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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
genes on a chromosome
of
 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?