Mendel and the Gene Idea Lecture Presentation PDF

Chapter 14 Mendel and the Gene Idea Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education, Inc. Kathleen Fitzpatrick...

Chapter 14 Mendel and the Gene Idea Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education, Inc. Kathleen Fitzpatrick Figure 14.1 © 2021 Pearson Education, Inc. Figure 14.1a © 2021 Pearson Education, Inc. CONCEPT 14.1: Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments © 2021 Pearson Education, Inc. Mendel’s Experimental, Quantitative Approach Mendel’s fresh approach to the study of heredity allowed him to deduce principles that had remained elusive to others A heritable feature that varies among individuals (such as flower color) is called a character Each variant for a character, such as purple or white color for flowers, is called a trait Peas were available to Mendel in many different varieties © 2021 Pearson Education, Inc.  Other advantages of using peas – Short generation time – Large numbers of offspring – Mating could be controlled; plants could be allowed to self-pollinate or could be cross-pollinated © 2021 Pearson Education, Inc. Figure 14.2 © 2021 Pearson Education, Inc.  Mendel chose to track only those characters that occurred in two distinct alternative forms He also started with varieties that were true- breeding (plants that produce offspring of the same variety when they self-pollinate) © 2021 Pearson Education, Inc.  In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are called the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2 generation is produced © 2021 Pearson Education, Inc. The Law of Segregation In the 1800s, the explanation of heredity was the “blending” hypothesis When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple This result was not predicted by the blending hypothesis © 2021 Pearson Education, Inc.  When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white Mendel discovered a ratio of about three purple flowers to one white flower in the F2 generation © 2021 Pearson Education, Inc. Figure 14.3 © 2021 Pearson Education, Inc. Video: Mendel’s Cross on Flower Color © 2021 Pearson Education, Inc.  Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids Mendel called the purple flower color a dominant trait and the white flower color a recessive trait The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation © 2021 Pearson Education, Inc.  Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits What Mendel called a “heritable factor” is what we now call a gene © 2021 Pearson Education, Inc. Table 14.1 © 2021 Pearson Education, Inc. Mendel’s Model Mendel developed a model to explain the 3:1 inheritance pattern he observed in F2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes © 2021 Pearson Education, Inc.  First: alternative versions of genes account for variations in inherited characters For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers These alternative versions of a gene are called alleles Each gene resides at a specific locus on a specific chromosome © 2021 Pearson Education, Inc. Figure 14.4 © 2021 Pearson Education, Inc.  Second: for each character, an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about chromosomes The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation Or the two alleles at a locus may differ, as in the F1 hybrids © 2021 Pearson Education, Inc.  Third: if the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance The other, the recessive allele, has no noticeable effect on appearance In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant © 2021 Pearson Education, Inc.  Fourth, the law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Thus, an egg or a sperm gets only one of the two alleles that are present in the organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis © 2021 Pearson Education, Inc.  The model accounts for the 3:1 ratio observed in the F2 generation of Mendel’s crosses Possible combinations of sperm and egg can be shown using a Punnett square A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele © 2021 Pearson Education, Inc. Figure 14.5 © 2021 Pearson Education, Inc. Useful Genetic Vocabulary An organism with two identical alleles for a gene is called a homozygote It is said to be homozygous for the gene controlling that character An organism with two different alleles for a gene is a heterozygote and is said to be heterozygous for the gene controlling that character Unlike homozygotes, heterozygotes are not true- breeding © 2021 Pearson Education, Inc.  An organism’s traits do not always reveal its genetic composition Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes © 2021 Pearson Education, Inc. Figure 14.6 © 2021 Pearson Education, Inc. The Testcross An individual with the dominant phenotype could be either homozygous dominant or heterozygous To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offspring display the recessive phenotype, the mystery parent must be heterozygous © 2021 Pearson Education, Inc. Figure 14.7 © 2021 Pearson Education, Inc. The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids, meaning that they were heterozygous for one character A cross between such heterozygotes is called a monohybrid cross © 2021 Pearson Education, Inc.  Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring together as a unit or independently © 2021 Pearson Education, Inc. Figure 14.8 © 2021 Pearson Education, Inc.  Using a dihybrid cross, Mendel developed the law of independent assortment It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together © 2021 Pearson Education, Inc. CONCEPT 14.2: Probability laws govern Mendelian inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability that apply to tossing coins or rolling dice When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles © 2021 Pearson Education, Inc. The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities Probability in an F1 monohybrid cross can be determined using the multiplication rule Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele © 2021 Pearson Education, Inc.  The addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous © 2021 Pearson Education, Inc. Figure 14.9 © 2021 Pearson Education, Inc. Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probability to predict the outcome of crosses involving multiple characters A multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied © 2021 Pearson Education, Inc. Figure 14.UN01 © 2021 Pearson Education, Inc. Figure 14.UN02 © 2021 Pearson Education, Inc. CONCEPT 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance © 2021 Pearson Education, Inc. Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: – When alleles are not completely dominant or recessive – When a gene has more than two alleles – When a gene produces multiple phenotypes © 2021 Pearson Education, Inc. Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways © 2021 Pearson Education, Inc. Figure 14.10 © 2021 Pearson Education, Inc.  The Relationship Between Dominance and Phenotype In the case of pea shape, the dominant allele codes for an enzyme that converts an unbranched form of starch in the seed to a branched form The recessive allele codes for a defective form of the enzyme, which leads to an accumulation of unbranched starch This causes water to enter the seed, which then wrinkles as it dries © 2021 Pearson Education, Inc.  Tay-Sachs disease is a fatal inherited disorder; a dysfunctional enzyme causes an accumulation of lipids in the brain – At the organismal level, the allele is recessive – At the biochemical level, the phenotype (that is, the enzyme activity level) is incompletely dominant – At the molecular level, the alleles are codominant © 2021 Pearson Education, Inc.  Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles One baby out of 400 in the United States is born with extra fingers or toes This condition, polydactyly, is caused by a dominant allele, found much less frequently in the population than the recessive allele © 2021 Pearson Education, Inc. Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme that attaches A or B carbohydrates to red blood cells: IA, IB, and i The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither © 2021 Pearson Education, Inc. Figure 14.11 © 2021 Pearson Education, Inc. Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease © 2021 Pearson Education, Inc. Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes In epistasis, one gene affects the phenotype of another due to interaction of their gene products In polygenic inheritance, multiple genes independently affect a single trait © 2021 Pearson Education, Inc. Epistasis In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in Labrador retrievers and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair © 2021 Pearson Education, Inc.  If heterozygous black labs (genotype BbEe) are mated, we might expect the dihybrid F2 ratio of 9:3:3:1 However, a Punnett square shows that the phenotypic ratio will be 9 black to 3 chocolate to 4 yellow labs Epistatic interactions produce a variety of ratios, all of which are modified versions of 9:3:3:1 © 2021 Pearson Education, Inc. Figure 14.12 © 2021 Pearson Education, Inc. Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype Height is a good example of polygenic inheritance; over 180 genes affect height Skin pigmentation in humans is also controlled by many separately inherited genes © 2021 Pearson Education, Inc. Figure 14.13 © 2021 Pearson Education, Inc. Nature and Nurture: The Environmental Impact on Phenotype Another departure from simple Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The phenotypic range is broadest for polygenic characters Traits that depend on multiple genes combined with environmental influences are called multifactorial © 2021 Pearson Education, Inc. Figure 14.14 © 2021 Pearson Education, Inc. A Mendelian View of Heredity and Variation An organism’s phenotype includes all aspects of its physical appearance, internal anatomy, physiology, and behavior An organism’s phenotype reflects its overall genotype and unique environmental history © 2021 Pearson Education, Inc. CONCEPT 14.4: Many Human Traits follow Mendelian Patterns of Inheritance Humans are not good subjects for genetic research – Generation time is too long – Parents produce relatively few offspring – Breeding experiments are unacceptable However, basic Mendelian genetics endures as the foundation of human genetics © 2021 Pearson Education, Inc. Pedigree Analysis In human genetics, geneticists analyze the results of human matings that have already occurred A pedigree is a family tree that describes the inheritance of a trait across generations © 2021 Pearson Education, Inc. Figure 14.15 © 2021 Pearson Education, Inc. Animation: Simplified Cross of One Character in Humans © 2021 Pearson Education, Inc.  Pedigrees can be used to make predictions about future offspring We can use the multiplication and addition rules to predict the probability of specific phenotypes © 2021 Pearson Education, Inc. Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to life-threatening © 2021 Pearson Education, Inc. The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal Most individuals with recessive disorders are born to carrier parents Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair © 2021 Pearson Education, Inc. Figure 14.16 © 2021 Pearson Education, Inc.  If a recessive allele that causes a disease is rare, it is unlikely that two carriers will meet and mate Consanguineous matings (that is, between close relatives) increase the chance that both parents of a child carry the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives © 2021 Pearson Education, Inc. Cystic Fibrosis Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes, leading to a buildup of chloride ions outside the cell Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine © 2021 Pearson Education, Inc.  Untreated, cystic fibrosis can cause death by the age of 5 Daily doses of antibiotics to stop infection and physical therapies can prolong life In the United States, more than half of those with cystic fibrosis now survive into their 40s © 2021 Pearson Education, Inc. Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications Sickle-cell disease affects one out of 400 African- Americans It is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells In homozygous individuals, all hemoglobin is abnormal (sickle-cell) Symptoms include physical weakness, pain, organ damage, and even paralysis © 2021 Pearson Education, Inc.  Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms About one out of ten African-Americans has sickle- cell trait, an unusually high frequency Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common © 2021 Pearson Education, Inc. Figure 14.17 © 2021 Pearson Education, Inc. Dominantly Inherited Disorders Some human disorders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondroplasia is a form of dwarfism caused by a rare dominant allele © 2021 Pearson Education, Inc. Figure 14.18 © 2021 Pearson Education, Inc.  The timing of onset of a disease significantly affects its inheritance Huntington’s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Once the deterioration of the nervous system begins, the condition is irreversible and fatal © 2021 Pearson Education, Inc.  There is a test that can detect the presence of the Huntington’s allele in an individual’s genome Some individuals with a family history of Huntington’s disease choose to be tested for the allele Others decide that it would be too stressful to find out © 2021 Pearson Education, Inc. Multifactorial Disorders Many diseases, such as heart disease, cancer, alcoholism, and mental illnesses, have both genetic and environmental components No matter what our genotype, our lifestyle has a tremendous effect on phenotype © 2021 Pearson Education, Inc. Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Fetal and newborn testing can also reveal genetic disorders © 2021 Pearson Education, Inc. Counseling Based on Mendelian Genetics and Probability Rules Suppose a couple both have a brother who died from the same recessively inherited disease A genetic counselor can help determine the risk that this couple will have a child with the disease It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings © 2021 Pearson Education, Inc.  If both members of the couple had a sibling with the recessively inherited illness, both of their parents were carriers Thus each has a ⅔ chance of being a carrier themselves If both are carriers, there is a ¼ chance of each child having the recessive illness The overall probability of them having a child with the illness is ⅔ × ⅔ × ¼ = 1/9 © 2021 Pearson Education, Inc. Tests for Identifying Carriers For a growing number of diseases, tests are available that identify carriers and help define the odds of having an affected child more accurately The tests enable people to make more informed decisions about having children However, they raise other issues, such as whether affected individuals fully understand their genetic test results, and how the test results are used © 2021 Pearson Education, Inc. Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested for certain genetic disorders In chorionic villus sampling (CVS), a sample of the placenta is removed and tested Other techniques, such as ultrasound, allow fetal health to be assessed visually in utero © 2021 Pearson Education, Inc. Figure 14.19 © 2021 Pearson Education, Inc. Video: Ultra Sound of Fetus © 2021 Pearson Education, Inc. Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000–15,000 births in the United States The number of conditions that can be tested in newborns is over 100 and ever-increasing © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 Pearson Education, Inc. © 2021 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