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How do linkage and gene interactions such as epistasis cause departures from the expected Mendelian ratios?

Explain autosomal linkage, recombination by crossing over, and epistasis as causes of departure from expected ratios

A focused answer to the H2 Biology Inheritance and Evolution outcome on linkage and gene interaction. Autosomal linkage and recombination by crossing over, why linked genes give non-Mendelian ratios, and epistasis where one gene masks another.

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What this dot point is asking

SEAB wants you to explain why real crosses sometimes depart from the expected Mendelian ratios, through autosomal linkage (genes on the same chromosome inherited together), recombination by crossing over, and epistasis (one gene masking another). This builds on dihybrid inheritance and connects to the sources of variation.

The answer

Autosomal linkage

Genes on the same chromosome are linked and tend to be inherited together rather than assorting independently. A cross between two double heterozygotes therefore does not give the 9:3:3:1 ratio; instead the parental combinations of alleles dominate the offspring, because the linked alleles travel together in the same gamete.

Recombination by crossing over

New combinations (recombinants) arise when homologous chromosomes exchange segments during crossing over in prophase of meiosis I, at points called chiasmata. A crossover between two linked genes produces recombinant gametes. The closer the genes, the rarer the crossover between them, so the fewer the recombinants. This is the basis of gene mapping.

Epistasis

In epistasis, one gene affects or masks the expression of another. This produces modified ratios. For example, if both genes must have a dominant allele for a phenotype to appear, the 9:3:3:1 ratio collapses into 9:7 (the 9 with both dominant versus the 7 lacking one). Other interactions give ratios such as 12:3:1 or 9:3:4.

Examples in context

Example 1. Gene mapping. Because recombination frequency increases with the distance between genes, measuring the percentage of recombinant offspring lets geneticists order genes and estimate distances along a chromosome. This was the original method for building chromosome maps.

Example 2. Coat colour epistasis. In some mammals one gene determines whether any pigment is made at all; if its recessive genotype blocks pigment, the animal is white regardless of a second gene controlling colour. The first gene is epistatic to the second, a classic example of one gene masking another.

Try this

Q1. State what is meant by autosomal linkage. [1 mark]

  • Cue. Two or more genes located on the same (non-sex) chromosome that tend to be inherited together.

Q2. Explain how recombinant gametes are produced from linked genes. [2 marks]

  • Cue. During crossing over in prophase of meiosis I, homologous chromosomes exchange segments at a chiasma; a crossover between the linked genes produces gametes with new allele combinations.

Q3. Define epistasis. [1 mark]

  • Cue. An interaction in which one gene affects or masks the phenotypic expression of another gene.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksTwo genes are located close together on the same chromosome. Explain why a cross between two double heterozygotes does not give the expected 9:3:3:1 ratio, and how recombinant offspring arise.
Show worked answer →

The answer should explain linkage and then crossing over.

Because the two genes are close together on the same chromosome, they are linked and tend to be inherited together rather than assorting independently. The parental combinations of alleles are therefore much more common in the offspring than the new combinations, so the 9:3:3:1 ratio (which assumes independent assortment) is not seen. Instead the offspring are mostly of the two parental phenotypes.

Recombinant offspring (with new allele combinations) arise by crossing over during meiosis. In prophase of meiosis I, homologous chromosomes pair and may exchange segments at a chiasma. If a crossover occurs between the two linked genes, it produces recombinant gametes carrying new combinations of the alleles. Because the genes are close together, crossovers between them are rare, so recombinants are a minority.

Markers reward linkage causing the genes to be inherited together, the parental types being commoner, the role of crossing over in producing recombinants, and the point that close linkage makes recombinants rare.

Original4 marksIn a particular plant, a cross that would be expected to give a 9:3:3:1 ratio instead gives a 9:7 ratio. Explain how epistasis could account for this.
Show worked answer →

The answer should define epistasis and apply it to the modified ratio.

Epistasis is an interaction in which one gene affects or masks the expression of another gene. In this example, two genes are needed for the dominant phenotype to appear, and a recessive genotype at either gene blocks it.

Only offspring with at least one dominant allele of both genes (the 9 portion of the sixteen) show the full phenotype. All the others (the 3, 3 and 1 portions, totalling 7) lack a functional allele of at least one gene and so show the alternative phenotype. This converts the 9:3:3:1 ratio into 9:7.

Markers reward the definition of epistasis, the requirement of both genes for the dominant phenotype, and the grouping of the 3:3:1 classes into a single phenotype to give 9:7.

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