Genetics The study of heredity - how traits are passed

from parent to offspring

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or

or

The study of heredity started with the work of Gregor Mendel and his

pea plant garden

Mendel was an Austrian Monk that lived in the mid 1800’s

Why did Mendel work with pea plants?

• Fast to grow• Easy to cross pollinate• Two distinct forms for traits

• Tall or short

• Yellow or green pods

• Smooth or pinched pods

• Yellow or green peas

• Round or wrinkled peas

• Colored or white seed coats

• Side flowers or end flowers

Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between short pea plants yielded all short pea plants.

Mendel’s cross between tall pea plants and short pea plants yielded all tall pea plants.

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x =

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Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being short.

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Mendel’s Factors • Mendel repeated this experiment

with many pea plants with many different traits

• For each, he discovered that there seem to be “factors” that disappeared in one generation and then reappeared in the next

• This went against the idea that an organism is a blend of their parents’ traits

Why was Mendel’s work so important?

• Mendel worked at time when microscopes were not very good and the cell theory was only just being thought of

• When scientists understood DNA and chromosomes, they understood how important his work with pea plants was

Dominant vs. Recessive Genes

There are TWO basic kinds of genes:

– Dominant - A gene that is always expressed and hides others

Represented with a capital letter

– Recessive - A gene that is only expressed when a dominant gene isn’t present

Represented with a lowercase letter

A Human Example • A dominant gene will always

mask a recessive gene.• A “widow’s peak” is dominant,

not having a widow’s peak is recessive.

• If one parent contributes a gene for a widow’s peak, and the other parent doesn’t, the off- spring will have a widow’s peak.

Eddie Munster

Latin Roots

All organisms have two copies of each gene: one contributed by the father and the other contributed by the mother.

Homozygous - Two copies of the same gene

“homo” - same

Heterozygous - Two different genes

“hetero” - different

Genotypes and Phenotypes

For the widow’s peak:

Genotypes: WW, Ww, or ww

Phenotypes: widow’s peak or no widow’s peak

WW - has a widow’s peak Homozygous dominant

Ww - has a widow’s peak Heterozygous

ww - no widow’s peak Homozygous recessive

Eddie Munster’s Parents

Father = Herman - No Widow’s Peak

Mother = Lilly - Has a Widow’s Peak

What can you determine about a genotype from a phenotype?

Since Herman has no widow’s peak, he must be “ww”, since Lilly has a widows peak she could be either “WW” or “Ww”

Definitely ww Homozygous

recessive

Either Ww Heterozygous

or WW Homozygous dominant

Punnett SquaresA Punnett Square is a tool used to predict the

possible gene combinations of the offspring. In this case, we can work backwards…

Ww ww

Ww www

w

W w

Assume Lilly is heterozygous Ww

Assume Herman is homozygous recessive ww

• A Punnet Square begins with a box 2 x 2

• One gene is called an “allele”

• One parents pair is split into alleles on top, the other along the side

• Each allele is crossed with the other allele to predict the traits of the offspring

How can you tell?

So which is true? Is Lilly homozygous dominant (WW) or is she heterozygous (Ww)?

Ww

Www

w

W W

Ww

Ww

Ww ww

Ww www

w

W w

More Information Needed

Recall that Herman and Lilly had another offspring, Marylin. She had no widow’s peak. Therefore, Lilly must be heterozygous.

Incomplete dominance

• Two forms blend

Co-dominant Traits

• both forms are shown

Multiple Alleles

• more than two forms of a trait

Polygenic Traits

• Controlled by many genes