Mendal,s Law

Mendel's First Law of Genetics (Law of Segregation)
Genetic analysis predates Gregor Mendel, but Mendel's laws form the theoretical basis of our understanding of the genetics of inheritance.

Mendel made two innovations to the science of genetics:



1. developed pure lines
2. counted his results and kept statistical notes
Pure Line - a population that breeds true for a particular trait [this was an important innovation because any non-pure (segregating) generation would and did confuse the results of genetic experiments]
Mendel's Law of Independent Assortment
To this point we have followed the expression of only one gene. Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment. First, a few terms are presented.

Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)

Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)

Again a dihybrid cross is not a cross between two dihybrids. Now, let's look at a dihybrid cross that Mendel performed.

Parental Cross: Yellow, Round Seed x Green, Wrinkled Seed

F1 Generation: All yellow, round

F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled

At this point, let's diagram the cross using specific gene symbols.

Choose Symbol Seed Color: Yellow = G; Green = g
Seed Shape: Round = W; Wrinkled = w


The dominance relationship between alleles for each trait was already known to Mendel when he made this cross. The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs.