In the garden pea, a tall stem (T) is dominant over a dwarf stem (t), and yellow seeds (Y) are…
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In the garden pea, a tall stem (T) is dominant over a dwarf stem (t), and yellow seeds (Y) are dominant over green seeds (y). The two genes assort independently. A true-breeding tall plant with green seeds is crossed with a true-breeding dwarf plant with yellow seeds, and the F1 plants are self-pollinated. The figure shows the Punnett square for the F2 generation, with the gametes of the F1 plants along its top and side.
- How many of the boxes hold a tall plant with green seeds?
- What percentage of the F2 plants show a combination of the two characters that neither parent of the F1 had?
- Out of F2 plants, how many are expected to be tall with yellow seeds and to breed true for both characters?
- Among the F2 plants that are tall with green seeds, what percentage are heterozygous for stem height?
- An F1 plant is test crossed with a dwarf plant with green seeds. What percentage of the offspring are dwarf with yellow seeds?
The worked solution
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The F1. TTyy (tall, green) × ttYY (dwarf, yellow) gives only TtYy plants: tall with yellow seeds. Each F1 plant makes four kinds of gametes in equal numbers, TY, Ty, tY and ty, because the genes assort independently.
(a) Tall with green seeds. These need at least one T and the genotype yy. In the square they are TTyy (box 6, Ty × Ty) and Ttyy (boxes 8 and 14, Ty × ty and ty × Ty): boxes.
(b) New combinations. The F2 phenotypes are tall yellow, tall green, dwarf yellow and dwarf green in the ratio . The parents had tall green and dwarf yellow, so the new combinations are tall yellow () and dwarf green (): .
(c) True-breeding tall yellow. Only TTYY breeds true for both characters. It fills box out of : plants.
(d) Heterozygous among tall green. Of the tall green boxes, is TTyy and are Ttyy: .
(e) Test cross. TtYy × ttyy gives TtYy, Ttyy, ttYy and ttyy in equal numbers. The dwarf plants with yellow seeds are ttYy: .
Answer: (a) boxes. (b) . (c) plants. (d) . (e) .
Check. Treat the two genes one at a time. Tall is and green is , so tall green is : boxes, as counted. Among tall plants, are Tt, whatever the seed colour.
Watch out. Which combinations are "new" depends on the parents. In Mendel's yellow round × green wrinkled cross the new ones are yellow wrinkled and green round; here the parents carried one dominant character each, so the new ones are the double dominant and the double recessive.
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