrinkled, green seeds, what is the phenotypic ratio of their offspring?

Beyond Dominant and Recessive Alleles

Beyond Mendel

• Despite the importance of Mendel’s work, there are important exceptions to most of his principles.

• Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance

• When one crosses two four o’clock plants (Miarabilis), the F1 generation cross between a red-flowered (RR) plant and a white-flowered (WW) plant, consists of pink-flowerd (RW) plants.

Incomplete Dominance

• Which allele is dominant?• Neither. Cases in which

one allele is not dominant over another are called incomplete dominance.

• The heterozygous phenotype is in between the two homozygous phenotypes.

Codominance• In codominance, both

alleles contribute to phenotype.

• Example: human bloodtypes have two alleles. One for “A” and one “B”

• People with Type A blood have two alleles for “A” or one “A” and one i.

Codominance• Codominance is when alleles share in

dominance.– A calico cat is part white cat and part colored cat.

• In shorthorn cattle, when a red bull (RR) is crossed with a white cow (WW), all the offspring are roan—a spotted, red and white or milky red color.

Multiple Alleles

• Many genes have more than two alleles are said to have multiple alleles.

• A common example is coat color in rabbits.• Their color is determined by a gene that has at

least four different alleles.• Human blood type is also multiple allelic,

meaning that there are three possible alleles, A, B, and i (ii causes O type blood)

Multiple Alleles

Polygenic Traits• Many traits are produced by the interaction of

several genes.• Traits controlled by two or more genes are

said to be polygenic traits.• Skin color in humans is caused by multiple

genes that code for melanin in the skin.• Many genetic disorders are polygenic such as

autism, diabetes, and cancer.

T.H. Morgan

• Lexingtonian Thomas Hunt Morgan worked on the fruit fly, Drosophila melanogaster.

• Morgan showed that Mendel’s principles applied to animals and not just pea plants.

• He was the first Kentuckian (and only… for now) to be awarded the Nobel Prize in Medicine.

• He was awarded this for determining that some traits were sex-linked and found on sex chromosomes. We’ll discuss this more later