Plant Genetics

This document is not meant to be a substitute for a formal laboratory report. The Lab Report

Assistant is simply a summary of the experiment's questions, diagrams if needed, and data tables

that should be addressed in a formal lab report. The intent is to facilitate students' writing of lab

reports by providing this information in an editable file which can be sent to an instructor.

Many modern theories of heredity find their origins in experiments, originally conducted by Gregor

Mendel, on the crossbreeding or hybridization of garden peas. See Figure 1. Like humans, these

are diploid organisms, containing pairs of alleles or genes for each trait, located on chromosomes.

Characteristics that are inherited are called traits, and Mendel was the first person to accurately

predict how and in what relative frequency traits would be transferred from one generation to

another.

Mendel started his research in genetics by using true breeding plants in which each generation

that self-pollinated looked exactly like the previous generation. It is easier to study genetics when

only one characteristic would change, such as purple versus white flowers. When plants are

crossbred and differ in only one trait, this is called a monohybrid cross. Tobacco seeds used in

this experiment are also monohybrids because only trait will be studied and that is color.

In the parent or true breeding generation of the tobacco seeds, both alleles are the same in each

plant--either both green or both albinos. In the first generation of the cross or F1 generation, all

seeds contain two different alleles: one green and one albino. Since the green allele is dominant,

the seeds will all appear green. In the next generation of cross, the F2 generation, there are four

possible combinations of alleles. Thus, because it is possible for some seeds to contain two recessive

alleles for albino, it is possible for these recessive alleles or genes to determine the phenotype or

appearance of the seed. Through his experiments, Mendel stated the rule of dominance that says

certain alleles are dominant and others are recessive. When both the dominant and the recessive

alleles