Patterns of Inheritance – Mendelian Genetics

Genes

Location of a gene on a chromosome is called its locus

Homologous chromosomes

What are alleles?

Homozygous Alleles vs. Heterozygous Alleles

Who Was Gregor Mendel?

nitro.biosci.arizona.edu

Secrets of Mendel’s Success

Self-fertilization Pea flowers can self-fertilize

Pollen from male structures transfers to eggs in female structures

True-breeding

Plants homozygous for a characteristic are true-breeding

Cross-fertilization

Mendel was able to mate two different plants by hand (cross-fertilization)

Female parts (carpels) were dusted with pollen from other selected plants

Secrets of Mendel’s Success

Mendel experimental design was simple and methodical

Chose a good organism to study

Studied characteristics that have unmistakably different forms (like purple versus white)

Studied one trait (characteristic) at a time

Genetic Cross

The mating of pollen and eggs (from same or different parents)

Genetic Cross Parents used in a cross are part of the

parental generation (known as P)

Offspring of the P generation are members of the first filial generation (F1)

Offspring of the F1 generation are members of the F2 generation, etc.

Alleles

Every cell in a pea plant carries 2 alleles per characteristic (either the same or different)

Genotype vs Phenotype

Genotype Particular combination of the 2 alleles carried

by an individual (e.g. PP, Pp, or pp)

Phenotype Physical expression of the genotype

(e.g. purple or white flowers)

Dominant and Recessive Alleles

How Meiosis Separates Genes

2 alleles for a characteristic separate during gamete formation (meiosis)

Homologous chromosomes separate in meiosis anaphase I

During meiosis anaphase II, each gamete receives one of each pair of homologous chromosomes and thus one of the two alleles per characteristic

How Meiosis Separates Genes

Separation of alleles in meiosis is known as Mendel’s Law of Segregation

Genetic Bookkeeping Punnett Square Method predicts offspring

genotypes from combinations of parental gametes

Genetic Bookkeeping From the sums of all the different kinds of

offspring genotypes, create a genotypic ratio ¼ PP, ½ Pp, ¼ pp is in the ratio 1PP: 2Pp:

1pp

Based on dominant and recessive rules, determine the phenotypic ratio A genotypic ratio of 1PP: 2Pp: 1pp yields 3 purple flower plants: 1 white flower

plant

The Test Cross

Traits Inherited Independently

Mendel performed genetic crosses in which he followed the inheritance of two traits at the same time

Traits Inherited Independently

Seed color (yellow vs. green peas) and seed shape (smooth vs. wrinkled peas) were characteristics studied

Allele symbols were assigned: Y = yellow (dominant), y = green (recessive) S = smooth (dominant), s = wrinkled (recessive)

Traits Inherited Independently

2 trait cross was between two true breeding varieties for each characteristic

P: SSYY x ssyy

Traits Inherited Independently

Mendel then allowed the F1 offspring to self fertilize: SsYy x SsYy

Mendel’s Genius Went Unrecognized