Genetics Review
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Test tomorrow…
List Mendel’s three proposed principles that are
recognized as the basis of inheritance.
• The Principle of Dominance
• The Principle of
Segregation
• The Principle of
Independent Assortment
Define: Homozygous
•Having identical alleles
or pairs of genes
•Ex: BB, bb, DD, dd
Define: Heterozygous
•One gene in the pair is
dominant, and the other
gene is recessive.
•Ex: Bb, Dd
Define: Zygote
•A fertilized ovum or egg
Define: Dominant
•Refers to genes that are
expressed whether an
allele is identical or
similar.
•Ex: D, B
Define: Recessive
• Refers to genes that are
only expressed when they
are paired with an identical
allele
• Ex: b, d
In swine, the gene for drooping ears (D) is dominant, while the gene for erect
ears (d) is recessive. If two swine are mated, one is heterozygous with
drooping ears and the other is homozygous with erect ears (Dd x dd), what
would be the expected offspring outcome?
d
d
D Dd Dd
d dd dd
Homozygous drooped ears= 0
Heterozygous drooped ears= 2
Homozygous erect ears=2
When two or more genes influence a
trait, an allele of one may have an
overriding effect on the phenotype,
such as for comb shapes in chickens.
This is known as:
•Epistasis
An exception to the principle of
dominance occurs when a
heterozygote exhibits traits found in
both associated homozygotes, such as
a roan coat color in horses. This is
known as:
•Codominance
When a pure, brown-eyed sheep is
crossed with a pure, green-eyed
sheep, blue-eyed off-spring are
produced. This is known as:
•Partial dominance
Sex-linked genes are carried on what?
•The X or Y
chromosome
Genetic improvement can take up
to how long to see an
improvement in a trait?
• It can take several
generations.
Is phenotype predetermined?
•NO!! Phenotype is not
predetermined by the
parents.
Can genotypic characteristics be
measured or observed?
•NO! Only phenotype
can be measured or
observed.