Blending hypothesis

• Originally, organisms were thought to be a blend of their parents’ characteristics.

• Pure-breeds were defined as organisms that looked identical to their parents, and they looked like their parents, etc.

• So now, using these two pieces of information, pretend you are a young scientist in the early 1800’s. You cross a purebred red flowering plant with a purebred white flowering plant. What do you expect to find?

THE ACTUAL RESULTS…

Mendelian Genetics

Father of Modern Genetics

• Austrian monk, high school teacher, and part-time garden keeper

• First to propose biological inheritance of traits

• Work not recognized until after his death

• Knew nothing about DNA

Why pea plants?

• It’s what he had• Pea plants are true

breeding• Gregor noticed that one

stock of seed would produce only tall plants and another only short plants

P generation

Cross-pollination

Next, Mendel wanted to find out if the recessive alleles had disappeared, orif were they still present in the F1 generation.

VOCAB BREAK…

• Traits• Genes• Alleles• Genotype• Phenotype

Segregation• Mendel crossed the F1

generation with itself• He found that the

recessive traits reappeared in some of the F2 generation

• He proposed this was due to two things: the principle of dominance and segregation of alleles during formation of gametes

Probability

• Mendel categorized and counted the many offspring of each of his experiments.

• He noticed that each time he repeated a particular cross he obtained similar results.

For example:Every time he crossed two

plants that were heterozygous for stem height (Tt), about ¾ of the offspring were tall and ¼ were short.

Probability and Punnett Squares

• Punnett squares are used to predict this probability

• Monohybrid crosses can be performed to determine probability of genotype for one particular trait

Genotypes

AAAa

aa

Homozygous Dominant

Heterozygous

Homozygous Recessive

Law of Segregation• Mendel crossed the F1

generation with itself• He found that the

recessive traits reappeared in some of the F2 generation

• He proposed this was due to two things: the principle of dominance and segregation of alleles during formation of gametes

Law of Independent Assortment

• Genes that segregate independently do not influence each other’s inheritance