Genetics Cheat Sheet by joanna_2201

This is because the recessive allele on the X chromosome will show in the phenotype in males, as they have XY, so there is no dominant allele to override it (whereas females have XX, so will always have the dominant allele).

Males and females carry a different dose of X, so males have dosage compen­sation – they produce double the amount of gene product (upreg­ulation of X in males)

Genes are physically linked when they are on the same chromo­some.

Genes that are farther away from each other are more likely to be separated during a process called homologous recomb­ina­tion, and are practi­cally unlinked

The percentage of recomb­inant offspring (recom­bin­ation frequency) = distance between genes on a chromosome

1. the lowest frequency is the double crossing over

3. find the pairs of the other crossing over/s

5. to calculate the distance between alleles, remember that it is the recomb­ination frequency between one cross and the recomb­ination frequency of another cross. This is calculated by adding together the frequency of both alleles in the pair, and the alleles in the double cross.

Linkage can only be detected when recomb­ination frequency is less than 50%

There is a difference between ‘physi­cally linked’ and ‘genet­ically linked’ – if asked how many linkage groups there are in an organism with x chromo­somes, you cannot tell.