CHAPTER 4: HEREDITY AND EVOLUTION Chapter Outline I.
Introduction A. It was not until the 20th century that scientists understood how selective breeding could increase the frequency of desirable characteristics. B. The genetic principles described by Mendel form the basis of modern genetics. 1. Anne Boleyn, mother of England’s Queen Elizabeth I and wife of Henry VIII had an extra little finger; a historic example of polydactyly. 2. The predominant belief centered on the blending of parental traits in the offspring, starting with the Greek philosophers; even Darwin held this belief.
II. The Genetic Principles Discovered by Mendel A. Historically, there have been examples of the effects of genetics, although the field of genetics did not emerge until the 19th century. B. Gregor Mendel (1822-1884) developed his theory of heredity while working with garden pea hybrids. C. Segregation 1. The parental (P) generation was crossed to produce the first filial (F1) generation. a. The F1 generation did not have intermediate traits. b. The F1 generation was then crossed to produce the F2 generation. (i) One expression of the trait, shortness of the stem, for example, disappeared in the F1 generation but reappeared in the F2 generation. (ii) The expression present in the F1 generation occurred more often in the F2 generation (in a 3:1 ratio). c. Mendel concluded that discrete units, occurring in pairs and separating into different sex cells, must control the traits. (i) This is Mendel’s first principle of inheritance, the principle of segregation. D. Dominance and Recessiveness 1. Mendel used these terms to account for the fact that the expression of one (recessive) trait in the F1 generation was masked by the expression of the other (dominant) trait. These principles of dominance and recessiveness are important concepts in genetics today. a. Variations of genes at a locus are termed alleles. b. Plant height in peas (but not in all plants) is controlled by two alleles at one locus. (i) When two copies of the same allele occur at one locus, the individual is homozygous. (ii) When two different alleles are paired at the same locus, the individual is heterozygous. c. The genotype refers to an organism’s entire genetic makeup. d. The phenotype is the observed physical manifestation of genes. 2. A Punnett square can be used to predict the proportions of F2 phenotypes and genotypes. E. Independent Assortment 1. Mendel also demonstrated that different characteristics aren’t necessarily inherited together by showing that plant height and seed color are independent of each other. a. Mendel stated this relationship as the principle of independent assortment. (i) According to this principle, the genes that code for different traits (e.g. plant height and seed color) sort out independently of each other during gamete formation.
Copyright © 2017 Cengage Learning. All Rights Reserved.
