Patterns of Inheritance introduction EVERY LIVING CREATURE IS A PRODUCT of the long evolutionary history of life on Earth. All organisms share this history, but as far as we know, only humans wonder about the processes that led to their origin and investigate the possibilities. We are far from understanding everything about our origins, but we have learned a great deal. Like a partially completed jigsaw puzzle, the boundaries of this elaborate question have fallen into place, and much of the internal structure is becoming apparent. In this chapter, we discuss one piece of the puzzle—the enigma of heredity. Why do individuals, like the children in this picture, differ so much in appearance despite the fact that we are all members of the same species? And, why do members of a single family tend to resemble one another more than they resemble members of other families? 12.4 Probability: Predicting the Results of Crosses
■ Mendel’s Principle of Independent Assortment explains dihybrid results concept outline 12.1 The Mystery of Heredity ■ Early plant biologists produced hybrids and saw puzzling results ■ Mendel used mathematics to analyze his crosses 12.2 Monohybrid Crosses: The Principle of Segregation ■ The F 1 generation exhibits only one of two traits, without blending ■ The F 2 generation exhibits both traits in a 3:1 ratio ■ The 3:1 ratio is actually 1:2:1 ■ Mendel’s Principle of Segregation explains monohybrid observations ■ The Punnett square allows symbolic analysis ■ Some human traits exhibit dominant/recessive inheritance 12.3 Dihybrid Crosses: The Principle of Independent Assortment ■ The F 1 generation displays two of four traits, without blending ■ The F 2 generation exhibits four types of progeny in a 9:3:3:1 ratio ■ Two probability rules help predict monohybrid cross results ■ Dihybrid cross probabilities are based on monohybrid cross probabilities 12.5 The Testcross: Revealing Unknown Genotypes 12.6 Extensions to Mendel ■ In polygenic inheritance, more than one gene can affect a single trait ■ In pleiotropy, a single gene can affect more than one trait ■ Genes may have more than two alleles ■ Dominance is not always complete ■ Genes may be affected by the environment ■ In epistasis, interactions of genes alter genetic ratios 219
rav65819_ch12_219-236.indd 219 rav65819_ch12_219-236.indd 219 1/2/07 6:04:21 PM 1/2/07 6:04:21 PM 12.1 The Mystery of Heredity As far back as written records go, patterns of resemblance among the members of particular families have been noted and commented on (figure 12.1), but there was no coherent model to explain these patterns. Before the 20th century, two concepts provided the basis for most thinking about heredity. The first was that heredity occurs within species. The second was that traits are transmitted directly from parents to offspring. Taken together, these ideas led to a view of inheritance as resulting from a blending of traits within fixed, unchanging
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