Mendel and His Discoveries

Chapter 11

Gregor Mendel

Gregor Mendel (1822-1884) Experimented with pea plants and developed fundamental rules of genetics and patterns of inheritance.

Punnett Squares

Crosses between parents that differ in only one trait are called MONOHYBRID CROSSES.

P= Parents F1= First Filial generation (kids) F2= Second Filial generation

(grandkids)

Mendel and Pea Plants

For his plant experiment he crossed a tall plant with a short plant.

All of the offspring looked tall. Why?

Because the tall plants had a dominant trait and the short plants had a recessive trait.

Alleles

Alleles are simply versions of genes

THE ALLELES FOR COW FUR COLOR ARE BLACK. BROWN, AND WHITE

Prediction of Genetic Crosses

The alleles for tallness of a plant: TT= Dominant (Tall) tt= Recessive (Short)

How do we know for sure that they would all be tall?

The Punnet Square

Punnett Squares

Tool to predict outcomes of genetic crosses

Make a tic-tac-toe board Place the parent alleles like below:

TALL PARENT PLANT SHORT PARENT PLANT

Let’s try another one…

If the trait is for feather color of parrots, then GG= Dominant and is green.

gg= recessive and is gold.

GG allele is homozygous dominant; (Homo- same; zygous-sex cell; dominant- dominant)

gg allele is homozygous recessive; (Homo- same; zygous- sex cell; recessive- recessive)

So from the cross above we get all Gg. So are they Green or gold or a mix of both?

They are all green. Gg is called heterozygous. Heterozygous means different.

If a G and a g are together to make a heterozygous trait, the trait looks like the dominant gene or G. So, ALL the parrots look green.

But what about the KIDS of these new, green parrots?

Genetics of the new, GREEN (F1) parrots:

Gg x GgG g

G GG Gg

g Gg gg

Ratios?

Phenotype: the “look” of the genes Genotype: the “letters” or alleles of

the organism

GENOTYPE RATIOS:

1 GG, 2Gg 1gg

PHENOTYPE

RATIOS:

3:1

Why we needed the math: The Dihybrid Cross

Lets say that in peas, We cross a Homozygous dominant Smooth, yellow seed with a Homozygous recessive wrinkled, green seed. What would be the genotype and phenotype of the F1 generation?

Setting it up

SS= Smooth YY= yellow ss= wrinkled yy= green

So we are crossing a SSYY x ssyy

Ready, set, go!

Most important question: What are all of the possible sperm and egg that each plant can produce with those alleles? (huh?)

SSYY Plant ssyy Plant

SY sy

The product of this cross will all be: SsYy, or all Smooth, Yellow peas

(F1)

But what about the F2 generation?

Ask the Important Question again!

Most important question: What are all of the possible sperm and egg that each plant can produce with those alleles?

SsYy

PEA SEED

SY

Sy

sY

sy

9:3:3:1

Incomplete Dominance

What do you get when you cross a homozygous dominant red rose with a homozygous white rose?

Incomplete Dominance

A Pink Rose!

Since all of the F1 are Rr= pink, what would be the F2 generation of all of the F1 pink roses?

1 Red: 2 Pink: 1 White

R r

R RR Rr

r Rr rrrr

Codominance

Codominance- when both alleles contribute to the phenotype of the organism

Multiple Alleles

When three or more alleles of the same gene exist in a population i.e. blood types

(A, B, O)

Continuous Variation

- The range of small differences of a single trait in a population.

It is usually where several genes effect a single trait.