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In roosters, two genes determine the shape of their comb. The R_pp produces a rose comb, rrP_ produces a pea comb. When both genes have dominant alleles (R_P_), a walnut comb is produced. When both genes are recessive (rrpp), a single comb is produced.
a. What F1 genotypic and phenotypic ratios are obtained from a cross between a pure breeding rose comb rooster (RRpp) and a hen with alleles for pure-breeding pea comb (rrPP).
b. What genotypic and phenotypic ratios are seen in a cross between two F2 individuals? Show your work for full credit.

Respuesta :

The genotype ratio is the proportion of different genotypes observed in the next generation from a genetic cross. The phenotypic ratio is the proportion of phenotypic traits observed from a genetic cross.

In this case, the genetic cross between a rose comb rooster (RRpp) and a pea comb rooster (rrPP) will produce the F1 generation:

  • Parental gametes: Rp; rP
  • Cross: Rp x rP

F1 Generation:

  1. Genotype: 100% RrPp
  2. Phenotype: 100% walnut comb (R_P_)

F1 gametes:

  • 1/4 RP; 1/4 Rp; 1/4 rP; 1/4 rp

Punnett square segregate alleles (F2):

                   1/4 RP             1/4  Rp               1/4 rP              1/4  rp

1/4 RP      1/16 RRPP        1/16 RRPp          1/16 RrPP         1/16 RrPp

1/4 Rp     1/16 RRPp         1/16 RRpp         1/16 RrPp         1/16 Rrpp

1/4 rP       1/16 RrPP          1/16 RrPp          1/16 rrPP          1/16 rrPp

1/4 rp       1/16 RrPp          1/16 Rrpp          1/16 rrPp           1/16 rrpp

Hence, the genotypic and phenotypic ratios in a cross between two F2 will be:

Genotypic ratio F2:

  • 1/16 RRPP
  • 1/8 RRPp
  • 1/16 RRpp
  • 1/8 RrPP
  • 1/4 RrPp
  • 1/8 Rrpp
  • 1/16 rrPP
  • 1/8 rrPp
  • 1/16 rrpp

Phenotypic ratio F2:

  • 9/16 walnut comb (R_P_)
  • 3/16 rose comb (R_pp)
  • 3/16 pea comb (rrP_)
  • 1/16 single comb (rrpp)

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