1. Chapter 15 173 The Chromosomal Basis of Inheritance
Thomas Hunt Morgan, an embryologist at Columbia University, was the first one to associate a specific gene with a specific chromosome.
Fruit fly: 8 chromosomes: 3 pairs of autosomes, and 1 pair of sex chromosomes (XX or XY).
Fruit fly was used because of the relatively simple chromosome composition, short generation time, large number of progeny, and the ease of caring in the laboratory.

2. 173
Morgan and his students invented a convention for symbolizing alleles.
w: white eye
w+: wild-type (red eye)
Cy: curled shape wing
Cy+: wild-type
Sex-linked genes are located on sex chromosome.
Linked genes are located on the same chromosome and are segregated into the same gamete.

3. Fig. 15-3
Figure 15.3 Morgan’s first mutant

4. 174 Chromosomal Basis of Sex: X-Y System:
Humans have one pair of sex chromosomes (XX or XY), and 22 pairs or 44 pieces of somatic chromosomes (also called autosomes).
In 1990, a British researcher identified a gene responsible for the development of testes. It is called the sex-determining region Y gene (Sry gene). In its absence, the gonads develop into ovaries. The gene codes for a protein that regulates other genes.
Holandric genes are located on Y chromosomes. They are responsible for male characteristics. Hairy pinna traits is believed to be determined by a gene on the Y chromosome.

5. Fig. 15-5 X Y
Figure 15.5 Human sex chromosomes

6. (a) The X-Y system 44 + XY 44 + XX Parents 22 + X 22 + X 22 + Y or +
Fig. 15-6a
44 + XY
44 + XX
Parents
22 + X
22 + X
22 + Y or +
Sperm
Egg
Figure 15.6 Some chromosomal systems of sex determination
44 + XX
44 + XY or
Zygotes (offspring)
(a) The X-Y system

7. 174
XO System:
Grasshoppers, crickets, roaches and some other insects do not have Y chromosome. A female has XX and a male has one X, and is thus XO.
Z-W System:
In birds, some fishes, and some insects (butterflies and moths), the female is the heterogametic sex. Their sex chromosomes are designated A and W to avoid confusion with the X-Y system. Male birds are ZZ and female birds are ZW. Sex is determined by whether the egg carries a Z or a W chromosome.

8. (b) The X-0 system 22 + XX 22 + X Fig. 15-6b
Figure 15.6 Some chromosomal systems of sex determination

9. (c) The Z-W system 76 + ZW 76 + ZZ Fig. 15-6c
Figure 15.6 Some chromosomal systems of sex determination
(c) The Z-W system

10. 174 Haplo-diploid System:
There are no sex chromosomes in most species of bees and ants. Fertilized eggs (diploid) develop into females, and unfertilized eggs (haploid) develop into males. So, females are diploid, and males are haploid.
Parthenogenesis is the development of an organism from an unfertilized egg.
In Drosophila, maleness is determined by the ratio of the gene product of X chromosome to certain gene products of the autosomes. A high ratio produces a female, and a low ratio results in a male. The Y chromosome does not determine sex. It contains genes for normal male fertility.

11. (d) The haplo-diploid system
Fig. 15-6d 32
(Diploid) 16
(Haploid)
Figure 15.6 Some chromosomal systems of sex determination
(d) The haplo-diploid system

12. 174 Sex-Linked disorders in Humans:
Duchenne’s muscular dystrophy is characterized by a progressive weakening of the muscles and loss of coordination. The patients rarely survive over 20. It is caused by a lack of a key muscle protein coded by a gene on the X chromosome.
Hemophilia is a sex-linked recessive trait. The patients bleed excessively because of a lack of a blood protein required for blood clotting.

13. 175
Barr body is the inactive X chromosome which contracts into a dense object. It is visible as a dark spot at the edge of the nucleus of female cells.
Number of Barr body
= Number of X chromosomes - 1

14. 175 The Recombination of Linked Genes: Crossing Over
The disproportional number of progeny with the parental traits of gray body and long wing in Drosophila can be explained by crossing over (Fig. 15.9, p. 299)
b+ b b b
vg+ vg x vg vg |
965 b+ b : b b : b+ b : b b
vg+ vg vg vg vg vg vg+ vg

15. 177 Chiasmata: the points where two chromosomes cross.
In 1911, T.H. Morgan proposed a hypothesis which states that the amount of crossing over is a function of the distance in the chromosome between the genes. The distance that would give on the average one crossover per 100 fertilized eggs as 1 unit (centimorgan, a map unit in honor of Morgan) of the genetic map.

16. 177 BbLl x bbll BL Bl bL bl bl BbLl Bbll bbLl bbll
If 7 out of 8 exhibited the parental characteristics, and 1 out of 8 exhibited the recombinant type, then the distance between B and L is derived as follows:
7 out of 8 is 87.5%, 1 out of 8 is 12.5%.
The genetic map between B and L is
12.5 units.
B units L

17. 177 In a cross between AaLl and aall, if the parental
type is 95%, then the recombinant type is 5%.
AL + al (parental type) %
Al + aL (recombinant type) %
The distance between A and L is 5 units.
5 units units
B A L A
12.5 units

18. 177
If a cross: AaBb x aabb yielded 92.5% parental type, and 7.5% recombinant type, then the distance between B and A is 7.5 units. So, the genetic map containing the genes A, B and L should be as follows:
7.5 units 5 units
B A L
12.5 units

19. 178
The chromosomal map (linkage or genetic map) does not disclose the actual locations of the genes on the chromosomes. It tells only the relative distance between the genes. The cytological map pinpoints the gene loci.

20. 178
Polyploidy is the condition of having more than two complete chromosome sets.
Triploidy has 3 sets (3n), and tetraploidy has 4 sets (4n).
Polyploidy is common in plants, and is very rare in animals. It plays an important role in the evolution of plants.

21. 178 Alterations of Chromosome Structure: Deletion: Duplication:
Inversion:
Translocation:

22. Reciprocal translocation
Fig
A B C D E F G H
A B C E F G H
Deletion
(a)
A B C D E F G H
A B C B C D E F G H
Duplication
(b)
A B C D E F G H
A D C B E F G H
(c)
Inversion
Figure Alterations of chromosome structure
A B C D E F G H
M N O C D E F G H
(d)
Reciprocal
translocation
M N O P Q R
A B P Q R

23. 179 Chromosomal nondisjuction occurs in female sex chromosomes: X Y
XX XXX XXY
(abnormal) (Klinefelter’s
syndrome)
O XO YO
(Turner’s (Inviable male)

24. 179 Chromosomal nondisjuction occurs in male sex chromosomes: XY O
X XXY XO
(Klinefelter’s (Turner’s
syndrome) syndrome)
X XXY XO

25. 179 Abnormal Chromosome Number:
Aneuploidy is the abnormal number of chromosomes. It may be caused by chromosomal nondisjuction in anaphase of meiosis I or meiosis II where sister chromatids fail to separate.
Monosomy is the presence of 2n-1 chromosomes of one type.
Trisomy is the presence of 2n + 1 chromosomes of one type. Down syndrome is an example of trisomy for chromosome 21.

26. 179 Aneuploidy of Sex Chromosomes:
Klinefelter’s syndrome: XXY, XXYY, XXXY, XXXXY and XXXXXY
male with small testes which produce few sperm; has female-like breasts; mentally retarded; susceptible to chronic pulmonary distress and diabetes.
XYY: taller than the average male; less severe abnormalities; poorly developed genitalia, subnormal intelligence, predisposed to violence (some scientists disagree).

27. 179-180 Turner’s syndrome: XO
ovaries fail to develop; uterus is small; with external genitalia; sterile and of short stature, usually have no mental deficiency; most live a normal life. It is the only known viable human monosomy.
Trisomy X (XXX) are metafemales who has limited fertility and may be mentally retarded.

28. 180 Aneuploidy of Somatic Chromosomes: Down’s syndrome (Mongolism):
trisomy 21 or translocation between chromosome 15 and 21. Patients are short and stocky; with folded eyelid; defective heart; susceptible to respiratory infection and mentally retarded. Most are sterile.

29. Fig a
Figure Down syndrome

30. Fig b
Figure Down syndrome

31. 180 Patau’s syndrome: trisomy 13
Patients have serious eye, brain and circulatory defects as well as harelip and cleft palate. They also have mental and developmental retardation. Most victims survive less than one year. Most result in abortion.
Edward’s syndrome: trisomy 18
Its symptoms are similar to those of Patau’s syndrome. Almost every system is affected.

32. 180
Cri Du Chat (cry of the cat) syndrome is caused by a specific deletion in chromosome 5. The victim is mentally retarded; with small head an unusual facial features; produces a cry that sounds like the mewing of a distressed cat. Death occurs during infancy or early childhood.
Chronic myelogenous leukemia (CML) is caused by chromosomal translocation between a large portion of chromosome 22 and a small fragment from a tip of chromosome 9, producing a much shortened, easily recognized chromosome 22 known as the Philadelphia chromosome.

33. Translocated chromosome 22 (Philadelphia chromosome)
Fig
Reciprocal
translocation
Normal chromosome 9
Translocated chromosome 9
Translocated chromosome 22
(Philadelphia chromosome)
Normal chromosome 22
Figure Translocation associated with chronic myelogenous leukemia (CML)

34. 181
The mitochondrial genes in most animals and plants that are passed on to a zygote come from the cytoplasm of the egg. The genes produce the protein complexes of the electron transport chain and ATP synthase. Defects of one or more of these proteins reduce the amount of ATP the cell can make. A patient with mitochrondrial myopathy suffers weakness, intolerance of exercise and muscle deterioration. Mitochondrial mutations inherited from a person’s mother contribute to diabetes, heart disease, Alzheimer’s disease, and aging process.