Warm-Up

1. What is the phenotypic ratio and genotypic ratio of the offspring of a monohybrid cross?

2. What is the phenotypic ratio of a dihybrid cross?

3. List all the gametes that could be made by a pea plant heterozygous for seed color, seed shape, and pod shape (YyRrIi).

14.2: The laws of probability govern Mendelian inheritance

Mendel’s laws of segregation and independent assortment reflect the rules of probability.

When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles.

The Multiplication Rule Applied to Monohybrid Crosses

The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities.

Probability in an F1 monohybrid cross can be determined using the multiplication rule.

Segregation in a heterozygous plant is like flipping a coin: each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele.

The Addition Rule Applied to Monohybrid Crosses The addition rule states that the probability that any one of

two or more exclusive events will occur is calculated by adding together their individual probabilities.

The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous.

As a class…Two organisms, with genotypes BbDD and BBDd, are mated. Assuming independent assortment, write the genotypes of all possible offspring and use the rules of probability to calculate the chance of each genotype occurring.

As a class…In sesame plants, the one-pod condition (P) is dominant to the three-pod condition (p), and normal leaf (L) is dominant to wrinkled leaf (l). Determine the genotypes for the two parents for all possible matings producing the following offspring:

150 one-pod, normal leaf147 one-pod, wrinkled leaf51 three-pod, normal leaf48 three-pod, wrinkled leaf

On your own…Three characters are considered in a cross between two pea plants (PpYyIi x ppYyii). What fraction of offspring are predicted to be homozygous recessive for at least two of the three characteristics?