The Work of Gregor Mendel

Essential Question:

Describe Mendel’s studies and conclusions about inheritance.

Gregor Mendel = Father of Genetics

Worked with garden pea plant

1857

Mendel’s Experiments

• Peas are a “model system.”

•

• Started with “true breeding” plants

• Trait: specific characteristic (e.g., seed color, plant height) of an individual

• Hybrid: created from cross of true-breeding individuals

• Small, easy to grow. • Makes them convenient to

study. • Help explain how organisms

inherit traits

Pollination and Fertilization in Plants

Male part

Female part

Plants have the ability to self-pollinate (self-fertilize) because they have both reproductive parts.

Cross-Pollination

Pollen

Taking pollen from one plant and using it to fertilize the eggs of another plant

Pea Characteristics of Mendel’s peas

Mendel studied these characteristics to observe patterns in traits across generations.

Mendel’s CrossesWhen Mendel crossed plants with contrasting traits, the hybrid offspring showed traits of only one parent.

Dominant and Recessive Traits

Yellow seed color is

to green seed color.

Green seed color is

to yellow seed color.

dominant

recessive

When a yellow pea plant is crossed with a green pea plant, the resulting offspring are yellow.

Principle of Dominance

• Some alleles are dominant, some recessive.

• An organism with at least one dominant allele will exhibit that trait.

• An organism with a recessive allele will exhibit the trait only in the absence of a dominant allele.

dominantrecessivedominant

Genes and AllelesTraits (seed shape, seed color) are determined by genes. •Genes: passed from one generation to the next; genetic code that determines an individual’s characteristics

Forms of traits (yellow/green, round/wrinkled) are called alleles. •Alleles: the different forms of a gene.

Characteristics such as pod color

are determined by .

Yellow and green pod colors are

determined by .

The allele for green pods is to the allele for yellow.

genes

alleles

recessive

The P1 and F1 Generation

P1 generation

F1 generation

True-breeding

Hybrids

Let’s figure out how we can predict the F1 gen from the P1 gen.

Monohybrid Cross = One Trait

“Height”

“True Breeding” = Homozygous

Tall Pea x Short PeaTT x tt

T = Dominantt = Recessive

X

“Homozygous Dominant”

“Homozygous Recessive”

Heterozygous and Homozygous

Homozygous: has two identical alleles for a gene

Heterozygous: has two different alleles for a gene

Which are:

homozygous?

heterozygous? 1 2 3

1, 4

2, 3

4

1st LAW OF HEREDITY

“Law of Segregation” = – The 2 alleles for each trait must separate when

gametes (sperm/egg) are formed.

-A parent passes on, at random, only one allele for each trait to each offspring

A a a

Segregation

Random Fertilization

The Formation of Gametes

Eggs and sperm

Fertilization

Making a Punnett Square

Combine gamete genotypes

Gametes for a parent along one side

Possible gametes

A cross of birds: Beak size (B, b)

Generations

P1 = Parent Plants

-True breed Tall x True breed Short

= First Generation (children)F1

TT x tt

What’s the probability that the offspring will be tall? ________ Short? _______

Genotype of offspring? ______________Genotype=genetic makeup of organism

Phenotype of offspring? _____________Phenotype=physical characteristics

Law of Probability“Rules of Chance”“Odds”

20:1 5%10:1 10%2:1 50%

Numerator = winnerDenominator = # horses1/12 1:12 “odd”

8% “chance”

Probability?

“Chance” of getting a tail? “Chance” of getting snake eyes on two dice?

Male: FemaleXY XX

½ 1:2 odd Boy 1:2 odd Girl50% chance

Survey Large Number1:1

“Chance” of getting a boy?

Dominant = Trait ObservedRecessive = Trait that disappeared

Tall = Dominant (T) Short = Recessive (t)

Alleles:

TT = TallTt = Talltt = Short

PhenotypeGenotype

Can’t always know an organisms genotype bylooking at itsphenotype?

TT tt

T t

Tt

Homozygous Dominant

Homozygous Recessive

Heterozygous

Tt x Tt

3 Tall and 1 ShortTT

tt

F1

F2

Tt

Combination of Letters/Alleles?

Tt

Change of colors to follow the parental alleles:

The F2 Generation – created from a cross of two F1 generation offspring.

What proportion of F2 offspring were short?

What proportion of F2

offspring were tall?

1/4

3/4

Dihybrid Crosses:Crossing for 2 characteristics/ traits

•2 TRAITS: Seed Shape and Seed Color

•Shape =Round (R) was dominant to Wrinkled (R)•Color = Yellow (Y) was dominant to Green (y)

EXAMPLE

Making a Punnett Square: Two Factors

Combine gamete genotypes A cross of pea plants: size (T, t) and pod color (G, g)

All possible gametes

All gametes for a parent along one side

Round Yellow Seeds Wrinkled Green SeedsHomozygous dominant for both traits Homozygous recessive for both traits

x

_RRYY_ ___rryy____(Genotype) 1. We need to first find all the possible

gamete combinations of

each parent

(Genotype)

Use FOIL to help you figure out the combinations

(RR)(YY)F(irst) RYO(uter) RYI(nner) RYL(ast) RY

Use FOIL to help you figure out the combinations

(rr)(yy)F(irst) ryO(uter) ryI(nner) ryL(ast) ry

gametes gametes

2. Place your gametes in a Punnett square. RY RY RY RY

ry

ry

ry

ry

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RY RY RY RY

ry

ry

ry

ry

3. Fill in the Punnett square.

So the P1 cross of true breeding plants:

Round Yellow Seeds Wrinkled Green Seeds

RRYY rryy

produced

RrYy

P1

F1 All plants Round Yellow Seeds

1. We need to first find all the

possible gamete combinations of

each parent

Now let’s cross two F1 generation plants.

RrYy RrYy

Use FOIL!RYRyrYry

RYRyrYry

RrYy

RY Ry rY ry

The two pairs of alleles segregate independently of each other.

The presence of one specific

allele for one trait has no impact

on the presence of a specific

allele for the second trait.

Four classes of gametes

(RY, Ry, rY, and ry) would be

produced in equal amounts.

2. Place your gametes in a Punnett square. RY

RyrYry

RYRyrYry

RY Ry rY ry

RY

Ry

rY

ry

RrYy RrYy

Round Yellow Round Green Wrinkled Yellow Wrinkled Green

2ND LAW OF HEREDITY

• Law of Independent Assortment = genes for different traits are inherited “independently” of each other.

•RrYy = the R and r will separate as well as the Y and y.

•Alleles can recombine in 4 different ways.