11-1 The Works of Gregor Mendel Chapter 11: Introduction to Genetics

Genetics is the scientific study of heredity. Modern genetics is based on experiments by Gregor Mendel (Austria, 1822), who worked with garden peas. He had seven true-breeding stocks (true-breeding: produce offspring identical to self-pollinating parents) Then, he cross pollinated plants from two true-breeding parents differing in the same trait (ex. tall x short)

Mendel’s Work Mendel studied seven different pea plant traits trait: a specific characteristic that varies from one individual to another hybrids: offspring of crosses between parents with different forms of a trait P generation: original true-breeding parental plants F1 generation: first filial generation produced by the parental generation F2 generation: second filial generation

Mendel’s Seven True Breeding Traits Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall

Mendel’s Results F1 hybrids were not blends but had the characteristic of only one parent He concluded that traits controlled by factors (genes) each trait controlled by only one gene that occurred in two contrasting forms alleles: different forms of a gene a dominant allele: is the characteristic always expressed by an organism a recessive allele: is only expressed when a dominant allele is not present We must remember that alleles occur in pairs

P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short Go to Section:

Segregation Mendel self pollinated the F1 hybrids to produce a F2 (second filial) generation the recessive trait reappeared in ¼ of the F2 offspring (and ¾ showed the dominant trait) Mendel determined that the recessive allele must have separated from the dominant allele and paired itself with another recessive allele during pollination Law of segregation: separation of alleles during gamete formation the number of alleles divides in half when gametes are made during meiosis (gametes: reproductive cells)

P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

F1 generation segregation gametes F2 generation Tt Tt T t T t TT Tt Tt Results: TT = Tall, Tt = Tall, tt = Short 75% Tall, 25% Short

Mendel’s Terminology- VERY IMPORTANT alleles: different forms of a gene a dominant allele is the characteristic always expressed by an organism…shown by capital letter a recessive allele is only expressed when a dominant allele is not present…shown by small letter

Mendel’s Terminology- VERY IMPORTANT homozygous: organisms with two identical alleles for a trait (ex. TT or tt) heterozygous: organisms with two different alleles for a trait (ex. Tt) phenotype: the manner in which the genes for a trait are expressed (what we physically see or what it looks like) genotype: the actual genetic makeup of the individual (the “code” or letters we do not see)