6.2.7 - Autosomal Linkage

• There are many more genes in a genome than there are chromosomes:

  • When two are more genes loci are on the same chromosome, they are said to be linked.
• The chromosome, not the gene, is the unit of transmission during sexual reproduction - therefore, genes are not free to undergo independent assortment and are usually inherited together as a unit.
• When a set of genes on the same autosome are inherited together as one unit, this is called autosomal linkage.
• In autosomal linkage, when gene loci are close together on the same chromosome, there is usually no crossing over in Prophase I of meiosis - the linked genes are inherited as one unit.
• If the gene loci are far apart, there may be crossing over and independent assortment, producing recombinant gametes.
• In sweet pea plants, the genes for flower colour and pollen grain shape are on the same chromosome - each gene has 2 alleles:

  • When true-breeding homozygous dominant plants are crossed with homozygous recessive plants, only two types of gamete are expected with no independent assortment.
• When members of the heterozygous F2 generation interbreed, only one type of gamete is expected, producing a ratio of 1:2:1 of dominant : heterozygous : recessive:

  • The ratios are the same as the Mendelian monohybrid inheritance ratios with only 2 phenotypes in the F2 generation.
  • If the genes were not linked, we would expected 4 different distinct phenotypes in the ratio of 9:3:3:1.
• If the gene loci are far apart, there may be some crossing over and independent assortment, producing some recombinant gametes but not all - crossing over is rare in linked genes.
• If crossing over occurs in meiosis, then plants can have a different combination of phenotypes.
• Four phenotypes can be produced, but do not show the expected 9:3:3:1 ratio that would be expected if the genes were not linked.
• In autosomal linkage with crossing over, there are also some recombinant gametes:

  • The further apart the gene loci on the chromosome, the greater the chance of recombinant gametes forming.