Chapter 11 Mendelian Patterns of Inheritance PDF

Because learning changes everything. ® Biology Sylvia S. Mader...

Because learning changes everything. ® Biology Sylvia S. Mader Michael Windelspecht Chapter 11 Mendelian Patterns of Inheritance Copyright 2022 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.  Outline 11.1 Gregor Mendel 11.2 Mendel’s Laws 11.3 Mendelian Patterns of Inheritance and Human Disease 11.4 Beyond Mendelian Inheritance 2  11.1 Gregor Mendel Blending concept of inheritance: Parents of contrasting appearance produce offspring of intermediate appearance: Mendel’s formulated the particulate theory of inheritance. Mendel proposed the laws of segregation and independent assortment. Inheritance involves reshuffling of genes from generation to generation. 3  Gregor Mendel’s Back ground Austrian monk , studied science and mathematics at the University of Vienna. Conducted breeding experiments on the garden pea Pisum sativum. Experiments on the inheritance of simple traits disproved the blending hypothesis. 4 Why the garden pea? Easy to cultivate. Short generation. Normally self-pollinating, but can be cross-pollinated by hand. Pollen was transferred from the male (anther) of one plant to the female (stigma) parts of another plant. True-breeding varieties available. Simple, objective traits. 5 Cross-Pollination in the Garden Pea Figure 11.2 6  11.2 Mendel’s Laws Mendel performed cross-pollination experiments. Used true-breeding (homozygous) plants. Chose varieties that differed in only one trait (monohybrid cross). Performed reciprocal crosses. Parental generation = P. First filial generation offspring =. Second filial generation offspring =. Formulated the law of segregation. 7 Single-Trait Cross Done by Mendel Figure 11.3 8 Figure 11.3 9 Relationship Between Observed Phenotype and F2 Offspring Figure 11.4 10  Law of Segregation Each individual has a pair of factors (alleles) for each trait. The factors (alleles) segregate (separate) during gamete (sperm and egg) formation. Each gamete contains only one factor (allele) from each pair of factors. Fertilization gives the offspring two factors for each trait. 11 Mendel’s Cross as Viewed by Modern Genetics Each trait in a pea plant is controlled by two alleles (alternate forms of a gene). Dominant allele (capital letter) masks the expression of the recessive allele (lowercase). Alleles occur on a homologous pair of chromosomes at a particular gene locus. Homozygous = identical alleles Heterozygous = different alleles 12 Homologous Chromosomes Figure 11.5 13  Genotype Versus Phenotype Genotype It refers to the two alleles an individual has for a specific trait. If identical, genotype is homozygous. If different, genotype is heterozygous. Phenotype It refers to the physical appearance of the individual. 14  Dominant and Recessive Alleles The dominant and recessive alleles represent DNA sequences that code for proteins. The dominant allele codes for the protein associated with the normal gene function within the cell. The recessive allele represents a “loss of function.” During meiosis I, the homologous chromosomes separate. The two alleles separate from each other. The process of meiosis explains Mendel’s law of segregation and why only one allele for each trait is in a gamete. 15 Mendel’s Law of Independent Assortment A dihybrid cross uses true-breeding plants differing in two traits. Mendel tracked each trait through two generations. It started with true-breeding plants differing in two traits. The plants showed both dominant characteristics. plants self-pollinated. He observed phenotypes among plants. Mendel formulated the law of independent assortment. The pair of factors for one trait segregate independently of the factors for other traits. All possible combinations of factors can occur in the gametes. 16 Dihybrid Cross Done by Mendel 17 Independent Assortment and Segregation During Meiosis Parent cellhas Parent cell has two pairs of of two pairs homologous homologous chromosomes. chromosomes. Figure 11.7 18  Figure 11.7 19  Figure 11.7 20 Independent Assortment and Segregation During Meiosis Figure 11.7 21  Mendel and the Laws of Probability It allows us to easily calculate probability of genotypes and phenotypes among the offspring. Punnett square in next slide shows a 50%12 (or) chance. The chance ofA = 12 The chance ofa = 12 An offspring will inherit The chance ofAA = 12  12  1 4 The chance ofAa = 1  1  1 2 2 4 The chance ofaA = 1  1  1 2 2 The chance ofaa = 1  1  1 4 2 2 4 The sum rule allows us to add the genotypes that produce the identical phenotype to find out the chance of a particular phenotype. 22 Punnett Square Example Figure 11.8 Access the text alternative for slide images. 23  Testcrosses Individuals with recessive phenotype always have the homozygous recessive genotype. However, individuals with dominant phenotype have indeterminate genotype. May be homozygous dominant TT, or. Heterozygous Tt. A testcross determines the genotype of an individual having the dominant phenotype. 24 One-Trait Testcrosses Figure 11.9 25  Two-Trait Testcross An individual with both dominant phenotypes is crossed with an individual with both recessive phenotypes. If the individual with the dominant phenotypes is heterozygous for both traits, the expected phenotypic ration is 1:1:1:1. 26  11.3 Mendelian Patterns of Inheritance and Human Disease Genetic disorders are medical conditions caused by alleles inherited from parents. Autosome is any chromosome other than a sex chromosome (X or Y). Genetic disorders caused by genes on autosomes are called autosomal disorders. Some genetic disorders are autosomal dominant. An individual with AA or Aa has the disorder. An individual with aa does NOT have the disorder. Other genetic disorders are autosomal recessive. An individual with AA or Aa does NOT have the disorder. Aa is a carrier. An individual with aa DOES have the disorder. 27 Autosomal Recessive Pedigree Figure 11.10 28  Autosomal Recessive Disorders If both parents carry one copy of a recessive gene they are unaffected but are capable of having a child with two copies of the gene who is affected. Methemoglobinemia. It is a relatively harmless disorder. Accumulation of methemoglobin in the blood causes skin to appear bluish- purple. Cystic Fibrosis Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous. 29 Methemoglobinemia Figure 11.11 Courtesy Division of Medical Toxicology, University of Virginia 30 Cystic Fibrosis Figure 11.12 Access the text alternative for slide images. 31  Autosomal Dominant Disorders Two parents with a dominantly inherited disorder will be affected by one copy of the gene. Osteogenesis Imperfecta. Characterized by weakened, brittle bones. Most cases are caused by mutation in genes required for the synthesis of type I collagen. Huntington Disease. Neurological disease that leads to progressive degeneration of brain cells. Caused by mutated copy of the gene for a protein called huntingtin. Hereditary Spherocytosis. It is caused by a mutation in the ankyrin-1 gene. Red blood cells become spherical, are fragile, and burst easily. 32 Autosomal Dominant Pedigree Figure 11.13 33  11.4 Beyond Mendelian Inheritance Some traits are controlled by multiple alleles The gene exists in several allelic forms, but each individual only has two alleles. ABO blood types: The alleles: antigen on red blood cells, anti-B antibody in plasma. antigen on red blood cells, anti-A antibody in plasma. i = Neither A nor B antigens on red blood cells, both anti-A and anti-B antibodies in plasma. The ABO blood type is also an example of codominance. More than one allele is fully expressed. Both and are expressed in the presence of the other. 34 ABO Blood Type 35  Incomplete Dominance Heterozygote has a phenotype intermediate between that of either homozygote. Homozygous red has red phenotype. Homozygous white has white phenotype. Heterozygote has pink (intermediate) phenotype. Phenotype reveals genotype without a testcross. 36 Incomplete Dominance Figure 11.14 37  Familial Hypercholesterolemia (FH) Homozygotes for the mutant allele develop fatty deposits in the skin and tendons and may have heart attacks during childhood. Heterozygotes may suffer heart attacks during early adulthood. Homozygotes for the normal allele do not have the disorder. 38  Familial hypercholesterolemia Homozygote Plasma cholesterol (milligrams/deciliter) 1000 900 800 700 600 Heterozygote 500 400 cholesterol 300 Normal deposits 200 100 0 © Mediscan/ Medical-On-Line Figure 21.17 The inheritance of familial hypercholesterolemia.  Pleiotropic Effects Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits. Marfan syndrome has been linked to a mutated gene FBN1 on chromosome 15 which codes for the fibrillin protein. Marfan syndrome is pleiotropic and results in the following phenotypes: Disproportionately long arms, legs, hands, and feet A weakened aorta Poor eyesight 40  Marfan Syndrome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Connective tissue defects Skeleton Heart and blood vessels Eyes Lungs Skin Chest wall deformities Mitral valve Enlargement Lens dislocation Stretch marks in skin Collapsed lungs Long, thin fingers, arms, legs prolapse of aorta Severe nearsightedness Recurrent hernias Scoliosis (curvature of the spine) Dural ectasia: stretching Flat feet of the membrane that Long, narrow face holds spinal fluid Loose joints Aneurysm Aortic wall tear (Left): © AP/Wide World Photos; (Right): © Ed Reschke; (Sickled cells, p. 203): © Phototake, Inc./Alamy 41  Polygenic Inheritance Occurs when a trait is governed by two or more sets of alleles. Each dominant allele has a quantitative effect on the phenotype. These effects are additive. It results in continuous variation of phenotypes within a population. The traits may also be affected by the environment. Examples Human skin color Height Eye color 42 The Polygenic Basis of Skin Color Figure 11.18 43 Polygenic Inheritance Example Figure 11.17 44  Multifactorial Inheritance Multifactorial traits are controlled by polygenes & subject to environmental influences. E.g.: Temperature, human skin and height -> For Himalayan rabbit, enzyme encoded by the gene involved in producing melanin is active only at low temperature -> black fur occurs at the extremity, where heat is lost.  X-Linked Inheritance The term X-linked is used for genes that have nothing to do with gender. X-linked genes are carried on the X chromosome. The Y chromosome does not carry these genes. Most sex linked experiments are performed on fruit flies They can be easily and inexpensively raised in simple laboratory glassware. Fruit flies have a similar sex chromosome pattern to humans. Morgan’s experiments with X-linked genes apply directly to humans. 46 Fruit Flies and X-Linked Inheritance Figure 11.19 47  Human X-Linked Disorders Color blindness. The allele for the blue-sensitive protein is autosomal. The alleles for the red- and green-sensitive pigments are on the X chromosome. Muscular dystrophy. Causes wasting away of the muscle. It is caused by the absence of the muscle protein dystrophin. Hemophilia. It is an absence or minimal presence of clotting factor VIII or clotting factor IX. An affected person’s blood either does not clot or clots very slowly. 48 X-Linked Recessive Pedigree Figure 11.20 49