| The female in this cross (top left) is heterozygous for the dominant mutation Antp and has also inherited a SNP from each of its parents, both of which abolish a restriction site (RS1 and RS2) in the DNA of the third chromosome. The male (top right) is wild type. Half of the progeny of the cross between these two flies have legs instead of antennae (left) and half have normal antennae (right). The length and number of restriction fragments produced by digesting the relevant region of the chromosome with RE1 (that cleaves DNA at RS1) and RE2 (that cleaves DNA at RS2) show that most of the progeny (166/200) inherit a maternal third chromosome in its original form and so retain either RS1 or RS2; most of the leg headed progeny (83/200) have RS1 (two lower molecular weight bands on gel after digestion of DNA with RE1) but not RS2 (single, higher molecular weight band on gel after digestion with RE2), whereas a similar proportion of flies with normal antennae inherit RS2 only (single, higher molecular weight band on gel after digestion with RE1; two lower molecular weight bands on gel after digestion with RE2). Some of the progeny carry both restriction sites (two lower molecular weight bands on gel after digestion with both RE1 and RE2), and some neither (single high molecular weight band on gel after RE1 and RE2 digestion). Since Antp is closer to SNP1 than to SNP2, a higher proportion of these will have leg heads and both restriction sites or normal antennae and neither restriction site (18/200), than will have normal antennae and both restriction sites or leg heads and neither restriction site (14/200). A small proportion of the progeny (2/200) have antennapedia and RS2, or normal antennae and RS1, as a result of recombination events between the maternal chromosomes during meiosis. |
