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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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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
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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.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Y Y
Y
Y
Y
Y
Y
Y
R R
R
R
R
R
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r
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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
10.5 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
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ame
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1
P
L
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l
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L
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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
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.