used to thinking about. Recessive mutations in zygoti-cally acting genes elicit mutant phenotypes only in ho-mozygous mutant animals. The alternative category— the maternal-effect genes—has already produced its gene products in the ovary of the mother. Thus, the maternal-effect proteins are already present in the oocyte at fertilization. In maternal-effect mutations, the phenotype of the offspring depends on the geno-type of the mother, not of the offspring, because the source of the gene products is the mother’s genes. A recessive maternal-effect mutation will produce mutant animals only when the mother is a mutant homozygote. Genes that contribute to the Drosophila body plan can be cloned and characterized at the molecular level with relative ease. Any Drosophila gene with a well-established chromosomal map location can be cloned by using recombinant DNA techniques such as those described in Chapter 11. The analysis of the cloned genes often provides valuable information on the func-
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.