Genetics PPT –Part 1Biology-Mrs. Flannery

In an Abbey Garden

• Mendel studied garden peas because they

– were easy to grow,

– came in many readily distinguishable varieties, had easily visible traits

– are easily manipulated

– can self-fertilize.

Gregor Mendel’s Peas

• Mendel had

– true-breeding pea plants, which were the basis of his experiments.

• True-breeding plants produce offspring that are identical to themselves, if allowed to self-pollinate.

• Mendel wanted to cross-pollinate his pea plants, in order to produce seeds that came from two different parents.

• He did this by cutting off the male parts of one flower and dusting it with the pollen from another flower.

Figure 9.3-3

Removedstamensfrom purpleflower.

Stamens

Transferred pollen fromstamens of white flower

to carpel of purple flower.

Parents

(P)

Carpel

Offspring

(F1)

Pollinated carpel

matured into pod.

Planted seeds

from pod.

4

3

2

1

• A character is a heritable feature that varies among individuals.

• A trait is a variant of a character.

• Each of the characters Mendel studied occurred in two distinct traits.

In an Abbey Garden

Character

WhitePurple

RecessiveDominant

Green Yellow

Terminal Axial

Wrinkled Round

Green Yellow

Seed

shape

Seed

color

Flower

position

Flower

color

Pod

color

RecessiveDominant

Pod

shape

Stem

length

Inflated Constricted

Tall Dwarf

Trait

What Mendel did…..

– created purebred varieties of plants – plants that when they self-fertilized only produced off spring similar to that of the parent.

– then he crossed two different purebred varieties.

In an Abbey Garden

• Hybrids are the offspring of two different purebred varieties.

– The parental plants are the P generation.

– Their hybrid offspring are the F1 generation.

– A cross of the F1

plants forms the F2 generation.

His Results???

Monohybrid Crosses

• A monohybrid cross is a cross between purebred parent plants that differ in only one character.

Figure 9.5-3

Purple flowers

F1 Generation

White flowers

P Generation(purebredparents)

All plants havepurple flowers

F2 Generation

Fertilization

among F1 plants

(F1 F1)

of plants

have purple flowers

of plants

have white flowers

34

14

Mendel’s Law of Segregation

• Lets examine his experiments and see how his results led him to develop 4 hypotheses.

• Mendel developed four hypotheses from the monohybrid cross, listed here using modern terminology (including “gene” instead of “heritable factor”).

1. The alternative versions of genes are called alleles.

Mendel’s 4 Hypothesis

Figure 9.7

Homologous

chromosomes

P

Genotype:

Gene loci

P

a

aa

b

B

Dominant

allele

Recessive

alleleBbPP

Homozygous

for the

dominant allele

Homozygous

for the

recessive allele

Heterozygous

with one dominant

and one recessive

allele

a

2. For each inherited character, an organism inherits two alleles, one from each parent.

– An organism is homozygous for that gene if both alleles are identical.

– An organism is heterozygous for that gene if the alleles are different.

Mendel’s Hypothesis

3. If two alleles of an inherited pair differ,

– then one determines the organism’s appearance and is called the dominant allele

– the other has no noticeable effect on the organism’s appearance and is called the recessive allele.

Mendel’s Hypothesis

4. Gametes (sperm and egg) carry only one allele for each inherited character.

– The two alleles for a character segregate (separate) from each other during the production of gametes (meiosis I = homologous chromosomes separate)

– This statement is called the law of segregation.

Mendel’s Hypothesis

• Do Mendel’s hypotheses account for the 3:1 ratio he observed in the F2 generation?

• A Punnett square highlights

– the four possible combinations of gametes

– the four possible offspring in the F2 generation.

Monohybrid Crosses

Figure 9.11

F1 Genotypes

F2 Genotypes

Bb female Bb male

Formation of eggs Formation of sperm

Male

gametes

Fem

ale

gam

ete

s

2

1

2

1

2

1

2

1

2

1

2

14

1

4

1

4

1

4

1

( )

B

BB B B

B

b

b

b b b b

Figure 9.6P Generation Genetic makeup (alleles)

Alleles carried

by parents

Gametes

Purple flowers

PP

White flowers

pp

All P All p

pP

F1 Generation

(hybrids)

F2 Generation

(hybrids)

Alleles

segregate

Gametes

Purple flowers

All Pp

21

21

Sperm from

F1 plant

Eggs from

F1 plant

Phenotypic ratio

3 purple : 1 white

Genotypic ratio

1 PP : 2 Pp : 1 pp

p

p

P

PPP Pp

Pp pp

Figure 9.6a

Purple flowers White flowers

P Generation Genetic makeup (alleles)

Gametes

Alleles carried

by parentsPP

P

pp

pAll All

Figure 9.6b

F1 Generation

(hybrids)

All Pp

Purple flowers

Gametes

Alleles segregate

P p2

1

2

1

Figure 9.6c

F2 Generation

(hybrids)Sperm from

F1 plant

Eggs from

F1 plant

Phenotypic ratio

3 purple : 1 white

Genotypic ratio

1 PP : 2 Pp : 1 pp

p

p

P

P

PP Pp

Pp pp

Warm Up

Freckles (F) in humans is dominant to no freckles (f).

Using the letters F and f, what are the genotypes for…

1. A person who is homozygous dominant.

2. A person who is heterozygous.

3. A person who is homozygous recessive.

What are the phenotypes for…..

1. Ff

2. ff

3. FF

Question:

What information goes inside of the Punnett Square?

What information goes on the outside?