Non-Mendelian Genetics
• i.e. exceptions to Mendel’s Rules
Recall Incomplete
Dominance
•pattern of gene
expression in which the
phenotype of a
heterozygous individual is
intermediate between
those of the parents.
Codominance
• the condition in which both alleles in a
heterozygous organism are expressed.
•
Ex. Roan horses or cows
Multiple Alleles
• genes with three or more
alleles
• Ex. Blood types- there is
an A, B, and O allele (IA,
IB, i)
• (More on this when we
discuss blood type
genetics)
Polygenic Trait• when several genes influence one trait.
• Ex. Eye color, height, hair, and skin color
Height is a
polygenic trait
Skin color is a polygenic trait. The greater the total number of
dominant alleles, the darker skin color an individual will have.
For this example, assume 3 genes control this trait as shown in
the chart:
6 dominant (ex. AABBCC) – extremely dark skin
5 dominant (ex. AABBCc)– very dark skin
4 dominant (ex. AaBBCc)- dark skin
3 dominant (ex. AaBbCc) - olive skin
2 dominant (ex. aaBbCc) - light skin
1 dominant (ex. aaBbcc)- very light skin
0 dominant (ex. aabbcc) – extremely pale skin
If a father with the genotype AABbcc has a child with a mother
whose genotype is aabbCc, what are the chances their child will
have the genotype AaBbCc? What would the phenotype of this
child be?
Pleiotropy
• one gene
that affects
more than
one
seemingly
unrelated
phenotypes
Anemia,
infections,
weakness,
impaired
growth, liver
and spleen
failure,
death.
Traits (phenotypes)
associated with the
sickle cell allele.
Epistasis
• one gene masks the
expression of a different
gene for a different trait
• Ex. The gene for albinism
masks the effects of genes
for skin, hair, and eye color.
Influence of Environment
• phenotype depends on conditions in the
environment.
• Ex. Arctic fox, Siamese cats, height, and skin
color
Temperature sensitive –
The cold extremities (ears,
nose, tail, and feet) express
pigmentation while the warm
body does not)
Practice Problems
INCOMPLETE DOMINANCE:
• Yellow coat color in guinea pigs is
produced by the homozygous
genotype YY
• Cream color by the heterozygous
genotype Yy
• White by the homozygous genotype
yy.
What genotypic and phenotypic ratios
are matings between cream-colored
individuals likely to produce?
1:2:1
yellow: cream: white
CO-DOMINANCE:
• In cattle, reddish coat color is
not completely dominant to
white coat color. Heterozygous
individuals have coats that are
roan colored (ie. reddish, but
with spots of white hairs).
• What would be the results of
the following crosses:
a. red x roan:
b. white x roan:
c. red x white:
d. roan x roan:
Genotype
1:1 RR:Rr
1:1 Rr:rr
All Rr
1:2:1 RR:Rr:rr
phenotype
1:1 Red: Roan
1:1 Roan:White
all Roan
1:2:1 Red:Roan:White
• Outline a breeding procedure whereby a pure
breeding strain of red cattle could be
established from a roan bull and a white cow:
– First cross a roan bull with a white cow; next cross
two roan progeny and 25% should be red.
Height in humans is polygenic. Each upper case letter adds 3”.
Males (aabbcc) are 5’ and females (aabbcc) are 4’ 7”.
How tall would a male with the genotype AaBbCc be?
5’9”
A female?
5’4”
Probability that mating with this male & female result in a child
that is homozygous recessive for all three traits?
(1/4)3 = 1/64
How tall would that child be if it were a girl?
4’7”
Epistasis
• In Labs, black is dominant
to chocolate (B or b).
Yellow is recessive epistatic
(when present, it blocks
the expression of the black
and chocolate alleles) E or
e.
• Determine the number of
chocolate labs produced
from a black female and a
yellow male (BbEe x bbee)
Phenotype
Possible Genotypes
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
BbEe x bbee
BE
Be
bE
be
be
BbEe
Bbee
bbEe
bbee
Color
Black
Yellow
Brown
Yellow
Pea Plants
• 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.
– If a plant with homozygous purple, controlled flowers(CC) is
crossed with a plant with white, non-controlled(cc) flowers,
diagram the Punnett square for the F1 and F2 generations and
calculate the phenotype ratios.
• First, what are the genotypes of the parents in the first generation?
PPCC and ppcc
• What are the genotypes of their offspring?
PpCc
• What is always the phenotypic ratio for a
dihybrid cross:
9: Dominant for both traits
3: Dominant for first trait and recessive for second
3: Recessive for first train and dominant for second
1: Recessive for both traits
P-CP-cc
ppCPpcc
What are the phenotypes?
purple
white
white
white
genotype
P-CP-cc
ppCppcc