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Modern Genetic Analysis Chapter 5. Recombination of Genes
Solved Problem

1. | Consider three yellow, round peas, labeled A, B, and C. Each was grown into a plant and crossed to a plant grown from a green, wrinkled pea. Exactly 100 peas issuing from each cross were sorted into phenotypic classes as follows: What were the genotypes of A, B, and C? (Use gene symbols of your own choosing; be sure to define each one.) See answer

Solution

Notice that each of the crosses is

Because A, B, and C were all crossed to the same plant, all the differences between the three progeny populations must be attributable to differences in the underlying genotypes of A, B, and C.

How much we can deduce from the data? What about dominance? The key cross for deducing dominance is B. Here, the inheritance pattern is

So yellow and round must be dominant phenotypes, because dominance is literally defined in terms of the phenotype of a heterozygote. Now we know that the green, wrinkled parent used in each cross must be fully recessive; we have a very convenient situation because it means that each cross is a testcross, which is generally the most informative type of cross.

Turning to the progeny of A, we see a 1:1 ratio for yellow to green. This is a demonstration of Mendel's first law (equal segregation) and shows that for the character of color, the cross must have been heterozygote × homozygous recessive. Letting Y = yellow and y = green, we have

For the character of shape, because all the progeny are round, the cross must have been homozygous dominant × homozygous recessive. Letting R = round and r = wrinkled, we have

Combining the two characters, we have

Now, cross B becomes crystal clear and must have been

because any heterozygosity in pea B would have given rise to several progeny phenotypes, not just one.

What about C? Here, we see a ratio of 50 yellow:50 green (1:1) and a ratio of 49 round:51 wrinkled (also 1:1). So both genes in pea C must have been heterozygous, and cross C was

which is a good demonstration of Mendel's second law (independent assortment of different genes).

How would a geneticist have analyzed these crosses? Basically, the same way we just did but with fewer intervening steps. Possibly something like this: "yellow and round dominant; single-gene segregation in A; B homozygous dominant; independent two-gene segregation in C."

See question

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