Mendel's Second Set of Experiments

Objectives

• Explain Mendel's second set of experiments
• Summarize the law of independent assortment
• Distinguish genotype from phenotype

After his first set of experiments, Mendel began to study two traits at a time. This is a dihybrid cross.

A dihybrid cross between yellow round seeds and green wrinkled seeds resulted in yellow round seeds (F1).

When F1 self-pollinated, F2 exhibited all possible combination of characteristics, even ones not present in F1 or F2.

This led to the law of independent assortment: the factors controlling different characteristics are inherited independently of each other.

Mendel published his results in 1866, but they were largely ignored.

In 1900 three other scientists independently arrived at the same conclusion.

The position of a gene on a chromosome is its locus.

In sexually reproducing organisms, each individual has two copies of the same gene because there are two version of the same chromosome (homologous chromosomes).

Different versions of a gene are called alleles, but they will always be in the same locus.

During meiosis, homologous chromosomes separate and go to different gametes.

Thus, the two alleles go to different gametes.

When gametes unite during fertilization, the resulting zygote obtains two alleles for each gene.

These alleles make up the genotype.

A homozygote is an organism with two alleles of the same type.

A heterozygote is an organism with two different alleles (e.g. Bb).

The expression of an organisms genotype is its phenotype. Multiple genotypes may result in the same phenotype.

When only one of two alleles is expressed, the dominant one is expressed. The one that isn't expressed is recessive.