Page 124 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

116 Chapter 4 The Chromosome Theory of Inheritance

Figure 4.20 A Drosophila eye color gene is located on the X chromosome. X-linkage explains the inheritance of alleles of
the white gene in this series of crosses performed by Thomas Hunt Morgan. The progeny of crosses A, B, and C outlined with green dotted
boxes are those used as the parents in the next cross of the series.
Cross A Cross B
w + w + w w + w w +
X X X Y X X X Y

w + w w + w + w + w + w w + w
X X X Y X X X X X Y X Y

All progeny red-eyed 3 red 1 white

Cross C Cross D w w w +
X w + X w X w Y X X X Y

w + w w
w + w w w w + w X X X Y
X X X X X Y X Y

Crisscross inheritance

results of the reciprocal crosses red female × white male shown in Fig. 4.20, Morgan’s assumption that the gene for
(cross A) and white female × red male (cross D) are not eye color is X-linked explains the results of his breeding
identical, again in contrast with Mendel’s findings. experiments. Crisscross inheritance, for example, occurs
From the data, Morgan reasoned that the white gene for because the only X chromosome in sons of a white-eyed
eye color is X-linked, that is, carried by the X chromo- mother (X X ) must carry the w allele, so the sons will be
w
w
some. (Note that while symbols for genes and alleles are white-eyed. In contrast, because daughters of a red-eyed
+
+
italicized, symbols for chromosomes are not.) The Y chro- (X Y) father must receive a w -bearing X chromosome
w
mosome carries no allele of this gene for eye color. Males, from their father, they should all have red eyes.
therefore, have only one copy of the gene, which they in-
herit from their mother along with their only X chromo-
some; their Y chromosome must come from their father. Validation of the chromosome theory from
Thus, males are hemizygous for this eye color gene, be- the analysis of nondisjunction
cause their diploid cells have half the number of alleles Although Morgan’s work strongly supported the hypothe-
carried by the female on her two X chromosomes. sis that the gene for eye color lies on the X chromosome, he
If the single white gene on the X chromosome of a himself continued to question the validity of the chromo-
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male is the wild-type w allele, he will have red eyes and a some theory until Calvin Bridges, one of his top students,
+
w
genotype that can be written X Y. [Here we designate the found another key piece of evidence. Bridges repeated the
chromosome (X or Y) together with the allele it carries, to cross Morgan had performed between white-eyed females
emphasize that certain genes are X-linked.] In contrast to and red-eyed males, but this time he did the experiment on
+
w
w
an X Y male, a hemizygous X Y male would have white a larger scale. As expected, the progeny of this cross con-
eyes. Females with two X chromosomes can be one of sisted mostly of red-eyed females and white-eyed males.
+
w
three genotypes: X X (white-eyed), X X (red-eyed However, about 1 in every 2000 males had red eyes, and
w
w
w
+
+
+
because w is dominant to w), or X X (red-eyed). As about the same small fraction of females had white eyes.
w
w