Beyond Dominant and Recessive Alleles

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
Beyond Mendel
• Despite the importance of Mendel’s work,
there are important exceptions to most of his
• 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 whiteflowered (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
• In codominance,
both alleles
contribute to
• 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 is when alleles share in
– 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

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