Mendelian Genetics

Genetics Problem Set -- Campbell

In the 1860's, Gregor Mendel developed a theory for inheritance based on statistical analysis of empirical data he collected for the garden pea plant.

Why study genetics?

Essential Genetic Definitions

Allele. a particular form of a gene at a given location on a chromosome.
Geneticists use the term character for a heritable feature, such as flower color, that varies among individual. Each variant for a character such as purple or white is called a trait.
Dihybrid cross. a cross where two different pairs of alleles are involved.
Genotype. the total combination of an organism's genes.
Homozygous. a genotype where both alleles are the same.
Heterozygous. a genotype where the alternate alleles are different.
Monohybrid cross. a cross where one pair of alternate alleles is under consideration.
Pedigree. A family tree describing the occurrence of heritable characteristics in parents and offspring across as many generations as possible. Take a look at what the symbols in a pedigree chart mean.
Phenotype. the combination of observed or measured traits, generally what is readily visible.
The symbols P1, F1 and F2 are used to designate first and subsequent generations in crosses.
True-breeding individuals are homozygous, alternate alleles are identical, offspring are identical to parents.

Some resources for Mendel's Genetics.

Visit Java Genetics for dynamic illustrations of various genetic problems.


Lecture Notes

Mendel's Crosses

Mendel used a quantitative approach (statistics) to reveal the effects of crossing different strains of the common garden pea.

He used seven different characteristics, each located on a different chromosome. These include seed form, color of seeds, color of seed pods, shape of ripe pods, length of stem, position of flowers, and color of flowers.

He developed techniques to cross pollinate true-breeding strains manipulating pollen by hand to avoid self-pollination. When a trait is true-breeding both alleles are the same and the individual is homozygous. Self-pollination results in offspring identical to the parent. Pea plants contain both male and female parts in the same flower.


What Mendel Discovered

  1. "Factors" or genes are responsible for all heritable characteristics. Phenotype is based on Genotype.
  2. Each trait is based on two genes one from the mother and the other from the father.
  3. True-breeding individuals are homozygous -- that is both alleles in question are the same.
  4. The Principle of Dominance. When different alleles for a characteristic are inherited (heterozygous) the trait of only one (the dominant one) will be expressed. The recessive trait's phenotype only appears in true-breeding (homozygous) individuals
  5. The Law of Segregation. Each genetic trait is produced by a pair of alleles which separate (segregate) during reproduction
  6. The Law of Independent Assortment. Each factor (gene) is distributed (assorted) randomly and independently of the genes for other characteristics.

Study Outline for Mendel's Genetics (Daniels)

Links to other genetics lectures

Links to Genetics self tests

Links to Genetic Simulations

  • Virtual Fly Lab - Bio 114
  • Note: the Cal. State LA Virtual Fly Lab is no longer available online.

Information about Gregor Mendel

  • MendelWeb - The life and works of Gregor Mendel

A link to many genetic links


Modified Dec. 6 2002