Autosomal linkage

Autosomal linkage is when two genes are on the same autosome, a chromosome that is not a sex chromosome. Because they sit on one chromosome, their alleles tend to be passed on together, instead of sorting independently. So a dihybrid cross gives far more of the parental combinations, and far fewer new (recombinant) combinations, than the usual 9:3:3:1 or 1:1:1:1 ratios. Recombinants only appear when crossing over in meiosis separates the alleles.

What is autosomal linkage?

Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Each chromosome carries many genes. Genes on the same chromosome are linked. If that chromosome is an autosome, it is autosomal linkage; if it is the X chromosome, it is sex linkage.

Genes on different chromosomes follow independent assortment: in meiosis, each pair of homologous chromosomes lines up independently, so any allele of one gene can end up with any allele of the other. Linked genes cannot do this, because their alleles are physically joined on one chromosome.

How crossing over makes recombinants

Take a plant that inherited one chromosome carrying R and H and a homologous chromosome carrying r and h. Without crossing over, its gametes are only RH or rh: the parental combinations.

During prophase I of meiosis, non-sister chromatids of homologous chromosomes can swap sections at points called chiasmata. If a swap happens between the two genes, it makes chromosomes carrying R with h and r with H. Gametes with these new combinations are recombinants, and they are always the smaller groups.

The further apart two genes are on a chromosome, the more likely a crossover is to fall between them, so the more recombinants there are. Genes very close together are almost always inherited together.

Autosomal linkage in a dihybrid cross

Example 1: a test cross

In a plant, the allele for red flowers (R) is dominant to white (r), and the allele for hairy leaves (H) is dominant to smooth (h). A plant heterozygous for both genes, carrying RH on one chromosome and rh on the other, is crossed with a white, smooth plant (rrhh). If the genes were unlinked, the four phenotypes would appear in a 1 : 1 : 1 : 1 ratio. The results were:

Two large classes and two small ones is the sign of linkage. The large classes have the parent's original combinations; the small ones come from crossing over. The recombination frequency is the percentage of recombinant offspring:

Example 2: testing for linkage with chi-squared

AQA A-level Biology uses the chi-squared test to compare observed ratios with expected ones. The null hypothesis is that the genes are not linked, so the 800 offspring should be 200 of each phenotype.

There are 4 categories, so 3 degrees of freedom. The critical value at p = 0.05 is 7.82. Since 438.4 is far bigger, the difference is significant: reject the null hypothesis. The results do not fit independent assortment, which is evidence that the genes are linked.

Example 3: crossing two heterozygotes

If two plants that are both RH / rh are crossed and no crossing over happens, each makes only RH and rh gametes. The offspring are RRHH, RrHh, RrHh and rrhh, giving 3 red, hairy : 1 white, smooth, not 9:3:3:1. In real crosses a few recombinants usually appear as well, from crossing over.

Practise predicting the odds from a Punnett square in this question from the Genetics and DNA course, then breed your own crosses in the Genetics Cross Lab.

Autosomal linkage vs sex linkage

How to spot autosomal linkage

1. Work out the ratio you would expect if the genes assorted independently (9:3:3:1 or 1:1:1:1).
2. Compare the observed numbers. Linkage gives two big classes (parental) and two small classes (recombinant).
3. Check the results are the same for males and females. If they are, it is not sex linkage.
4. Use a chi-squared test to show the difference from the expected ratio is significant.
5. Explain the recombinants by crossing over between the genes in prophase I.

Check your understanding.

Common mistakes

• Confusing autosomal linkage with sex linkage. Autosomal linkage is two genes on one autosome; sex linkage is a gene on the X or Y.
• Saying linked genes are never separated. Crossing over can separate them, which is where recombinants come from.
• Mixing up parental and recombinant classes. Parental are the large groups; recombinant are the small groups.
• Writing crossing over happens in mitosis. It happens in prophase I of meiosis.
• Wrong degrees of freedom. It is the number of categories minus one: 4 phenotypes gives 3.
• Writing genotypes without showing linkage. Show which alleles are together, for example RH / rh, in genetic diagrams.

Practise with a short timed paper

Inheritance, including autosomal linkage and chi-squared, is in the second year of A-level Biology. The daily A-level Biology: Year 2 paper is a short timed paper of 8 questions, with a new one every day. For single-gene crosses, see the Punnett square calculator guide, and for how crossing over happens, meiosis.

Read next

Biology · Genetics and inheritanceDihybrid cross: Punnett square, 9:3:3:1 ratio, examplesHow to do a dihybrid cross: working out the gametes, the 16-box Punnett square, why it gives 9:3:3:1, other ratios, and testing results with chi-squared.