Genetics 11.1 & 11.2

The scientific study of heredity.

What is Heredity ?• The passing of traits from parents

to their offspring

1860s: Gregor Mendel• “Father of Genetics”

• Mendel 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

The Role of Fertilization

Male part of each flower makes pollen, which contains sperm—the plant’s male reproductive cells.

Female portion of each flower produces reproductive cells called eggs.

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 generationpurple

X

purple

white

purple purple

purple

Genes and AllelesAll of Mendel’s crosses had the characteristics of only one of their parents. An individual’s characteristics are determined by factors (genes) that are passed from parent to offspring

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@

FrecklesDimples

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 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

Ex: Eye color traits will not affect a person’s hair color.

The Two-Factor Cross: F2Mendel’s experimental results were very close to the 9:3:3:1 ratio

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 TtPhenotypes: 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 : 1ttPhenotypes: 3 Tall: 1 Short.

• http://www.youtube.com/watch?v=CBezq1fFUEA