Mendelian Genetics

What Came Before? • Blending Inheritance

• Inheritance of Acquired Characteristics

Mendel’s Model Organism•Advantages of pea plants for genetic study:

o There are many varieties with distinct heritable features, or traits (such as peacolor)

o Mating of plants can be controlled

o Quick generation time; lots of offspring

o Can isolate true-breeding lines for particular traits

Fig. 14-2a

StamensCarpel

Parentalgeneration(P)

TECHNIQUE

1

2

3

4

Fig. 14-3-3

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

Table 14-1

CONCEPT 1: There are alternative versions of “heritable factors”

Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

CONCEPT 2: For each trait, an organism inherits two alleles, one from each parent

Fig. 14-4

CONCEPT 3: If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

Fig. 14-3-3

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

Misconceptions About Dominant vs. Recessive• Dominant alleles are not necessarily more

common in populations than recessive alleles

• Dominant alleles are not necessarily “better” / adaptive

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

CONCEPT 4: Law of Segregation

Fig. 14-5-3

P Generation

Appearance:Genetic makeup:

Gametes:

Purple flowers White flowersPP

P

pp

p

F1 Generation

Gametes:

Genetic makeup:Appearance: Purple flowers

Pp

P p1/21/2

F2 Generation

Sperm

Eggs

P

PPP Pp

p

pPp pp

3 1

How can you tell the genotype of an individual with the dominant phenotype?

CONCEPT 5:

Law of Independent Assortment

Fig. 14-8

EXPERIMENT

RESULTS

P Generation

F1 Generation

Predictions

Gametes

Hypothesis ofdependentassortment

YYRR yyrr

YR yr

YyRr

Hypothesis ofindependentassortment

orPredictedoffspring ofF2 generation

Sperm

Sperm

YR

YR

yr

yr

Yr

YR

yR

Yr

yR

yr

YRYYRR

YYRR YyRr

YyRr

YyRr

YyRr

YyRr

YyRr

YYRr

YYRr

YyRR

YyRR

YYrr Yyrr

Yyrr

yyRR yyRr

yyRr yyrr

yyrr

Phenotypic ratio 3:1

EggsEggs

Phenotypic ratio 9:3:3:1

1/21/2

1/2

1/2

1/4

yr

1/41/4

1/41/4

1/4

1/4

1/4

1/43/4

9/163/16

3/161/16

Phenotypic ratio approximately 9:3:3:1315 108 101 32

Fig. 14-3-3

EXPERIMENT

P Generation

(true-breeding parents) Purple

flowers Whiteflowers

F1 Generation

(hybrids) All plants hadpurple flowers

F2 Generation

705 purple-floweredplants

224 white-floweredplants

Mendel’s Laws Translated Into Math

• Multiplication rule = the probability that two or more independent events will occur together is the product of their individual probabilities (AND)

• Addition rule = the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities (OR)

Fig. 14-9

Rr RrSegregation of

alleles into eggs

Sperm

R

R

R R

R

R r

rr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4

1/4

1/41/4

Example: BbEe x BbEe

• What is the probability the offspring will be homozygous recessive for both traits?

• What is the probability the offspring will be homozygous recessive for at least one trait?

Example: BbEe x BbEe

PRACTICE PROBLEMS!