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Summarizing Mendel’s Experiments and the Laws of Probability

Summary

Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced F1 offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P0 parent. Reciprocal crosses generated identical F1 and F2 offspring ratios. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.

Two rules in probability can be used to find the expected proportions of offspring of different traits from different crosses. To find the probability of two or more independent events occurring together, apply the product rule and multiply the probabilities of the individual events. The use of the word “and” suggests the appropriate application of the product rule. To find the probability of two or more events occurring in combination, apply the sum rule and add their individual probabilities together. The use of the word “or” suggests the appropriate application of the sum rule.

Glossary

blending theory of inheritance

hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance

continuous variation

inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them

discontinuous variation

inheritance pattern in which traits are distinct and are transmitted independently of one another

dominant

trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait

F1

first filial generation in a cross; the offspring of the parental generation

F2

second filial generation produced when F1 individuals are self-crossed or fertilized with each other

hybridization

process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring

model system

species or biological system used to study a specific biological phenomenon to be applied to other different species

P0

parental generation in a cross

product rule

probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone

recessive

trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed

reciprocal cross

paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross

sum rule

probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities

trait

variation in the physical appearance of a heritable characteristic