Extensions of Mendelian Analysis
Interactions between alleles of one gene.
Multiple alleles,incomplete dominance,codominance
Interactions between alleles of two genes.
New phenotypes,epistasis
Relationship between genotype & phenotype
lethals,variable penetrance,conditional alleles
Interactions between alleles of one gene
Dominance is a relationship between two alleles of the
same gene.
Several relationships are possible:
complete dominance
partial dominance/ incomplete dominance
codominance
overdominance
Each relationship has particular characteristics.
Dominance is relative,an allele can be dominant to
one allele and recessive to a third allele.
w+/ wa = red eye (wa is recessive)
w+/ w1 = red eye (w1 is recessive)
wa/ w1 = light orange (wa is partially dominant to w1),
The white gene has many different alleles,each
producing a different amount of pigment,eosin,
apricot,buff,etc.
Incomplete
dominance,
heterozygotes are
intermediate.
Heterozygous
snapdragons (R/r)
have an intermediate
color,pink.
Crossing two pink
snapdragons gives a
1 R/R,2R/r,1r/r
progeny ratio.
4.1
Incomplete dominance in dihybrid crosses
One gene with partial dominance,one gene with complete
dominance,F2 = 3:6:3:1:2:1
3/4 B/- (1) A/A B/- = 3/16
1/4 A/A
1/4 b/b (2) A/A b/b = 1/16
3/4 B/- (3) A/a B/- = 6/16
1/2 A/a
1/4 b/b (4) A/a b/b = 2/16
3/4 B/- (5) a/a B/- = 3/16
1/4 a/a
1/4 b/b (6) a/a b/b = 1/16
Incomplete dominance in dihybrid crosses
Two genes with partial dominance,F2 = 1:2:1:2:4:2:1:2:1
1/4 B/B A/A B/B = 1/16
1/4 A/A 1/2 B/b A/A B/b = 2/16
1/4 b/b A/A b/b = 1/16
1/4 B/B A/a B/B = 2/16
1/2 A/a 1/2 B/b A/a B/b = 4/16
1/4 b/b A/a b/b = 2/16
1/4 B/B a/a B/B = 1/16
1/4 a/a 1/2 B/b a/a B/b = 2/16
1/4 b/b a/a b/b = 1/16
Dominance is a relationship between two alleles.
Alleles show different degrees of dominance,
4.2
Codominant alleles
Codominant alleles show both parental phenotypes.
ex,ABO blood type,one gene with three alleles.
Different alleles = different antigens
IA = produces the A antigen
IB = produces the B antigen
i (or Io) = produces neither antigen.
Individuals produce antibodies to the antigen they do
not have:
IA/ IA = anti-B antibodies
IB/ IB = anti-A antibodies
i/ i = anti-B and anti-A antibodies
Lethal Alleles
Mutations of vital genes kill
the individual,
Lethals modify the
genotypic ratio,depending
on when the offspring are
scored.
Example,the Yellow allele
in mice has a recessive
lethal,dominant visible
phenotype.
4.17
Huntington Disease
Huntington disease is a dominant
lethal in humans.
Phenotype,central nervous system
degeneration.
Death occurs after reproductive age.
Interactions can:
Generate new phenotypes
Change phenotypic ratios
Suppress or enhance phenotypes
Interactions between alleles of two genes,
Green = b/b,G/-
Blue = b/b,g/g
Brown = B/-,-/-
Eye color in humans
Eye color in humans is thought to be controlled by
two sets of genes,Brown eyed individuals must have at
least one copy of the B gene
Blue eyed indviduals do not have any B gene or G gene.
Green eyed individuals must have at least one G
gene and no B gene,
Interactions can give new phenotypes
In Mendel’s crosses
the double mutant
had both
phenotypes,
Interactions can
give the double
mutants novel
phenotypes.
9/16 R/- P/-
3/16 R/- p/p
3/16 r/r P/-
1/16 r/r p/p
Eye color in
Drosophila
bwbw or ryry =
brown eye ++ ++ =
red eyes.
F2 Ratio,9:7
4.13
Interactions change the number of F2 phenotypic
classes,but not the number of genotypic classes.
Parents brown (+/+ bw/bw) X rosy (ry/ry +/+)
F1 red (+/bw +/ry)
F2 =
3/4 +/- +/- +/- = 9/16 red
3/4 +/-
1/4 ry/ry +/- ry/ry = 3/16
3/4 +/- bw/bw +/- = 3/16 7/16 brown
1/4 bw/bw
1/4 ry/ry bw/bw ry/ry = 1/16
Epistasis
The expression of one gene
masks the expression of
another gene.
Recessive epistasis,
individuals homozygous for
the recessive allele have the
same phenotype,i.e..:
aa BB = aa Bb = aa bb
Example,coat color in mice,
Albino is epistatic to coat
color genes.
white---->black---->agouti
4.14
Epistasis results in phenotpes that are
not predicted by the genotypes
The key is to determine which
classes have which phenotype.
3/4 B/- 9/16 A/- B/- = agouti
3/4 A/-
1/4 b/b 3/16 A/- bb = black
3/4 B/- 3/16 a/a B/- = albino
1/4 a/a
1/4 b/b 1/16 a/a b/b = albino
Penetrance and Expressivity
Penetrance,the percentage of individuals with a mutant
genotype that show the phenotype.
Ex,Bpt = Blackpatch,eye degeneration mutation that gives
black eye spots.
Blackpatch has incomplete penetrance.
Bpt/Bpt females = 95% have eye spots
Bpt/Bpt males = 45% have eye spots.
Penetrance = 95% in females,45% in males.
Expressivity,the range of the phenotype.
Ex,w67g/w67g = all individuals have pure white eyes.
white has 100% penetrance and constant expressivity
Ex,Bpt/Bpt = some individuals have small spots,some
have large spots.
Bpt has variable expressivity.
4.16
Conditional alleles
Conditional alleles give different phenotypes in different
environments,
Gene expression is affected by the external environment.
Gene expression is affected by other alleles in the,genetic
background”,or the internal environment.
Temperature sensitive
Auxotrophic
Light sensitive
Interactions between alleles of one gene.
Multiple alleles,incomplete dominance,codominance
Interactions between alleles of two genes.
New phenotypes,epistasis
Relationship between genotype & phenotype
lethals,variable penetrance,conditional alleles
Interactions between alleles of one gene
Dominance is a relationship between two alleles of the
same gene.
Several relationships are possible:
complete dominance
partial dominance/ incomplete dominance
codominance
overdominance
Each relationship has particular characteristics.
Dominance is relative,an allele can be dominant to
one allele and recessive to a third allele.
w+/ wa = red eye (wa is recessive)
w+/ w1 = red eye (w1 is recessive)
wa/ w1 = light orange (wa is partially dominant to w1),
The white gene has many different alleles,each
producing a different amount of pigment,eosin,
apricot,buff,etc.
Incomplete
dominance,
heterozygotes are
intermediate.
Heterozygous
snapdragons (R/r)
have an intermediate
color,pink.
Crossing two pink
snapdragons gives a
1 R/R,2R/r,1r/r
progeny ratio.
4.1
Incomplete dominance in dihybrid crosses
One gene with partial dominance,one gene with complete
dominance,F2 = 3:6:3:1:2:1
3/4 B/- (1) A/A B/- = 3/16
1/4 A/A
1/4 b/b (2) A/A b/b = 1/16
3/4 B/- (3) A/a B/- = 6/16
1/2 A/a
1/4 b/b (4) A/a b/b = 2/16
3/4 B/- (5) a/a B/- = 3/16
1/4 a/a
1/4 b/b (6) a/a b/b = 1/16
Incomplete dominance in dihybrid crosses
Two genes with partial dominance,F2 = 1:2:1:2:4:2:1:2:1
1/4 B/B A/A B/B = 1/16
1/4 A/A 1/2 B/b A/A B/b = 2/16
1/4 b/b A/A b/b = 1/16
1/4 B/B A/a B/B = 2/16
1/2 A/a 1/2 B/b A/a B/b = 4/16
1/4 b/b A/a b/b = 2/16
1/4 B/B a/a B/B = 1/16
1/4 a/a 1/2 B/b a/a B/b = 2/16
1/4 b/b a/a b/b = 1/16
Dominance is a relationship between two alleles.
Alleles show different degrees of dominance,
4.2
Codominant alleles
Codominant alleles show both parental phenotypes.
ex,ABO blood type,one gene with three alleles.
Different alleles = different antigens
IA = produces the A antigen
IB = produces the B antigen
i (or Io) = produces neither antigen.
Individuals produce antibodies to the antigen they do
not have:
IA/ IA = anti-B antibodies
IB/ IB = anti-A antibodies
i/ i = anti-B and anti-A antibodies
Lethal Alleles
Mutations of vital genes kill
the individual,
Lethals modify the
genotypic ratio,depending
on when the offspring are
scored.
Example,the Yellow allele
in mice has a recessive
lethal,dominant visible
phenotype.
4.17
Huntington Disease
Huntington disease is a dominant
lethal in humans.
Phenotype,central nervous system
degeneration.
Death occurs after reproductive age.
Interactions can:
Generate new phenotypes
Change phenotypic ratios
Suppress or enhance phenotypes
Interactions between alleles of two genes,
Green = b/b,G/-
Blue = b/b,g/g
Brown = B/-,-/-
Eye color in humans
Eye color in humans is thought to be controlled by
two sets of genes,Brown eyed individuals must have at
least one copy of the B gene
Blue eyed indviduals do not have any B gene or G gene.
Green eyed individuals must have at least one G
gene and no B gene,
Interactions can give new phenotypes
In Mendel’s crosses
the double mutant
had both
phenotypes,
Interactions can
give the double
mutants novel
phenotypes.
9/16 R/- P/-
3/16 R/- p/p
3/16 r/r P/-
1/16 r/r p/p
Eye color in
Drosophila
bwbw or ryry =
brown eye ++ ++ =
red eyes.
F2 Ratio,9:7
4.13
Interactions change the number of F2 phenotypic
classes,but not the number of genotypic classes.
Parents brown (+/+ bw/bw) X rosy (ry/ry +/+)
F1 red (+/bw +/ry)
F2 =
3/4 +/- +/- +/- = 9/16 red
3/4 +/-
1/4 ry/ry +/- ry/ry = 3/16
3/4 +/- bw/bw +/- = 3/16 7/16 brown
1/4 bw/bw
1/4 ry/ry bw/bw ry/ry = 1/16
Epistasis
The expression of one gene
masks the expression of
another gene.
Recessive epistasis,
individuals homozygous for
the recessive allele have the
same phenotype,i.e..:
aa BB = aa Bb = aa bb
Example,coat color in mice,
Albino is epistatic to coat
color genes.
white---->black---->agouti
4.14
Epistasis results in phenotpes that are
not predicted by the genotypes
The key is to determine which
classes have which phenotype.
3/4 B/- 9/16 A/- B/- = agouti
3/4 A/-
1/4 b/b 3/16 A/- bb = black
3/4 B/- 3/16 a/a B/- = albino
1/4 a/a
1/4 b/b 1/16 a/a b/b = albino
Penetrance and Expressivity
Penetrance,the percentage of individuals with a mutant
genotype that show the phenotype.
Ex,Bpt = Blackpatch,eye degeneration mutation that gives
black eye spots.
Blackpatch has incomplete penetrance.
Bpt/Bpt females = 95% have eye spots
Bpt/Bpt males = 45% have eye spots.
Penetrance = 95% in females,45% in males.
Expressivity,the range of the phenotype.
Ex,w67g/w67g = all individuals have pure white eyes.
white has 100% penetrance and constant expressivity
Ex,Bpt/Bpt = some individuals have small spots,some
have large spots.
Bpt has variable expressivity.
4.16
Conditional alleles
Conditional alleles give different phenotypes in different
environments,
Gene expression is affected by the external environment.
Gene expression is affected by other alleles in the,genetic
background”,or the internal environment.
Temperature sensitive
Auxotrophic
Light sensitive
