The Chromosomal basis of inheritance provides an

understanding of the pattern of passage (transmission) of

genes form parent to offspringHeredity Part 1

Ideas about inheritance

Blending Inheritance Particulate Inheritance

The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)

• The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)

• Mendel documented a particulate mechanism through his experiments with garden peas

Gregor Mendel

Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

https://www.youtube.com/watch?v=2xpTz7SUbnc

Why study pea plants?

◦There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits

◦Mating of plants can be controlled

◦Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)

◦Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

TECHNIQUE

RESULTS

Parentalgeneration(P) Stamens

Carpel

1

2

3

4

Firstfilialgener-ationoffspring(F1)

5

•Mendel chose to track only those characters that varied in an either-or manner

•He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

Table 14-1

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

Fig. 14-4

Allele for purple flowers

Homologouspair ofchromosomes

Locus for flower-color gene

Allele for white flowers

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

P Generation Yellow-roundseeds (YYRR)

Y

F1 Generation

Y

R R

R Y

r

r

r

y

y

y

Meiosis

Fertilization

Gametes

Green-wrinkledseeds ( yyrr)

All F1 plants produceyellow-round seeds (YyRr)

R R

YY

r ry y

Meiosis

R R

Y Y

r r

y y

Metaphase I

Y Y

R Rrr

y y

Anaphase I

r r

y Y

Metaphase IIR

Y

R

y

yyy

RR

YY

rrrr

yYY

R R

yRYryrYR1/41/4

1/41/4

F2 Generation

Gametes

An F1 F1 cross-fertilization

9 : 3 : 3 : 1

LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologouschromosomes assortindependently during gameteformation.

LAW OF SEGREGATIONThe two alleles for each geneseparate during gameteformation.

1

2

33

2

1

Segregation and Independent Assortment of chromosomes result in genetic variation

Vocab

DominantRecessiveHomozygousHeterozygousGenotypePhenotypeMonohybrid CrossTest CrossDihybrid Cross

Fig. 14-7

TECHNIQUE

RESULTS

Dominant phenotype, unknown genotype:

PP or Pp?

Predictions

Recessive phenotype, known genotype: pp

If PP If Ppor

Sperm Spermp p p p

P

P

P

p

Eggs Eggs

Pp

Pp Pp

Pp

Pp Pp

pp pp

or

All offspring purple 1/2 offspring purple and1/2 offspring white

Test Cross: used to determine genotype of individuals with dominant phenotype

Fig. 14-9

Rr RrSegregation of

alleles into eggs

Sperm

R

R

R RR

R rrr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4

1/4

1/41/4

Rules of Probability can be applied

Rule of multiplication◦The probability of a compound event (like

flipping 2 coins and both coins landing heads up) is the product of the separate probabilities of the independent events ½ x ½ = ¼

Rule of addition◦The probability that an event can occur in two

or more alternative ways (like the chance of rolling a 1 or a 2 when rolling a die) is the sum of the events 1/6 + 1/6 = 2/6 = 1/3

We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters

A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously

In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

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

Example

Dihybrid Cross: TtGg X ttGG

What is the probability of producing offspring that is heterozygous for both traits.

You could do a dihybrid Punnett square.You could do 2 monohybrid Punnett squares

then use the rule of multiplication.

Example

2 monohybrids…◦Tt X tt probability of Tt offspring = ½

◦Gg X GG probability of Gg offspring = ½

◦Probability of both TtGg = ½ X ½ = ¼

Or do dihybrid – 16 box Punnett Square