Chapter 4 43 Chapter 4 The Chromosome Theory of Inheritance Synopsis: Chapter 4 is extremely critical for understanding basic genetics because it connects Mendel's Laws with chromosome behavior during meiosis. While you may have learned mitosis and meiosis in your basic biology class, now is the time to make sure you understand these processes in the context of inheritance. The physical basis for inheritance is chromosome segregation during meiosis. You should have an increased understanding of the importance of meiosis for genetic diversity through both independent assortment and recombination. Genes are located on chromosomes and travel with them during cell division and gamete formation. In the first division of meiosis, homologous chromosomes in germ cells segregate from each other, so each gamete receives one member of each matched pair, as predicted by Mendel's first law and as seen in Figure 4.13. Also, during the first meiotic division the independent alignment of each pair of homologous chromosomes results in the independent assortment of genes carried on different chromosomes, as predicted by Mendel's second law. The second meiotic division generates gametes with a haploid number of chromosomes (n). Fertilization of an egg and a sperm restores the diploid number of chromosomes (2n) to the zygote. The experiments that showed the correlation between chromosome behavior and inheritance using X-linked genes in Drosophila are described in this chapter. X-linked traits have characteristic inheritance patterns recognized in pedigrees or in results of reciprocal crosses (Table 4.5). Significant Elements: After reading the chapter and thinking about the concepts you should be able to: ♦ Understand homologs and alleles in meiosis as seen in Figure 4.17. Think of a good analogy. For example, think of the road or street that you live on as a copy of a chromosome. Nearby there is another copy of the same street (homologous chromosome). The 2 copies of the street are very similar, but not identical. For instance, any building (gene) found on one copy will be found in the same position on the other copy (alleles). The 2 copies of your residence are identical on both homologous streets (your gene is homozygous). Your next-door neighbor's residence has minor differences between the 2 copies - the front door is green on one and yellow on the other (heterozygous). What will happen to these 2 streets during meiosis? During mitosis? ♦ Draw chromosome alignments during metaphase of mitosis, meiosis I and meiosis II.44 Chapter 4 ♦ Describe how chromosome behavior explains the laws of segregation and independent assortment. ♦ Identify sex-linked inheritance patterns - see 3EQ#3 below. Determine genotypes in sex-linked pedigrees and probabilities of specific genotypes and phenotypes. ♦ If you truly understand meiosis, you can explain how the results seen in the genotype of a child enable you to figure out if non-disjunction occurred in meiosis I or meiosis II and the parent in which it occurred. Your explanation will include sister and non-sister chromatids. ♦ Understand the differences between sex determination in humans and Drosophila, see Table 4.1. Problem Solving Tips: ♦ Keep clear the distinction between sister chromatids (identical, replicated copies of a chromosome) and homologs (chromosomes carrying the same genes but different alleles). ♦ Compare and contrast mitosis and meiosis as in Table 4.3. ♦ Two features that lead you to consider X-linked inheritance are criss-cross inheritance (inheritance of a characteristic from mother to son and father to daughter) and a sex-dependent phenotypic difference in the progeny of a cross (see Problem 4-33). Problem Solving - How to Begin: THREE ESSENTIAL QUESTIONS (3EQ): 1. How many genes are involved in the cross? 2. For each gene involved in the cross: what are the phenotypes associated with the gene? Which phenotype is the dominant one and why? Which phenotype is the recessive one and why? 3. For each gene involved in the cross: is it X-linked or autosomal? From this point on, all 3 questions are valid. Hints: For 3EQ#1. look for the number of phenotypic classes in the F2 progeny. For 3EQ#2. if the parents of a cross are true-breeding, look at the phenotype of the F1 individuals. Also, look at the monohybrid ratios for each gene in the F2 progeny. If there is a 3:1 ratio then the phenotype associated with the 3/4 portion is the dominant one. For 3EQ#3 the determination of whether or not a gene is X-linked is more subtle. In general X-linkage is seen as a clear phenotypic difference between the sexes in one generation's progeny of a cross. This is NOT a difference in the absolute numbers of males and females of a certain phenotype, but instead a phenotype that is present in one sex and totally absent in the other sex. This difference between the sexes will be seen in either the F1 generation or the F2 generation but not in bothChapter 4 45 generations in the same cross. It is not possible to make a definitive conclusion about X-linkage based on just one generation of a cross - you must see the data from both the F1 and F2 progeny. If the sex difference is seen in the F1 generation then the female parent had the X-linked phenotype. If the sex difference is seen in the F2 generation then the male parent had the X-linked trait. X-linked genes usually show a 1:1 monohybrid ratio. After you answer questions 3EQ#1, #2 and #3 to the best of your ability, use the answers to diagram the cross, i.e. assign genotypes to the parents of the cross. Then follow the cross through, figuring out the expected genotypes and phenotypes in the F1 and F2 generations. Remember to assign the expected phenotypes based on those initially assigned to the parents. Next, compare your predicted results to the observed data you were given. If the 2 sets of information match then your initial genotypes were correct! In many cases there may be two possible set of genotypes for the parents. If your predicted results do not match the data given, try the other possible set of genotypes for the parents. Solutions to Problems: Vocabulary 4-1. a. 13; b. 7; c. 11; d. 10; e. 12; f. 8; g. 9; h. 1; i. 6; j. 15; k. 3; l. 2; m. 16; n. 4; o. 14; p. 5. Section 4.1 – Chromosomes: The Carriers of Genes 4-2. A diploid number of 46 means there are 23 homologous pairs of chromosomes. a. A child receives 23 chromosomes from