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Chapter 6 Patterns of Inheritance

Genetics Explains and Predicts Inheritance Patterns Genetics can explain how these poodles look different. Section 10.1

Genetics Explains and Predicts Inheritance Patterns Analyzing their genes can also help predict the appearance of their offspring. Section 10.1

Genetics Explains and Predicts Inheritance Patterns But most genes encode proteins that have nothing to do with outward appearance. The enzymes essential to these poodles lives are also the products of genetics. Section 10.1

Genetics Explains and Predicts Inheritance Patterns Studying genetics also allows scientists to breed superior crops and doctors to track genetic illnesses. Section 10.1

Chromosomes Are Packets of Genetic Information Recall that DNA is wound tightly into chromosomes. Section 10.1 Figure 10.1

Chromosomes Are Packets of Genetic Information Cells with only one set of chromosomes, such as sex cells, are haploid. Section 10.1 Figure 10.1

Chromosomes Are Packets of Genetic Information When two haploid cells fuse during fertilization, a diploid zygote with two full sets of chromosomes is formed. Section 10.1 Figure 10.1

Chromosomes Are Packets of Genetic Information Most cells of a mature individual are diploid. Section 10.1 Figure 10.1

Chromosomes Are Packets of Genetic Information Homologous chromosomes have the same genes, but might have different versions (alleles) of those genes. Section 10.1 Figure 10.1

Chromosomes Are Packets of Genetic Information Diploid cells therefore have two alleles for each gene. These alleles might be identical (gene A) or different (gene B). Section 10.1 Figure 10.1

Chromosomes Are Packets of Genetic Information Each gene s locus is its location on a chromosome. Section 10.1 Figure 10.1

10.1 Mastering Concepts How do meiosis, fertilization, diploid cells, and haploid cells interact in a sexual life cycle? 1996 PhotoDisc, Inc./Getty Images/RF

Mendel Uncovered Basic Laws of Inheritance Gregor Mendel used pea plants to study heredity. Section 10.2 Figure 10.2

Mendel Uncovered Basic Laws of Inheritance Hand-pollinating plants allowed Mendel to control plant breeding experiments. Section 10.2 Figure 10.3

Mendel Uncovered Basic Laws of Inheritance Self-fertilizing and cross-fertilizing in different combinations allowed Mendel to deduce the principles of inheritance. Section 10.2 Figure 10.4

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 F 1 generation When F 1 individuals self-pollinate or crosspollinate with other F 1 hybrids, the F 2 generation is produced 2011 Pearson Education, Inc.

EXPERIMENT Figure 14.3-1 P Generation (true-breeding parents) Purple flowers White flowers

EXPERIMENT Figure 14.3-2 P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) Self- or cross-pollination All plants had purple flowers

EXPERIMENT Figure 14.3-3 P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) Self- or cross-pollination All plants had purple flowers F 2 Generation 705 purpleflowered plants 224 white flowered plants

The Law of Segregation When Mendel crossed contrasting, truebreeding white- and purple-flowered pea plants, all of the F 1 hybrids were purple When Mendel crossed the F 1 hybrids, many of the F 2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2 generation 2011 Pearson Education, Inc.

Mendel reasoned that only the purple flower factor was affecting flower color in the F 1 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 F 2 generation 2011 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 2011 Pearson Education, Inc.

Table 14.1

Mendel Uncovered Basic Laws of Inheritance True-breeding plants produce offspring identical to themselves. Section 10.2 Figure 10.4

Mendel Uncovered Basic Laws of Inheritance Dominant alleles exert their effects whenever they are present. Crossing a yellow-seed plant with a green-seed plant always yields some yellow seeds. Yellow seed color is therefore dominant. Section 10.2 Figure 10.4

Mendel Uncovered Basic Laws of Inheritance A recessive allele is one whose effect is masked if a dominant allele is also present. Recessive alleles usually encode nonfunctional proteins. Section 10.2 Figure 10.4

Mendel Uncovered Basic Laws of Inheritance If yellow seed color is dominant, why are some seeds green when a yellow-seed plant is crossed with a green-seed plant? We need more information before we can fully answer this question. Section 10.2 Figure 10.4

Mendel Uncovered Basic Laws of Inheritance But the answer has to do with each plant having two alleles for each gene (because of their homologous pairs of chromosomes). Section 10.2 Figures 10.4, 10.5

Mendel Uncovered Basic Laws A genotype represents an individual s two alleles for one gene. The genotype confers a phenotype, or observable characteristic. of Inheritance Section 10.2 Figures 10.4, 10.5

Mendel Uncovered Basic Laws of Inheritance Homozygous dominant individuals have two dominant alleles for a gene. Heterozygous individuals have one dominant and one recessive allele. Homozygous recessive individuals have two recessive alleles. Section 10.2 Figures 10.4, 10.5

Mendel Uncovered Basic Laws of Inheritance It is possible to look at offspring to determine the genotype of the parent. As we ll see, Punnett squares help solve these puzzles. Section 10.2 Figures 10.4, 10.5

Mendel s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F 2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes 2011 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 now called alleles Each gene resides at a specific locus on a specific chromosome 2011 Pearson Education, Inc.

Figure 14.4 Allele for purple flowers Locus for flower-color gene Pair of homologous chromosomes Allele for white flowers

Second: 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 particular locus 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 F 1 hybrids 2011 Pearson Education, Inc.

Third: 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 F 1 plants had purple flowers because the allele for that trait is dominant 2011 Pearson Education, Inc.

Fourth (now known as 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 2011 Pearson Education, Inc.

Mendel s segregation model accounts for the 3:1 ratio he observed in the F 2 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 2011 Pearson Education, Inc.

10.2 Mastering Concepts Distinguish between dominant and recessive; heterozygous and homozygous; phenotype and genotype; wild-type and mutant. 1996 PhotoDisc, Inc./Getty Images/RF

Punnett Squares Represent Gamete Formation and Fertilization A Punnett square uses the genotypes of the parents to reveal which alleles the offspring may inherit. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization In this example, a female parent that is heterozygous for seed color is crossed with a male parent that is also heterozygous for seed color. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization This is a monohybrid cross since both parents are heterozygous for the one gene being evaluated. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization Genotype Yy indicates that all diploid cells, including germ cells, in these parents have both dominant and recessive seed color alleles. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization When germ cells divide by meiosis, chromosomes (and the alleles on those chromosomes) are randomly distributed among gametes. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization A gamete from the female parent and a gamete from the male parent then unite at fertilization. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization If both gametes carry dominant alleles, the offspring will inherit two dominant alleles. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization If one gamete carries a dominant allele and the other carries a recessive allele, the offspring will be heterozygous. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization If both gametes carry recessive alleles, the offspring will inherit two recessive alleles. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization This Punnett square therefore represents all possible offspring that might result from these parents. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization This Punnett square also shows the relative proportion of the offspring phenotypes and genotypes. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization On average, three offspring will have yellow seeds for every one with green seeds. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization On average, one offspring will have genotype YY for every two with Yy and for every one with yy. Section 10.3 Figure 10.6

Punnett Squares Represent Gamete Formation and Fertilization Punnett squares allow us to determine the genotypes of these yellow-seed pea plants. Section 10.3 Figures 10.4, 10.5

Punnett Squares Represent Gamete Formation and Fertilization Punnett squares also help us answer this question: If yellow seed color is dominant, why are some seeds green when a yellow-seed plant is crossed with a green-seed plant? Section 10.3 Figures 10.4, 10.5

Punnett Squares Represent Gamete Formation and Fertilization If a cross between a yellowseed pea plant (YY or Yy) and a green-seed pea plant (yy) yields all yellow seeds, the yellow-seed parent is homozygous dominant. Section 10.3 Figures 10.5, 10.8

Punnett Squares Represent Gamete Formation and Fertilization If the cross yields some green seeds, the yellow-seed parent is heterozygous. Section 10.3 Figures 10.5, 10.8

Meiosis Explains Mendel s Law of Segregation Punnett squares summarize meiosis and fertilization. Section 10.3 Figure 10.9

Meiosis Explains Mendel s Law of Segregation The two alleles for the Y gene are packaged into separate gametes, which then combine at random. Section 10.3 Figure 10.9

Meiosis Explains Mendel s Law of Segregation Can you create a Punnett square representing the information in this figure? Section 10.3 Figure 10.9

Mendel s Law Applied to Humans Punnett squares are also useful for tracking the inheritance of genetic disorders, such as cystic fibrosis. Section 10.3 Figure 10.10

Clicker Question #1 Cystic fibrosis is caused by a recessive allele. If a healthy carrier and an affected individual have a child, what is the chance the child will be affected? A. 1/4 B. 1/3 C. 1/2 D. 3/4 E. 1 1996 PhotoDisc, Inc./Getty Images/RF

10.3 Mastering Concepts What is a monohybrid cross, and what are the genotypic and phenotypic ratios expected in the offspring of the cross? 1996 PhotoDisc, Inc./Getty Images/RF

The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F 1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross 2011 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 F 1 generation, heterozygous for both characters A dihybrid cross, a cross between F 1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently 2011 Pearson Education, Inc.

Dihybrid Crosses Track the Inheritance of Two Genes at Once Two genes on different chromosomes can be combined into one large Punnett square. Section 10.4 Figure 10.11

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 or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together 2011 Pearson Education, Inc.

Alleles Separate During Meiosis Based on dihybrid crosses, Mendel proposed the law of independent assortment, which states that the segregation of alleles for one gene does not influence the segregation of alleles for another gene. Section 10.4 Figure 10.12

Clicker Question #2 Why is it impossible for one of the female gametes to have genotype rr? A. Each germ cell only has one r allele. B. Each gamete can only receive two alleles, and one must be a y. C. The r alleles separate during meiosis. 1996 PhotoDisc, Inc./Getty Images/RF

The Product Rule Replaces Complex Punnett Squares Tracking two or more genes on one Punnett square is challenging and time-consuming. The product rule simplifies these problems. Section 10.4 Figure 10.13

The Product Rule Replaces Complex Punnett Squares The chance that two independent events will both occur, equals the product of the individual chances that each event will occur. Section 10.4 Figure 10.13

The Product Rule Replaces Complex Punnett Squares For example, the probability that an offspring inherits genotype Rr Yy Tt is equal to the probability of Rr (1/2) times the probability of Yy (1/2) times the probability of Tt (1/2). Section 10.4 Figure 10.13

Clicker Question #3 A male with genotype Qq Bb Dd is crossed with a female with genotype qq bb dd. What proportion of the offspring will be homozygous recessive for all three genes? A. 1/2 B. 1/3 C. 1/4 D. 1/6 E. 1/8 1996 PhotoDisc, Inc./Getty Images/RF

10.4 Mastering Concepts How does the law of independent assortment reflect the events of meiosis? 1996 PhotoDisc, Inc./Getty Images/RF

Genes on the Same Chromosome Are Linked The product rule cannot be used if genes are linked, because inheriting one allele influences the likelihood of inheriting a linked allele. Section 10.5 Figure 10.14

Genes on the Same Chromosome Are Linked However, because of crossing over, linked alleles are not always inherited together. Section 10.5 Figure 10.14

Genes on the Same Chromosome Are Linked The probability of a crossover event occurring between two linked alleles is proportional to the distance between the genes. Section 10.5 Figure 10.15

Genes on the Same Chromosome Are Linked The letters below the linkage map of this chromosome represent alleles. The numbers above represent crossover frequencies relative to y. Section 10.5 Figure 10.15

Genes on the Same Chromosome Are Linked Crossing over frequently separates y and r but rarely separates y and w. Therefore, even without this diagram, one could infer that y is nearer to w than to r. Section 10.5 Figure 10.15

Summary of Mendel s Laws The law of independent assortment: each pair of alleles segregates independently of each other pair of alleles during gamete formation The law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes The principle of dominance: 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

10.5 Mastering Concepts Explain how to use crossover frequencies to make a linkage map. 1996 PhotoDisc, Inc./Getty Images/RF

Gene Expression Can Appear to Alter Mendelian Ratios So far we ve discussed genes with two alleles, in which the dominant allele masks the recessive allele. But gene expression does not always follow that pattern. Section 10.6 Figure 10.16

Incomplete Dominance and Codominance Add Phenotype Classes In incomplete dominance, the heterozygote has an intermediate phenotype. Section 10.6 Figure 10.16

Incomplete Dominance and Codominance Add Phenotype Classes The recessive allele (r 2 ) still encodes a nonfunctional protein. Section 10.6 Figure 10.16

Incomplete Dominance and Codominance Add Phenotype Classes The heterozygote is pink because it receives half the dose of the red pigment conferred by the dominant allele. Section 10.6 Figure 10.16

Incomplete Dominance and Codominance Add Phenotype Classes In codominance, more than one allele encodes a functional protein. Section 10.6 Figure 10.17

Incomplete Dominance and Codominance Add Phenotype Classes If two dominant alleles are present, both proteins encoded by those alleles will be represented in the phenotype. Section 10.6 Figure 10.17

Incomplete Dominance and Codominance Add Phenotype Classes In human blood types, both I A and I B are dominant alleles. Genotype I A I B confers red blood cells with both A and B molecules. Section 10.6 Figure 10.17

Incomplete Dominance and Codominance Add Phenotype Classes The I gene also has a recessive allele, i, which encodes a nonfunctional protein. But the two dominant alleles, I A and I B, make the I gene codominant. Section 10.6 Figure 10.17

One Gene, Many Phenotypes In pleiotropy, one gene has multiple effects on the phenotype. For example, a gene might affect more than one biochemical pathway. Gene Protein (enzyme) Section 10.6 A 1 A 2 A 3 Phenotype A + X B 1 B 2 B 3 Phenotype B + C 1 C 2 C 3 Phenotype C Biochemical pathways

One Gene, Many Phenotypes In this example, a gene encodes a protein that catalyzes reactions in two biochemical pathways and blocks another. Gene Protein (enzyme) Section 10.6 A 1 A 2 A 3 Phenotype A + X B 1 B 2 B 3 Phenotype B + C 1 C 2 C 3 Phenotype C Biochemical pathways

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 2011 Pearson Education, Inc.

Figure 14.13 AaBbCc AaBbCc Sperm 1 / 1 8 / 1 8 / 1 8 / 1 8 / 1 8 / 1 8 / 1 8 / 8 1 / 8 1 / 8 1 / 8 Eggs 1 / 8 1 / 8 1 / 8 1 / 8 1 / 8 Phenotypes: 1 / 64 6 / 64 15 / 64 20 / 64 15 / 64 6 / 64 1 / 64 Number of dark-skin alleles: 0 1 2 3 4 5 6

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, depending on soil acidity 2011 Pearson Education, Inc.

Figure 14.14

Figure 14.14a

Figure 14.14b

Norms of reaction are generally broadest for polygenic characters Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype 2011 Pearson Education, Inc.

10.6 Mastering Concepts How do incomplete dominance and codominance increase the number of phenotypes? 1996 PhotoDisc, Inc./Getty Images/RF

Sex-Linked Genes Have Unique Inheritance Patterns In humans, females have two X chromosomes. Males have one X chromosome and one Y chromosome. Section 10.7 Figure 10.18

Sex-Linked Genes Have Unique Inheritance Patterns This Punnett square shows that each fertilization event has a 50% chance of producing a female and a 50% chance of producing a male. Section 10.7 Figure 10.18

Sex-Linked Genes Have Unique Inheritance Patterns Which gamete, the sperm or the egg, determines the sex of the offspring? Section 10.7 Figure 10.18

Sex-Linked Genes Have Unique Inheritance Patterns The egg will always carry an X chromosome. The sex chromosome in the sperm therefore determines if the offspring is female or male. Section 10.7 Figure 10.18

Sex-Linked Genes Have Unique Inheritance Patterns X-linked recessive disorders affect more males than females. Section 10.7 Figure 10.19

Sex-Linked Genes Have Unique Inheritance Patterns Females must receive a recessive allele on both X chromosomes to express an X-linked recessive disorder. Section 10.7 Figure 10.19

Sex-Linked Genes Have Unique Inheritance Patterns Males only have one X chromosome. To express a recessive disorder, they only need to inherit one X-linked recessive allele. Section 10.7 Figure 10.19

Clicker Question #4 Hemophilia is a X-linked recessive disorder. If an affected female and an unaffected male have a boy, what is the chance he will have hemophilia? A. 0 B. 1/4 C. 1/2 D. 3/4 E. 1 1996 PhotoDisc, Inc./Getty Images/RF

Sex-Linked Genes Have Unique Inheritance Patterns Section 10.7 Table 10.2

Sex-Linked Genes Have Unique Inheritance Patterns X-inactivation prevents double-dosing of gene products. Each cell in an XX individual, such as these female cats, randomly inactivates one X chromosome. Section 10.7 Figure 10.20

Sex-Linked Genes Have Unique Inheritance Patterns If one X chromosome has an allele for orange fur and the other has an allele for black fur, color patterns emerge when X chromosomes are randomly inactivated. Section 10.7 Figure 10.20

10.7 Mastering Concepts Why do males and females express recessive X-linked alleles differently? 1996 PhotoDisc, Inc./Getty Images/RF

Pedigrees Show Modes of Inheritance A pedigree depicts family relationships and phenotypes. Section 10.8 Figure 10.21

Pedigrees Show Modes of Inheritance This pedigree tracks an autosomal dominant disorder. Section 10.8 Figure 10.21

Pedigrees Show Modes of Inheritance This pedigree tracks an autosomal recessive disorder. Section 10.8 Figure 10.21

Pedigrees Show Modes of Inheritance This pedigree tracks an X- linked recessive disorder. Note that more males are affected than females. Section 10.8 Figure 10.21

Clicker Question #5 This pedigree tracks an autosomal dominant disorder. What is the genotype of I-2? A. homozygous dominant B. heterozygous C. homozygous recessive 1996 PhotoDisc, Inc./Getty Images/RF

10.8 Mastering Concepts How are pedigrees helpful in determining a disorder s mode of inheritance? 1996 PhotoDisc, Inc./Getty Images/RF

The Environment Can Alter Phenotype Many genes are affected by the environment. For example, the enzyme responsible for pigment production in Siamese cat fur is active only in cool body parts. Section 10.9 Figure 10.22

Some Traits Depend on Multiple Genes Skin color is a polygenic trait; it is affected by more than one gene. Section 10.9 Figure 10.23

10.9 Mastering Concepts How can the environment affect a phenotype? 1996 PhotoDisc, Inc./Getty Images/RF

Investigating Life: Heredity and the Hungry Hordes Bollworm larvae devastate cotton crops. But some bollworms are susceptible to Bt toxin. Biologists have inserted the gene encoding this toxin into the cotton genome. Section 10.10 Figure 10.24

Investigating Life: Heredity and the Hungry Hordes In a mating between two Bt-resistant bollworms, all of the offspring will also be resistant. Section 10.10 Figure 10.24

Investigating Life: Heredity and the Hungry Hordes However, if a resistant bollworm mates with a susceptible bollworm, only some and sometimes none of the offspring will be resistant. (Would you guess Bt resistance is conferred by a dominant or a recessive allele?) Section 10.10 Figure 10.24

Investigating Life: Heredity and the Hungry Hordes To avoid 100% resistance among bollworms of future generations, farmers must plant some crops without the toxin gene. Crops with the Bt toxin Section 10.10 Crops without the Bt toxin Figure 10.24

Investigating Life: Heredity and the Hungry Hordes This arrangement increases the chance that some susceptible bollworms will remain in the population. Crops with the Bt toxin Section 10.10 Crops without the Bt toxin Figure 10.24

10.10 Mastering Concepts Explain the logic of planting non-bt crop buffer strips around fields planted with Bt crops. 1996 PhotoDisc, Inc./Getty Images/RF