Biology. Slide 1 of 31. End Show. Copyright Pearson Prentice Hall

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1 Biology 1 of 31

2 11 3 Exploring Mendelian 11-3 Exploring Mendelian Genetics Genetics 2 of 31

3 Independent Assortment Independent Assortment To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross. 3 of 31

4 Independent Assortment The Two-Factor Cross: F 1 Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). RRYY x rryy All of the F 1 offspring produced round yellow peas (RrYy). 4 of 31

5 Independent Assortment The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). 5 of 31

6 Independent Assortment The Two-Factor Cross: F 2 Mendel crossed the heterozygous F 1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F 2 generation. RrYy RrYy 6 of 31

7 Independent Assortment The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F 2 generation. Represents: Independent Assortment 7 of 31

8 Independent Assortment The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment. Genes that segregate independently do not influence each other's inheritance. 8 of 31

9 Independent Assortment The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. 9 of 31

10 A Summary of Mendel's Principles A Summary of Mendel's Principles Genes are passed from parents to their offspring. If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. 10 of 31

11 A Summary of Mendel's Principles In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. 11 of 31

12 Beyond Dominant and Recessive Alleles Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. 12 of 31

13 Beyond Dominant and Recessive Alleles Incomplete Dominance When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes. 13 of 31

14 Beyond Dominant and Recessive Alleles A cross between red (RR) and white (WW) four o clock plants produces pinkcolored flowers (RW). WW RR 14 of 31

15 Beyond Dominant and Recessive Alleles Codominance In codominance, both alleles contribute to the phenotype. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. 15 of 31

16 Beyond Dominant and Recessive Alleles Multiple Alleles Genes that are controlled by more than two alleles are said to have multiple alleles. An individual can t have more than two alleles. However, more than two possible alleles can exist in a population. A rabbit's coat color is determined by a single gene that has at least four different alleles. 16 of 31

17 Beyond Dominant and Recessive Alleles Different combinations of alleles result in the colors shown here. KEY C = full color; dominant to all other alleles c ch = chinchilla; partial defect in pigmentation; dominant to c h and c alleles c h = Himalayan; color in certain parts of the body; dominant to c allele Full Chinchilla: Himalayan: AIbino: color: cc CC, cc chh c, h Cc, or c ch, c Cc hch c, h or, or c ch Ccc c = albino; no color; recessive to all other alleles 17 of 31

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19 Beyond Dominant and Recessive Alleles Polygenic Traits Traits controlled by two or more genes are said to be polygenic traits. Skin color in humans is a polygenic trait controlled by more than four different genes. 19 of 31

20 LE Fraction of progeny 11 3 Exploring Mendelian Genetics AaBbCc AaBbCc aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC 20 /64 15 /64 6 /64 1 /64 20 of 31

21 Applying Mendel's Principles Applying Mendel's Principles Thomas Hunt Morgan used fruit flies to advance the study of genetics. Morgan and others tested Mendel s principles and learned that they applied to other organisms as well as plants. 21 of 31

22 11 3 Continue to: - or - Click to Launch: 22 of 31

23 11 3 In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F 2 generation is evidence for a. the two traits being inherited together. b. an outcome that depends on the sex of the parent plants. c. the two traits being inherited independently of each other. d. multiple genes being responsible for each trait. 23 of 31

24 11 3 Traits controlled by two or more genes are called a. multiple-allele traits. b. polygenic traits. c. codominant traits. d. hybrid traits. 24 of 31

25 11 3 In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have a. pink flowers. b. white flowers. c. half white flowers and half red flowers. d. red flowers. 25 of 31

26 11 3 A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of a. incomplete dominance. b. multiple alleles. c. codominance. d. a polygenic trait. 26 of 31

27 11 3 Mendel's principles apply to a. pea plants only. b. fruit flies only. c. all organisms. d. only plants and animals. 27 of 31

28 END OF SECTION