614 Chapter 18 NEL
Mendel then produced an F 2 generation by allowing the F 1 progeny to self-fertilize. He
recorded the phenotypes of all the F 2 progeny and then calculated the ratio of each phenotype
he observed. The F 2 generation had the following phenotypic ratios: 196 yellow, round
seeds; 136 green, round seeds; 136 yellow, wrinkled seeds; and 116 green, wrinkled seeds.
Figure 4shows the expected genotypes from this cross when we assume that inde-
pendent assortment occurred. The parents would produce four types of gametes. The
genotypes in nine of the 16 cells would determine yellow, round seeds (YYRR,YyRR,
YYRr, and YyRr); three of the 16 cells would determine green, round seeds (yyRRand
yyRr); three more cells would determine yellow, wrinkled seeds (YYrrand Yyrr); and
one cell would determine green, wrinkled seeds (yyrr). Since the predicted phenotypic
ratio is the same as the ratio that Mendel observed, this cross also provides evidence for
independent assortment.GametesYYRR YyRR YYRr YyRrYyRRYYRrYyRryyRRYyRryyRrYyRrYYrrYyrryyRrYyrryyrrYRyRYryrYR yR Yr yrFigure 4
From the Punnett square analysis,
self-fertilization of the F 1 generation will
result in an F 2 generation with a 9:3:3:1
ratio. This ratio can only result if
segregation of alleles and independent
assortment of chromosomes occurs.Probability and Dihybrid Crosses
We can determine the probability of particular phenotypes and genotypes in the progeny
of dihybrid crosses in much the same way as for monohybrid crosses. Probability values can
be used to predict the chances of getting a particular genotype or phenotype in an offspring,
or to tell us whether two genes are likely to be located on different chromosomes. In dihybrid
crosses, however, we are interested in finding out the probability that two outcomes will occur
at the same time.Recall that probability (P) is given byP
number of ways that a given outcome can occur
total number of possible outcomes
Purpose Design Analysis
Problem Materials Evaluation
Hypothesis Procedure Synthesis
Prediction EvidenceTo perform this investigation, turn to page 620.
Genetics of Corn
Use Punnett squares and phenotypic ratios to analyze the
inheritance of two traits in corn.
INVESTIGATION 18.2Introduction Report Checklist