Download Objective 6 Polygenic Inheritance

OBJECTIVE 6 NOTES:
POLYGENIC INHERITANCE
Objective 6 - Describe a simple model for
polygenic inheritance and explain why most
polygenic characters are described in
quantitative terms.
Polygenic traits – more than two alleles and
multiple genes control the expression of a
trait
TWO TYPES OF TRAITS
Discrete or discontinuous traits: traits occur in distinct
categories: Trait is there or it is not (examples:
cystic fibrosis, Huntington’s disease) Mendelian inheritance,
single genes, complete dominance)
Continuous traits: Distribution of phenotypes in the population
varies along a continuum. Individuals differ by small degrees.
(examples include height, blood pressure, reaction time,
learning ability) Polygenic quantitative or multifactorial
inheritance. Genes act additively.
Examples of Discontinous
Polygenic Traits
Cardiac defects (most types)
Cleft lip (alone)
Cleft lip, with or without cleft palate
Diaphragmatic hernia
Hip dislocation
Limb reduction defect (most forms)
Neural tube defects (anencephaly, spina bifida)
Height is a continuous trait
Crow, 1997 Genetics 147:1
INHERITANCE OF CONTINUOUS TRAITS
A simple case with two loci, two alleles each
A
A1
B
B1
An allele with a prime 1, adds one unit of measure
to the phenotype
If a trait, say height, is controlled by two loci,
A and B, and each locus has two alleles, one regular
and one prime allele, what are the possible genotypes
and phenotypes?
AABB
AA1BB
AABB1
AA1BB1
A1A1BB
AAB1B1
A1A1B1B
A1AB1B1
A1A1B1B1
0----------short
1----------semi-short
1----------semi-short
2----------average
2----------average
2----------average
3----------semi-tall
3----------semi-tall
4----------tall
semishort short average
semitall
tall
AABB AA1BB AA1BB1 A1A1B1B A1A1B1B1
AABB1 A1A1BB A1AB1B1
AAB1B1
0
1
2
3
4
Two loci,
Two alleles
Three loci,
Two alleles
Characteristics of polygenic inheritance:
1. A substitution at one locus usually produces the
same effect on the phenotype as a substitution at
another.
2. Many loci with small, additive effects.
3. Tall parents can produce a short child, tall child or
in between, with limits.
4. Average parents can produce a tall, average
or short child
Why are the traits continuous?
Because of the environment.
In height, for example, nutritional differences
can play a major role in variation
For skin color, exposure to sun can modify
the phenotype
How do we know how much of the variability
we see among people is due to genetic differences
between them as opposed to environmental
differences?
Fig 14.12
Polygenic inheritance of skin tone
3 loci: each has two possible alleles: A,a B,b C,c,
each capital allele adds one unit of darkness
each lower case allele adds nothing
Parents with intermediate tone
aabbcc
Offspring can have tone darker or lighter than either
parent.
Can two light toned people have a darker child?
AABBCC
Fig 14.12
0
1
2
3
4
5
Number of ‘darker’ alleles
6
Another example of a polygenic trait:
• Hair Color
– Hair color is
controlled by alleles
on chromosomes 3, 6,
10, and 18.
– The more dominant
alleles that appear in
the genotype, the
darker the hair!
Hypothetical mechanism for determination of eye color in Humans
Gene for melanin production B (produce) dominant to b (none)
2 Modifier loci affect amount of pigment deposited
CC’ and DD’ each non prime allele contributes one unit of deposition
G’type at B
Modifier loci
Phenotype
B_
CCDD
Dk brown (+4)
B_
CCDD’
Med. Brown (+3)
B_
CC’DD’
Lt Brown (+2)
B_
CC’D’D’
Hazel (+1)
BB
C’C’D’D’
Green
Bb
C’C’D’D’
Greenish blue
bb
any g’type
Blue
TO CONFUSE EYE COLOR FURTHER:
University of Queensland geneticist Rick Strum suggests that the
genetics are not so clear.
“There is no single gene for eye color,” he says, “but the biggest effect is
the OCA2 gene.” (THE ONE CALLED B IN THE PREVIOUS SLIDE)
This gene Accounts for about 74 percent of the total variation in people’s
eye color.
Sturm found that how OCA2 is expressed—and how much pigment a
person has—is strongly linked to three single nucleotide polymorphisms
(SNPs), or single letter variations, in a DNA sequence near the OCA2
gene.
For example, among individuals carrying the SNP sequence “TGT” at all
three locations on both copies of the gene, 62 percent were blue-eyed,”
By contrast, only 21 percent of individuals carrying only one TGT copy
at each location and 7.5 percent of those lacking the TGT entirely had
blue eyes.
Depending on the particular combination of SNPs inherited, a person can
have a range of OCA2 activity that lands them on the spectrum between
blue and brown eyes. What about green eyes? “Green eyes probably
represent the interaction of multiple variants within the OCA2 and in
other genes, including perhaps the red-hair gene,” Duffy says.
Hypothetical mechanism for determination of eye color in
Humans
A possible cross:
P
bbCCDD
(Blue)
X
BbC’C’D’D”
(Gr-blue)
Gametes
bCD
BC’D’,
F1
BbCC’DD”
(Lt. Brown)
bbCC’DD’
(Blue)
bC’D’
James Bond Height Chart