Two true-breeding white strains of the plant Illegitimati noncarborundum were mated, and the F1 progeny were all white. When the F1 plants were allowed to self-fertilize, 126 white-flowered and 33 purple-flowered F2 plants grew. a. Using the ratio of these offspring, determine how flower color is inherited? b. Define allele symbols.. Indicate (using these symbols) the genotypic classes and the corresponding phenotypic ratios of the F2. c. A white F2 plant is a allowed to self-fertilize. Of the progeny ¾ are white-flowered, and ¼ are purple-flowered. What is the genotype of the white F2 plant? Briefly explain how you determined your answer.

Answer and Explanation:

Suppression: refers to an allele of a gene that can annul the expression of the other gene. The suppressor gene might be associated with its own particular phenotype (such as color) or it might not have any detectable effect other than suppressing the other gene expression.

Available data:

a. Using the ratio of these offspring, determine how flower color is inherited?

There are two genes involved in the color expression, we might define them as C gene for color, and W gene for suppression of the color. So, W inhibits the expression of C. The dominant C allele expresses purple, while the recessive allele does not express color.

As there are two genes interacting, there are 16 possible offspring produced. So, if the total individuals in the F2 are 159 (126 + 33), then:

159 individuals in the F2 100% progeny 16

126  white-flowered individuals79.25% 12.68

33 purple-flowered individuals 3.32 % 3.32

These proportions approximate to the proportion 13:3, corresponding to the genetic interaction "suppression".

b. Define allele symbols. Indicate (using these symbols) the genotypic classes and the corresponding phenotypic ratios of the F2.

As we mentioned before, the proposed symbols for these genes are C for the color gene and W for the suppressor gene. The dominant allele C expresses purple, while the recessive allele c does not express any color. W allele inhibits C, and w allele does not inhibit c.

1st Cross:

Parental)  WWCC      x      wwcc

Gametes) WC  WC   WC  WC

                 wc   wc   wc   wc

Punnet square)     WC       WC      WC       WC

                    wc   WwCc  WwCc  WwCc  WwCc

                    wc   WwCc  WwCc  WwCc  WwCc

                    wc   WwCc  WwCc  WwCc  WwCc

                    wc   WwCc  WwCc  WwCc  WwCc

F1) 100% WwCc

2nd Cross:

Parental)  WwCc      x      WwCc

Gametes) WC  Wc   wC  cc

                WC  Wc   wC  cc

Punnet square)      WC         Wc        wC        wc

                    WC   WWCC  WWCc   WwCC  WwCc

                    Wc    WWCc   WWcc   WwCc   Wwcc

                    wC   WwCC   WwCc   wwCc   wwCc

                    wc    WwCc    Wwcc    wwCc   wwcc

F2)  White-flowered plants 13/16 = 9/16 + 3/16 + 1/16

      W-C- 9/16

      W-cc 3/16

      wwcc 1/16

      Purple-flowered plants 3/16

      wwC-

c. A white F2 plant is allowed to self-fertilize. Of the progeny ¾ are white-flowered, and ¼ are purple-flowered. What is the genotype of the white F2 plant? Briefly explain how you determined your answer.

3/4 = 12/16 white

1/4 = 4/16 purple

To get the proportion 3:1 we should self-fertilize and individual that is heterozygous for one of the genes and homozygote for the other gene. As we have 1/4 of purple-flowered plants, we can assume that there must be present the dominant allele C for purple color. So, the homozygote dominant allele should belong to the C gene, and the W gene should be heterozygous one needed to get the required proportion. The cross would be like:

Parental)   WwCC    x    WwCC

Gametes)  WC   wC   WC   wC

                 WC   wC   WC   wC

Punnet square)    

                 WC      wC         WC         wC

  WC     WWCC  WwCC  WWCC  WwCC

  wC     WwCC   wwCC   WwCC  wwCC

  WC     WWCC  WwCC  WWCC  WwCC

  wC     WwCC   wwCC   WwCC  wwCC

F3) White-flowered plants

      4/16 WWCC

      8/16 WwCC

      Purple-flowered plants

      4/16 wwCC