The Work of Gregor Mendel

Genetics

• Heredity – transmission of traits from one generation to the next

• Genetics – study of heredity

Gregor Mendel’s Peas

• Augustinian monk named Gregor Mendel in 1800s worked with peas to demonstrate the basic genetic principles we know today

• Mendel’s garden peas were true-breeding, meaning that if they were allowed to self-pollinate, they would produce offspring identical to themselves

• Mendel selected the pea plants that he would breed with each other.

• Why pea plants?– Easy to care for, grow quickly, take up little space, etc

Genes and Dominance• Mendel studied several different pea plant traits• Trait – specific characteristic, such as seed color or plant height,

that varies from one individual to another• Each trait Mendel studied had two contrasting characters – Ex.

Seed color – yellow or green

Seed Shape

Flower Position

Flower Color

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Purple

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Purple Smooth Green Axial Tall

Genes and Dominance (continued)

• Mendel crossed plants with each of the contrasting characters and studied their offspring

• Each original pair of plants were the P (parental) generation. The offspring from the cross were called the F1 generation

• Offspring of crosses between parents with different traits are called hybrids

• F1 generation of pea plants had the character of only ONE of the parents. The other character seemed to have disappeared…

Genes and Dominance (continued)

• Mendel then came to a few hypotheses:– There are alternative versions of genes (Mendel

termed genes “factors) that account for variations in inherited characters (different versions of a gene = alleles)

– An organism inherits two alleles - one allele from each parent – homozygous (same alleles)/heterozygous (different alleles)

– Principle of Dominance – some alleles are dominant and others are recessive

– (the fourth we’ll talk about in a moment)

Principle of Dominance

• Dominant allele – an organism with a dominant allele for a particular form of a trait will always have that form (Ex. Aa, AA = dominance)

• Recessive allele – an organism with a recessive allele for a particular trait will only have that form when the dominant allele isn’t present (Ex. aa = recessive)

Law of Segregation

– Mendel wanted to know what happened to the recessive traits that seemed to have disappeared

– Crossed F1 plants with themselves to produce F2 (second generation) plants – this caused the recessive traits to reappear

– Roughly 1/4th of the F2 plants showed the recessive trait

Segregation (continued)

– Reappearance indicated that at some point, the recessive allele separated from the dominant allele = Segregation

– Fourth Hypothesis: Law of Segregation - Two alleles are segregated from each other so that each gamete (sex cell – sperm or egg) carries only a single copy of each gene

Example: Tallness vs. Shortness in Pea Plants

P generation cross (true breeding)

TT x tt (tall) x (short)

F1 generation

Tt(all tall plants produced)

F1 generation cross (self-pollinated)

Tt x Tt (tall) x (tall)

F2 generation

TT, Tt, Tt, tt(3 tall, 1 short plant produced)

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Go to Section:

Principle of Dominance

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Go to Section:

Principle of Dominance

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Go to Section:

Principle of Dominance

concluded that

which is called the

which is called the

GregorMendel

Law ofDominance

Law ofSegregation

Peaplants

“Factors”determine

traits

Some alleles are dominant,

and some alleles are recessive

Alleles are separated during gamete formation

experimented with

Gregor Mendel’s Work