LINKED GENES

=Linked Genes= Chromosomes carry many genes. '''Genes on the same chromosome are called linked genes''' and often move together during meiosis because they are physically tethered together. [image:http://i.imgur.com/V2dACoH.png?1] Early genetic experiments showed that there were some traits that did not follow the normal expected patterns of inheritance. The expected ratios of 9:3:3:1 (for a heterozygous cross) or 1:1:1:1 (for a back or test cross) were not seen at all. In fact, in some cases only two different phenotypes were produced! If we look at the diagram (left), we can see that this is because the genes are linked. As a result only two different types of gamete can be produced. Independent assortment does not affect linked genes as it only results in a random combination of chromosomes and does not alter the combination of alleles within a chromosome itself. [image:http://i.imgur.com/3NlHLky.png?1] Recombination (crossing over) between homologous chromosomes can result in a new combination of alleles within a chromosome. The chromosomes / offspring that carry these new combinations of alleles are referred to as '''recombinants'''. The diagram (left) shows how these recombinants are produced. It's worth noting that the further apart two genes are, from each-other, the greater the chance of crossing over occurring somewhere between the two. Likewise, if two genes are very close to each other, the chance of crossing over occurring in-between them is small. This is why in some cases the recombinant are rarely seen -the genes are simply so close together that recombination (crossing over) doesn't often occur between them.