Free Autosomal Linkage and Sex Linkage OCR A Level Biology revision notes – covering specification points 6.1.2(bi) and 6.1.2(bii).
Diploid organisms have two types of chromosomes:
- Autosomes: Fully homologous chromosomes with all of the same gene loci.
- Allosomes*: Partially homologous chromosomes, where some gene loci are missing on one type.
*More commonly known as sex chromosomes.
Diploid organisms have two homologous chromosomes* of each type, which carry many different genes at different loci.
*With sex chromosomes being the exception.
Genes located on the same chromosome are inherited together, because the chromosome (not the individual genes) is the unit of transmission during reproduction.
Autosomal Linkage
When two or more genes are located on the same autosome, they are described as being autosomally linked.
Worked Example
Flower colour and pollen grain shape in sweet pea plants are determined by two genes located on the same chromosome:
- Locus 1: P produces purple flowers, p produces red flowers.
- Locus 2: L produces long pollen grains, l produces short pollen grains.
A homozygous purple-flowered, long-grained plant is crossed with a homozygous red-flowered, short-grained plant to produce the F1 generation:
The F1 generation is then interbred to produce the F2 generation:
The 3:1 phenotypic ratio arising from a dihybrid cross between two heterozygous individuals can be indicative of autosomal linkage.
Sex Linkage
Genes for traits located on one of the sex chromosomes are described as being sex linked.
Sex linkage causes the pattern of inheritance for traits to vary between males and females.
In humans, sex is determined by the 23rd pair of chromosomes, composed of X and Y.
Most sex linked traits are located on the X chromosome, and affect the inheritance patterns of females and males differently:
- Females inherit two copies of the gene, and so can inherit one recessive allele, and have it masked by a dominant allele.
- Males inherit one copy of the gene (from their mother), and will express that gene.
Sex-linked genotypes are written with the alleles shown as superscripts on the X chromosome (e.g. XH and Xh), to denote that the gene is sex-linked.
Worked Example
Haemophilia A is a blood-clotting disorder caused by an abnormal gene on the X chromosome, which codes for factor VIII (8), a blood-clotting protein:
- XH codes for a normal, functioning factor VIII.
- Xh codes for non-functioning factor VIII.
The phenotypes produced depend on the sex of the individual and the alleles inherited:
- A female who is XHXH only produces functional factor 8, so her blood clots normally.
- A female who is XHXh produces enough factor 8 to clot normally, but is a carrier of the disease.
- A female who is XhXh has no functioning allele and suffers from haemophilia A.
- A male who is XHY produces enough factor 8 to clot normally.
- A male who is XhY has no functioning allele and suffers from haemophilia A.
A symptomless carrier mother and an unaffected father are crossed:
