Mendelian Patterns of Inheritance PDF

Chapter 11 14 Mendelian Patterns Geneticsof Inheritance Lecture Presentations by...

Chapter 11 14 Mendelian Patterns Geneticsof Inheritance Lecture Presentations by Nicole Tunbridge and © 2021 Pearson Education Ltd. Kathleen Fitzpatrick  CONCEPT 11.1: Mendel and his Laws Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments The blending concept of inheritance: Parents of contrasting appearance always produce offspring of intermediate appearance. It was thought that a cross between plants with red flower and white flower always produced offspring of pink flower. Particulate concept of inheritance: Existence of minute particles, or hereditary units. Now we call genes. © 2021 Pearson Education Ltd. 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 Ltd.  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 Ltd.  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 Ltd.  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 Ltd. 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 Ltd.  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 Ltd.  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 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 Ltd. Table 14.1 © 2021 Pearson Education Ltd. 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 Ltd.  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 Ltd.  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 Ltd.  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 Ltd.  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 Ltd.  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 Ltd. 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 Ltd.  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 Ltd. Figure 14.6 © 2021 Pearson Education Ltd. 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 Ltd. Figure 14.7 © 2021 Pearson Education Ltd.  Mendel’s Laws 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 The law of segregation:. Each individual has two factors for each trait.. The factors segregate during the formation of the gametes.. Each gamete contains only one factor from each pair of factors.. Fertilization gives each new individual two factors for each trait. © 2021 Pearson Education Ltd.  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 Ltd. Figure 14.8 © 2021 Pearson Education Ltd.  Using a dihybrid cross, Mendel developed the law of independent assortment. 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 The law of independent assortment:. It states that each pair factors segregates independently of the other pairs during gamete formation. All possible combination of factors can occur in the gametes. © 2021 Pearson Education Ltd.  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 Ltd. 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 Ltd.  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 Ltd. Figure 14.9 © 2021 Pearson Education Ltd. 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 Ltd. Figure 14.UN01 © 2021 Pearson Education Ltd. CONCEPT 11.3: Mendelian patterns of inheritance and human disease Patterns of inheritance are 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 Ltd.  Autosomal patterns of inheritance Autosomal recessive inheritance: If a parent has an autosomal recessive trait, they'll show no symptoms. In order to pass it on to their children, both parents need to carry the trait. But because they don’t have any symptoms, they often don’t even know they have it. Both parents need to pass an altered gene onto their child for their child to inherit the genetic condition or trait in an autosomal recessive pattern. One quarter of children will get an autosomal recessive gene if both parents have it. Only changes that occur in the DNA of the sperm or egg can be passed on to children from their parents. © 2021 Pearson Education Ltd. 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 Ltd. Figure 14.16 © 2021 Pearson Education Ltd.  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 Ltd. 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 Ltd.  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 Ltd. 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 Ltd.  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 Ltd. Figure 14.17 © 2021 Pearson Education Ltd.  Methemoglobinemia: Is a very rare blood disorder, sometimes called “blue baby syndrome,” which affects how red blood cells deliver oxygen to cells and tissues. Not everyone has symptoms, but nearly all people with this condition have skin, nails or lips that are a distinctive shade of blue or purple. In some cases, methemoglobinemia can be life threatening. © 2021 Pearson Education Ltd.  Phenylketonuria also called PKU is a rare inherited disorder that causes an amino acid called phenylalanine to build up in the body. PKU is caused by a change in a gene. This gene helps create the enzyme needed to break down phenylalanine. Without the enzyme necessary to break down phenylalanine, a dangerous buildup can develop when a person with PKU eats foods that contain protein or eats aspartame, an artificial sweetener. This can eventually lead to serious health problems. For the rest of their lives, people with PKU — babies, children and adults — need to follow a diet that limits phenylalanine, which is found mostly in foods that contain protein. Newer medications may allow some people with PKU to eat a diet that has a higher or an unrestricted amount of phenylalanine. © 2021 Pearson Education Ltd.  Autosomal dominant inheritance: Autosomal dominant is one way that genetic traits pass from one parent to their child. When a trait is autosomal dominant, only one parent needs to have an altered gene to pass it on. Half of the children of a parent with an autosomal trait will get that trait. Only changes that occur in the DNA of the sperm or egg can be passed on to children from their parents. © 2021 Pearson Education Ltd. 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 Ltd. Figure 14.18 © 2021 Pearson Education Ltd.  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 Ltd.  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 Ltd.  Osteogenesis imperfecta (OI) Is a genetic or heritable disease in which bones fracture (break) easily, often with no obvious cause or minimal injury. OI is also known as brittle bone disease, and the symptoms can range from mild with only a few fractures to severe with many medical complications. © 2021 Pearson Education Ltd.  Huntington's disease Is an inherited condition that causes brain cells to slowly lose function and die. It affects the cells in parts of the brain that regulate voluntary movement and memory. Common symptoms include uncontrollable movements and changes to thinking, behaviour and personality. © 2021 Pearson Education Ltd.  Hereditary spherocytosis Is an inherited blood disorder. It happens because of a problem with the red blood cells (RBCs). Instead of being shaped like a disk, the cells are round like a sphere. These red blood cells (called spherocytes) are more fragile than disk-shaped RBCs. © 2021 Pearson Education Ltd. 11.4 Beyound Mendelian Inheritance 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 Ltd. 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 Ltd. Figure 14.10 © 2021 Pearson Education Ltd.  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 Ltd.  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 Ltd. 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 Ltd. Figure 14.11 © 2021 Pearson Education Ltd. 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 Ltd. 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 Ltd. 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 Ltd.  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 Ltd. Figure 14.12 © 2021 Pearson Education Ltd. 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 Ltd. Figure 14.13 © 2021 Pearson Education Ltd. 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 Ltd. Figure 14.14 © 2021 Pearson Education Ltd.  X-linked Inheritance: X-linked genes are genes that are located on the X chromosome and have nothing to do with sex determination Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors © 2021 Pearson Education Ltd.  Several characteristics make fruit flies a convenient organism for genetic studies: - They breed at a high rate - A generation can be bred every two weeks - They have only four pairs of chromosomes Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes He crossed these organisms for 2 years in order to see naturally occurring mutants. © 2021 Pearson Education Ltd. © 2021 Pearson Education Ltd.  In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) The F1 generation all had red eyes The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome Morgan’s finding supported the chromosome theory of inheritance © 2021 Pearson Education Ltd.  Fig. 12-4c CONCLUSION w+ w P X X Generation X  Y w+ w Sperm Eggs F1 w+ w+ w+ Generation w w+ Sperm Eggs w+ w+ w+ F2 Generation w w w w+ © 2021 Pearson Education Ltd. 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 Sex-linked genes follow specific patterns of inheritance For a recessive sex-linked trait to be expressed – A female needs two copies of the allele – A male needs only one copy of the allele Sex-linked recessive disorders are much more common in males than in females (why?) © 2021 Pearson Education Ltd. Figure 14.15 © 2021 Pearson Education Ltd.  Some disorders caused by recessive alleles on the X chromosome in humans: – Color blindness – Duchenne muscular dystrophy- weakening of the muscles and loss of coordination; missing a muscle protein called dystrophin – Hemophilia- proteins for clotting factors are missing – Menkes syndrome – Adrenoleukodystrophy © 2021 Pearson Education Ltd.