Mendel’s Experiment

& Punnett Squares

Mendel Knew � Mendel found true breeding pea plants

with respect to pea color

�  Yellow Male X Yellow Female = Yellow Offspring

� Green Male X Green Female = Green Offspring

�  –

His Experiment… �  For his parent test generation Mendel

Crossed:

�  Yellow Pea Male X Green Pea Female

�  All offspring (F1 generation) were yellow

� Green trait had disappeared!

His Experiment… � Mendel decided to see if the trait was truly

gone or just hidden

� Mendel took Male and Female parts from various F1 plants and crossed them (F2 generation)

�  F2 = F1 yellow X F1 yellow = Some green and some yellow.

�  The green trait returned

Conclusions � Mendel completed thousands of

experiments

� Dominant: The form of the allele that appeared in the F1 generation

�  Recessive: The form of the allele that was hidden in the F1 generation

�  In our example: � Yellow seed color= dominant � Green seed color= recessive

�  Pure breeding yellow peas would have to have: �  Both yellow alleles �  We call this homozygous Dominant

�  Pure breeding green peas would have to have: �  Both green alleles �  We call this homozygous Recessive

�  The F1 generation would have: �  1 green allele and 1 yellow allele �  We call this heterozygous

�  If we used the letter Y to represent the alleles then: �  A dominant allele would be Y �  A recessive allele would be y

�  The genotype (or combination of genes) for the yellow parent would be: �  YY

�  The genotype for the green parent would be: �  yy

�  The genotype for the F1 generation would be �  Yy

�  The phenotypes would then be the trait that is expressed from the genotype

�  YY would express as: � Yellow

�  yy would express as: � Green

�  Yy would express as: � Yellow

Mendel’s Law and how we show it

�  The Punnett Square! �  Developed by Dr.

Reginald Punnett

�  P generation for pea color

�  1 Parent goes on top

�  2nd parent goes on side

�  Parents sort to show chance of each outcome

Y

Y

Y

Y

YY

y

y

y

y y

y

Genotype: 100% Yy Phenotype: 100% Yellow

Mendel’s Law �  The Punnett Square!

�  F1 generation cross for pea color

�  1 Parent goes on top

�  2nd parent goes on side

�  Parents sort to show chance of each outcome

Y y

Y

y

Genotype: Phenotype:

Now you try… �  Straight hair is

dominant to curly.

�  Cross a heterozygous parent with a homozygous recessive parent.

�  What is the genotypic and phenotypic ratio?

More practice… �  Blue eyes are

dominant to green eyes.

�  Cross a Heterozygous male with a Homozygous Dominant female.

What is the genotypic and phenotypic ratio?

Even more practice… �  Round seeds are

dominant to wrinkled seeds.

�  Cross a heterozygous male with a homozygous recessive female.

�  What is the genotypic and phenotypic ratio?