Genetics The Principles of Mendel. Beginnings 1886- Mendel proposes the particulate model of inheritance. This model replaces the earlier “blending”

Genetics

The Principles of Mendel

Beginnings

1886- Mendel proposes the particulate model of inheritance.

This model replaces the earlier “blending” inheritance theory.

2 laws –Law of segregation Law of independent assortment.

Mendel used the pea plant & picked plant morphology to study inheritance.

Law of Segregation

Law of Segregation

Law of Independent Assortment

Phenotype vs. Genotype

The genetic make-up of an organism, i.e. the whole, entire genome of an organism is called genotype.

The morphology of an organism is called its phenotype. Pea plants carrying the homozygous alleles tt for plant

height, will be short as compared to plants carrying TT or Tt, which will be tall.

Note that TT & Tt are distinct genotypes, but the same phenotype (tall)

Mendelian Genetics Simple Dominance: when one allele of a gene dominates

phenotypic expression; e.g.Tt for plant height will still give a tall plant although an allele for a short phenotype (t) is present.

The cross between two individuals produces the F1 or first filial generation

The cross between two F1 individuals produces F2 generation.

The cross between the F1 & one of the parents is called back-cross

Mendelian Genetics

Phenotypic ratio: 3 tall plants & 1 short (3:1) Genotypic ratio: 1:2:1

X T t

T TT Tt

t Tt tt

Non-Mendelian Genetics

Co-dominance Incomplete Dominance Lethality Epistasis

Incomplete Dominance

Both alleles, when present, will contribute to the phenotype with intermediate characteristics developing.

A plant with red flowers: RR crossed with a plant with white flowers, rr will give, a plant with pink flowers- Rr.

Co-Dominance

Both alleles, when present, will contribute equally to the phenotype.

Blood typing in humans- the ABO system; Rh factor incompatibility (erythrobalstosis feotalis).

Terms: antigen, antibody, agglutination, Rh factor.

Co-Dominance

Co-Dominance

Lethality

Inheritance of a lethal gene. Often a homozygous recessive trait.

E.g. an albino plant that cannot synthesize plant pigment, or humans suffering from Cystic Fibrosis

TransposonsDiscovered by Barbara McClintock

“Jumping genes”

Corn cob color is inherited by transposons

Some Genetic Disorders

Cystic Fibrosis Huntington’s Chorea Phenylketonuria