Autosomal linkage
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of monohybrid and dihybrid genetic crosses, allele interaction, and inheritance patterns as introduced in basic Mendelian genetics. You can test your knowledge on these below.
What is a gene locus?
The specific, fixed position of a gene on a chromosome.
How many alleles does an individual have for a single gene?
Two alleles - one inherited from each parent.
What process exchanges genetic material between non-sister chromatids?
Crossing over
Topic Explainer Video
Check out this @JoeDoesBiology video that explains autosomal linkage or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Autosomal Linkage and Allele Combinations in Gametes
Autosomal Linkage
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Autosomes are all chromosomes excluding sex chromosomes (22 pairs in humans).
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Autosomal linkage occurs when two or more genes are located on the same autosome (non-sex chromosome).
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These genes are said to be linked and do not assort independently, they stay together when the gametes are produced
Allele Combinations in Gametes
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Linked genes tend to be inherited together during meiosis.
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This limits the variety of gametes produced, reducing the combinations compared to if the genes were on separate chromosomes.
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Crossing over during meiosis can still separate them - but if they are close together, this is less likely.
Crossing Over, Linked Genes & How Autosomal Linkage Affects Inheritance of Traits
Crossing Over and Linked Genes
- Crossing over can separate linked alleles, producing recombinant gametes. For example, if genes A and B are linked and an individual is heterozygous for both genes - AaBb - you would expect only gametes with AB and ab to be produced. However, due to crossing over, gametes with Ab or aB may be produced as well.
- The closer two genes are on a chromosome, the less likely they are to be separated by crossing over.
- This is why linked genes tend to be inherited together, but recombinants still occur, albeit at a lower frequency.
How Does It Affect Inheritance of Traits?
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Fewer phenotypic combinations than expected are observed in offspring.
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Ratios from dihybrid crosses involving autosomal linkage differ from standard 9:3:3:1 ratios when two heterozygous are crossed together.
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Offspring more often inherit parental combinations (due to no recombination). Some non-parental phenotypes may occur due to recombination but at a lower frequency.
Example
If genes A/a (tall/short) and B/b (black/white) are linked on the same chromosome and parents are AaBb (tall, black), most gametes will be AB or ab — few Ab or aB gametes result from crossing over. Most offspring will be tall / black or short / white there may be a few tall, white or short, black due to recombination.
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How to Identify Autosomal Linkage
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Use genetic cross data:
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If the observed phenotypic ratio in offspring is significantly different from the expected ratio, linkage may be present.
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Apply a Chi-squared test:
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A significant difference between observed and expected suggests genes are not assorting independently and are therefore linked.
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Parental combinations occur at a higher frequency than recombinants.
Key Terms
- Autosome: A non-sex chromosome.
- Linked genes: Genes on the same chromosome that are usually inherited together.
- Recombinant: An individual or gamete with a new combination of alleles due to crossing over.
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Exam Tips
Always check phenotypic ratios — autosomal linkage will skew the ratio toward parental types.
Learn the 9:3:3:1 ratio for a heterozygous cross where genes are not linked, it will make spotting linkage easier.
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In fruit flies a gene controls the colour of the eye, the dominant allele (R) produces red eyes and the recessive allele (r) produces white eyes. A second gene controls wing development, the dominant allele (W) leads the production of wings and the recessive allele (w) leads to wingless flies. A geneticist crosses two heterozygous fruit flies (RrWw). The following offspring phenotypes were observed:
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Red with wings: 45
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White without wings: 42
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Red without wings: 7
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White with wings: 6
Explain the results and suggest what they show about the inheritance of these genes. (3 marks)
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Genes R and W are linked as they are located on the same autosome.
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Parental phenotypes (Red with wings and white without wings) occur more frequently than recombinants (Red without wings and white with wings).
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This is because crossing over occurs infrequently between these genes, producing fewer gametes with Rw rW alleles.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!