Chapter 13

Summary		Questions		Media Resources
13.1 Mendel solved the mystery of heredity.
• Koelreuter noted the basic facts of heredity a century before Mendel. He found that alternative traits segregate in crosses and may mask each other's appearance. Mendel, however, was the first to quantify his data, counting the numbers of each alternative type among the progeny of crosses. • By counting progeny types, Mendel learned that the alternatives that were masked in hybrids (the F1 generation) appeared only 25% of the time in the F2 generation. This finding, which led directly to Mendel's model of heredity, is usually referred to as the Mendelian ratio of 3:1 dominant-to-recessive traits. • When two genes are located on different chromosomes, the alleles assort independently. • Because phenotypes are often influenced by more than one gene, the ratios of alternative phenotypes observed in crosses sometimes deviate from the simple ratios predicted by Mendel.		1.  Why weren't the implications of Koelreuter's results recognized for a century? 2.  What characteristics of the garden pea made this organism a good choice for Mendel's experiments on heredity? 3.  To determine whether a purple-flowered pea plant of unknown genotype is homozygous or heterozygous, what type of plant should it be crossed with? 4.  In a dihybrid cross between two heterozygotes, what fraction of the offspring should be homozygous recessive for both traits?		• Introduction to Classic Genetics •Monohybrid Cross •Dihybrid Cross •Beyond Mendel • Probability and Hypothesis Testing in Biology •The Monk in the Garden by Henig •Mendel's Experiment Results •Testcross •Dihybrid Cross •Continuous Variation •Incomplete Dominance •Epistasis
13.2 Human genetics follows Mendelian principles.
• Some genetic disorders are relatively common in human populations; others are rare. Many of the most important genetic disorders are associated with recessive alleles, which are not eliminated from the human population, even though their effects in homozygotes may be lethal.		5.  Why is Huntington's disease maintained at its current frequency in human populations?		•Heredity in Families •Gene Segregation Within Families •Cystic Fibrosis • Advances in Gene Therapy • Lethal Mutations in Populations (Muller) •Patterns of Inheritance •Multiple Alleles-ABO Blood Groups •Sickle Cell Allele and Malaria
13.3 Genes are on chromosomes.
• The first clear evidence that genes reside on chromosomes was provided by Thomas Hunt Morgan, who demonstrated that the segregation of the white-eye trait in Drosophila is associated with the segregation of the X chromosome, which is involved in sex determination. • The first genetic evidence that crossing over occurs between chromosomes was provided by Curt Stern, who showed that when two Mendelian traits exchange during a cross, so do visible abnormalities on the ends of the chromosomes bearing those traits. • The frequency of crossing over between genes can be used to construct genetic maps. • Primary nondisjunction results when chromosomes do not separate during meiosis, leading to gametes with missing or extra chromosomes. In humans, the loss of an autosome is invariably fatal.		6.  When Morgan crossed a white-eyed male fly with a normal red-eyed female, and then crossed two of the red-eyed progeny, about 1_4 of the offspring were white-eyed-but they were ALL male! Why? 7.  What is primary nondisjunction? How is it related to Down syndrome? 8.  Is an individual with Klinefelter syndrome genetically male or female? Why?		•Constructing a Genetic Map •Making a Restriction Map - Down Syndrome •Introduction to Chromosomes •Recombination •Sex Chromosomes •Abnormal Chromosomes *Activity: Genetic Recombination •Genes are Located on Chromosomes (Morgan) •Genetic Recombination Involves Physical Exchange (McClintock/Stern) •The First Genetic Map (Sturtevant) •Barr Bodies •Nondisjunction and Sex Chromosomes

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