Gregor Mendel

“The Father of Genetics” Studied peas: easy and

quick to breed, can control mating

Mendel’s Pea PlantsPlant Height

Tall Short

Pod color

Seed ShapePod Shape

Seed Color

Green Yellow Green Yellow

RoundWrinkledSmooth Pinched

Mendel’s ExperimentsMendel mated “purebred” plants. (Homozygous)

Offspring always looked like parents.

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Purebred Short Parents

Purebred Tall Parents

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Short Offspring

Tall Offspring

Mendel’s First ExperimentMendel crossed purebred plants with opposite forms of a trait (like tall x short).

Called parental generation , or P generation.

Parent TallP generation

Parent ShortP generation

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Offspring TallF1 generation

All of the offspring grew to be tall plants. None resembled the short parent. He called this generation of offspring the first filial , or F1 generation, (The word filial means “son” in Latin.)

Mendel’s Second ExperimentMendel then crossed two of the offspring from the F1 generation in his first experiment.

TallF1 generation

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3⁄4 Tall & 1⁄4 ShortF2 generation

Mendel called this second generation of plants the second filial, F2, generation. It had both tall and short plants in a 3:1 ratio

Parent Plants Offspring

TT Tt

Tt

tt

T t

T

t

Genotype:

Tt x Tt 1 TT, 2 Tt, 1 tt

Mendel’s Law of Segregation

•Traits exist in different forms and one individual contains 2 factors (alleles) for each trait.

•When egg or sperm are formed, the two alleles separate (segregate) so only one factor is passed down -- at random.

Dominant and Recessive GenesMendel concluded that one factor masked the other factor.

Alleles that mask or hide other alleles, such as the “tall” allele, are said to be dominant.

A recessive allele, such as the short allele, is masked, or covered up, whenever the dominant allele is present.

What happens if you cross plants heterozygous for 2 traits?

Green Allele Yellow Allele

Round seed Allele Wrinkled seed allele

Yellow seeds are dominant over green and round seeds are dominant over wrinkled seeds.

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Law of Independent AssortmentEach pair of alleles separate independently of each other in the production of eggs and sperm.

The process is random for each—

So if there are 4 possible outcomes in a monohybrid cross (cross of heterozygotes for 1 trait)

There are 4 x 4 = 16 possible outcomes in a dihybrid cross (cross of heterozygotes for 2 traits)

Most traits are not so simple

Incomplete dominance

A heterozygous individual has an intermediate phenotype

Codominance

Characteristics of both traits show up in the phenotype

Multiple Alleles

Each allele at multiple sites contributes to a phenotype

THE END