Intro to Genetics What is an allele? –one of the alternative forms of a gene that governs a characteristic, such as hair color.

Law of Dominance Dominant – trait that is always expressed Recessive – trait that is hidden Phenotype – physical appearance of trait; what you can see (eye color, hair color) Genotype – set of alleles (genes) for a trait Introduction to Heredity

Language of Genetics Genotypes written for a trait (eye color) B – dominant (brown) b – Recessive (blue) BB homozygous dominant Bb heterozygous bb homozygous recessive

Phenotypes for eye color: -BB brown eyes (dominant, dominant) -Bb brown eyes (dominant, recessive) -bb blue eyes (recessive, recessive)

-Homozygous (purebred) – individual has two identical alleles for a given trait (BB, bb) -Heterozygous (hybrid) – individual has two different alleles for a given trait (Bb)

Mendel’s experiments He bred (cross-pollinated) different varieties of pea plants. One plant had purple flowers. The other plant had white flowers. The offspring resulted only in purple plants.

Predictions… Which color was dominant? What would be the result if two of the purple plants (offspring) were crossed? (known as F2 generation) What were the genotypes of the two plants in the first cross? (known as the F1 generation) What are the genotypes of the plants in the second cross?

Hints… Pure breeding Purple: PP Pure breeding White: pp Mixed Purple: Pp F2 results: Purple plants genotypes PP and Pp

Notes… Homozygous dominant: PP, two dominant alleles Homozygous recessive: pp, two recessive alleles Heterozygous: Pp, a dominant and a recessive allele

Exception to Mendelian genetics: codominance: –Both alleles are dominant/both phenotypes show Red flower + white flower = white flower with red tips RR WW RW Incomplete dominance: –Neither allele is dominant/ a new phenotype is shown Red flower + white flower = pink flower RR rr Rr –Incomplete and Co-dominanceIncomplete and Co-dominance

A Punnett square predicts the results of a genetic cross between individuals of known genotype. Punnett Square Fun