A gene has the alleles A and a. The egg and the two polar nuclei of an embryo sac always carry the…
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A gene has the alleles and . The egg and the two polar nuclei of an embryo sac always carry the same allele, and the two male gametes carry the same allele. A female plant is crossed with a male plant . Each of the four combinations of a female allele and a male allele has the same chance.
- What per cent of the seeds have an endosperm of genotype ?
- What per cent of the seeds have an embryo of genotype ?
- How many different endosperm genotypes are possible?
Need a reminder? The key ideaShow
One male gamete fuses with the egg and makes the zygote, . The other fuses with the two polar nuclei and makes the primary endosperm nucleus (PEN), . The two polar nuclei and the egg come from the same megaspore, so they are alike, and the two male gametes are alike. The PEN divides by mitosis, first with free nuclei and later with walls.
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. Female , male .
Plan. Enumerate the four equally likely seeds. The embryo is the female allele plus the male allele, and the endosperm is the female allele twice plus the male allele.
Step 1. The four seeds.
- female , male : embryo , endosperm .
- female , male : embryo , endosperm .
- female , male : embryo , endosperm .
- female , male : embryo , endosperm .
Step 2. The endosperm . One of the four seeds, so .
Step 3. The embryo . Two of the four seeds, so .
Step 4. The endosperm types. , , and , which are types.
Answer: ; ; .
Check. The four seeds each have a chance of , and they add up to .
Watch out. The endosperm of an female is never with two different female alleles, because the two polar nuclei are alike.
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