This worksheet will help you to work through the cross for the cinnabar (cn) mut
ID: 149866 • Letter: T
Question
This worksheet will help you to work through the cross for the cinnabar (cn) mutation. The cross is symbolized in this fashion: / cn bw vg; st/st The semicolons are used to separate the genotype for each chromosome and the chromosomes are listed in X; 2; 3 order from left to right. The "/"sign is used to separate the genotype for each homologue. For the Fl generation, this means that chromosome two has one homologue that carries cn, bw and vg' while its counterpart carries cnt, bw and vg. Because the third chromosome and X chromosome are all homozygous and wild-type in this cross, we will ignore them. 1. What is the expected ratio for sex in the F2 generation? 2. What are the two types of gametes that will be produced by the F1 male for just the second chromosome? 3. What are the four types of gametes that will be produced by the Fl female for just the second chromosome?Explanation / Answer
1. As sex is related only with X chromosome, so only X chromosome alleles from F1 parents will be taken into consideration. F1 male contains one v+ and one Y chromosome; F1 female contains only one type of allele v+ in X chromosome. So the possible genotypes of F2 individuals for X chromosome are v+/v+ (female) and v+/Y (male). So the ratio of male and female is 1:1.
2. Two types of gametes produced by F1 male for second chromosome are cn bw vg+ and cn+ bw vg.
3. Four types of gametes produced by F1 female are cn bw vg+ , cn+ bw vg, cn bw vg and cn+ bw vg+.
4. There are only two types of gametes in males because Drosophila males show no crossing-over. Because in prophase I of meiosis I, no synaptonemal complex and chiasmata form in males, so homologous chromosomes can not pair to produce recombinant alleles. Moreover, some proteins required for crossing over are also absent in males.
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