Testing Mendelian inheritance patterns in pea plants

Genetics

Based on the resulting F1 and F2 phenotypes, the dominant allele is yellow and the recessive green. This was the only possible conclusion given that the parents were yellow and green, the F1 phenotypes were all yellow, and the 3:1 F2 ratio of yellow to green. In contrast, incomplete dominance would have resulted in yellowish-green peas.

If 'C' represents the yellow allele and 'c' the green, then the parents were CC and cc, and the F1 generation were all Cc. Since the F1 generation was all Cc and predicted to segregate in accordance with Mendelian inheritance, the predicted ratio would be 3:1 yellow-dominant.

Trait

Observed

Expected

Obs - Exp

(O-E)^

[(O-E)^2]/E

Yellow

Green

Chi Square

Sum =

When the Chi Square test was performed on the resulting numbers, the result was zero. This suggests that there was no difference between the results and the ratio predicted by Mendelian inheritance for a dominant-recessive set of two alleles.

Experiment 2

The result of crossing PP and pp parents, and then crossing the F1 generation with each other, revealed that the dominant allele is axial (P) and the recessive is terminal (p). This conclusion is supported by getting all F1 axial flowers and an axial/terminal ratio of 3:1 for the F2 generation. If these alleles segregated according to Mendelian genetics, then the F2 generation would be predicted to result in a 3:1 ration of axial to terminal phenotypes.

Trait

Observed

Expected

Obs - Exp

(O-E)^2

[(O-E)^2]/E

Axial

75

75

0

0

0

Terminal

25

25

0

0

0

Chi Square

Sum =

0

To confirm that the axial and terminal alleles are segregating according to Mendelian genetics, a chi square test was performed on the results (see above table). The resulting chi square is zero, which means there is no difference between the results and what would be predicted according to Mendelian genetics.

If the alleles from experiments 1 and 2 were found to exist in natural world at the same ratios, then the C (yellow) and P (axial) alleles would be considered wild-type. Although many mutations result in a loss of function or expression, there are exceptions. For example, the gap-junction protein connexin 26 is encoded by a gene susceptible to both germ-line and somatic mutations leading to deafness (Welch, Marin, Pandya, and Amos 1567). Most of the 100 deafness-linked mutations in this gene are autosomal recessive, but a few are dominant.

Experiment 4

Incomplete dominance for flower color is supported non-pink parents resulting in an all pink F1 generation and 50% pink F2 generation. Complete dominance would not have resulted in any pink flowers. The genotypes of all the phenotypes are as follows: Red = FRFR; Pink = FRFW, FWFR; White = FWFW. Incomplete dominance can be shown by comparing the results of the F1 generation with what would be predicted if complete dominance were controlling the inheritance of this trait. The results are shown in the table below.

Trait

Observed

Expected

Obs - Exp

(O-E)^2

[(O-E)^2]/E

Red

28

75

-47

29.45

White

26

25

1

1

0.04

Pink

46

0

46

0

Chi Square

Sum =

29.49

The chi square obtained from this comparison is off the charts in terms of significance, which suggests there is no similarity between what would be predicted by Mendelian inheritance and these results.