1. 11-1 The Works of Gregor Mendel
Chapter 11: Introduction to Genetics

2. Genetics is the scientific study of heredity.
Modern genetics is based on experiments by Gregor Mendel (Austria, 1822), who worked with garden peas.
He had seven true-breeding stocks (true-breeding: produce offspring identical to self-pollinating parents)
Then, he cross pollinated plants from two true-breeding parents differing in the same trait (ex. tall x short)

3. Mendel’s Work Mendel studied seven different pea plant traits
trait: a specific characteristic that varies from one individual to another
hybrids: offspring of crosses between parents with different forms of a trait
P generation: original true-breeding parental plants
F1 generation: first filial generation produced by the parental generation
F2 generation: second filial generation

4. Mendel’s Seven True Breeding Traits
Seed Shape
Seed Color
Seed Coat
Color
Pod
Shape
Pod Color
Flower Position
Plant Height
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
Round
Yellow
Gray
Smooth
Green
Axial
Tall

5. Mendel’s Results
F1 hybrids were not blends but had the characteristic of only one parent
He concluded that traits controlled by factors (genes)
each trait controlled by only one gene that occurred in two contrasting forms
alleles: different forms of a gene
a dominant allele: is the characteristic always expressed by an organism
a recessive allele: is only expressed when a dominant allele is not present
We must remember that alleles occur in pairs

6. P Generation
F1 Generation
F2 Generation
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short

7. P Generation
F1 Generation
F2 Generation
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short
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8. Segregation
Mendel self pollinated the F1 hybrids to produce a F2 (second filial) generation
the recessive trait reappeared in ¼ of the F2 offspring (and ¾ showed the dominant trait)
Mendel determined that the recessive allele must have separated from the dominant allele and paired itself with another recessive allele during pollination
Law of segregation: separation of alleles during gamete formation
the number of alleles divides in half when gametes are made during meiosis
(gametes: reproductive cells)

9. P Generation
F1 Generation
F2 Generation
Tall
Short
Tall
Tall
Tall
Tall
Tall
Short

10. F1 generation segregation gametes F2 generation Tt Tt T t T t TT Tt Tt
Results: TT = Tall, Tt = Tall, tt = Short
75% Tall, 25% Short

11. Mendel’s Terminology- VERY IMPORTANT
alleles: different forms of a gene
a dominant allele is the characteristic always expressed by an organism…shown by capital letter
a recessive allele is only expressed when a dominant allele is not present…shown by small letter

12. Mendel’s Terminology- VERY IMPORTANT
homozygous: organisms with two identical alleles for a trait (ex. TT or tt)
heterozygous: organisms with two different alleles for a trait (ex. Tt)
phenotype: the manner in which the genes for a trait are expressed (what we physically see or what it looks like)
genotype: the actual genetic makeup of the individual (the “code” or letters we do not see)