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The Chromosomal Basis of Inheritance
Chapter 15, pp. 286 - 304
Lecture Outline Chapter 14 • Pedigree analysis • Human inherited disorders Chapter 15 • Chromosome theory of inheritance • Morgan’s fruit fly experiments and evidence for the theory • Sex-linked genes, examples and evidence
Fig. 15-2
P Generation Yellow-round seeds (YYRR)
Y
F1 Generation
Y R R
R Y
× r r
r
y
y
y
Meiosis
Fertilization
Gametes
Green-wrinkled seeds ( yyrr)
All F1 plants produce yellow-round seeds (YyRr)
R R
Y Y r r
y y
Meiosis
R R
Y Y
r r
y y
Metaphase I
Y Y
R R r r
y y Anaphase I
r r
y Y
Metaphase II R
Y
R
y
y y y
R R
Y Y r r r r
y Y Y R R
yR Yr yr YR 1/4 1/4 1/4 1/4
F2 Generation
Gametes
An F1 × F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
LAW OF SEGREGATION The two alleles for each gene separate during gamete formation.
1
2
3 3
2
1
Parent
Bud
0.5 mm
Fig. 15-3
Wild type w+
Mutant w
P Generation
Generation F1 All offspring had red eyes ×
EXPERIMENT
F2 Generation
Fig. 15-4
Morgan’s cross
3:1 ratio
X
Y
Sex chromosomes
Homology
Fig. 15-4
Eggs F1
CONCLUSION
Generation
Generation
P X X
w
Sperm
X Y
+
+
+ + +
Eggs Sperm
+
+ + +
+
Generation F2
×
w
w
w
w w
w w
w
w w
w
w
w
w w
(a) (b) (c)
Eggs XN
XN
Eggs XN
Xn
Eggs XN
Xn
Fig. 15-7
XNXN XnY XNXn XNY XNXn XnY
N = dominant allele for normal color vision n = recessive allele for color blindness
Unaffected Carriers Color-blind
× × ×
Color-blindness is an X-linked trait in humans. Which of the following is true?
A. If a woman is a carrier, all of her sons will be color-blind B. If a man is a carrier, all of his sons will be color-blind C. If a man is color blind, all of his daughters will be carriers D. The probability that a color-blind man and a normal-sighted, non-
carrier woman will have a color-blind son is 1/2.
(a) (b) (c)
Eggs XN
XN
Eggs XN
Xn
Eggs XN
Xn
Fig. 15-7
XNXN XnY XNXn XNY XNXn XnY
N = dominant allele for normal color vision n = recessive allele for color blindness
Unaffected Carriers Color-blind
× × ×
Color-blindness is an X-linked trait in humans. Which of the following is true?
A. If a woman is a carrier, all of her sons will be color-blind B. If a man is a carrier, all of his sons will be color-blind C. If a man is color blind, all of his daughters will be carriers D. The probability that a color-blind man and a normal-sighted, non-
carrier woman will have a color-blind son is 1/2.
Laws governing X-linked traits
• Fathers pass X-linked genes to all their daughters, but none to their sons – fathers pass their Y chromosome to their sons
• Mothers can pass X-linked genes to both daughters and sons
• Males are hemizygous (XY)