Genetics

*transmission of traits – heredity *variation *genetics

Two main hypotheses on how traits were transmitted:

*blending inheritance *particulate inheritance

Gregor Johann Mendel 1822-1884

The father of transmission genetics:

Mendel tracked heritable characters for three generations

*P - parental generation*F1 – first filial generation*F2 – second filial generation

-Example:

X

Tall Dwarf

P

F1 – all Tall

Tall

F2

Mendel’s hypotheses (to explain his results)

*genes and alleles

1. Alternative versions of genes (alleles) account for variation in inherited characters

2. For each character, an organism inherits two alleles, one from each parent

3. If two alleles differ, one is dominant, the other recessive

X

Tall Dwarf

P:

DD dd

F1 – all Tall

TallDd

4. The two alleles for each character segregate (separate) during gamete production.

Mendel’s Law of Segregation

Punnett Square predicts the results of a genetic cross between individuals of known genotype

D d d

Tall DwarfP:

DD ddX

Gamete formation: D

*genotype

*phenotype

*Homozygous

*Heterozygous

Mendel’s Law of Independent Assortment

*What happened when he looked at two characters?

If they segregate together: If they segregate independently:

Dihybrid cross- A genetic cross between two individuals involving two characters

F1

All yellow, round

GGWW ggww

Example:

P1

yellow, round

green, wrinkled

X

Punnett square and the law of independent assortment:

GW GW GW GW

gw

gw

gw

gw

GgWw

F1

All yellow, roundGgWw

F1

All yellow, roundGgWw

X

F2

9/16 yellow, round

3/16 yellow, wrinkled

3/16 green, round

1/16 green, wrinkled

Punnett square and the law of independent assortment:GW Gw gW gw

gW

GW

gw

Gw

9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:

1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw1/16 GGww, 2/16 Ggww

1/16 ggWw, 2/16 ggWw

1/16 ggww

Mendelian inheritance is based on probability

Example- coin toss

*1/2 chance landing heads

*Each toss is an independent event*Coin toss, just like the distribution of alleles into gametes

*The rule of multiplication – determines the chance that two or more independent events will occur together

½ x ½ = ¼