Topic G

Question 1

Allelic Interactions

Answer 1

An allelic interaction is the interaction between two alleles of the same gene in a diploid, and this interaction dictates phenotype

The allelic interactions we will be exploring occur in heterozygotes who have two different alleles of a single gene
- Dominance/Complete dominance
- Incomplete dominance
- Codominance

Question 2

Allelic Interactions

Mendel’s monohybrid cross:

Light purple offspring

Answer 2

Purple parent X white parent = light purple offspring

PP purple > Pp light purple > pp white

• The allelic interaction between P and p in the heterozygote is incomplete dominance (also called haploinsufficiency, having 1 copy of the allele is not enough to get back to parental phenotype)
• What phenotypic ratio will we observe in the F2 of this monohybrid cross?
  It’d be 1:2:1 (dark purple:light purple:white. This is a modified Mendelian ratio

Question 3

Allelic Interactions

Mendel’s monohybrid cross:

White and purple offspring

Answer 3

PP purple = Pp purple and white = pp white

• The allelic interaction between P and p in the heterozygote is one of codominance
• What phenotypic ratio will we observe in the F2 of this monohybrid cross?

1:2:1. homo purple:hetero purple and white: homo white

Question 4

Allelic Interactions

Mendel’s monohybrid cross:

Purple offspring

Answer 4

F1 Heterozygote (Pp) purple = P1 (PP) purple

• The F1 heterozygote displays an allelic interaction of complete dominance between P and p (also called haplosufficiency)
• F2: PP:2Pp:pp will have a phenotypic ratio of 3:1

Question 5

Most genes have multiple alleles

Answer 5

Genes have multiple alleles and individual alleles can show different allelic interactions

• As we saw with the ABO blood type gene where we saw allelic interactions of complete dominance between some alleles, and codominance between others

Question 6

Pleiotropy and allelic interactions

Answer 6

Pleiotropy and the white gene in Drosophila
- Eye pigmentation
- Less successful mating for males
- Shorter life expectancy
- Neurological difficulties

Most genes are pleiotropic

Because allelic interactions are determined by comparing the phenotype of the heterozygote to that of the homozygotes, pleiotropy creates an additional consideration; which phenotype?

Two alleles of a single gene can demonstrate different allelic interactions depending on which phenotype you consider

Allelic interactions must always be defined on the basis of a single specific phenotype.

Allelic interactions will determine whether a 3:1 or 1:2:1 phenotypic ratio is observed in the F2 of a monohybrid cross

Question 7

Gene interactions - Epistasis

Answer 7

Genes do not act in isolation

• The link between genotype and phenotype is rarely simple, it is usually complex
• Interactions between alleles of different genes adds to the complexity in the link between genotype and phenotype
• Put simple, interactions between the alleles of different genes can be described as epistasis

Question 8

Epistasis

Answer 8

Classic definition (Bateson & Punnet)
- When the allele(s) of one gene mask the effects of alleles of another gene
— The allele(s) that mask = Epistatic (standing on)
— The allele(s) being masked = Hypostatic

Modern definition
- Term epistasis used to describe anytime genes interact

Question 9

Gene ‘A’ determines the way pigment is distributed in the fur:

Answer 9

A_ = agouti

aa = solid (black)

Gene ‘C’ determines whether or not there can be color

C_ = color

cc = albino

F1 AaCc x AaCc (both Agouti)

F2 gen phen
9 A_C_ agouti
3 A_cc albino
3 aaC_ black/solid
1 aacc albino

We see a modified phenotypic ratio of 9:4:3 in the F2 which suggests epistasis

Hint – always look at the ‘combined’ phenotypic category to identify the epistatic relationship. Here we see that whenever one gene is cc it does not matter what the genotype of the other gene is (A_ or aa)

cc is epistatic to A and aa

Question 10

Gene interactions - Epistasis

Coloured squash example

Answer 10

• Colored squash can be yellow (A_) or green (aa)
• A second gene, B, either allows pigment to be produced (bb) or blocks the production of pigment resulting in white squash (B_)

F2 gen phen
9 A_B_ white
3 A_bb yellow
3 aaB_ white
1 Aabb green

We see a modified phenotypic ratio of 12:3:1 in the F2.

In this case B is epistatic to A and aa.

Question 11

Gene interactions - Epistasis

Bateson and Punnett

Biochemical pathway – pea color

Answer 11

Colourless precursor—EnzC—> coulourless intermediate —EnzP—> purple pigment

Gene C goes to EnzC
Gene P goes to EnzP

What phenotypes and ratios will you observe in
the F2?

F2 gen phen
9 C_P_
3 C_pp
3 ccP_
1 ccpp

Complementation: Each recessive allele (c and p) is complemented by a wild-type allele (C and P).
This phenomenon indicates that the recessive alleles are in different genes.

Epistasis: Homozygosity for the recessive allele of either gene results in a white phenotype, thereby masking the purple (wild-type) phenotype.

Both gene products encoded by the wild-type alleles (C and P) are needed for a purple phenotype.

Question 12

Gene interactions - Epistasis

Triangle seed example

Answer 12

In this case a dominant allele of either T or V is sufficient to produce the triangular seed phenotype

F2 gen phen
9 T_V_
3 T_vv
3 ttV_
1 ttvv

Question 13

Modified Mendelian Ratios

Answer 13

Allelic interactions other than complete dominance result in modified 3:1 phenotypic ratios in the F2 of a monohybrid cross (1:2:1 and 2:1)

Gene interactions result in modified 9:3:3:1 phenotypic ratios in the F2 generation of a dihybrid cross

What if all you are given is phenotypes and frequencies of an F2 cross?

Question 14

Variable phenotype

Answer 14

As we have already seen, the relationship between genotype and phenotype is not always straightforward

Sometimes we see unusual relationships between genotype and phenotype that may be due to complex interactions between genes and between genes and the environment

Example – polydactyly is an Autosomal Dominant condition that results in extra fingers and/or toes

Question 15

Variable phenotype

Purple oval example

Answer 15

All of the ovals below have the same genotype that produces a purple phenotype

Reduced penetrance and variable expressivity

Question 16

Review of Concept

Answer 16

• Individuals with a particular genotype may not express the corresponding phenotype
  — Reduced or incomplete penetrance
• Multiple individuals with a particular genotype may express varying degrees of the corresponding phenotype
  — Variable expressivity
• A trait may demonstrate both reduced penetrance and variable expressivity in a population

Question 17

Summary

Answer 17

Many traits are determined by a single gene showing simple mendelian inheritance.

A single gene may have many alleles that can interact in different ways.

By comparing the phenotype of a heterozygote to the homozygotes, we can determine if the allelic interaction is an example of complete dominance, incomplete dominance or codominance.

The latter two will produce a 1:2:1 phenotypic ratio in the F2 generation of a monohybrid cross rather than the 3:1 ratio we see in the case of complete dominance.

Many traits are determined by multiple genes which can interact in different ways to influence phenotype.

In some cases multiple genes may carry out redundant or overlapping function or may mask the effects of other genes in an epistatic relationship.

Interactions between two genes will also impact the phenotypic ratios we observe in the F2 generation of a dihybrid cross.

More complex polygenic traits are more difficult to understand and these types of traits may also be influenced by the environment.

Incomplete penetrance and variable expressivity are likely the result of complex interactions between genotype and environment that we may have yet to fully understand.