Page 123 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.6 Validation of the Chromosome Theory 115

Mendel’s laws of segregation and independent assort- A gene determining eye color
ment. If Mendel’s genes for pea shape and pea color are on the Drosophila X chromosome
assigned to different (that is, nonhomologous) chromo- Thomas Hunt Morgan, an American experimental biolo-
somes, the behavior of chromosomes can be seen to paral- gist with training in embryology, headed the research group
lel the behavior of genes. Walter Sutton’s observation of whose findings eventually established a firm experimental
these parallels led him to propose that chromosomes and base for the chromosome theory. Morgan chose to work
genes are physically connected in some manner. Meiosis with the fruit fly Drosophila melanogaster because it is
ensures that each gamete will contain only a single chro- extremely prolific and has a very short generation time,
matid of a bivalent and thus only a single allele of any taking only 12 days to develop from a fertilized egg into a
gene on that chromatid (Table 4.4a). The independent be- mature adult capable of producing hundreds of offspring.
havior of two bivalents during meiosis means that the genes Morgan fed his flies mashed bananas and housed them in
carried on different chromosomes will assort into gametes empty milk bottles capped with wads of cotton.
independently (Table 4.4b). In 1910, a white-eyed male appeared among a large
From a review of Fig. 4.17a, which follows two dif- group of flies with brick-red eyes. A mutation had appar-
ferent chromosome pairs through the process of meiosis, ently altered a gene determining eye color, changing it from
you might wonder whether crossing-over abolishes the the normal wild-type allele specifying red to a new allele
clear correspondence between Mendel’s laws and the that produced white. When Morgan allowed the white-eyed
movement of chromosomes. The answer is no. Each chro- male to mate with its red-eyed sisters, all the flies of the F 1
matid of a homologous chromosome pair contains only generation had red eyes; the red allele was clearly dominant
one copy of a given gene, and only one chromatid from to the white (Fig. 4.20, cross A).
each pair of homologs is incorporated into each gamete. Establishing a pattern of nomenclature for Drosophila
Because alternative alleles remain on different chromat- geneticists, Morgan named the gene identified by the ab-
ids even after crossing-over has occurred, alternative al- normal white eye color the white gene, for the mutation that
leles still segregate to different gametes as demanded by revealed its existence. The normal wild-type allele of the white
Mendel’s first law. gene, abbreviated w , is for brick-red eyes, while the coun-
+
Furthermore, because the orientations of nonhomolo- terpart mutant w allele results in white eye color. The
gous chromosomes are completely random with respect to superscript + signifies the wild type. By writing the gene
each other during both meiotic divisions, the genes on dif- name and abbreviation in lowercase, Morgan symbolized
ferent chromosomes assort independently even if crossing-over that the mutant w allele is recessive to the wild-type w . (If
+
occurs, as demanded by Mendel’s second law. In Fig. 4.17a, a Drosophila mutation results in a dominant non-wild-type
you can see that without recombination, each of the two phenotype, the first letter of the gene name or of its abbre-
random alignments of the nonhomologous chromosomes viation is capitalized; thus the mutation known as Bar eyes
results in the production of only two of the four gamete is dominant to the wild-type Bar allele. (See the Appendix
+
types: AB and ab for one orientation, and Ab and aB for the Guidelines for Gene Nomenclature.)
other orientation. With recombination, each of the align- Morgan then crossed the red-eyed males of the F 1 gen-
ments of alleles in Fig. 4.17a may in fact generate all four eration with their red-eyed sisters (Fig. 4.20, cross B) and
gamete types. (Imagine a crossover switching the posi- obtained an F 2 generation with the predicted 3:1 ratio of
tions of A and a nonsister chromatids in Fig. 4.17a). Thus, red to white eyes. But there was something askew in the
both the random alignment of nonhomologous chromo- pattern: Among the red-eyed offspring, there were two fe-
somes and crossing-over contribute to the phenomenon of males for every one male, and all the white-eyed offspring
independent assortment. were males. This result was surprisingly different from
the equal transmission to both sexes of the Mendelian
Specific Traits Are Transmitted traits discussed in Chapters 2 and 3. In these fruit flies,
with Specific Chromosomes the ratio of eye colors was not the same in male and
female progeny.
The fate of a theory depends on whether its predictions By mating F 2 red-eyed females with their white-eyed
can be validated. Because genes determine traits, the pre- brothers (Fig. 4.20, cross C), Morgan obtained some fe-
diction that chromosomes carry genes could be tested by males with white eyes, which then allowed him to mate a
breeding experiments that would show whether transmis- white-eyed female with a red-eyed wild-type male (Fig. 4.20,
sion of a specific chromosome coincides with transmis- cross D). The result was exclusively red-eyed daughters
sion of a specific trait. Cytologists knew that one pair of and white-eyed sons. The pattern seen in cross D is known
chromosomes, the sex chromosomes, determines whether as crisscross inheritance because the males inherit their
an individual is male or female. Would similar correla- eye color from their mothers, while the daughters inherit
tions exist for other traits? their eye color from their fathers. Note in Fig. 4.20 that the