GENETICS

Medelian Genetics

• Gregor Mendel• 1800’s

Medelian Genetics

all purple flowers result

Pollen transferred from white flower to stigma of purple flower

self-pollinate

?

F2generation

3:175%purple-flower peas

25%white-flower peas

Looking closer at Mendel’s work

Parents

100%F1generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

We use letters to represent the alleles or genes.

P = purplep = white

Purple is dominant over white. White is recessive.

Genotype Phenotype

PPPp

pp

Purple

PurpleWhite

Homozygous dominant

Heterozygous or hybrid

Homozygous recessive

Predicting Inheritance

Let’s cross a totally dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.

TT x tt

T T t t

Predicting Inheritance

Let’s cross a totally dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.

TT x tt

T T t t

Mendels’ “Law” of Segregation

Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait.

Punnett Squares

Tt Tt

Tt Tt

The genes from one parent go here.

The genes from the other parent go here.

Punnett Squares

Tt Tt

Tt Tt

Punnett squaresPp x Pp

P pmale / sperm

P

pfem

ale

/ eg

gs

PP

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

PP Pp

Pp pp pp

Pp

Pp

1st

generation(hybrids)

Aaaaah,phenotype & genotypecan have different ratios

What if you didn’t know the genotype of an individual?

• TEST CROSS

Let’s say Gray is dominant over white in mice.

If we have a gray individual, how could we determine if it is homozygous or heterozygous gray?

Cross the gray mouse by a white mouse.

What do you get?

If you get

100% all gray, then the genotype must have been

GG

1 white individual, then the genotype must have been

Gg

DihybridsMore than 2 traits are crossed at a time.

T = tallt = short

G = greeng = yellow

What is the genotype for a heterozygous tall and green pea plant?

TtGgWhat gametes can it produce?

TtGgFirstOuter

InnerLast

TG Tg tG tg

TG Tg tG tgTGTgtGtg

Dihybrid Cross