The Work of Gregor Mendel

Biology Honors

8.1-8.3

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 F1

generation BUT… Missing trait showed up again in

the F2 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