Autosomal Linkage (AQA A-Level Biology): Revision Notes
Autosomal Linkage
What is autosomal linkage?
Humans possess 23 pairs of chromosomes that carry thousands of genes responsible for different characteristics. Since each chromosome contains many genes, it follows that some genes must be located on the same chromosome.
Autosomal linkage occurs when two or more genes are located on the same autosome (non-sex chromosome). These genes are described as linked genes, and all genes on a single chromosome form a linkage group. In humans, there are 22 autosomes, so genes carried on these chromosomes show autosomal linkage, while genes on sex chromosomes show sex-linkage.
With thousands of genes distributed across only 23 pairs of chromosomes, it's inevitable that many genes will share the same chromosome. This physical proximity on chromosomes is what creates the phenomenon of genetic linkage.
How linked genes inherit differently
When genes are linked on the same chromosome, they behave very differently during inheritance compared to genes on separate chromosomes. The key difference lies in how they segregate during meiosis.
Linked genes remain together during meiosis and pass into gametes as a unit, rather than separating independently. This means they do not follow Mendel's Law of Independent Assortment, which only applies to genes on different chromosomes.
Critical Concept: Linked genes violate Mendel's Law of Independent Assortment because they are physically connected on the same chromosome. They travel together during meiosis like passengers on the same bus, rather than taking separate routes.
Comparing linked and non-linked inheritance
The inheritance patterns differ dramatically depending on whether genes are linked or not:
Genes on separate chromosomes (not linked)
When genes A and B are on different chromosomes and both parents are heterozygous (AaBb):
- Four different types of gametes are produced: AB, Ab, aB, ab
- All combinations of alleles can occur in the offspring
- This follows independent assortment
Genes on the same chromosome (linked)
When genes A and B are linked on the same chromosome, assuming no crossing over occurs:
- Only two types of gametes are produced: AB and ab
- The allele combinations that were present in the parents stay together
- This creates a much simpler inheritance pattern
The reduction from four possible gamete types to just two is the hallmark of genetic linkage. This dramatic simplification makes linked inheritance patterns much more predictable than independent assortment.
Example: fruit fly characteristics
A clear example involves fruit flies (Drosophila melanogaster) with two linked characteristics:
Worked Example: Fruit Fly Linkage Cross
Body colour gene:
- G allele = grey body (dominant)
- g allele = black body (recessive)
Wing size gene:
- N allele = normal wings (dominant)
- n allele = vestigial wings (recessive)
The Cross: Two heterozygous flies (both grey-bodied with normal wings) Genotype: GgNn × GgNn
Expected Result with Linkage:
- 3 grey body, normal wings offspring (GN/GN, GN/gn, GN/gn)
- 1 black body, vestigial wings offspring (gn/gn)
Ratio: 3:1 (much simpler than independent assortment!)
This 3:1 ratio is much simpler than the complex ratios seen when genes assort independently.
Why linkage matters in genetics
Understanding autosomal linkage is essential because:
- It explains why certain characteristics tend to be inherited together
- It affects the outcomes of genetic crosses and breeding programmes
- It influences genetic variation in populations
- It helps predict inheritance patterns in families and species
Distance and Linkage Strength: The closer two genes are on a chromosome, the more tightly linked they are, and the less likely they are to be separated by crossing over during meiosis. Think of it like two towns on a highway - the closer they are, the more likely travellers will visit both.
Remember!
Key Points to Remember:
- Autosomal linkage occurs when genes are located on the same non-sex chromosome
- Linked genes travel together during meiosis and pass into gametes as a unit
- Fewer gamete types are produced compared to independent assortment (typically 2 instead of 4)
- Inheritance patterns are simpler with linked genes, often showing clear dominant/recessive ratios
- Crossing over can separate linked genes, but this is covered in more advanced topics