History of

Genetics

Biology

Genetics

• How characteristics are transmitted from parents to offspring

Gregor Mendel

• worked with Pisum satium (pea plant)

Gregor Mendel (1822-1884)

• Flower color • Flower position along stem

• Seed color• Seed shape• Pod shape• Pod color• Stem length

Mendel studied 7

characteristics

Mendel’s Experiments

Mendel’s First Cross

• Crossed “purebred” purple with “purebred” white + P (parent) generation

- F1 – purple

• Crossed F1 hybrids + produced F2 generation

- 3 purple : 1 white

• Alternative versions of genes (alleles) account for variations in inherited characters. + each gene resides at a particular location of a chromosome and can exist in variable forms (two different forms for flower color)

Mendel’s Ideas of Inheritance

P

p

Pp

Mendel’s Ideas of Inheritance• For each characteristic, an organism inherits two alleles, one from each parent. + an organism is either homozygous (two of the same alleles) or heterozygous (two different alleles)

Homozygous Heterozygous

• If the two alleles differ, the dominant allele is fully expressed and the recessive allele has no noticeable affect on appearance. + heterozygotes express the dominant trait

Mendel’s Ideas of Inheritance

RR rr

Rr

• The two alleles for each character segregate duringgamete production.

+ ovum/sperm get only one of the two alleles (haploid)

Mendel’s Law of Segregation

P generation Phenotype: Purple/White Genotype: PP/pp

F1 generation Phenotype: Purple Genotype: Pp

F2 generation? Genotype/Phenotype can be predicted using a Punnet Square

Mendel’s Law of Independent Assortment

Each pair of alleles segregates into gametes independently.

WH wHwh Wh

Mendel’s Law of Independent Assortment

Parental Genotypes GGWW ggww

Parental Gametes? GW gw

GgWw

GW Gw gW gw

F1 Genotypes?

F1 Gametes?

Beyond Mendel...

Different Types of Inheritance:

Complete Dominance Incomplete Dominance Mendel’s Traits Snapdragon Color

Incomplete DominanceOne allele does not dominant the other in the heterozygous condition. Both alleles are expressed but a new phenotype appears that is intermediate.

RR rrRr

Codominance

Both alleles in the Heterozygous condition are dominant.

CC Cc cc

Codominance & Multiple Alleles