Transcript Slide 1
The Work of Gregor Mendel Genetics • Heredity – transmission of traits from one generation to the next • Genetics – study of heredity Gregor Mendel’s Peas • Augustinian monk named Gregor Mendel in 1800s worked with peas to demonstrate the basic genetic principles we know today • Mendel’s garden peas were true-breeding, meaning that if they were allowed to selfpollinate, they would produce offspring identical to themselves • Mendel selected the pea plants that he would breed with each other. • Why pea plants? – Easy to care for, grow quickly, take up little space, etc Genes and Dominance • Mendel studied several different pea plant traits • Trait – specific characteristic, such as seed color or plant height, that varies from one individual to another • Each trait Mendel studied had two contrasting characters – Ex. Seed color – yellow or green Flower Color Pod Shape Pod Color Smooth Green Seed Shape Seed Color Round Yellow Wrinkled Green White Constricted Round Yellow Purple Smooth Purple Flower Position Plant Height Axial Tall Yellow Terminal Short Green Axial Tall Genes and Dominance (continued) • Mendel crossed plants with each of the contrasting characters and studied their offspring • Each original pair of plants were the P (parental) generation. The offspring from the cross were called the F1 generation • Offspring of crosses between parents with different traits are called hybrids • F1 generation of pea plants had the character of only ONE of the parents. The other character seemed to have disappeared… Genes and Dominance (continued) • Mendel then came to a few hypotheses: – There are alternative versions of genes (Mendel termed genes “factors) that account for variations in inherited characters (different versions of a gene = alleles) – An organism inherits two alleles - one allele from each parent – homozygous (same alleles)/heterozygous (different alleles) – Principle of Dominance – some alleles are dominant and others are recessive – (the fourth we’ll talk about in a moment) Principle of Dominance • Dominant allele – an organism with a dominant allele for a particular form of a trait will always have that form (Ex. Aa, AA = dominance) • Recessive allele – an organism with a recessive allele for a particular trait will only have that form when the dominant allele isn’t present (Ex. aa = recessive) Law of Segregation – Mendel wanted to know what happened to the recessive traits that seemed to have disappeared – Crossed F1 plants with themselves to produce F2 (second generation) plants – this caused the recessive traits to reappear – Roughly 1/4th of the F2 plants showed the recessive trait Segregation (continued) – Reappearance indicated that at some point, the recessive allele separated from the dominant allele = Segregation – Fourth Hypothesis: Law of Segregation - Two alleles are segregated from each other so that each gamete (sex cell – sperm or egg) carries only a single copy of each gene Example: Tallness vs. Shortness in Pea Plants P generation cross (true breeding) TT (tall) x x tt (short) F1 generation Tt (all tall plants produced) F1 generation cross (self-pollinated) Tt (tall) x x Tt (tall) F2 generation TT, Tt, Tt, tt (3 tall, 1 short plant produced) Principle of Dominance P Generation Tall Go to Section: Short F1 Generation Tall Tall F2 Generation Tall Tall Tall Short Principle of Dominance P Generation Tall Go to Section: Short F1 Generation Tall Tall F2 Generation Tall Tall Tall Short Principle of Dominance P Generation Tall Go to Section: Short F1 Generation Tall Tall F2 Generation Tall Tall Tall Short Gregor Mendel’s Work Gregor Mendel concluded that experimented with Pea plants “Factors” determine traits Some alleles are dominant, and some alleles are recessive which is called the Law of Dominance Alleles are separated during gamete formation which is called the Law of Segregation