Genetics

The scientific study of heredity.

What is Heredity ?

• The passing of traits from parents to their

offspring

1860s: Gregor Mendel

• “Father of Genetics” (my favorite Austrian monk)

• He figured out how traits are passed on before we

knew about DNA, or chromosomes or meiosis

Mendelstudied heredity with pea

plants • Why pea plants? (was it luck?)

– 1. Small

– 2. Easy to grow

– 3. Produces large numbers of offspring

– 4. Matures quickly

– 5. Plants can self or cross fertilize

– 6. Many varieties

– 7. Each trait has two forms

• Tall/short purple/white flowers

• green/yellow seeds colored/white seed coat

• green/yellow seed pods inflated/narrowed seed pod

• wrinkled/smooth seed

To begin, Mendel self fertilized to

create pure breeding plants

• Purple

First Cross:

pure purple with pure white

• Cross fertilized P (Parental) generation

– Pure purple with pure white

– P Generation F1 Generation

purple white

X

All purple

Second Cross:

Cross 2 F1 plants

• Cross fertilized two of the offspring of the F1

generation

purple

X

purple

white

purple purple

purple

Mendel’s Experiments led him to

3 Laws of Inheritance

• The Law of Dominance

• The Law of Segregation

• The Law of Independent Assortment

The Law of Dominance:

• Each individual has TWO factors for each trait

known as alleles

– Some alleles dominate over others – the

dominant allele is expressed in the offspring

– Homologous chromosomes-same traits -different forms

Freckles

Dimples

Tongue

roller From Mom

From Dad

No Freckles

No dimples

Non-roller

These are

alleles

Alleles are:

• Represented by a letter of the alphabet

Capital = dominant allele A

lowercase = recessive allele a

• Each allele corresponds to a gene on a

chromosome

- the alleles represent different forms of

the gene

Law of Segregation – The two alleles for each trait separate and

move into different gametes – meiosis

– Since only one egg or one sperm will

contribute to the new offspring, only one allele

for a trait is passed on

– The chance that any allele will be passed on is

50%

Law of Independent Assortment

• the inheritance of alleles for one trait

doesn’t affect the inheritance of alleles for

another trait

- occurs in meiosis during metaphase 1

Genotype:

• Genotype is the combination of alleles for a

particular expressed with letters

• Homozygous dominant:

– TT - two capital letters

• Heterozygous

– Tt - one of each letter (big and small)

• Homozygous recessive

– tt - two lower case letters

Phenotype:

• Phenotype is physical expression of the

genotype

Genotype Phenotype

TT tall plant

Tt tall plant

(dominant allele always is expressed over

the recessive allele)

tt short plant

(recessive trait is expressed only if there

is no dominant gene)

If R = red, and r = white

• What is the genotype of homozygous

dominant?

– RR

• What is the heterozygous genotype?

– Rr

• What is the homozygous recessive

genotype?

– rr

If R=red and r =white

• What is the phenotype of of RR?

– Red

• What is the phenotype of Rr?

– Red

• What is the phenotype of rr?

– White

You can determine phenotypes using a Punnet

square.

Tt Tt

Tt Tt

If two parents are crossed (TT X tt), what

are the resulting offspring?

T T

t

t

parent

parent

Genotypes: 4 Tt

Phenotypes: 4 Tall

This is the first or parent generation

Now try on your paper a cross of two of the

offspring from the parent generation. This is called

the first generation or the F1.

Tt

Tt

T

T

t

t

TT

tt

Genotypes: 1TT: 2Tt : 1tt

Phenotypes: 3 Tall: 1 Short.

Vocabulary Heredity

Law of Dominance

Law of Segregation

Law of Independent Assortment

Allele

Genotype

Phenotype