Download Lethal Alleles Example

1. LETHAL ALLELES
2. MULTIPLE ALLELES
3. PENETRANCE AND
EXPRESSIVITY
DIFFERENT DOMINANT RELATIONSHIP
Two alleles with recessive lethal
Some
alleles
may
cause
lethality
Inheritance
patterns
in three crosses
involving the
wild-type agouti
allele (A) and
the mutant
yellow allele
(AY) in mice.
Note that the
mutant allele
behaves as a
homozygous
lethal allele, and
the genotype
AYAY does not
survive.
LETHAL ALLELES
Many alleles that cause genetic
diseases are called "dominant"
because heterozygotes are
affected. A common example is
ACHONDROPLASIA, the
most common form of dwarfism,
with a normal length body
trunk but shortened limbs.
Another in the Manx cat,
which doesn't have a tail.
 In fact, these genes would be
better described as partially
dominant, because the
homozygotes are quite different
from the heterozygotes:
homozygotes are lethal.

LETHAL ALLELES
Example: Manx cat
ML = tailless,
lethal in homozygote
m = tail
Tailless male x Tailless female
ML m
x
ML m
2/3 tailless + 1/3 tails
½ ML
¼
ML ML
½ ML
dies
X
½m
¼ ML m
tailless
½m
¼ ML m
tailless
¼ mm
tail
MORE LETHAL ALLELES
Lethal alleles give an unusual inheritance ratio.
Consider a mating between two Manx cats. Each is
heterozygous Tt, with T the dominant tailless allele and
t the recessive normal tail allele.
 Using Mendel's Law of Segregation, we see that zygotes
form in the ratio of 1/4 TT, 1/2 Tt, and 1/4 tt.
 However, all the TT embryos die at a very early stage,
and only the Tt (tailless) and tt (tailed) cats are born.
 Because there are twice as many Tt as tt, the ratio of
offspring in the Tt x Tt cross is 2/3 Tt (tailless) to 1/3 tt
(tailled).
 Note that pure breeding lines of Manx cats (and
achondroplastic dwarves) can't exist, because 1/3 of their
offspring are of the incorrect type.

Multiple Alleles:
when more than two different
alleles exist for the same trait.
*
(Remember: each individual will only have two
alleles for a trait but there are several alleles to
choose from.)
In humans, blood type has multiple
alleles. IA and IB are codominant
and i (type O blood) is recessive.
Blood Type
Type A
Type B
Type AB
Type O
Type of Antigen
Possible Genotypes
Blood Type
Type A
Type B
Type AB
Type O
Type of Antigen
A antigen
Possible Genotypes
Blood Type
Type A
Type B
Type AB
Type O
Type of Antigen
Possible Genotypes
A antigen
IAIA, IAi
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
Type AB
Type O
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
Type O
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
Type O
A and B antigens
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
A and B antigens
IAIB
Type AB
Type O
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
A and B antigens
IAIB
Type AB
Type O
No antigens
Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
A and B antigens
IAIB
No antigens
ii
Type AB
Type O
Cross a heterozygous type A
with a heterozygous type B.
IAi x ___
Cross a heterozygous type A
with a heterozygous type B.
IAi x IBi
A
I
B
I
i
i
A
I
B
I
i
A
B
I I
i
B
I
i
A
I
i
A
B
I I
B
I i
A
I
i
B
I
A
B
I I
B
I i
i
IAi
A
I
i
B
I
A
B
I I
B
I i
i
IAi
ii
Genotypic Ratios:
1 IAi : 1 IAIB : 1 IBi : 1 ii
Phenotypic Ratios:
1 Type A: 1 Type AB: 1 Type B:
1Type O
Cross a person with type AB
blood with another person
with type O blood.
IAIB x ___
Cross a person with type AB
blood with another person
with type O blood.
IAIB x ii
A
I
i
i
B
I
A
I
i
i
A
I i
B
I
i
i
A
I
B
I
A
I i
B
I i
A
I
B
I
i
A
I i
B
I i
i
IAi
A
I
B
I
i
A
I i
B
I i
i
IAi
IBi
Genotypic Ratios:
2
A
I i
:2
B
I i
Phenotypic Ratios:
2 Type A: 0 Type AB: 2 Type B:
0 Type O
PENETRANCE AND EXPRESSIVITY
 Penetrance
= percentage of individuals with a
given genotype who exhibit the phenotype
 Expressivity = extent to which genotype is
expressed at the phenotypic level (may be due to
allelic variation or environmental factors)
PENETRANCE AND EXPRESSIVITY
Expression of many genes is affected by the
environment or by "background" genetic influences.
Two closely related concepts are used to describe this.
 Penetrance is the percentage of offspring with the
mutant genotype that express the mutant phenotype.
 Expressivity is the degree to which the mutant
phenotype is expressed.
 Example. Polydactyly is having extra fingers and toes.
There are several forms of this condition. For one
form, polydactyly is 65% penetrant: 65% of those who
carry the dominant polydactyly allele have extra
digits. Examining these people, there is a range of
expression: some have 1 extra digit, some have 2, etc.
Also, some of the digits are functional: have proper
bones, muscles and nerves, while others are missing
vital components or connections.

POLYDACTYLY
 Alfredo
Alfonseca
"The Six Shooter",
former Chicago
Cubs relief pitcher.
 Six fingers and toes
on each hand, all
functional.
ENVIRONMENTAL EFFECTS
Many traits are affected
by the environment as
well as by genetics.
 For example, the
hydrangea flower color is
controlled first by flower
color genes similar to
those in the pea: purple
vs. white with complete
dominance. But, pink vs.
purple is controlled by
the acidity of the soil in
which the plants grow.
