OBJECTIVE 6 NOTES: POLYGENIC INHERITANCE

Polygenic traits – more than two alleles and multiple genes control the expression of a trait

Objective 6 - Describe a simple model for polygenic inheritance and explain why most polygenic characters are described in quantitative terms.

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 populationvaries 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.

TWO TYPES OF TRAITS

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

AA1

BB1

An allele with a prime 1, adds one unit of measure to the phenotype

INHERITANCE OF CONTINUOUS TRAITS

A simple case with two loci, two alleles each

If a trait, say height, is controlled by two loci,A and B, and each locus has two alleles, one regularand one prime allele, what are the possible genotypes and phenotypes?

AABB 0----------shortAA1BB 1----------semi-shortAABB1 1----------semi-shortAA1BB1 2----------averageA1A1BB 2----------averageAAB1B1 2----------averageA1A1B1B 3----------semi-tallA1AB1B1 3----------semi-tallA1A1B1B1 4----------tall

AABBAA1BBAABB1

AA1BB1

A1A1BBAAB1B1

A1A1B1BA1AB1B1

A1A1B1B1

shortsemi-short average semi-

tall tall

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 thesame effect on the phenotype as a substitution atanother.

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 differencescan play a major role in variation

For skin color, exposure to sun can modify the phenotype

How do we know how much of the variabilitywe see among people is due to genetic differencesbetween them as opposed to environmental differences?

Fig 14.12 Polygenic inheritance of skin tone

aabbcc AABBCC

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

Offspring can have tone darker or lighter than either parent.

Can two light toned people have a darker child?

Fig 14.12

0 1 52 63 4Number of ‘darker’ alleles

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 HumansGene 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.

P bbCCDD X BbC’C’D’D” (Blue) (Gr-blue)

Gametes bCD BC’D’, bC’D’

F1 BbCC’DD” bbCC’DD’(Lt. Brown) (Blue)

Hypothetical mechanism for determination of eye color in HumansA possible cross:

James Bond Height Chart