Question

One gene determines red vs. white flowers in pea plants, and another determines axial vs. terminal flowers. Two true-breeding stocks of pea plants are crossed. One parent has red, axial flowers and the other has white, terminal flowers; all F1 individuals have red, axial flowers. If you perform a cross between two of these F1 individuals, approximately what fraction of the offspring should have red, axial flowers?

Answer 1

Answer:

9/16.

Explanation:

Let us suppose R gene controls flower color while A gene controls flower position.

Given:

One parent has = red, axial flowers with genotype RRAA

Other parent has = white, terminal flowers with genotype rraa

F1 individuals = all are red, axial flowers.

Since all F1 progeny are red, axial flowers then it means that red is dominant over white color and axial position is dominant over terminal position.

The parent which has 'RRAA genotype' will produce 4 identical gametes with 'RA allelic' combination while the parent which has 'rraa genotype' will produce 4 identical gametes with 'ra allelic' combination.

When both these gametes will fuse, they may produce 16 progeny with same 'genotype RrAa'. The cross is depicted as under:

Parental genotype:      RRAA  x   rraa

                                         I              I                    

Gametes:                        RA           ra

F1 genotype:                      RrAa

When these F1 individuals will be crossed with each other, again 16 progeny will be produced but this time they will have genotypes which are different from each other. The cross is attached:

So it is clear from the punette square diagram of this dihybrid cross that out of the 16 probable progeny, there will be 9 progeny which will have red and axial flowers. So, the fraction of progeny will be 9/16.

These are the 9 genotypes RRAA, RRAa, RrAA, RrAa, RRAa, RrAa, RrAA, RrAa & RrAa of progeny which will have red and axial flowers.

Answer 2

Answer:

Fraction of the offspring with red, axial flowers is $\frac{9}{16}$

Explanation:

Let the allele for red flower be represented by "R" and the allele for white flower be represented by "r" and R is a dominant trait over r.

Let the allele for axial flower be represented by "A" and the allele for terminal flower be represented by "a" and A is a dominant trait over a

F1 cross

RRAA * rr aa

RrAa

F2 Cross

RrAa * RrAa

RrAa

RA         Ra         rA     ra

RA RRAA RRAa RrAA RrAa

Ra RRAa RRaa RrAa Rraa

rA RrAA RrAa rrAA rrAa

ra RrAa Rraa rrAa    rraa

Possible genotypes

RRAA - 1

RRAa- 2

RrAA- 2

RrAa- 4

RRaa- 1

Rraa- 2

rrAA- 1

rrAa – 2

rraa- 1

Possible Phenotypes –  

Red & Axial – RRAA, RRAa, RrAA, RrAa

Red & Twisted – Rraa, Rraa

White and Axial – rrAA, rrAa

White and Twisted - rraa

Fraction of the offspring with red, axial flowers is $\frac{9}{16}$