RECAP
• Manx cat
M
m
M
MM
Mm
m
Mm
mm
Manx
Death
Normal
PLEIOTROPY
• Is multiple effects of a single gene on the
phenotype
• Occurs for several reasons
– suppression of single gene can affect cell function
in more than one way
– A gene may be expressed in different cell types in a
multicellular organism
– A gene may be expressed at different stages of
development
– Eg cystic fibrosis, inability to recycle salt causing
extra salty sweat
5- GENE INTERACTIONS
Compiled by
Siti Sarah Jumali
Level 3 Room 14
Ext 2123
Polygene interactions
• Mendel studied one gene for height but he
did not realize that there are other genes
that also affect height
• i.e.
– Tall strain KK LL MM NN OO PP QQ RR SS TT
– Dwarf strain KK LL MM NN OO PP QQ RR SS tt
EPISTASIS
• An inheritance pattern in which the alleles
of one gene mask the phenotypic effects of
the alleles of a different gene
EPISTASIS
• Occurs when: gene at one locus affects the expression
of an allele at a different locus b.
• eg: fur color in mice
• 1) B is allele for black fur
2) b is allele for brown fur
3) BUT, must have allele C at a different locus to have
colored fur
4) if different locus has cc, then fur is colorless even if B
or b is present
5) see what happens when BbCc mouse is crossed with
BbCc mouse
(a) expect a (9:3:3:1 ratio) (b) get a 9:3:4
Chicken comb (recessive epistasis)
WT
Rose
Pea
Walnut
Single
R_pp
rrP_
R_P_
rrpp
R is dominant to r
P is dominant to p
RP is codominant
(walnut)
rrpp single comb
Epistasis on Chicken Comb
• Alleles of one gene mask the phenotype of
alleles on another gene. In this case, rrpp
masks a walnut comb
Epistasis on Flower
• 9:7 ratio is a modification of phenotypic but not
genotypic Mendelian ratios.
• The C and P genes independently assort, the presence of
a recessive genotype at one locus (i.e., cc or pp) masks
the effects of the alleles at the other locus.
• There are 9 combinations of alleles in the F1 generation
that feature at least 1 dominant C and 1 dominant
P allele, which would yield a purple flower phenotype
• Conversely, there are seven combinations that result in
either a cc or a pp, which would yield the white flower
phenotype-hence, the 9:7 ratio of purple to white
flowers.
9:7 phenotypic ratio
• In this case, white masks purple petal
CP
Cp
cP
cp
CP
CCPP
CCPp
CcPP
CcPp
Cp
CCPp
CCpp
CcPp
Ccpp
cP
CcPP
CcPp
ccPP
ccPp
cp
CcPp
Ccpp
ccPp
ccpp
9:3:4 ratio
• Agouti x Agouti
– BbCc x BbCc
– Brown agouti &
albino mask agouti
– c epistatic to B;
b epistatic to C
Ratio
Description
Name(s) of Relationship
(Used by Some Authors)
9:3:3:1
Complete dominance at both gene pairs; new phenotypes result
from interaction between dominant alleles, as well as from interaction between
both homozygous recessives
Not named because the ratio
looks likeindependent assortment
9:4:3
Complete dominance at both gene pairs; however, when 1
gene is homozygous recessive, it hides the phenotype of the other gene
Recessive epistasis
9:7
Complete dominance at both gene pairs; however, when either
gene is homozygous recessive, it hides the effect of the other gene
Duplicate recessive epistasis
12:3:1
Complete dominance at both gene pairs; however, when one gene is dominant, it
hides the phenotype of the other gene
Dominant epistasis
15:1
Complete dominance at both gene pairs; however, when either gene is dominant, it Duplicate dominant epistasis
hides the effects of the other gene
13:3
Complete dominance at both gene pairs; however, when eithergene is dominant, it
hides the effects of the other gene
Dominant and recessive epistasis
9:6:1
Complete dominance at both gene pairs; however, when eithergene is dominant, it
hides the effects of the other gene
Duplicate interaction
7:6:3
Complete dominance at one gene pair and partial dominance at the other;
when homozygous recessive, the first gene is epistatic to the second gene
No name
3:6:3:4
Complete dominance at one gene pair and partial dominance at the other;
when homozygous recessive, either gene hides the effects of the other gene;
when both genes are homozygousrecessive, the second gene hides the effects of
the first
No name
11:5
Complete dominance for both gene pairs only if both kinds ofdominant alleles are
present; otherwise, the recessivephenotype appears
No name
• All of these are also called as gene modifier
effect by other geneticists
Question
• Manx cats are heterozygous for a dominant mutation that
results in no tails (or very short tails), large hind legs, and a
distinctive gait. The mating of two Manx cats yields two Manx
kittens for each normal, long-tailed kitten, rather than three-toone as would be predicted from Mendelian genetics. Therefore,
the mutation causing the Manx cat phenotype is likely a(n)
__________ allele.
•
•
•
•
•
A. pleiotropic
B. codominant
C. epistatic
D. lethal
E. sex-linked