Mendelian Genetics The Basics

Gregor Mendel Mendel was an Austrian monk who published his research on the inheritance of pea plant characteristics in 1866.

Mendel studied the inheritance of several traits by crossing true-breeding plants. He studied –Flower color –Plant height –Seed color –Seed shape –Pod shape –Pod color –Flower position –And many others.

He carefully controlled each cross by manipulating the reproductive structures of flowers And carefully observed the results. In this case, all of the F 1 generation were purple, which contributed to the development of the rule of dominance.

Dominant and recessive alleles Different forms of a gene are called alleles. In some cases, one form is dominant to the other, and will be expressed in the organisms appearance. In the example here, purple flowers are dominant to white flowers.

When two F 1 plants are crossed, we get a ratio of individuals that look like either one of the original parent plants. This led to the development of Mendel’s Law of Segregation

Mendel’s Law of Segregation This is the idea that every organism has two copies of each trait. When reproducing, the offspring receives one copy from each parent, but not both. We now know that this is the result of meiosis and production of haploid gametes.

Genotype and Phenotype Based on the laws of dominance and segregation, it is possible for different combinations of genes (genotypes) to produce the same physical appearance (phenotype).

In this example, plants with the genotype PP or Pp have purple flowers, while the genotype pp produces plants with white flowers.

Organisms with two of the same alleles are called homozygous, those with different alleles are called heterozygous. Homozygotes can be Homozygous dominant (PP) or homozygous recessive (pp).

Punnett Squares Punnett squares can be used to predict the outcome of genetic crosses. The possible gametes from each parent are placed on adjacent sides of the grid. The intersecting grids will then show possible genotypic results of the cross.

Probable results In this cross we get a probable result of 3 purple flowers to 1 white flower. –The phenotype ratio is 3:1 However, we have one that is homozygous dominant, two that are heterozygous, and one that is homozygous recessive. –The genotype ratio is 1:2:1

Punnet Square Practice In Pea plants, tall (T) is dominant to short (t). Use a punnett square to predict the genotype and phenotype ratios for a cross between a pure breeding tall plant (TT) and a pure breeding short plant (tt).

All Tall, All Heterozygous TT tTt t

Perform a cross between two heterozygous tall plants.

TT TTTTt t tt Genotype Ratio = 1:2:1 Phenotype ratio 3:1

How about a heterozygous tall plant with a homozygous short plant?

Tt tTttt tTttt

Law of Segregation States that each organism has two copies of every gene, but that only one of those