10.4 Genes of Different Chromosomes Are Inherited Independently

A. Dihybrid Crosses Track the Inheritance of Two Genes at Once

• P generation true-breeding for 2 traits• F1 crossed to yield F2

• Four phenotypes in ratio 9:3:3:1– All possible combinations found

rr yyRR YY

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R Y

R Y

r y r y

Mal

e g

amet

es

F1 generation

rr yy

Female gametes

RR YY

Yy

YyRrRr Yy

Rr Yy Rr

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Y Y

R Y R y r Y r y

R Y

R y

r Y

r y

9331

F2 generation

Female gametes

Mal

e g

amet

es

Phenotypic ratio 9:3:3:1

Smooth, yellowSmooth, greenWrinkled, yellowWrinkled, green

RR

RR

Rr

Rr

Rr Yy

YyRrYYRrYyRR

Yy RR yy Rr Yy yyRr

YY Rr Yy rr YY rr Yy

Yy Rr yy rr Yy rr yy

YyRr

10.4 Genes of Different Chromosomes Are Inherited Independently

B. Meiosis Explains Mendel’s Law of Independent Assortment

• During gamete formation, segregation of alleles for one gene does not influence alleles for another gene

• Provided genes are on different chromosomes

Figure 10.9 Mendel’s Law of Independent Assortment.

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Y Y

Y

Y

Y

Y

Y

Y

R R

R

R

R

R

R

R

rr

r

rr

r

r

r

y

y

y

y

y

y

y

Y

Y

RR

r

r

y

y

y

R

r

R

r

YR

Y

yr

y

yR

y

Y

r

Y

Diploid cell

Alternative 1

Alternative 2

Metaphase I

MEIOSIS I

MEIOSIS I

MEIOSIS II

MEIOSIS II

MEIOSIS II

MEIOSIS II

HaploidgametesMetaphase II

10.4 Genes of Different Chromosomes Are Inherited Independently

B. Meiosis Explains Mendel’s Law of Independent Assortment

• Punnet squares for more than 2 genes cumbersome

• Product rule– Chance that two independent events will both

occur equals the product of the individual chances that each event will occur

Figure 10.10 The Product Rule.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

R

R r

r

× ×

Y

Y y

y

T

T t

t

RR Rr

Rr rr

1/2 Rr

Tt

ttTt

TTYy

yyYy

YY

1/2 Tt

Probability that off spring is Rr Yy Tt = 1/8

1/2 Yy

10.4 Mastering Concepts

Explain how meiosis contributes to independent assortment of alleles.

10.5 Genes on the Same Chromosome Many Be Inherited Together

A. Genes on the Same Chromosome Are Linked• Unlike genes on different chromosomes, they

do not assort independently during meiosis• First noticed in early 1900s

Figure 10.11 Gene Linkage Changes the Results of a Dihybrid Cross.

Ma

le g

ame

tes

3

1

P

L

p

l

P

L

p

l

×

b. Pp Ll self-cross, genes linked

FemaleMale

Crimson flowers,long pollen

Crimson flowers,long pollen

Crimson flowers,long pollen

Red flowers,round pollen

p l

P L

Pp Ll pp ll

PP LL Pp Ll

P L p l

Female gametes

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Ma

le g

ame

tes

9

3

3

1

P pP p

L lL l

×

a. Pp Ll self-cross, genes not linked

Male

Crimson flowers,long pollen

Crimson flowers,Long pollen

Female

P L P l P L P l

Red flowers,round pollen

Red flowers,long pollen

Crimson flowers,round pollen

Crimson flowers,long pollen

p l

p L

P l

P L

Pp Ll

Pp LL

PP Ll

PP LL

Pp ll

Pp Ll

PP ll

PP Ll

pp Ll

pp LL

Pp LL

Pp Ll

pp ll

pp Ll

Pp ll

Pp Ll

Female gametes

10.5 Genes on the Same Chromosome May Be Inherited Together

A. Genes on the Same Chromosome Are Linked• Thomas Hunt Morgan• Drosophila melanogaster – fruit fly• Linkage groups are on the same chromosome

10.5 Genes on the Same Chromosome Many Be Inherited Together

A. Genes on the Same Chromosome Are Linked• Sometimes did see combinations not found in

parents– Due to crossing over

• Parental chromatid- no crossing over• Recombinant chromatid

Figure 10.12 Crossing Over.

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p p

l

lL

PL

pl

Pl

P P

L

pP

L L

P p

l

pL

Homologous chromosomescross over in prophase I

MEIOSIS I

MEIOSIS II

MEIOSIS II

Homologs part

Recombinant

Recombinant

Gametes

Parental

Parental

10.5 Genes on the Same Chromosome Many Be Inherited Together

B. Studies of Linked Genes Have Yielded Chromosome Maps

• Some crosses had a higher proportion of recombinant offspring

• Alfred Sturtevant, Morgan’s undergraduate assistant, proposed linkage maps

• Genetic markers used now

Figure 10.13 Breaking Linkage.

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AGenes A and B are far apart; crossing overis more likely to separate these alleles.

Genes B and C are close together; crossingover is less likely to separate these alleles.

B

C

a

b

cy w v m r

54.536.133.00 1.5

a. b.

Clicker Question

• What is the order of genes on a chromosome if g and m are 4 units apart, k and h are 6 units apart and m and h are 2 units apart?

A.ghmkB.gmkhC.hkmg