Introduction to Genetics

Introduction to Genetics

Who was Gregor Mendel?• He was known as the “FATHER OF

GENETICS” • He discovered how traits were inherited

Genetics: The study of heredity, how traits are passed from parent to offspring

x =or

or

Mendel’s Peas• Mendel did his study on pea plants• Pea plants have many traits

(tall/short, purple flowers/white flowers)

• Pea plants can beself-fertilized or cross-fertilized

• Mendel crossed (bred) tall pea plants with short pea plants. Predict what the offspring were like.

Tall x Short

?

Mendel allowed the F1 plants to self-pollinate

Types of Plants1.True-Breeding - these plants always create plants that look like themselves

2.Hybrids – offspring of true-breeding plants

Tall x Short = Hybrid

DominanceSome traits are dominant over others.

Tall x Short = all tall offspring (hybrids)

*Tall is the dominant trait* Short is recessive

Genes and Alleles

Mendel discovered that each trait is controlled by two factors (alleles)

Genes – factors that determine your traits

There are three basic kinds of genes:–Dominant - A gene that is always

expressed and hides others–Recessive - A gene that is only

expressed when a dominant gene isn’t present

–Codominant - Genes that work together to produce a third trait

GENOTYPE  -  what genes, letters, the organism has (TT, Tt, tt)

PHENOTYPE   - what it looks like (tall or short)

Dominant and Recessive Genes• A dominant gene will always mask a recessive

gene.• A “widows peak” is dominant, not having a widows peak is recessive.• If one parent contributes a gene for a widows peak, and the other parent doesn’t, the offspring will have a widows peak.

Widows Peak

Punnett Square - A tool we use for predicting the traits of an offspring•Shows all of the possible combinations of alleles from the parents

Alleles: different forms of the same gene •Organisms have two alleles for each gene (one from each parent)

– Letters are used as symbols to designate genes

– Capital letters are used for dominant genes

– Lower case letters are used for recessive genes

– Genes always exist in pairs

Ex) Tall pea plants x short pea plants (TT) (tt)

genotype TALL

SHORT offspring 100% tall (tT)

T T

t tT tT

t tT tT

Ex) Tall pea plants x tall pea plants (tT) (tT)

genotype TALL

TALL offspring ¾ tall – 75% ¼ short – 25%

T t

T TT tT

t tT tt

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

Homozygous – same alleles (TT, tt)Heterozygous – different alleles (tT)

A Widows Peak, dominant, would be symbolized with a capital “W”, while no widows peak, recessive, would be symbolized with a lower case “w”.

Father - No Widows Peak - w

Mother - Has a Widows Peak - W

For the widows peak:

WW - has a widows peak Homozygous dominantWw - has a widows peak Heterozygousww - no widows peak Homozygous recessive

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

Definitely ww Homozygous

recessiveEither Ww

Heterozygous or WW

Homozygous dominant

We can use a “Punnet Square” to determine what pairs of genes Lilly has

Ww ww

Ww www

w

W w

Assume Lilly is heterozygous Ww

Assume Herman is homoozygous 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

Notice that when Lilly is crossed with Herman, we would predict that half the offspring would be “Ww”, the other half would be “ww”

Half “Ww”, Heterozygous, and will have a widows peak

Half “ww”, Homozygous, and will not have a widows peak

Ww ww

Ww www

w

W w

GeneticsAnother possibility is that Lilly might be

“WW”, homozygous dominant.

Ww

Www

w

W W

Assume Lilly is homozygous dominant WW

Assume Herman is homoozygous ww

Ww

Ww

Notice that all the offspring are heterozygous and will have a widows peak

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

Ww

Www

w

W W

Ww

Ww

Ww ww

Ww www

w

W w

If Lilly were heterozygous, then 1/2 of their offspring should have a widows peak, 1/2 shouldn’t

If Lilly were homozygous, all of their children will have a widows peak

Recall that Herman and Lilly had another offspring, Marylin. She had no widows peak, therefore, Lilly must be heterozygous.

Hair color is a perfect example of a trait

Prince Charming is blond

Snow White has dark hair

What color hair should their children have?

Hair color is different from widows peak, no color is truly dominant. – Brown and blond are the two, true

traits– Homozygous conditions produce

either brown or blond hair– Heterozygous conditions produce

red hair

For Snow White to have brown hair she must be homozygous dominant, “BB”, a blond Prince Charming must be homozygous recessive, “bb”.

Bb

Bbb

b

B B

Bb

Bb

All the offspring from Prince Charming and Snow White will therefore be

heterozygous, “Bb”, and since hair color is codominant….. all their children will

have red hair.

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