Chapter 14 Overview: Drawing from a deck of Genes -What principles account for the passing of traits? -The ?blending? hypothesis suggests that genetic material from two parents blends together -The ?particulate? hypothesis is the idea that parents pass on discrete heritable units (genes) -Mendel documented a particulate mechanism through his experiments with garden peas Gregor Mendel did experiments to show that inheritance was the result of the particulate model of gene idea. An organism?s collection of genes is more like a deck of cards than a pail of paint. Blending predicts that traits would tend to become homogeneous, all humans would have the same color eyes for example. However, that is not what we observe. In addition, the blending hypothesis cannot explain how traits can skip a generation and return. The alternative particulate hypothesis or gene hypothesis suggests that parents pass on heritable units that are discrete and retain their identity in offspring. Genes can be passed along, generation after generation in an undiluted form. 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 -Mendel discovered the law of segregation and the law of independent assortment Mendel discovered the principles of inheritance in breeding garden peas in carefully controlled experiments in 1857. He used the pea plant as a model system. It is easy to see the scientific process in these experiments. Pea plant was particularly good for the experiments. All of the reproductive organs are in the flower. The pollen (sperm) is made in stamens and the eggs are in the carpel. Since both organs and therefore both gametes are usually present in the same flower, peas usually self fertilize (pollinate). Crossing, or hybridization can be carefully controlled by removing the stamens from a flower to prevent self fertilization, then collecting the pollen and brushing it on to the carpels of a different flower. The zygote is the pea in the pea pod. When these are planted, they develop into the offspring plant and the traits in the offspring can be observed and counted. When Mendel crossed a true breeding purple flower plant with a true breeding white flower plant all of the offspring had purple flowers. This was true for the cross of purple pollen with white eggs and vice versa. After hundreds of experiments, Mendel deduced the law of segregation and the law of independent assortment. Mendel?s Experimental, Quantitative Approach Advantages of pea plants for genetic study: There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits Mating of plants can be controlled Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another Mendel chose peas because they were available in many varieties that differed in an easily identifiable way. You call a characteristic that is being observed a character. Good examples of a character are flower color or seed shape. You call the different appearances of a character a trait. For the character of flower color, the traits are purple and white. So, a heritable feature is called a character and each variant of a character is called a trait. Mendel started with true-breeding traits and easily distinguished traits with no intermediate states. True breeding traits are those that will appear generation after generation when the plant is self fertilized (it means homozygous). Breeding Experiments In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate, the F2 generation is produced In a typical breeding experiment, Mendel cross-pollinated two contrasting true breeding pea varieties. The mating or crossing is called hybridization. The parents are called the P generation and the hybrid offspring are the F1 generation (first filial). If the F1 individuals are self pollinated, they produce an F2 generation. Mendel?s quantitative analysis of the F2 generations of thousands of crosses allowed him to deduce two principles of heredity. He called these the law of segregation and the law of independent assortment. The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple (result if blending?) When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white (how could white reappear?) Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation When Mendel crossed a purple flowered plant with a white flowered plant, the F1 generation had all purple flowers. Note that if inheritance were a blending, the flowers would have been light purple. If the white trait disappeared completely, the F2 generation would also be all purple, but Mendel showed that the F2 generation was a 3:1 ratio of purple and white. The white trait was not lost, instead, it was somehow masked in the F1 generation and reappeared in the F2 generation. Mendel reasoned that the purple trait was carried by a factor that was dominant over the factor carrying the recessive white trait. The reappearance of the white trait in the F2 proves that the white factor was not diluted or destroyed. Dominant and Recessive Traits 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 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 Alternative versions of genes account for variations in inherited characteristics. For example, the gene for flower color exists in a purple version and a white version. These alternate versions are called alleles. Each gene is a sequence of DNA at a particular locus along a particular chromosome. The DNA sequence at a locus can vary slightly and hence its information content. The purple flower allele and the white flower allele are to DNA variations possible at the flower color locus. Mendel?s Model Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes The first concept is that alternative versions of genes account for variations in characters These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome The second concept is that for each character an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel?s P generation Alternatively, the two alleles at a locus may differ, as in the F1 hybrids The third concept is that if the two alleles at a locus differ, then one (the dominant allele) determines the organism?s appearance, and 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 The fourth concept, now known as the law of segregation, states that 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 somatic cells of an organism This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis For each character, an organism inherits two alleles, one from each parent. In Mendel?s true breeding pea plants, the alleles are identical. The alleles of the F1 generation of the purple and white cross are different. Figure 14.4 Alleles, alternative versions of a gene. Alternative versions of genes account for variations in inherited characteristics. For example, the gene for flower color exists in a purple version and a white version. These alternate versions are called alleles. Each gene is a sequence of DNA at a particular locus along a particular chromosome. The DNA sequence at a locus can vary slightly and hence its information content. The purple flower allele and the white flower allele are two DNA variations possible at the flower color locus. There are two homologous chromosomes in each individual. An individual can have the same allele on each of the two chromosomes or can have one of each different allele. If you have two copies of the dominant allele you will show the dominant trait. If you have one copy of each allele, you will show the dominant trait. If you have two copies of the recessive allele, you will have the recessive trait. For each character, an organism inherits two alleles, one from each parent. In Mendel?s true breeding pea plants, the alleles are identical. The alleles of the F1 generation of the purple and white cross are different. If two alleles at locus differ, then one, called the dominant allele, determines the organism?s appearance; the other allele, called the recessive allele has no noticeable effect on the organism?s appearance. Call the appearance of the trait the phenotype. Law of segregation states that the two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one copy of the allele and there are two alleles in the somatic cell. In terms of chromosomes, this segregation corresponds to the distribution of two members of a homologous pair of chromosomes to different gametes. Note that if an organism has identical alleles for a particular character, that is, the organism is true-breeding for that character, then that allele is present in all gametes. But if different alleles are present, as in the F1 hybrids, then 50% of the gametes have the dominant allele and 50% have the recessive allele. Use a punnet square to map the possibilities. Following Mendel?s Results Mendel?s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele Useful Genetic Vocabulary An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character Unlike homozygotes, heterozygotes are not true-breeding Because of the different effects of dominant and recessive alleles, 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 An organism that has a pair of identical alleles for a character is called homozygous. Homozygous organisms are true breeding. An organism that has two different alleles for a gene is called heterozygous. The appearance of an organism is called the phenotype. The genetic make up is called the genotype. An organism that has a pair of identical alleles for a character is called homozygous. An organism that has two different alleles for a gene is called heterozygous. The appearance of an organism is called the phenotype. The genetic make up is called the genotype. The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous The answer is to 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 To determine a genotype, can do a test cross. Perform the test cross by crossing the unknown genotype to a homozygous recessive. If the unknown is homozygous, all progeny in the F1 will have the dominant phenotype. If the unknown genotype is heterozygous, the progeny of the test cross will be half dominant phenotype and half recessive phenotype. 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, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Dihybrid Cross 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 as a package or independently Mendel crossed individuals that differed in only one characteristic and produced heterozygotes in the F1. This is called a monohybrid cross. He identified his second law, the law of independent assortment by following two characters. This is called a dihybrid cross. For example, peas seeds can be either yellow or green. They can also be either round or wrinkled. Yellow is dominant and round is dominant. Mendel crossed yellow round seeds (both dominant) with green wrinkled seeds (both recessive). The F1 was heterozygous for both characters, so were 100% yellow round. The question is whether both dominant alleles will travel together or whether they travel independently. If they travel together, the F2 will give 3:1 yellow round:green wrinkled. If they travel independently, will see 9:3:3:1 yellow round:yellow wrinkled:green round:green wrinkled. Note that the law of independent assortment only holds for genes that are located on different chromosomes. The Law of Independent Assortment Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together 14.2 ? The laws of probability govern Mendelian inheritance Mendel?s laws of segregation and independent assortment reflect the rules of probability 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 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 chance of carrying the dominant allele and a chance of carrying the recessive allele The easiest way to solve genetics problems is to apply the rules of probability. For example, if something is a sure thing the probability is 1, if something is impossible the probability is 0. Everything else has a fractional probability. The probability of flipping heads on one toss of a coin is 0.5. The probability of drawing the ace of spades from a deck of cards is 1/52. The probability of drawing any card other than the ace of spades from a deck of cards is 51/52. Each toss of a coin or each draw from a deck is an independent event. In other words, the outcome of the previous toss does not change the probability of the current toss. No matter what the outcome of previous tosses, the probability of flipping a head will be 0.5. When there are independent events, the probability of getting a particular outcome from two events is the product of each independent event. The probability of flipping two heads in two tosses is 0.5 x 0.5 = 0.25. In a monohybrid cross of round and wrinkled all the offspring of the F1 are heterozygous (Rr), the probability of getting the round allele (R) in a gamete is 0.5 and the probability of getting the wrinkled allele (r) in a gamete is 0.5. Therefore, the probability of getting an offspring of rr in F2 is 0.5 x 0.5 = 0.25. The probability of getting RR in the F2 is 0.5 x 0.5 = 0.25. The probability of getting Rr in the F2 must be calculated differently because it can happen in two ways. The probability of getting Rr from an R sperm and an r egg is 0.25, but the probability of getting Rr from an r sperm and an R egg is also 0.25. Therefore, the probability of getting an Rr is 0.25 + 0.25 = 0.5. So, the probability of getting a round seed is 0.25 (RR) + 0.5 (Rr) = 0.75. Rule of Addition The rule of addition states that the probability that any one of two or more 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 Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other 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 together For a dihybrid cross can use the probabilities to solve any problem. For example, consider the dihybrid cross of YyRr x YyRr. Can treat this as two independent monohybrid crosses of Yy x Yy and Rr x Rr. Using probabilities, the F2 will be 0.25 YY, 0.5 Yy and 0.25 yy. Using the probabilities, the F2 will be 0.25 RR, 0.5 Rr and 0.25 rr. So, the probability of an F2 offspring being YYRR is 0.25 x 0.25 = 0.0625 (1/8). The probability of YYRr is 0.25 x 0.5 = 0.125. The probability of YyRR is 0.5 x 0.25 = 0.125. The probability of YyRr is 0.5 x 0.5 = 0.25. The probability of a yellow round plant is the sum of probabilities of YYRR and YyRR and YYRr and YyRr = 0.0625 + 0.125 + 0.125 +0.25 = 0.5625 = 9/16. Consider a trihybrid cross of PpYyRr (purple, yellow, round) x Ppyyrr (purple, green, wrinkled). Treat this as three independent monohybrid crosses. PP = 0.25, Pp = 0.5, pp = 0.25, Yy = 0.5, yy = 0.5, Rr = 0.5, rr = 0.5. Now can solve for any combination. What fraction of the F2 will exhibit the recessive character for at least two of the characters? This would be ppyyrr, ppyyRr, ppYyrr, Ppyyrr, PPyyrr. ppyyrr = 0.25 x 0.5 x 0.5 = 0.0625. ppyyRr = 0.25 x 0.5 x 0.5 = 0.0625. ppYyrr = 0.25 x 0.5 x 0.5 = 0.0625. Ppyyrr = 0.5 x 0.5 x 0.5 = 0.125. PPyyrr = 0.25 x 0.5 x 0.5 = 0.0625. Total = 0.0625+ 0.0625 + 0.0625 + 0.125 +0.0625 = 0.375 = 6/16 =3/8. 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 Mendel?s laws were derived from the study of genes that had a very straightforward pattern of inheritance. They were single gene traits with a clear dominant and recessive relationship. Not all characters are so simply explained. The relationship between genotype and phenotype is rarely simple. Some common complications occur when alleles are not completely dominant, when a particular gene has more than two alleles and when a single gene produces multiple phenotypes. 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 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 For some genes, neither allele is completely dominant and the F1 hybrids have an intermediate phenotype. This is called incomplete dominance. Red snapdragons crossed with white snapdragons yield pink snapdragons. There is also codominance where both alleles affect the phenotype in separate distinguishable ways. The MN blood group is an example. M and N are two different molecules that are found on the surface of blood cells. M and N are coded by alleles of the same gene. In homozygotes, only one of the two molecules is found on red blood cells. But in the heterozygote, both molecules are found. Note that this phenotype is not intermediate, rather both phenotypes are found together. The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele; alleles don?t interact Don?t respond to each other, completely separate Alleles are simply variations in a gene?s nucleotide sequence For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype An allele is not termed dominant because it somehow subdues the other allele. Alleles are simply variations of a genes nucleotide sequence. When a dominant allele coexists with a recessive allele, they do not actually interact at all. It is in the pathway from genotype to phenotype that the dominance and recessiveness come into play. The characterization of an allele as dominant or recessive can depend on the level that you observe the phenotype. Using Tay Sachs as an example, a recessive allele that codes for a defective lipid metabolizing enzyme causes a disease phenotype at the organismal level only when it is present as a homozygote, so the allele is recessive at the organismal level. In the heterozygote, the dominant allele producing the functional enzyme produces only half the enzyme activity of a homozygote, so at the cellular level, the alleles are incompletely dominant. At the level of the presence of the enzyme protein, both alleles produce protein, so they behave as codominant. Just because an allele is dominant does not mean it is more prevalent in the population. There are many examples where there are more than two alleles for a gene. Most genes influence more than one phenotype, which is called pleiotropy. Tay Sachs Example Tay-Sachs disease is fatal; 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 (i.e., the enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant Dominance does not Mean Frequent Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent than the population?s dominant allele 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 (I) 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 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 You have a gene that makes an inactive enzyme so you don?t get the appropriate ion transportation across the membrane. Cystic fibrosis can?t digest food, enzymes in pancreas don?t work. They are also sterile and unable to reproduce because they don?t make gametes correctly. They also produce excessive salt in their perspiration. Pleiotropic genes that have alleles that control a large number of phenotype Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in dogs 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 For many characters two or more genes are involved in determining particular phenotype. In epistasis (Greek for standing upon), a gene at one locus influences the phenotypic expression of a gene at another locus. 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 Skin color in humans is an example of polygenic inheritance Quantitative characters are those that vary in the population along a continuum such as height or skin color. Quantitative characters are usually a result of polygenic inheritance where an additive effect of two or more genes influence a single character. This is the converse of pleiotropy. Nature and Nurture: The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The norm of reaction is the phenotypic range of a genotype influenced by the environment For example, hydrangea flowers of the same genotype range from blue-violet to pink, Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype Phenotype can depend upon environmental conditions, which produces behavior that departs from simple Mendelian genetics. Good examples are leaf size on a tree that varies according to position with respect to sun and wind, nutrition influencing size in humans, exercise changing body build, and experience improving performance on tests. Even identical twins, which have an identical genetic component can differ in some characters as a results of the environment. Genotype is not associated with a rigidly defined phenotype, but rather with a range of phenotypic possibilities. The phenotypic range is called the norm of reaction for a genotype. For some phenotypes (like the ABO blood group) the norm of reaction has no breadth, either the gene is there is a phenotype or not. A given genotype mandates a particular phenotype. Generally, the norm of reaction will be broadest for polygenic characters. Environment has an influence on the quantitative nature of the character. Geneticists refer to these characters as multifactorial, meaning that many factors, both genetic and environmental, collectively influence phenotype. 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 Pedigree Analysis A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Study human genetics by assembling a family pedigree. Can determine genotype for some characters by studying the pedigree. Thousands of genetic disorders are inherited as simple recessive traits. For the majority, the recessive allele codes for a nonfunctional protein or does not code for the protein while the dominant allele codes for a normal protein. The heterozygote has one copy of the gene that codes for a functional protein, so there is sufficient protein and no disease phenotype. Although phenotypically normal, heterozygotes are carriers. Most people that have recessive disorders are born from parents that are phenotypically normal but heterozygous carriers. The chance of two carriers mating will be increased if there is consanguineous (same blood) matings in a family that has the trait in their history. People with recent common ancestors are more likely to be carrying the same recessive alleles than people that are more distantly related. 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 (i.e., pigmented) Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Consanguineous Matings If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele Most societies and cultures have laws or taboos against marriages between close relatives 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 Huntington?s Disease 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 Multifactorial Disorders Many diseases, such as heart disease and cancer, have both genetic and environmental components Polygenic diseases Little is understood about the genetic contribution to most multifactorial diseases Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Fetal Testing In amniocentesis, the liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS), a sample of the placenta is removed and tested Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
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