In a plant, two genes lie so close together on one chromosome that crossing over never separates…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
In a plant, two genes lie so close together on one chromosome that crossing over never separates them. A plant is heterozygous for these two genes, with A and B on one chromosome and a and b on the other. It pollinates itself, and F2 plants grow. A and B are dominant.
- How many F2 plants show both dominant traits?
- How many show one dominant trait and one recessive trait?
- How many show both recessive traits?
For comparison, the genes on different chromosomes would give , , and .
Need a reminder? The key ideaShow
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. AB over ab, no crossing over, F2 plants.
Plan. Find the gametes that the plant can make. Then pair them.
Step 1. Gametes. With no crossing over, the plant makes only the parental gametes AB and ab, half each.
Step 2. The square. Pairing gives AB with AB (AABB), AB with ab (AaBb, twice) and ab with ab (aabb). So three boxes are AB and one box is ab.
Step 3. (a) Both dominant: plants.
Step 4. (b) Kinds like Aabb or aaBb need a recombinant gamete, which cannot form: plants.
Step 5. (c) Both recessive: plants.
Answer: plants; plants; plants.
Check. . The ratio is , and that is a monohybrid ratio, as if there were only one gene.
Watch out. Complete linkage is rare. In real crosses a few recombinants usually appear.
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