Mendelian Genetics Gregor Mendel (1822 – 1884) performed one of the classic scientific experiments on inheritance during the mid-19 th century. Mendel performed careful experiments and interpreted quantitative data applying mathematical concepts to biology

Mendel’s Experiments Mendel examined specific characteristics of pea plant that were expressed in one of two ways: Seed shape, seed colour, pod shape, pod colour, flower colour, flower position on stem and stem length Mendel’ experiments came before knowledge of chromosomes or genes and he referred to what would be later known as genes as factors

Summary of Mendel’s Experiments 1.Inherited characteristics are controlled by factors (called genes) that occur in pairs. Each member of a pair of genes is called an allele. During cross-fertilization each parent contributes one of its alleles. 2.One factor, or allele, masks the effect or expression of another. An uppercase letter indicates the dominant allele; the recessive allele is indicated by a lowercase letter.

3.A pair of factors, or alleles, separate or segregate during the formation of sex cells. This is the Law of Segregation. Sex cells must contain only one member of the pair of factors, or alleles. Mendel was very fortunate in his selection of a species and traits to examine because these traits displayed a dominance that is not always found in most organisms.

Mendel’s Laws of Heredity 1.Each parent contributes one allele during cross-fertilization. If a pure- breeding plant self-fertilizes, each offspring receives two copies of the same allele. 2.The dominant allele is always expressed when the recessive allele is present. 3.Each pair of alleles segregates during the formation of sex cells.

Offspring of Cross Pollination

F 1 and F 2 Generations Mendel began his experiments by crossing two pure breeding plants for each form of the trait (homozygous for a particular trait) to produce a generation of hybrid plants all of which expressed the dominant form of an allele (heterozygous for a trait) This generation is called the F 1 or filial generation one

He then crossed two of the hybrid plants from the F 1 generation to produce an F 2 generation Unlike the initial cross between the two pure breed plants to produce the F 1 generation the results of the F 2 generation did not express only the dominant form of the allele

Mendel found that in the F 2 generation of an initial dominant/recessive pure breed cross for a single trait that: 1.75% of the offspring expressed the dominant trait 2.25% of the offspring expressed the recessive trait

Punnett Square A special chart is used to organize the result of a cross between the sex cells of two individuals is called a Punnett Square This chart can be used to predict the genotypes and phenotypes of the offspring Punnett squares are named for an English geneticist, Reginald Punnett.

To use a Punnett square you must: 1. Identify the genotype of the parents in a cross. 2. List the alleles for gametes produced by each parent based upon the parental genotypes. 3. Place the possible gametes of each parent along the top and sides of the Punnett square. 4. Note all possible allele combinations for the next generation within the squares.