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11-3 Exploring Mendelian Genetics 11–3 Mendel and beyond 11-3 Exploring Mendelian Genetics Copyright Pearson Prentice Hall

Independent Assortment The Two-Factor Cross: F1   Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). All of the F1 offspring produced round yellow peas (RrYy). Copyright Pearson Prentice Hall

Independent Assortment The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation. Copyright Pearson Prentice Hall

Independent Assortment The Two-Factor Cross: F2  Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F2 generation. RrYy × RrYy Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles What inheritance patterns exist aside from simple dominance? Copyright Pearson Prentice Hall

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. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles RR A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Codominance  In codominance, both alleles contribute to the phenotype. With codominance a cross between a white flower and a red flower produces seeds that will grow into plants that are red and white, not pink. Copyright Pearson Prentice Hall

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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. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Different combinations of alleles result in the colors shown here. KEY C = full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele c = albino; no color; recessive to all other alleles Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc. Full color CC Ccch Cch Cc Chinchilla: cchch, cchcch, or cchc Himalayan: chc, or chch Full color: CC, Ccch, Cch, or Cc Copyright Pearson Prentice Hall

Chinchilla cch cch cch ch cch c Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Himalayan chch chc Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles Different combinations of alleles result in the colors shown here. Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc. AIbino: cc Himalayan: chc, or chch Full color: CC, Ccch, Cch, or Cc Chinchilla: cchch, cchcch, or cchc Copyright Pearson Prentice Hall

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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. Copyright Pearson Prentice Hall

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Copyright Pearson Prentice Hall Epistasis Sometimes one gene can influence the expression of another gene In mice the allele for black fur is dominant and the allele for brown fur is recessive There is another separate gene that determines if the fur will have any color at all. If this gene is homozygous no color (recessive) the fur will be white, no matter what other color genes are present Copyright Pearson Prentice Hall

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc Fig. 14-12  BbCc BbCc Sperm 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs 1/4 BC BBCC BbCC BBCc BbCc 1/4 bC BbCC bbCC BbCc bbCc 1/4 Bc Figure 14.12 An example of epistasis BBCc BbCc BBcc Bbcc 1/4 bc BbCc bbCc Bbcc bbcc 9 : 3 : 4

Epistasis and polygenic inheritance can be hard to tell apart Copyright Pearson Prentice Hall

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. Copyright Pearson Prentice Hall

Applying Mendel's Principles Mendel’s principles can be used to study inheritance of human traits and to calculate the probability of certain traits appearing in the next generation. Copyright Pearson Prentice Hall

Genetics and the Environment Characteristics of any organism are determined by the interaction between genes and the environment. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Sex-Linked Genes The X chromosome and the Y chromosomes determine sex. Genes located on these chromosomes are called sex-linked genes. More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes X Chromosome Duchenne muscular dystrophy The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes. Melanoma X-inactivation center X-linked severe combined immunodeficiency (SCID) Colorblindness Hemophilia Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes. Y Chromosome Testis-determining factor Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Why are sex-linked disorders more common in males than in females? Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11–3 Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11–3 In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F2 generation is evidence for the two traits being inherited together. an outcome that depends on the sex of the parent plants. the two traits being inherited independently of each other. multiple genes being responsible for each trait. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall Sex-Linked Genes Sex-Linked Genes The X chromosome and the Y chromosomes determine sex. Genes located on these chromosomes are called sex-linked genes. More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11–3 Traits controlled by two or more genes are called multiple-allele traits. polygenic traits. codominant traits. hybrid traits. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11–3 In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have pink flowers. white flowers. half white flowers and half red flowers. red flowers. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 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 incomplete dominance. multiple alleles. codominance. a polygenic trait. Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall 11–3 Mendel's principles apply to pea plants only. fruit flies only. all organisms. only plants and animals. Copyright Pearson Prentice Hall

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