Introduction Introduction to Geneticsto GeneticsThe Work of Gregor MendelThe Work of Gregor Mendel

Genetics• Scientific study of heredity

• Gregor Mendel–Born in 1822 in Czech Republic

–Worked in monastery garden while teaching H.S. (math & science)

–Worked w/ true-breeding pea plants• If allowed to self-pollinate, produce offspring identical to themselves

Genes and Dominance

• Mendel studied 7 traits–Specific characteristic that varies

from one individual to another– color, height

–Each trait had two contrasting characters – greed seeds vs. yellow

–Crossed true-breeding plants of each type to opposing character

• True-breeding yellow seed X True-breeding green seed

• Original pair of plants = P (parental) generation

• Offspring = F1 (first filial)

• Offspring of crosses between parents w/ different traits = hybrids

Genes and Dominance

• What did the F1 hybrids look like? Did they look like a blend of both parents? No!

• All F1 offspring only showed the character of one parent.

Mendel’s Two Conclusions

• 1. Biological inheritance is determined by factors that are passed on from one generation to the next–Genes – chemical factors that determine traits

• Each trait that Mendel studied was controlled by one gene that occurred in two contrasting forms–Ex: Gene for Plant Height = One form for tall one for short

• Alleles – different forms of a gene

• 2. Principle of Dominance – some alleles are dominant and others are recessive–If there is a dominant present, that form is shown

–If there are two recessive present, the recessive is shown

Segregation• Did the recessive alleles disappear,

or were they still present ?–To find the answer, he allowed F1

generation plants to self-pollinate to create an F2 generation

• What happened? –Traits reappeared. ¼ of the F2

plants showed the recessive allele

Segregation, cont’d

• Law of Segregation–Two alleles separate from each other so that each gamete carries only a single copy of each gene

–Ex: Fig. 11-5

Introduction Introduction to Geneticsto Genetics

Genetics and Genetics and ProbabilityProbability

Genetics and Probability

• Probability–The likelihood that a particular event

will occur• Ex: tossing a quarter

–Heads = 1 in 2

–Tails = 1 in 2

–3 tails in a row ½ x ½ x ½ = 1/8

–Principles can be used to predict outcome of genetic crosses

Genetic Vocabulary

• Character– Detectable

inheritable feature of an organism

• Trait– Variant of inheritable

character

= allele / alleles

• Dominant– Capital letter (SS)

• Recessive– Lower case (ss)

• Two alleles for each character– Mom and dad

Genetic Vocabulary

• Genotype–The genetic makeup of an organism

–Written in letter format

–PP, pp, Pp

• Phenotype

–The physical traits of an organism

Genetic Vocabulary

• Homozygous–Having two

identical alleles for a given trait• PP

• ss

• Heterozygous–Having two

different alleles for a given trait• Pp

• Ss

Basic Crosses

Recessive/Dominant

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