The Work of Gregor Mendel Biology Honors

What is inheritance?  Heredity: Passing of characteristics from parents to offspring  Genetics Study of how those characteristics are passed on

200+ years ago  People knew that we resemble our ancestors Traits are passed on from generation to generation The question became – HOW?

Gregor Mendel  Father of Genetics 1860’s  Austrian monk  Studied genetic traits of peas and how traits are passed on (parent  offspring)

Why study peas?!? 1. Many varieties (traits): 7 traits of focus Traits showed complete dominance 2. Control reproduction: cross-pollinate/hybrid or self-pollinate/purebred 3. Short lifespan: Three generations in only three years P-generation, F1-generation, F2-generation

1. Traits

2. Control Reproduction  Pea flowers have both male (stamen) and female (carpel) parts  Plant can self-pollinate Its own stamen fertilizes its own carpel Mendel controlled this by removing stamen  Plant can cross-pollinate Stamen from one plant fertilizes the carpel of another Mendel used a paintbrush to control which plant bred with which so he could follow specific traits

3. Short lifespan  3 generations: P, F1, F2  One year per generation

Mendel’s Experiment #1 (P generation  F1 generation)  Trait: plant height Tall, short  Self-pollinated pea plants for many generations  Phenotypes: Purebred tall, purebred short  Genotypes: Purebred tall (TT), purebred short (tt)

Remember Alleles…  Organisms contain 2 alleles for each trait One from mom, one from dad (2n zygote) Only pass on 1 allele to offspring (1n gamete)

 Tall x short  All offspring were tall Hybrids (Tt) Tall alleles are dominant to short alleles Mendel’s Experiment #1 (P generation  F1 generation)

Mendel’s Experiment #2 (F1 generation  F2 generation)  Hybrid x Hybrid  Tall (Tt) x Tall (Tt)  Phenotype 3 tall 1 short  Genotype 1 TT 2 Tt 1 tt

Mendel’s Experiments  Notice the ratios!  F1 – genotype is always 100% heterozygous; phenotype is always 100% dominant trait  F2 – genotype is 1:2:1 homozygous : heterozygous : recessive; phenotype is always 3:1 dominant : recessive

Review:  When Mendel crossed pure (homozygous) plants with two different traits (ex: purple x white):  He always found the same pattern – Only one trait showed in the F 1 generation BUT… Missing trait showed up again in the F 2 generation in a 3:1 ratio

Principle (Law) of Dominance  Some alleles are dominant, others are recessive  Dominant alleles are always expressed  Recessive alleles are “hidden” in the presence of a dominant allele

Principle (Law) of Segregation  When F1 plants made gametes, their alleles for purple and for white separated  When these gametes recombined to make the F2 generation, the recessive trait reappears in ¼ of the offspring

Principle (Law) of Independent Assortment  Alleles for different traits separate during meiosis independently (randomly)

Test Cross  How can you tell if a plant that shows the dominant trait is homozygous (pure) genotype or heterozygous (hybrid) genotype?  Cross it with one you know the genotype for – a recessive trait  Look at your results