Page 143 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

5.1 Gene Linkage and Recombination 135

w y / Y represents the genotype of a male with an X chro- derived from each parent, the dominance relations of each
mosome bearing w and y, as well as a Y chromosome; pair of alleles determine the female phenotype.
phenotypically this male has white eyes and a yellow body. Now comes the significant cross for answering our
question about the assortment of genes on the same chro-
mosome. If these two Drosophila genes for eye and body
Detecting linkage by analyzing the gametes color assort independently, as predicted by Mendel’s sec-
produced by a dihybrid ond law, the dihybrid F 1 females should make four kinds of
In a cross between a female with mutant white eyes and a gametes, with four different combinations of genes on the
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wild-type brown body (w y /w y ) and a male with wild- X chromosome—w y , w y, w y , and w y. These four
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type red eyes and a mutant yellow body (w y / Y), the F 1 types of gametes should occur with equal frequency, that
offspring are divided evenly between brown-bodied females is, in a ratio of 1:1:1:1. If it happens this way, approxi-
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with normal red eyes (w y /w y) and brown-bodied males mately half of the gametes will be of the two parental
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with mutant white eyes (w y /Y) (Fig. 5.2). Note that the types, carrying either the w y allele combination seen in
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male progeny look like their mother because their pheno- the original female of the P generation or the w y allele
type directly reflects the genotype of the single X chromo- combination seen in the original male of the P generation.
some they received from her. The same is not true for the F 1 The remaining half of the gametes will be of two recombi-
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females, who received w and y on the X from their mother nant types, in which reshuffling has produced either w y
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and w y on the X from their father. These F 1 females are or w y allele combinations not seen in the P generation
thus dihybrids. With two alleles for each X-linked gene, one parents of the F 1 females.
We can see whether the 1:1:1:1 ratio of the four kinds
of gametes actually materializes by counting the different
Figure 5.2 When genes are linked, parental types of male progeny in the F 2 generation, as these sons
combinations outnumber recombinant types. Doubly receive their only X-linked genes from their maternal gam-
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heterozygous w y / w y F 1 females produce four types of male
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offspring. Sons that look like the father (w y / Y) or mother (w y / Y) ete. The bottom part of Fig. 5.2 depicts the results of a
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of the F 1 females are parental types. Other sons (w y / Y or w y / Y) breeding study that produced 9026 F 2 males. The relative
are recombinant types. For these closely-linked genes, many more numbers of the four X-linked gene combinations passed on
parental types are produced than recombinant types. by the dihybrid F 1 females’ gametes reflect a significant
departure from the 1:1:1:1 ratio expected of independent
P
assortment. By far, the largest numbers of gametes carry
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the parental combinations w y and w y. Of the total 9026
male flies counted, 8897, or almost 99%, had these geno-
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w y /w y + w y / Y types. In contrast, the new combinations w y and w y
made up little more than 1% of the total.
F 1 We can explain why the two genes fail to assort indepen-
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dently in one of two ways. The w y and w y combinations
could be preferred because some intrinsic chemical affinity
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w y /w y w y / Y exists between these particular alleles. Alternatively, these
combinations of alleles might show up most often because
they are parental types. That is, the F 1 female inherited w and
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F males y together from her P generation mother, and w and y to-
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4484 w y / Y gether from her P generation father; the F 1 female is then more
likely to pass on these parental combinations of alleles, rather
than the recombinant combinations, to her own progeny.
Parental types =
4484 + 4413 100 99%
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4413 w y / Y 9026
Linkage: A preponderance of parental
classes of gametes
A second set of crosses involving the same genes but with
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76 w y / Y a different arrangement of alleles explains why the dihy-
brid F 1 females do not produce a 1:1:1:1 ratio of the four
Recombinant types = possible types of gametes (see Cross Series B in Fig. 5.3).
76 + 53 100 1% In this second set of crosses, the original parental genera-
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53 w y / Y tion consists of red-eyed, brown-bodied females (w y /
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w y ) and white-eyed, yellow-bodied males (w y / Y), and
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Total 9026 the resultant F 1 females are all w y / w y dihybrids. To