1. Linkage Aims: Must be able to outline what linkage is and how it is brought about. Should be able to explain the detection of linkage between genes using crossing. Could be able to estimate the distance between genes and predict the outcome of linked crosses.

2. Linkage - Basics Linkage occurs when two genes are located close together. The closer the loci are the stronger the link. The further apart the gene loci the more chance of recombination separating them.

3. Linkage - Example RH controls rhesus blood type and EL1 controls shape of blood cells – these genes are linked. Diagram:

4. Linkage - Detection Linkage can be detected by looking at the results of a double heterozygous cross with a double homozygous recessive. If the genes are not linked then the genes assort independently – 4 possible offspring genotypes with equal proportions (25%) If the genes are linked then the cross results will show an increased number of parental gametes over recombinant gametes.

5. Linkage – Estimating Distance Results of a cross are used. Based on the percentage of recombinant offspring produced. Distance between loci = 100 x (number of recombinant offspring / Total number of offspring) The percentage of recombinant offspring corresponds to the number of map units separating genes (20% recombinant offspring = 20 map units between gene loci).

6. Linkage – Predicting Outcomes When a cross is done with two known linked genes then the outcome can be predicted. If the two linked genes are separated by 8 map units then a test cross involving these genes will produce 8% of recombinant offspring and 92% of the parental type of offspring.

7. Recombination The term given to offspring that show new combinations of characteristics from those in the parents. Caused by crossing over during meiosis in parents. Offspring have genotypes unlike either parent. E.g.

8. Linkage - Activity Check information on p326/7 in the textbook. Biozone pages 291-294.