Linked Genes and Linkage Mapping
If two genes are found on different chromosomes, or if they are far enough apart on the same chromosome that the chance of a crossover between them is very high, the genes are considered to be unlinked. Unlinked genes follow Mendel’s law of independent assortment.
If, however, two genes tend to “travel together” because they are near one another on the same chromosome, they are said to be linked. Linked genes do not follow Mendel’s law of independent assortment.
In this tutorial, you will compare the inheritance patterns of unlinked and linked genes.
Part A - Independent assortment of three genes
A wild-type tomato plant (Plant 1) is homozygous dominant for three traits: solid leaves (MM), normal height (DD), and smooth skin (PP).
Another tomato plant (Plant 2) is homozygous recessive for the same three traits: mottled leaves (mm), dwarf height (dd), and peach skin (pp).
In a cross between these two plants (MMDDPP x mmddpp), all offspring in the F1 generation are wild type and heterozygous for all three traits (MmDdPp).
Now suppose you perform a testcross on one of the F1 plants (MmDdPp x mmddpp). The F2 generation can include plants with these eight possible phenotypes:
•solid, normal, smooth
•solid, normal, peach
•solid, dwarf, smooth
•solid, dwarf, peach
•mottled, normal, smooth
•mottled, normal, peach
•mottled, dwarf, smooth
•mottled, dwarf, peach
Assuming that the three genes undergo independent assortment, predict the phenotypic ratio of the offspring in the F2 generation.
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