A mutational screen for developmental mutants in Drosophila :
differentiating zygotic versus maternal-effect genes
Zygotic Genes [Top] :
In a typical genetic cross
between two organisms heterozygous (m+m) for a recessive allele m, 3/4 of the
offspring are (m+m+)
or (m+m)
and will show
the wild-type phenotype (m+),
and 1/4 have the mutant genotype and phenotype (mm).
Observation of the 3:1 ratio
indicates that the trait is Mendelian,
the
result of the interactions of maternal and paternal alleles in the
zygote.
Maternal-Effect Genes [Below]: If a particular locus has a maternal effect, all offspring of a mother will show the maternal phenotype, irrespective of the genotype of the paternal parent. Thus the result is non-Mendelian. In the first cross, the result is indistinguishable from the typical Mendelian pattern. In the second cross, however, half of the offspring are genotypically (mm), but all show the wild-type (m+) phenotype of the mother. In the third cross, all of the offspring are (m+m), but all show the m phenotype. In the fourth cross, half of the offspring are (m+m), but all show the m phenotype. Note that the second and fourth crosses are genetically identical (mm x m+m), but produce different results according to the phenotype of the female parent. Note as well that the third and fourth crosses are also consistent with the mutant phenotype as a Mendelian dominant trait, however this is ruled out by the first and second crosses.
Maternal-Effect Genes [Below]: If a particular locus has a maternal effect, all offspring of a mother will show the maternal phenotype, irrespective of the genotype of the paternal parent. Thus the result is non-Mendelian. In the first cross, the result is indistinguishable from the typical Mendelian pattern. In the second cross, however, half of the offspring are genotypically (mm), but all show the wild-type (m+) phenotype of the mother. In the third cross, all of the offspring are (m+m), but all show the m phenotype. In the fourth cross, half of the offspring are (m+m), but all show the m phenotype. Note that the second and fourth crosses are genetically identical (mm x m+m), but produce different results according to the phenotype of the female parent. Note as well that the third and fourth crosses are also consistent with the mutant phenotype as a Mendelian dominant trait, however this is ruled out by the first and second crosses.
All text material © 2008 by Steven M. Carr