Not so easy-peasy genetics
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Not all genes follow Mendel’s rules.
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Incomplete dominance: when one allele can not completely mask the expression
over the other. Results in a blended phenotype
o In snap dragon flowers the domaint allele that produces a red color can
not completely dominant the recessive allele that produces a white color.
o In the lethal disease Tay-Sachs, homozygous dominant individuals are
able to break down a protein that slows brain damage, homozygous
recessive individuals are not able to break it down at all. Heterozygous
individuals are able to break down the protein at a 50% efficiency and
show no symptoms of the disease.
Red snap dragon x white snap dragon
o Parent genotypes:
R
R
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RW
RW
RW
RW
W
W
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Self cross a pink snap dragon
o Parent genotypes:
R
W
RR
RW
RW
WW
R
W
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Co dominance: when one allele isn’t dominant over another. Instead both
phenotypes are expressed.
 To keep from getting confused you write then genotype as
superscripts
o Roan cattle exhibit Co dominance. The color roan is produced by white
hairs and red hairs. To get roan cattle, you cross white cattle (CW CW)
with red cattle (CR CR)
o The tabby coloration in cats is produced by crossing a black cat (C BCB)
with a tan cat(CTCT) and results in a black and tan striped cat (CBCT)
Roan bull x white cow
P1 genotype: CrCw x CwCw
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Cross a two tabby cats
P1 genotype: CBCT x CBCT
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A locus is the place on a chromosome that an allele is found.
o The simple rules of dominance recessive, co domaince and incomplete
dominance only factor in genes at one loci. Some phenotypic expressions
are controlled by genes at more than one loci.
Epistasis is the masking of the phenotypic effect of alleles at one gene by alleles of
another gene. A gene is said to be epistatic when its presence suppresses the
effect of a gene at another locus. Epistatic genes are sometimes called inhibiting
genes because of their effect on other genes.
Mice color is an example of epistasis. It involves 2 coat-color loci.
o At loci A, black is dominant over albino.
o At locus B, agouti (bands of light and black, looks tabby or grizzled) is
dominant over black.
o A mouse that is homozygous for the albino gene will show no pigment
regardless of the genotype at the other locus.
 BbAA – agouti
bbAA- albino
Bbaa-black
o Remember!! it is actually proteins, not the genes, that interact. After
identifying interacting loci, the next step is determining the proteins that
the genes at those loci encode, and the properties of those proteins.
In sweet peas, purple flower color (P) is dominant over white (p), but there is
also a control gene such that if the plant has a “C”, the purple has “permission” to
express itself. If the plant is “cc,” the purple does not “have permission” to express
itself and the flower will be white anyway.
diagram the Punnett square for the F1 generations with their phenotypic
ratios for the following cross.
P1 generation: PPcc x ppCC
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