Linkage, Recombination Frequency and Genetic Maps
Genes on the same chromosome are inherited together unless a crossover separates them, so the proportion of recombinant offspring measures how far apart they sit. Frequencies from several crosses can be assembled into a map order.
What a learner can do afterwards
- Calculates recombination frequency from offspring counts and states it in map units.
- Orders three genes from pairwise frequencies, and explains why the outer distance comes out short.
- Recognises a frequency near fifty per cent as unlinked rather than as a very long distance.
1 · Read
Genes that sit close together on one chromosome tend to travel into gametes as a unit. Crossing over can swap segments between the pair and break that unit apart. Count the recombinant offspring and you measure how often crossovers fall between the two genes. A frequency of 1 percent equals one map unit, also called a centimorgan.
A test cross gives 1000 offspring and 120 show new allele mixes. Divide 120 by 1000 to get 0.12, which is 12 percent. The two genes sit about 12 map units apart.
With three genes, pairwise distances reveal their order along the chromosome. The gene found in both short distances sits in the middle. The longest distance usually comes out shorter than the two short ones added up, because double crossovers pass unseen.
A frequency near 50 percent means the genes are unlinked, on different chromosomes or very far apart. Do not read it as a very long mapped distance. Only values below 50 percent signal linkage.
Count recombinants to get map units, order three genes by pairwise distances, and read near 50 percent as unlinked.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.