Suppose you cross two plants that are both heterozygous for a given trait. On average, what fraction of their offspring will be heterozygous for the trait?

Question

BlackTom AI · Accepted Answer

Aa × Aa produces gametes A and a from each parent; a Punnett square gives the genotypes of offspring.

1) Relevant concepts
- Heterozygous means two different alleles at a locus (e.g., Aa).
- Mendelian single-gene inheritance with two alleles (A and a) and random fertilization applies.
- A Punnett square enumerates all equally likely gamete combinations.

2) Step-by-step reasoning
Write the gametes from each heterozygous parent: A and a. Set up a 2×2 Punnett square with one parent's gametes across the top and the other down the side. Fill in the four boxes by combining alleles.

3) Calculations / intermediate steps
Top gametes: A | a
Side gametes:
A → box1: A × A = AA
a → box2: a × A = aA (same as Aa)
A → box3: A × a = Aa
a → box4: a × a = aa

List of outcomes (4 boxes): AA, Aa, aA, aa.
Count heterozygotes: Aa and aA are the same genotype (heterozygous), so 2 out of 4 outcomes are heterozygous → 2/4 = 1/2 = 50%.

4) Why this answer is correct
The Punnett square enumerates all equally probable zygotes from random union of gametes; two of four possible zygote genotypes are heterozygous, so half the offspring are expected to be heterozygous on average.

5) Why other answers would be incorrect
- 25% would describe the probability of a specific homozygous genotype (AA or aa), not heterozygotes. 
- 75% might refer to the proportion showing a dominant phenotype (AA + 2×Aa = 3/4) but not the heterozygous genotype frequency.
- 0% or 100% are inconsistent with Mendelian segregation for Aa × Aa.

6) Conclusion
Therefore, the fraction of offspring expected to be heterozygous is 50% (half), matching the provided answer.

Suppose you cross two plants that are both heterozygous for a given trait. On average, what fraction of their offspring will be heterozygous for the trait?单项选择题

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