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Genetics Unit 2: Extensions of Mendel: Dominance, Epistasis and Linkage

Real traits often break Mendel's simple ratios. This unit explains why, using dominance types, multiple alleles, lethal alleles, gene interaction, penetrance and polygenic traits, and shows how to read the modified ratios.

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Covers: Incomplete dominance and codominance, Multiple alleles and ABO blood groups, Lethal alleles, Epistasis and modified dihybrid ratios, Penetrance, expressivity and pleiotropy, Polygenic (quantitative) traits, First look at linkage

Study guide

Complete dominance is only one possibility. In incomplete dominance the heterozygote is intermediate: red × white snapdragons give pink, and pink × pink gives 1 red : 2 pink : 1 white, so phenotype ratios equal genotype ratios. In codominance both alleles are fully expressed in the heterozygote, as in AB blood, where red cells carry both A and B antigens. Dominance describes how alleles of one gene interact in producing a phenotype; it says nothing about how common an allele is. Often the molecular reason is dosage: one working copy of an enzyme gene may make enough product (complete dominance) or only half as much (incomplete dominance).

A gene can have many alleles in a population even though each diploid person carries only two. The ABO gene has alleles Iᴬ, Iᴮ and i: Iᴬ and Iᴮ are codominant, and both are dominant to i. With n alleles, there are n(n + 1)/2 possible genotypes. Some alleles are lethal. The yellow allele in mice is dominant for coat color but kills homozygous embryos, so yellow × yellow gives 2 yellow : 1 non-yellow among survivors. Whenever you see a 2:1 ratio, suspect a lethal homozygous class and remember to calculate probabilities among survivors.

Epistasis is an interaction between different genes in which one gene masks or modifies another. It changes the 9:3:3:1 ratio by merging classes. Recessive epistasis (9:3:4) occurs when a homozygous recessive at one gene hides the other, as when ee Labradors are yellow regardless of B. Dominant epistasis (12:3:1) occurs when a dominant allele hides the other gene, as in squash. Complementary genes (9:7) occur when both genes are needed for a phenotype, as in a two-step pigment pathway. Duplicate genes (15:1) occur when either gene alone is enough. Ratios that sum to 16 tell you two genes are assorting independently.

Genotype does not always predict phenotype precisely. Penetrance is the percentage of individuals with a genotype who show the phenotype at all, and expressivity is how strongly it shows among those who do. Pleiotropy is one gene affecting several traits, as in Marfan syndrome or sickle-cell disease. Polygenic (quantitative) traits like height are controlled by many genes plus the environment, producing continuous bell-shaped variation. In a simple additive model, the fraction of F2 as extreme as one parent is (1/4)ⁿ for n genes, and the classes follow a binomial distribution.

Mendel's independent assortment holds only for genes on different chromosomes or far apart on the same one. Genes close together on a chromosome are linked and tend to be inherited together. In a dihybrid test cross, linked genes produce two large parental classes and two small recombinant classes instead of 1:1:1:1. The fraction of recombinant offspring is the recombination frequency, which never exceeds 50%. Unit 4 develops this into gene mapping; here, the key skill is recognizing that unequal test-cross classes mean linkage rather than epistasis.

Key ideas

Formulas and rules

Recessive epistasis 9:3:4; dominant epistasis 12:3:1; complementary 9:7; duplicate 15:1
Lethal homozygote among survivors of Aa × Aa: 2 Aa : 1 aa
Genotypes with n alleles = n(n + 1)/2
P(affected) = P(inherit allele) × penetrance
Additive polygenic: fraction as extreme as one parent = (1/4)ⁿ; P(k capital alleles) = C(2n, k)/4ⁿ
Recombination frequency = recombinant offspring / total offspring

Common mistakes

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Words to know

Incomplete dominance
The heterozygote has a phenotype intermediate between the two homozygotes.
Codominance
Both alleles are fully expressed in the heterozygote.
Multiple alleles
More than two alleles of a gene exist in a population.
Lethal allele
An allele that causes death, often only when homozygous.
Epistasis
One gene masks or modifies the phenotypic effect of another gene.
Recessive epistasis
A homozygous recessive genotype at one gene masks another gene; F2 ratio 9:3:4.
Dominant epistasis
A dominant allele at one gene masks another gene; F2 ratio 12:3:1.
Complementary genes
Two genes are both required for a phenotype; F2 ratio 9:7.
Duplicate genes
A dominant allele at either of two genes gives the same phenotype; F2 ratio 15:1.
Penetrance
The percentage of individuals with a genotype who show the associated phenotype.
Expressivity
The degree or severity of a phenotype among individuals that show it.
Pleiotropy
One gene affects several different traits.
Polygenic trait
A trait controlled by many genes, usually with continuous variation.
Complementation
Offspring of two recessive mutants are wild type because the mutations are in different genes.
Linkage
Genes close together on a chromosome tend to be inherited together.
Recombinant
An offspring or gamete with a new combination of alleles not found in either parental chromosome.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.

Question 1 (level 1)

In codominance, a heterozygote:

  1. Fully expresses the phenotypes of both alleles
  2. Shows only the phenotype of the dominant allele
  3. Fails to survive to adulthood
  4. Shows a phenotype intermediate between the two homozygotes
Show the answer and steps

Answer: A) Fully expresses the phenotypes of both alleles

  1. Codominance means both alleles are expressed side by side.
  2. The AB blood type, with A and B antigens together on red cells, is the classic example.
  3. An intermediate phenotype is incomplete dominance, a different pattern.
Question 2 (level 1)

Epistasis is best defined as:

  1. Many separate genes each adding a small effect to one continuous trait
  2. One allele masking another allele of the same gene
  3. A single gene affecting several unrelated traits
  4. An interaction in which one gene masks or modifies the effect of another gene
Show the answer and steps

Answer: D) An interaction in which one gene masks or modifies the effect of another gene

  1. Dominance is an interaction between alleles of one gene.
  2. Epistasis is an interaction between different genes.
  3. Pleiotropy (one gene, many traits) and polygenic inheritance are separate ideas.
Question 3 (level 3)

In summer squash, F2 from a dihybrid cross gives 12 white : 3 yellow : 1 green. What type of gene interaction does this ratio indicate?

  1. Complementary gene action
  2. Recessive epistasis
  3. Duplicate dominant genes
  4. Dominant epistasis
Show the answer and steps

Answer: D) Dominant epistasis

  1. 12 = 9 + 3, so a dominant allele of one gene (W_) gives white no matter what the other gene is.
  2. Among ww, the second gene gives yellow (Y_) or green (yy).
  3. A dominant allele masking another gene is dominant epistasis; recessive epistasis gives 9:3:4.
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