Page 125 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.6 Validation of the Chromosome Theory 117

Figure 4.21 Nondisjunction: Rare mistakes in meiosis help confirm the chromosome theory. (a) Rare events of nondisjunction
in an XX female produce XX and O eggs. The results of normal disjunction in the female are not shown. XO males are sterile because the
missing Y chromosome is needed for male fertility in Drosophila. (b) In an XXY female, the three sex chromosomes can pair and segregate
in two ways, producing progeny with unusual sex chromosome complements.
(a) Nondisjunction in an XX female (b) Segregation in an XXY female
White-eyed Red-eyed
White-eyed w w w + Red-eyed
P w w w + X X Y X Y
XX X Y Meiosis
Meiosis w w w w w +
Gametes Gametes
Non- w w w + Normal X Y X XX Y X Y
disjunction segregation
XX O X Y F 1

More X w + Y
F
1 w + frequent
X Y
w w X w Y w + w w
XX w w w + ww X X Y X Y Y
XX X XX Y red white
dies white
O w + X w
red X O O Y X w X w + X w Y
sterile dies Less red white
frequent
w
XX w w w w + w w
XX X XX Y
dies white
Y
w +
X Y Y Y
red dies

Bridges hypothesized that these exceptions arose behavior of X chromosomes during rare meiotic mistakes,
through rare events in which the X chromosomes fail to indicating that the X chromosome carries the gene for eye
separate during meiosis in females. He called such failures color. These results also suggested that zygotes with the two
in chromosome segregation nondisjunction. Mistakes lead- other abnormal sex chromosome karyotypes expected from
ing to nondisjunction can occur during either meiosis I or nondisjunction in females (XXX and OY) die during embry-
meiosis II, but in either case nondisjunction would result in onic development and thus produce no progeny.
some eggs with two X chromosomes and others with none. Because XXY white-eyed females have three sex chro-
As Fig. 4.21a shows, fertilization of these chromosomally mosomes rather than the normal two, Bridges reasoned
abnormal eggs could produce four types of zygotes: XXY they would produce four kinds of eggs: XY and X, or XX
(with two X chromosomes from the egg and a Y from the and Y (Fig. 4.21b). You can visualize the formation of
sperm), XXX (with two Xs from the egg and one X from these four kinds of eggs by imagining that when the three
the sperm), XO (with the lone sex chromosome from the chromosomes pair and disjoin during meiosis, two chro-
sperm and no sex chromosome from the egg), and OY (with mosomes must go to one pole and one chromosome to the
the only sex chromosome again coming from the sperm). other. With this kind of segregation, only two results are
When Bridges examined the sex chromosomes of the possible: Either one X and the Y go to one pole and the
rare white-eyed females produced in his large-scale cross, he second X to the other (yielding XY and X gametes), or the
found that they were indeed XXY individuals who must two Xs go to one pole and the Y to the other (yielding XX
have received two X chromosomes and with them two w al- and Y gametes). The first of these two scenarios occurs
leles from their white-eyed X X mothers. The exceptional more often because it comes about when the two similar X
w
w
red-eyed males emerging from the cross were XO; their eye chromosomes pair with each other, ensuring that they will
color showed that they must have obtained their sole sex go to opposite poles during the first meiotic division. The
+
chromosome from their X Y fathers. In this study then, second, less likely possibility happens only if the two
w
transmission of the white gene alleles followed the predicted X chromosomes fail to pair with each other.