Genetics

The scientific study of heredity.

Vocabulary: Make sure to DEFINE these words as you go through the notes

Heredity PhenotypeVariation GenotypeDominant DNARecessive AllelePurebred MeiosisHybridHomozygousHeterozygous

What is Heredity ?

• The passing of traits from parents to their offspring

• Traits are passed to you through your parent’s sperm and egg– 50% of DNA from mom, 50% from dad

• Variation (differences in people’s traits) are due to the combination you inherit

1860s: Gregor Mendel• “Father of Genetics”

(he was an Austrian monk)

• He figured out how traits are passed on before we knew about DNA, or chromosomes or meiosis

To begin, Mendel created 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

Meiosis: division of the sex cells

Genotype:

• Genotype is the combination of alleles for a trait expressed with letters

• Homozygous dominant (purebred)– TT - two capital letters

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

• Homozygous recessive (purebred)– tt - two lower case letters

Phenotype:• Phenotype is physical expression of the

genotype. (how it looks) 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 : 1ttPhenotypes: 3 Tall: 1 Short.