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)