Linkage.docx

Linkage changes BOTH the phenotype and the genotype. Independent assortment only works if genes are on different chromosomes. Genes on the same chromosome are linked together= linkage group. May show dependent assortment. May show independent assortment. Often show an intermediate situation which de...

Linkage changes BOTH the phenotype and the genotype. Independent assortment only works if genes are on different chromosomes. Genes on the same chromosome are linked together= linkage group. May show dependent assortment. May show independent assortment. Often show an intermediate situation which depends on how far genes are apart on chromosome. (very close together means more likely to link) Synapsis- non-sister chromatids of homologous chromosomes line up side by side. Crossing over- exchange of DNA between homologous chromosomes. Allows mixing of paternal and maternal DNA to create new combinations of alleles. Parental is the most frequently observed in F2 as it requires no recombination- chromosomes are directly inherited. Genes for different phenotypes e.g. eye colour etc are located on the same chromosome (X) and are most likely to be inherited together. How does the non-parental combinations arise? This is due to crossing over of the homologous X chromosome in the females that causes the alleles to be exchanged therefore create new non-parental combinations. The chance of having crossing over depends on: The distance= Bigger the distance between the genes is, the more likely to cross over. Closer the genes in distance are less likely to recombinate. Double cross over is very rare. Linkage mapping Helps to determine the number and location of genes on chromosomes. Distance between 2 genes is directly proportional to the chance of crossing-over between them. The frequency of crossing-over between two genes is directly proportional to the distance between them. Units= mu Constructing the map Consider each pair of genes separately. Identify recombinants. Add up the total recombinants and use the formula. Draw the results on a map, use the shortest distances on the map. To get frequency of double cross over, times together the single cross over probabilities. (product rule) Interference Interference is when a single crossover event affects the probability of a second crossover occurring in that vicinity. Can be quantified. Interference is 1- coefficient of coincidence. Positive interference= + value Negative interference= - value and it’s when a cross over enhances the probability of a second cross over occurring nearby. There can be multiple crossovers between the genes. 2 cross overs go back to parental genes. 3 cross overs is recombinant 4 cross over goes back to parental 5 cross overs is recombinant …… The further the genes are, the more likely is for them to cross over multiple times.