Gregor Mendel

What is Genetics? the study

of heredity

Gregor Mendel’s Peas

Pollen: plant’s sperm

Egg Cells: plants

reproductive cells

Fertilization: joining of pollen

+ egg cells develops into

embryo in a seed

Born in 1822. His work with pea

plants laid the foundation for

Genetics.

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Working with pea plants…

Self-pollinating: pollen fertilizes egg cells in the

SAME flower (single parent reproduction)

True-breeding: offspring genetically identical to

parents due to self-pollination

Cross-pollination: combining reproductive cells

from 2 DIFFERENT parent plants

Mendel could cross- breed a purple flower with a white flower…

What do you think is the color of the offspring?

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Genes and Dominance

TRAIT: specific characteristic (seed color, plant height, etc)

What did Mendel do in his pea plant experiments?

Studied 7 different traits each with contrasting characteristics (ex) Height, short or tall

He crossed the plants (with contrasting characters) and looked at their offspring Website upload 2014

P = parental generation = original pair of plants

F1 = first filial generation= first generation

Hybrids: offspring from parents with different

traits Website upload 2014

Tracking Generations

Parental generation P

mates to produce

First-generation offspring F1

mate to produce

Second-generation offspring F2

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CROSS-POLLINATION: Mendel cut the male parts of one

flower and dusted the female parts

with pollen from another flower.

P GENERATION: purple x

white flowers

F1 GENERATION: all

purple flowers

HYBRID

PLANTS

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What happened in Mendel’s crosses?

All the offspring only had one of the parent’s

characters…the other parent’s character

disappeared!

Mendel’s Conclusions:

1. Inheritance is determined by factors that are

passed down

2. GENES: the factors that determine traits

3. Contrasting characters are different forms of a

gene called ALLELES

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some alleles are dominant, some are recessive

DOMINANT ALLELE: form of trait that will

always be exhibited; usually expressed in

capitals

RECESSIVE ALLELE: form of trait is only

exhibited when the dominant allele is NOT

present

(ex) Allele for tall is dominant for and the allele

for short is recessive

Mendel’s Principle of Dominance

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What happened to the

recessive allele?

Mendel wanted to

know if the

recessive allele

disappeared from

the F1 plants.

F1 CROSS: He

self-crossed the F1

generations to

make F2 offspring Website upload 2014

THE F2 GENERATION…

The recessive traits reappeared!!

~¼ plants had white flowers, the recessive trait

Summary of Crosses:

tall plants X short plants tall plants

P P F1

tall plants self-pollinating ¼ short, ¾ tall

F1 Cross F2 Website upload 2014

Explaining the F1 Cross

Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominant allele. This is called…

SEGREGATION: separation of alleles occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis.

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Explaining the F1 Cross

F1 plants inherited 1 tall allele & 1 short allele from parents

When gametes are formed, the two alleles segregate from each other each gamete has 1 copy of each gene

So, 2 different types of gametes are formed (one w/ tall allele, one w/ short allele)

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SEGREGATION

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Punnett Squares

What is a Punnett Square?

A diagram showing the possible genetic combinations from a particular cross

Can be used to predict and compare the genetic variations that will result from a cross

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Punnett Squares

What do the letters represent in a Punnett square?

Letters represent alleles: capital = dominant lowercase = recessive

Homozygous: has two identical alleles for a trait (ex) TT or tt

Heterozygous: has two different alleles for the same trait (ex) Tt

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Punnett Square

for TT x Tt

Punnett

Square

for YY x yy

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Genotype vs Phenotype

GENOTYPE: the genetic

makeup of an organism

(ex) TT

PHENOTYPE: the

physical characteristics

exhibited (ex) tall plant

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Probability and Segregation

F2 generation from Tall F1 plants

¾ tall, ¼ short

3:1 ratio of tall to short plants

Punnet squares work to predict outcomes, so Mendel’s ideas about segregation are accurate!

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Exploring Mendelian Genetics

Mendel performed TWO-FACTOR CROSSES:

crossing 2 different genes and following traits

as they pass from one generation to the next

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Two-Factor Cross: F1

Two Genes: shape of pea & color of pea

The Cross:

Round yellow peas x wrinkled green peas

RRYY x rryy

What are the possible alleles parent 1 can pass? RY

What are the possible alleles parent 2 can pass? ry

All F1 were RrYy (round and yellow) or HYBRIDS

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Two-Factor Cross: F2

F1 Generation = RrYy

How would these alleles segregate when F1 self-

pollinated?

RrYy x RrYy

Do the two dominant alleles stay together?

What are the possible alleles each parent can pass

on?

There are 4 possible combinations: Ry, RY, rY, ry

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The F1 Hybrid cross produces a 9:3:3:1 phenotype ratio

Mendel found that the 2 alleles (seed shape & seed color) don’t influence each other’s inheritance

This is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes

INDEPENDENT

ASSORTMENT

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Independent Assortment

Metaphase I:

Metaphase II:

Gametes:

1/4 AB 1/4 ab 1/4 Ab 1/4 aB

A A A A

A A A A

A A A A

B B

B B

B B

B B

B B B B

a a a a

a a a a

a a a a

b b b b

b b b b

b b b b

OR

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Summary of Mendel’s Principles

A. Inheritance of characteristics is determined by genes which are passed to offspring

B. If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive

C. Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation

D. Alleles for different genes segregate independently

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Beyond Dominant & Recessive Alleles

Genetics is more complicated

Some alleles are neither dominant nor

recessive

Many traits are controlled by multiple alleles

or multiple genes

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Other Inheritance Patterns…

1. Incomplete Dominance

2. Codominance

3. Multiple Alleles

4. Polygenic Traits

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X Homozygous

parent (RR)

Homozygous

Parent (rr)

All F1 are

heterozygous

X

F2 shows three phenotypes in 1:2:1 ratio

Incomplete

Dominance

When one allele is not completely dominant; recessive allele is not totally masked

Heterozygous phenotype is in between the two homozygous phenotypes Website upload 2014

X Homozygous

parent (RR)

Homozygous

Parent (rr)

All F1 are

heterozygous

X

F2 shows three phenotypes in 1:2:1 ratio

Incomplete

Dominance

(ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers pink hybrid flowers (Rr)

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X homozygous parent homozygous parent

All F1 offspring

heterozygous for

flower color:

Cross two of the F1

plants and the F2

offspring will show

three phenotypes in

a 1:2:1 ratio:

Incomplete Dominance

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Codominance

Both alleles contribute to the phenotype

Heterozygous genotype expresses both phenotypes

(ex) Feather colors in chickens: white feathers & black feathers speckled chicken

(ex) Horse coats: red & white roan coat

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Codominance:ABO Blood Types

Alleles that controls blood type are codominant

Two alleles A & B are both exhibited when paired, a third allele (i) is recessive to others

AA or Ai = Type A Blood

BB or Bi = Type B Blood

AB = Type AB Blood

ii = Type O Blood Website upload 2014

Multiple Alleles

> 2 possible alleles for a gene Individuals can still only have 2 alleles each

but more than 2 alleles exist in a population

(ex) coat color in rabbits lots of options due

to 4 different alleles

(ex) blood type is determined by multiple

alleles

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Polygenic Traits

Traits controlled by the interaction of 2+ genes

(ex) Fruit fly eye color (3+ different genes)

(ex) Skin color in humans (4+ different genes), eye color, height, weight

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