1) In tomatoes, stem color andfruit color are determined by two independently as

Question

1)      In tomatoes, stem color andfruit color are determined by two independently assorting genes. Atone gene, purple stems (P) is dominant to green stems(p). At the other gene, yellow fruit (Y) isdominant to red fruit (y). Consider a cross between twotomato plants heterozygous for both these alleles:

a.       Specify the genotypes ofall gametes that can be made by one of theseplants, and in what proportions they will occur. (Note: Give thegenotypes of the gametes, not the offspring.)

b.      What proportion of theoffspring are expected to be heterozygous for both genes?

c.       What proportion areexpected to have genotype pp Yy?

d.      What proportion are expected tohave green stems?

e.       What proportion areexpected to have yellow fruit?

f.       What proportion areexpected to have purple stems and red fruit?

g.      What proportion are expected tohave purple stems and yellow fruit?

h.      If a double heterozygous plantwas crossed with a plant that has green stems and red fruit, whatoffspring phenotypes would be expected and in what proportions?

Explanation / Answer

Consider a cross between two tomato plants heterozygousfor both these alleles:

purple stems (P) is dominant to green stems(p).

heterozygous-Pp

yellow fruit (Y) is dominant to red fruity

heterozygous-Yy

                                   Pp Yy       x              Pp Yy       

P Y     Py      pY  py                       P Y     P y     pY  py                       -gametes

¼    ¼            1/4      1/4              ¼               1/4                           1/4                    1/4

gametes

P Y

P y

pY

py

P Y

P Y P Y purple yellow

P Y P y

purple yellow

P Y pY

purple yellow

P Y py

purple yellow

P y

P Y P y

purple yellow

P y P y

purple yellow

P y pY

purple yellow

P y py

purple yellow

pY

P Y pY

purple yellow

P y pY

purple yellow

pY pY

green yellow

pY py

green yellow

py

P Y py

purple yellow

P y py

purple red

pY py

green yellow

py py

green red

Specify the genotypes of all gametes that canbe made by one of these plants, and in what proportions they willoccur. (Note: Give the genotypes of the gametes, not theoffspring.)

P Y     Py      pY  py           x           P Y     P y     pY  py                       -gametes

¼    ¼            1/4      1/4              ¼            1/4                  1/4                 1/4

b.           What proportion of the offspring are expected to be heterozygousfor both genes?

                       = 3/16

c.            What proportion are expected to have genotype pp Yy?2/16

d.      What proportion are expected tohave green stems? 4/16

e.       What proportion areexpected to have yellow fruit?14/16

f.       What proportion areexpected to have purple stems and red fruit?1/16

g.      What proportion are expected tohave purple stems and yellow fruit? 11/16

gametes

P Y

P y

pY

py

P Y

P Y P Y purple yellow

P Y P y

purple yellow

P Y pY

purple yellow

P Y py

purple yellow

P y

P Y P y

purple yellow

P y P y

purple yellow

P y pY

purple yellow

P y py

purple yellow

pY

P Y pY

purple yellow

P y pY

purple yellow

pY pY

green yellow

pY py

green yellow

py

P Y py

purple yellow

P y py

purple red

pY py

green yellow

py py

green red

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