INTRODUCTION TO GENETICS: THE WORK OF GREGOR MENDEL

Honors BiologyUnit 5 / Chapter 11Powerpoint #1

•Gregor Mendel was born in 1822 in the Czech Republic

•Studied Science and Mathematics at the University of Vienna

•Spent the next 14 years teaching high school and working at a monastery in the garden

Mendel noticed that the SAME type of plant could have VARIATIONS, or differences, these are the

variations he noticed:

Seed Shape Seed Color Seed Coat Color

Round Yellow Gray

Wrinkled Green White

Pod Shape Pod Color Flower Position Plant Height

Smooth Green Axial Tall

Constricted Yellow Terminal Short

AFTER MENDEL NOTICED THESE VARIATIONS HE DECIDED TO DO AN EXPERIMENT OF HIS OWN. Mendel wanted to breed the pea

plants together in different combinations to see what the offspring (seedlings) would look like.

TECHNIQUES OF MENDEL

Describe how Mendel prevented self-pollination and controlled cross-pollination in pea plants: He cut off the male parts of one flower, then dusted the stigma with pollen from a second flower.

Why did he want to do this? He wanted plants with specific traits to breed with one another.

TERMINOLOGY

Genetics: study of heredity (how traits pass from one generation to the next)

True-breeding: if allowed to self-pollinate, produce offspring identical to self

Self-pollination: pollen from one flower fertilizes egg cells in same flower

Cross-pollination: pollen from one flower fertilizes egg cells in a different flower

TERMINOLOGY

Trait: specific characteristic that varies from one individual to another

Example: Hair color, Eye color, height

P generation: parental generation

TERMINOLOGY

F1 generation: first generation of offspring (first filial generation) (AKA kids of parents)

F2 generation: second generation of offspring (offspring of F1 plants) (AKA grandkids of parents)

Hybrid: offspring of crosses between parents with different traits. Example: Cross pollinate a red flower and a white flower. Seedlings=hybrid

X =

HYBRID CARS

Gas + Electric

TERMINOLOGY

Gene: segment of DNA that determines a trait

Alleles: different forms of a gene

(ex: Height: tall gene, or short gene)

Allele

PRINCIPLE OF DOMINANCE:

dominant trait always seen when dominant allele is present; recessive trait only seen when no dominant allele is present

TERMINOLOGY

Recessive allele: allele that is masked (hidden) when dominant allele is present

Allele for blue eyes

Dominant allele: allele that is always expressed when it is present

Allele for brown eyes

NOTATION

A capital letter represents the dominant allele.

A lower case letter represents the recessive allele.

Example: Pea plant height –

Tall: TShort: t

MECHANICS

Segregation of alleles: separation of alleles during gamete production (meiosis)

Gametes: sex cells (egg, sperm)

Note: all body cells contain two copies of each chromosome (and therefore two copies of each gene) – one copy is from the male parent and one copy is from the female parent

During meiosis, each gamete (egg or sperm cell) receives only one copy of each chromosome (and gene (allele)), thus the two copies of each gene are separated.

PEA PLANT TRAITS

Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant HeightPodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth GreenAxial Tall

MENDEL’S OBSERVATIONS

• 1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics?

• 2. How do the characteristics of the second generation compare to the characteristics of the first generation?

Parents

Long stems short stems

Purple flowers white flowers

Green pods yellow pods

Round seeds wrinkled seeds

Yellow seeds green seeds

First Generation

All long

All purple

All green

All round

All yellow

Second Generation

787 long: 277 short

705 purple: 224 white

428 green: 152 yellow

5474 round: 1850 wrinkled

6022 yellow: 2001 green

P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Mendel’s Experiment

TT Tt Tttt TT Tt Tt tt

MECHANICS

Why are there no white flowers in the F1 generation? The purple parent has 2 purple alleles and the white parent has 2 white alleles. The F1 plants each have one purple and one white allele – the purple allele is dominant so it masks the white allele.

Why are there no light purple flowers in the F1 generation? Traits do not blend.

Explain why the offspring of a true-breeding pea plant with white flowers and a true-breeding pea plant with purple flowers are all purple

MENDEL’S EXPERIMENT

How did Mendel show that the recessive allele had not disappeared in the F1 generation? When he cross-pollinated plants from the F1 generation, the recessive trait showed up again in the F2 generation.

This showed Mendel that there must be two factors (genes) controlling each trait – one is dominant and masks the other (which is recessive).

MENDEL DID HIS WORK BEFORE DNA!

Pea Plant DNA (Electron

Microscope)