CHAPTER 4 Concept check questions (in figure legends) FIGURE 4.1 Concept check: What is the difference between the X-Y and X-0 systems of sex determination? Answer: In the X-Y system, the presence of the Y chromosome causes maleness, whereas in the X-0 system, it is the ratio between the number of X chromosomes and number of sets of autosomes that determines sex. A ratio of 0.5 is male and 1.0 is female. FIGURE 4.2 Concept check: Is the male bee produced by sexual reproduction? Explain. Answer: A male bee is not produced by sexual reproduction because it is produced from an unfertilized egg. Sexual reproduction involves the union of gametes. FIGURE 4.3 Concept check: How might global warming affect alligator populations? Answer: A high temperature would favor a high percentage of female alligators. This might limit population size if there are insufficient numbers of males for reproduction. On the other, a high number of females might favor a larger population size if males are not limiting. FIGURE 4.4 Concept check: Which are the opposite sexes in dioecious plants, the sporophytes or the gametophytes. Answer: The sporophytes are the opposite sexes in dioecious plants. FIGURE 4.5 Concept check: Why is the Barr body more brightly staining in a cell nucleus? Answer: The Barr body is more brightly staining because it is very compact. FIGURE 4.6 Concept check: At which stage of development does X inactivation initially occur? Answer: X inactivation initially occurs during embryonic development. FIGURE 4.7 Concept check: Which of these phases occurs in an adult female? Answer: Only the maintenance phase occurs in an adult female. FIGURE 4.8 Concept check: Why are the homologous regions of the X and Y chromosome important during meiosis? Answer: Homologous regions of the X and Y chromosome are important for chromosome synapsis (pairing) during meiosis. FIGURE 4.9 Concept check: What is the key result that suggests an X-linked inheritance pattern? Answer: In the F2 generation, only the males had white eyes. FIGURE 4.10 Concept check: What features of this pedigree indicate that the allele for Duchenne muscular dystrophy is X-linked. Answer: The key pedigree feature that points to X-linked inheritance is that only males are affected with the disorder. Also, carrier females often have affected brothers. FIGURE 4.11 Concept check: Explain why the reciprocal cross yields a different result compared to the first cross. Answer: The reason why the reciprocal cross yields a different result is because females carry two copies of an X-linked gene whereas males have only one.