Autosomal Linkage and Crossing Over

Mendel's laws predict that genes for two different traits assort independently during meiosis, producing the classic 9:3:3:1 ratio in a dihybrid cross. But Mendel got lucky — he chose seven pea traits each controlled by a gene on a different chromosome. Real life is not so tidy. Genes that happen to lie on the same chromosome are said to be linked, and they do not assort independently — they tend to be inherited together. Crossing over partly breaks this linkage, and the frequency of recombinant offspring can even be used to map gene order on a chromosome. OCR A-Level Biology A specification module 6.1.2(d) requires you to understand autosomal linkage, crossing over and how linkage modifies expected genetic ratios.

Key Definitions:

• Linked genes — genes that lie on the same chromosome and tend to be inherited together.
• Autosomal linkage — linkage of genes on any chromosome other than a sex chromosome.
• Recombination — the production of new allele combinations via crossing over.
• Crossing over — the exchange of corresponding segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis.
• Chiasma (plural: chiasmata) — the visible point of crossing over.
• Parental (non-recombinant) offspring — carry the same allele combinations as the parents.
• Recombinant offspring — carry new combinations produced by crossing over.
• Cross-over value (COV) — the percentage of recombinant offspring among all offspring.

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Independent Assortment — and its Exceptions

In meiosis I, homologous chromosome pairs line up at the metaphase plate and are distributed randomly to the two daughter cells. For genes on different chromosomes, this randomness means that one gene's inheritance has no effect on the other's — this is Mendel's law of independent assortment. A heterozygous dihybrid AaBb can make four equally frequent gametes: AB, Ab, aB, ab.

But if A and B are on the same chromosome, then the dominant alleles start out together on one chromosome and the recessive alleles together on the other. Meiosis simply separates the whole chromosomes, so the gametes will nearly all be AB or ab — the parental combinations. Only crossing over during prophase I can produce recombinant Ab or aB gametes.

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How Crossing Over Works

During prophase I, homologous chromosomes pair up (synapsis) and form a structure called the bivalent (or tetrad — four chromatids). Non-sister chromatids exchange corresponding pieces at points called chiasmata. The exchange is reciprocal: the segment lost from one chromatid is replaced by an identical segment from the other.

flowchart LR
    A[Parental: ABC and abc] --> B[Crossing over between B and C]
    B --> C[Recombinants: ABc and abC]

The further apart two genes are on a chromosome, the more likely a chiasma will fall between them, and the higher the cross-over value. Genes very close together are rarely separated; genes very far apart may cross over so often that they effectively assort independently.

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Expected vs Observed Ratios

Consider a dihybrid test cross in fruit flies. Let:

• B = grey body, b = black body
• V = normal wings, v = vestigial wings

Both genes are on chromosome 2 (linked). Cross a dihybrid BbVv heterozygote (from a pure-bred cross BBVV × bbvv, so B and V are on one chromosome and b and v on the other) with a homozygous recessive bbvv.

If the genes were unlinked (Mendel's expectation)

• BV : Bv : bV : bv = 1 : 1 : 1 : 1

If the genes were completely linked with no crossing over

• BV : bv = 1 : 1 (parental types only)

If the genes are linked with some crossing over (the real result)

• Mostly BV and bv (parental), plus a smaller number of Bv and bV (recombinant).
• Thomas Hunt Morgan's original 1911 data: grey normal 965, black vestigial 944, grey vestigial 206, black normal 185.

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Calculating Cross-Over Value