1. Mendelian Genetics

2. + + = Genetics and heredity
For a long time, general ideas of inheritance were known + + =

3. + =
What was really lacking was a quantitative understanding of how particular traits were passed down to their offspring.

4. Gregor Mendel (1822-1884) German monk born in 1822
Lived in what is now the Czech Republic
Tended the garden at his monastery, conducted experiments with pea plants
Studied peas for 7 years, published results 1866; ignored until 1900
Now considered the “Father of Genetics”

5. What is a trait?
A trait is a characteristic that can vary from one individual to the next (e.g., eye color)
Mendel chose 7 pea plant traits to study:
seed shape, seed color, flower color, pod shape, pod color, flower position, plant height
He was lucky that each trait happened to be located on different chromosomes (people didn’t know about chromosomes back then)

6. Mendel's experiments First, he created true (pure) breeding pea plants
Then he experimented with different traits (one at a time)
He cross-pollinated plants with different traits, and observed the offspring
(Normally done by bees… Mendel used scissors and a paint brush)

7. Important vocabulary
True (pure) breeding = offspring always have same trait as parent (every time, no matter what!)
Self-pollination = plant fertilizes itself
Cross-pollination = one plant fertilizes another
P generation = Parent generation
F1 generation = 1st generation offspring (“filius” is Latin for “son”; offspring of P generation)
F2 generation = 2nd generation offspring (offspring of F1 generation)

8. …what size were the offspring (F1 generation)?
Mendel’s crosses
When Mendel crossed true-breeding tall plants with true-breeding dwarf plants (P generation)…
…what size were the offspring (F1 generation)?

9. The F1 generation were all tall

10. Mendel was surprised! Mendel had expected the traits to blend:
tall + dwarf = medium
Instead, the tallness of parent had masked the shortness of the other parent
tall + short = tall????

11. …what size were the offspring (F2 generation)?
Mendel then let the F1 generation self-pollinate to create the F2 generation
…what size were the offspring (F2 generation)?

12. Some of the F2 offspring were tall, and some were dwarf
The ratio was always 3 tall to 1 dwarf (3:1)

13. Mendel's findings
When Mendel crossed true-breeding plants with different traits, all of the F1 plants showed only one trait (e.g., all tall); the F2 plants showed a 3:1 ratio
He did not get “medium” plants!
He called the traits dominant (showed in F1 generation) or recessive (didn’t show up in the F1 generation)

16. Mendel’s conclusions Mendel realized that:
Inheritance of traits is controlled by genes contributed by each parent (e.g., flower color)

17. Mendel’s conclusions
These genes come in different forms, called alleles (e.g., purple or white flowers)

18. Mendel’s conclusions
Some alleles are dominant while others are recessive
When both types of alleles are present, dominant alleles mask (cover up) recessive alleles
Mendel’s
Discoveries
& Results

19. Mendel’s Law of Dominance:
Generally, one allele is dominant to another
Dominant allele always expressed, even if combined with recessive allele
written as uppercase letter of the trait
e.g., tall = dominant: TT, tT, or Tt
Recessive allele expressed only if dominant allele is not present.
written as a lowercase letter of the dominant trait
e.g., dwarf = recessive: tt

20. More vocabulary…
Genotype = genetic combination of alleles (e.g., TT, Tt, tt)
Phenotype = physical characteristic or trait (e.g., tall, short)
Homozygous = both alleles are the same (e.g., TT or tt) → “true breeding”
Heterozygous = alleles are different (e.g., Tt)
Hybrid = offspring of crosses of parents with different traits (e.g., offspring of TT and tt)

21. Mendel’s Law of Segregation
When gametes form, the alleles from each parent are segregated (separated) from each other
Each gamete carries a single allele for each gene

22. Probability of F2 generation being TT?1 2 3 4 5 6 8 7 9 10 11 12 13 14
Parents (P)
gametes F1
gametes
Probability of F2 generation being TT? F2
Circle #
Genotype
Hetero/Homo-zygous
Phenotype
Generation
Gamete?
(y / n) 1 3 7 14 TT
homozygous
tall no P T
n/a
n/a
n/a
yes Tt
heterozygous
tall F1 no tt
homozygous
short F2 no

23. Punnett Squares
Instead of doing all of that hard work for each of the possible outcomes under the Law of Segregation, we can do a Punnett Square (geneticist Reginald Punnett created it)
Mother’s Gametes
Father's Gametes
T t T t TT Tt
There is a 1 in 4 chance of the offspring being TT
There is a 2 in 4 chance of the offspring being Tt
There is a 1 in 4 chance of the offspring being tt
3 out of 4 plants = tall
1 out of 4 plants = dwarf
Tall to dwarf = 3:1

24. Mendel’s Law of Independent Assortment
Genes for two different traits are inherited independently
There is no connection between them (e.g., plant height and seed color)
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