Genetics

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

Mendel studied 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

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.

VocabularyHeredityLaw of DominanceLaw of SegregationLaw of Independent AssortmentAlleleGenotypePhenotype