Mendelian Genetics 101

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?

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.”

Background• Organisms usually

resemble their parents because they inheritcertain traits from them.

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

• Genetic information = segments of DNA = genes

Gregor Mendel

Father of 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 Artificially Cross-Pollinated Pea Plants

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

-He focused on 7

Mendel studied pea traits, each

with two distinct phenotypesCharacteristic

Mendel’s Experimental Design

Mendel’s Experimental Design

Mendel’s Experimental Design

TERMINOLOGY

PHENOTYPE - appearance ("pheno-

"=visible, as in "phenomenon")

GENOTYPE - genetic make-up;

combination of alleles

- not always visible, but

detectable by performing

crosses

ALLELES - variants of a gene.

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.

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

Mendel’s Experimental Design: What we know now

Mendel’s Experimental Design: What we know now

Mendel’s Experimental Design: What we know now

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

Production of gametes: Meiosis

• The number of chromosomes is reduced (by half).

• Alleles for genes are separated into the gametes.

• Gametes then combine during sexual reproduction to form zygotes with complete sets of chromosomes.

• Mendel’s flowers examples

• Dog examples handout

Gametes for Single Characteristic

Two characteristicsHeterozygous for both characteristics

_____

_____

__________

Mendel’s Laws of Inheritance

• “LAW OF DOMINANCE” – One factor in a pair may prevent the other from having an effect; that factor dominates or masks the other.

in modern terms:

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

Mendel’s Laws of Inheritance

• “LAW OF SEGREGATION” – paired factors separate during the formation of reproductive cells

in modern terms:

• "LAW OF SEGREGATION" - alleles separate from one another during meiosis in the formation of gametes.

Mendel’s Laws of Inheritance

• “LAW OF INDEPENDENT ASSORTMENT” – factors for individual characteristics are not paired together.

in modern terms:

• "LAW OF INDEPENDENT ASSORTMENT“ - genes at different locations are separated independently of one another during gamete formation.– Far away from each other on the same chromosome

– On separate chromosomes.