Genetics and Heredity

Inheritance• Parents and offspring often

share observable traits.

• Grandparents and grandchildren may share traits not seen in parents.

• Why do traits disappear in one generation and reappear in another?

BackgroundOrganisms usually resemble their parents because they inherit certain characteristics from them.

These characteristics, called traits, are determined by genetic information on chromosomes.

Genetic information = segments of DNA = genes

Definitions

Genetics = the branch of biology that studies heredity

Heredity = the passing on of characteristics from parents to offspring

***from the Latin word hered-, meaning “heir.”

Gregor Mendel

Father of modern genetics

Gregor Mendel, an Austrian monk, carried out the first important studies on heredity (1800s).

History

Mendel was the first person to succeed in predicting how traits would be transferred from one generation to the next.

Earlier observers looked at many traits at once-- Mendel focused on one at a time

Mendel Combined:

• Plant breeding

• Statistics

• Careful record keeping

Mendel’s findings of transmission of traits are

now considered the Laws of Inheritance.

Mendel’s Experiments

Mendel studied the pea plant Pisum sativum

- easy to cultivate and a short life cycle

- easy to control pollination- keep unwanted pollen out- cross-fertilize artificially

-had discontinuous characteristics -Ex: flower color, seed texture-knew of at least 34 such traits

Mendel Artificially Cross-Pollinated Pea Plants

Mendel studied pea traits, each with two distinct phenotypes

trait

TERMINOLOGY

PHENOTYPE - appearance ("pheno-"=visible, as in "phenomenon")

GENOTYPE - genetic make-up, not always visible, but detectable by performing crosses

ALLELES - variants of a gene.

Gene Seed shape Seed color Flower color

Flower position

Pod shape Pod color

Plant height

Dominant allele

Round (R) Yellow (Y) Purple (P)

Axial (A) Inflated (I)

Green (G)

Tall (T)

Recessive allele

Wrinkled (r)

Green (y) White (p)

Terminal (a)

Constricted (i)

Yellow (g)

Short (t)

Mendel‘s Experiments

Three important written conventions for writing genotypes:

The same letter is used for different alleles of the same gene.

Uppercase letters are used for dominant alleles and lowercase letters are used for recessive alleles.

The letter for the dominant allele is always written first.

TERMINOLOGY

HOMOZYGOUS - having two alleles that are alike

HETEROZYGOUS - having two unlike alleles

DOMINANT - showing a phenotypic effect in heterozygous form

RECESSIVE - showing a phenotypic effect only when homozygous

Mendel’s Laws of Inheritance

"LAW OF SEGREGATION" - dominant and recessive alleles of heterozygote separate from one another during meiosis

"LAW OF INDEPENDENT ASSORTMENT" for 2 genes at a time: genes at different locations are chosen (sampled) independently of one another during gamete formation.

“LAW OF DOMINANCE” – recessive alleles will always be masked by dominant alleles