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Chapter Summary
10.1 Chromosomes Are Packets of Genetic Information: A Review
10.2 Mendel's Experiments Uncovered Basic Laws of Inheritance
  1. Why Peas?

    • Gregor Mendel studied inheritance patterns in pea plants because they are easy to grow, develop quickly, and produce abundant offspring. It is also easy to control crosses between pea plants.

  2. Dominant Alleles Appear to Mask Recessive Alleles

    • An individual is true breeding for a trait if self-fertilization always yields offspring identical to the parent for that trait.

    • A dominant allele is always expressed if it is present; a recessive allele is masked by a dominant allele. Dominant does not always mean “most common.”

  3. For Each Gene, a Cell's Two Alleles May Be Identical or Different

    • The combination of alleles for a gene is the individual's genotype. A heterozygote has two different alleles for a gene. A homozygous recessive individual has two recessive alleles, and a homozygous dominant individual has two dominant alleles.

    • A phenotype is any observable characteristic of an organism.

    • A wild-type allele is the most common in a population. A change in a gene is a mutation and may result in a mutant phenotype.

  4. Every Generation Has a Name

    • In genetic crosses, the purebred parental generation is designated P; the next generation is the first filial generation, or F1; and the next is the second filial generation, or F2.

10.3 The Two Alleles of Each Gene End Up in Different Gametes
  1. Monohybrid Crosses Track the Inheritance of One Gene

    • A monohybrid cross is a mating between two individuals that are heterozygous for the same gene.

    • Punnett squares are useful for calculating the probability of each possible genotype and phenotype among the offspring of a genetic cross.

    • A test cross reveals an unknown genotype by breeding the individual to a homozygous recessive individual.

  2. Meiosis Explains Mendel's Law of Segregation

    • Mendel's law of segregation states that the two alleles of the same gene separate into different gametes. Each individual receives one allele of each gene from each parent.

10.4 Genes on Different Chromosomes Are Inherited Independently
  1. Dihybrid Crosses Track the Inheritance of Two Genes at Once

    • A dihybrid cross is a mating between individuals that are heterozygous for two genes.

  2. Meiosis Explains Mendel's Law of Independent Assortment

    • According to Mendel's law of independent assortment, the inheritance of one gene does not affect the inheritance of another gene on a different chromosome. This law reflects meiosis, in which homologous pairs of chromosomes (and the genes they carry) align randomly during metaphase I.

    • Page 223
    • The product rule is an alternative to Punnett squares for following inheritance of two or more traits at a time.

10.5 Genes on the Same Chromosome May Be Inherited Together
  1. Genes on the Same Chromosome Are Linked

    • Linked genes are located on the same chromosome. Dihybrid crosses for pairs of linked genes produce genotypic and phenotypic ratios that are different from those predicted by the law of independent assortment.

    • Linkage groups are collections of genes that are often inherited together because they are on the same chromosome.

    • The farther apart two linked genes are on a chromosome, the more likely crossing over is to separate their alleles. If crossing over occurs between two alleles, some offspring will have recombinant genotypes; otherwise, offspring will have parental genotypes for the two genes.

  2. Studies of Linked Genes Have Yielded Chromosome Maps

    • Breeding studies reveal the crossover frequencies used to create linkage maps—diagrams that show the order of genes on a chromosome.

10.6 Gene Expression Can Appear to Alter Mendelian Ratios
  1. Incomplete Dominance and Codominance Add Phenotype Classes

    • Heterozygotes for alleles with incomplete dominance have phenotypes intermediate between those of the two homozygotes. Codominant alleles are both expressed in a heterozygote.

  2. Some Inheritance Patterns Are Especially Difficult to Interpret

    • A pleiotropic gene has multiple effects on the body and therefore produces many different phenotypes.

    • Many biochemical reactions require multiple proteins. Mutated genes encoding any of the proteins can stop the pathway, producing the same phenotype.

    • In epistasis, one gene masks the effect of another.

10.7 Sex-Linked Genes Have Unique Inheritance Patterns
  • Genes controlling sex-linked traits are located on the X or Y chromosomes.

  1. X and Y Chromosomes Determine Sex in Humans

    • In humans, the male has X and Y sex chromosomes, and the female has two X chromosomes. The SRY gene on the Y chromosome controls other genes that stimulate development of male structures and suppress development of female structures.

  2. X-Linked Recessive Disorders Affect More Males Than Females

    • An X–linked gene passes from mother to son because the male inherits his X chromosome from his mother and his Y chromosome from his father. Scientists know of many more X-linked disorders than Y-linked disorders.

  3. X Inactivation Prevents “Double Dosing” of Proteins

    • X inactivation shuts off all but one X chromosome in the cells of female mammals, equalizing the number of active X-linked genes in each sex.

10.8 Pedigrees Show Modes of Inheritance
  • Pedigrees trace phenotypes in families and reveal modes of inheritance.

  • An autosomal dominant disorder affects both sexes and can be inherited from one affected parent. An autosomal recessive disorder can appear in either sex, must be inherited from both parents (who are either carriers or are affected), and can skip generations. X-linked recessive disorders affect mostly males.

10.9 Most Traits Are Influenced by the Environment and Multiple Genes
  1. The Environment Can Alter the Phenotype

    • Unlike traits that Mendel studied, most traits have environmental as well as genetic influences.

  2. Polygenic Traits Depend on More Than One Gene

    • A polygenic trait varies continuously in its expression, and the frequencies of the phenotypes form a bell-shaped curve.

10.10 Investigating Life: Heredity and the Hungry Hordes
  • Researchers have developed a way to test for the presence of recessive alleles in insect pests of cotton. This genetic test allows researchers to monitor caterpillars for resistance to Bt, an insecticidal toxin produced in genetically modified cotton.

Multiple Choice Questions
  1. Which of the following is a difference between an autosome and a sex chromosome?

    1. An autosome has more DNA.

    2. A sex chromosome is only present in the germ cells.

    3. Only autosomes can be diploid.

    4. There are more autosomes than sex chromosomes in a cell.

    answer

  2. According to Mendel, if an individual is heterozygous for a gene, the phenotype will correspond to that of

    1. the recessive trait alone.

    2. the dominant trait alone.

    3. a blend of the dominant and recessive traits.

    4. a wild-type trait.

    answer

  3. If an individual is homozygous for a gene, then the genotype will contain

    1. only the recessive allele.

    2. only the dominant allele.

    3. both a dominant and a recessive allele.

    4. Either a or b could be true.

    answer

  4. What can you conclude if all of the many offspring of a test cross show the phenotype associated with the dominant allele?

    1. One parent was homozygous dominant.

    2. One parent was heterozygous.

    3. The offspring are all homozygous dominant.

    4. Both b and c are correct.

    answer

  5. Which of the following is a possible gamete for an individual with the genotype PP rr?

    1. PP

    2. Pr

    3. pr

    4. rr

    answer

  6. Use the product rule to determine the chance of obtaining an offspring with the genotype Rr Yy from a dihybrid cross between parents with the genotype Rr Yy.

    1. ½

    2. ¼

    3. 1/8

    4. 1/16

    answer

  7. Recombination is most likely to occur between

    1. closely linked genes.

    2. genes on nonhomologous chromosomes.

    3. linked genes that are far apart.

    4. parental chromosomes.

    answer

  8. How does incomplete dominance affect the phenotype of a heterozygote?

    1. It results in a blend of the dominant and recessive phenotypes.

    2. It results in the expression of only the recessive phenotype.

    3. The dominant phenotype is still expressed, but only in patches.

    4. The trait is not observed in the individual.

    answer

  9. Page 224
  10. White cats have at least one dominant allele of the W gene, which masks the expression of all other genes that contribute to coat color. The W gene is an example of

    1. pleiotropy.

    2. independent assortment.

    3. codominance.

    4. epistasis.

    answer

  11. Suppose a woman is a symptomless carrier of a recessive X-linked disease. If her husband has the disease, what is the chance that their first child is a girl who also has the disease?

    1. 100%

    2. 50%

    3. 25%

    4. 0

    answer

  12. How does X inactivation contribute to genetic diversity?

    1. It controls the number and kind of genes expressed on an X chromosome.

    2. It determines which X chromosome is expressed in a male.

    3. It allows for expression of either the maternal or paternal X in different cells.

    4. It enhances expression of Y-linked genes in males.

    answer

  13. What is a polygenic trait?

    1. A trait that reflects expression of both dominant and recessive alleles

    2. A trait that reflects the influence of the environment

    3. A trait that reflects the expression of many alleles of the same gene

    4. A trait that reflects the expression of many different genes

    answer

Write It Out
  1. What advantages do pea plants and fruit flies have for studies of inheritance? Why aren't humans equally suitable?

    answer

  2. Some people compare a homologous pair of chromosomes to a pair of shoes. Explain the similarity. How would you extend the analogy to the sex chromosomes for females and for males?

    answer

  3. In an attempt to breed winter barley that is resistant to barley mild mosaic virus, agricultural researchers cross a susceptible domesticated strain with a resistant wild strain. The F1 plants are all susceptible, but when the F1 plants are crossed with each other, some of the F2 individuals are resistant. Is the resistance allele recessive or dominant? How do you know?

    answer

  4. Given the relationship between genes, alleles, and proteins, how can a recessive allele appear to “hide” in a heterozygote?

    answer

  5. Chapter 8 explained the roles of oncogenes and tumor suppressor genes in cancer. Both types of genes encode proteins that regulate the cell cycle. Determine the function of each type of protein, then explain why oncogenes are dominant and mutant alleles of tumor suppressor genes are recessive.

    answer

  6. Many plants are polyploid (see chapter 9); that is, they have more than two sets of chromosomes. How would having four (rather than two) copies of a chromosome more effectively mask expression of a recessive allele?

    answer

  7. Springer spaniels often suffer from canine phospho-fructokinase (PFK) deficiency. The dogs lack an enzyme that is crucial in extracting energy from glucose molecules. Affected pups have extremely weak muscles and die within weeks. A DNA test is available to identify male and female dogs that are carriers. Why would breeders wish to identify carriers if these dogs are not affected?

    answer

  8. How did Mendel use evidence from monohybrid and dihybrid crosses to deduce his laws of segregation and independent assortment? How do these laws relate to meiosis?

    answer

  9. In a dihybrid cross, the predicted phenotype ratio is 9:3:3:1. The “9” represents the proportion of plants expressing at least one dominant allele for both traits. How would you use test crosses to determine whether these plants are homozygous dominant or heterozygous for one or both genes?

    answer

  10. A white woman with fair skin, blond hair, and blue eyes marries a black man with dark brown skin, hair, and eyes. They have fraternal twins. One twin has blond hair, brown eyes, and light skin, and the other has dark hair, brown eyes, and dark skin. What Mendelian law does this real-life case illustrate?

    answer

  11. The radish has nine groups of traits. Within each group, dihybrid crosses do not yield a 9:3:3:1 phenotypic ratio. Instead, such crosses yield an overabundance of phenotypes like those of the parents. What does this information reveal about the chromosomes of this plant?

    answer

  12. How does gene linkage interfere with Mendel's law of independent assortment? Why doesn't the inheritance pattern of linked genes disprove Mendel's law?

    answer

  13. How does crossing over “unlink” genes?

    answer

  14. If two different but linked genes are located very far apart on a chromosome, how may the inheritance pattern create the appearance of independent assortment?

    answer

  15. Explain how each of the following appears to disrupt Mendelian ratios: incomplete dominance, codominance, pleiotropy, epistasis.

    answer

  16. Suppose a single trait is controlled by a gene with four codominant alleles. A person can inherit any combination of two of the four alleles. How many phenotypes are possible for this trait?

    answer

  17. What is the role of the Y chromosome in human sex determination?

    answer

  18. Do you agree with the statement that all alleles on the Y chromosome are dominant? Why or why not?

    answer

  19. Suppose a fetus has X and Y chromosomes but lacks receptors for the protein encoded by the SRY gene. Will the fetus develop as a male or as a female? Explain your answer.

    answer

  20. How are X-linked genes inherited differently in male and female humans?

    answer

  21. What does X inactivation accomplish?

    answer

  22. Rett syndrome is a severe X-linked recessive disorder that affects mostly female children. How does X inactivation explain this observation?

    answer

  23. The cells of a track runner are collected before an Olympic competition. Technicians examining the cells discover two Barr bodies in each nucleus. How is this unusual? What would you expect to find if you constructed a karyotype of the runner's chromosomes?

    answer

  24. A family has an X-linked dominant form of congenital generalized hypertrichosis (excessive hairiness). Although the allele is dominant, males are more severely affected than females. Moreover, the women in the family often have asymmetrical, hairy patches on their bodies. How does X chromosome inactivation explain this observation?

    answer

  25. Why are male calico cats rare?

    answer

  26. In the following pedigree, is the disorder's mode of inheritance autosomal dominant, autosomal recessive, or X-linked recessive? Explain your reasoning.

    answer

  27. Page 225
  28. Pedigree charts can sometimes be difficult to construct and interpret. People may refuse to supply information, and adoption or serial marriages can produce blended families. Artificial insemination may involve anonymous sperm donors. Many traits are strongly influenced by the environment. How does each of these factors complicate the use of pedigrees?

    answer

  29. Explain the following “equation”:

    Genotype + Environment = Phenotype

    answer

  30. Mitochondria and chloroplasts contain DNA that encodes some proteins essential to life. These organelles are inherited only via the female parent's egg cell. Do you expect these genes to follow Mendelian laws of inheritance? Explain your answer.

    answer

Genetics Problems

See pages 226 and 227 for step-by-step guides to solving genetics problems.

  1. Holstein cattle suffer from the condition citrullinemia, in which homozygous recessive calves die within a week of birth because they cannot break down ammonia that is produced when amino acids are metabolized. If a cow that is heterozygous for the citrullinemia gene is inseminated by a bull that is homozygous dominant, what is the probability that a calf inherits citrullinemia?

  2. Wild-type canaries are yellow. A dominant mutant allele of the color gene, designated W, causes white feathers. Inheriting two dominant alleles is lethal to the embryo. If a yellow canary is crossed to a white canary, what is the probability that an offspring will be yellow? What is the probability that it will be white?

  3. In humans, more than 100 forms of deafness are inherited as recessive alleles on many different chromosomes. Suppose that a woman who is heterozygous for a deafness gene on one chromosome has a child with a man who is heterozygous for a deafness gene on a different chromosome. Does the child face the general population risk of inheriting either form of deafness or the 25% chance that Mendelian ratios predict for a monohybrid cross? Explain your answer.

  4. A man and a woman each have dark eyes, dark hair, and freckles. The genes for these traits are on separate chromosomes. The woman is heterozygous for each of these genes, but the man is homozygous. The dominance relationships of the alleles are as follows:

    B = dark eyes; b = blue eyes

    H = dark hair; h = blond hair

    F = freckles; f = no freckles

    1. What is the probability that their child will share the parents' phenotype?

    2. What is the probability that the child will share the same genotype as the mother? As the father?

    Use the product rule or a Punnett square to obtain your answers. Which method do you think is easier?

  5. Genes J, K, and L are on the same chromosome. The crossover frequency between J and K is 19%, the crossover frequency between K and L is 2%, and the crossover frequency between J and L is 21%. Use this information to create a linkage map for the chromosome.

  6. A particular gene in dogs contributes to coat color. The two alleles exhibit incomplete dominance. Dogs with genotype mm have normal pigmentation; genotype Mm leads to “dilute” pigmentation; genotype MM produces an all-white dog. If a breeder mates a normal dog with a white dog, what will be the genotypes and phenotypes of the puppies? If two Mm dogs are mated, what is the probability that a puppy will be all white?

  7. Three babies are born in the hospital on the same day. Baby X has type AB blood; Baby Y has type B blood; Baby Z has type O blood. Use the information in the table below to determine which baby belongs to which couple. (Assume that all individuals are homozygous dominant for the H gene.)

    Couple

    Mother

    Blood type

    Father

    Blood type

    1

    Abby

    B

    Seth

    AB

    2

    Carol

    A

    Sam

    A

    3

    Nancy

    AB

    Bill

    O

  8. Consider a woman whose brother has hemophilia A but whose parents are healthy. What is the chance that she has inherited the hemophilia allele? What is the chance that the woman will conceive a son with hemophilia?

Pull It Together
  1. Which cells in the human body are haploid? Which cells are diploid?

    answer

  2. What is the difference between a genotype and a phenotype?

    answer

  3. What is the difference between a dominant and a recessive allele?

    answer

  4. Add meiosis, gametes, mutations, incomplete dominance, codominance, pleiotropy, and epistasis to this concept map.

    answer

Page 226
How to Solve a Genetics Problem: One Gene

Sample problem: Phenylketonuria (PKU) is an autosomal recessive disorder. If a man with PKU marries a woman who is a symptomless carrier, what is the probability that their first child will be born with PKU?

  1. Write a key. Pick ONE letter to represent the gene in your problem. Use the capital form of your letter to symbolize the dominant allele; use the lowercase letter to symbolize the recessive allele.

    Sample: The dominant allele is K; the recessive allele is k.

  2. Summarize the problem's information. Make a table listing the phenotypes and genotypes of both parents.

    Sample:

     

    Male

    Female

    Phenotype

    Has PKU

    No PKU (carrier)

    Genotype

    kk

    Kk

  3. Sketch the parental chromosomes and gametes. Use the genotypes in your table to draw the alleles onto chromosomes. Then draw short arrows to show the homologous chromosomes moving into separate gametes for each parent.

  4. Page 227
  5. Make a Punnett square. Arrange the gametes you sketched in step #3 along the edges of the square, and fill in the genotypes of the offspring.

  6. Calculate the genotypic ratio. Count the number of squares that contain each offspring genotype.

    Sample: 2 Kk; 2 kk

  7. Calculate the phenotypic ratio. Count the number of squares that contain each offspring phenotype.

    Sample: 2 PKU carriers; 2 PKU sufferers

  8. Calculate the probability of each phenotype. Divide each number in step #6 by 4 (the total number of squares) and multiply by 100.

    Sample: 50% probability that a child will be a carrier; 50% probability that a child will have PKU

How to Solve a Genetics Problem: Two Genes

Sample problem: A student collects pollen (male sex cells) from a pea plant that is homozygous recessive for the genes controlling seed form and seed color. She uses the pollen to fertilize a plant that is heterozygous for both genes. What is the probability that an offspring plant has the same genotype and phenotype as the male parent? Assume the genes are not linked.

  1. Write a key. Pick ONE letter to represent each of the genes in your problem. Use the capital form of your letter to symbolize the dominant allele; use the lowercase letter to symbolize the recessive allele.

    Sample: For seed form, the dominant allele (round) is R; the recessive allele (wrinkled) is r; for seed color, the dominant allele (yellow) is Y; the recessive allele (green) is y.

  2. Summarize the problem's information. Make a table listing the phenotypes and genotypes of both parents.

    Sample:

     

    Male

    Female

    Phenotype

    Wrinkled, green

    Round, yellow

    Genotype

    rr yy

    Rr Yy

  3. Sketch the parental chromosomes and gametes. Use the genotypes in your table to draw the alleles onto two sets of chromosomes, one for each parent. The law of independent assortment means that you need to draw all possible configurations. So redraw the chromosomes, this time switching the order of the alleles in one pair. Then draw short arrows to show the chromosomes separating, and sketch the four possible gametes for each parent. Depending on the parents' genotypes, some of the gametes produced by a parent may have the same genotype.

  4. Make a Punnett square. Arrange the gametes you sketched in step #3 along the edges of the square, and fill in the genotypes of the offspring.

  5. Calculate the genotypic ratio. Count the number of squares that contain each offspring genotype.

    Sample: 4 Rr Yy; 4 rr yy, 4 Rr yy, 4 rr Yy

  6. Calculate the phenotypic ratio. Count the number of squares that correspond to each possible phenotype combination.

    Sample: 4 round, yellow; 4 wrinkled, green; 4 round, green; 4 wrinkled, yellow

  7. Calculate the probability of each phenotype. Divide each number in step #6 by 16 (the total number of squares) and multiply by 100.

    Sample: 25% probability that an offspring has the same genotype and phenotype as the male parent.

NOTE: The product rule is a simpler way to solve the same problem and eliminates the need for a large Punnett square. To use the product rule in this case, first calculate the probability that the parents (rr×Rr) produce an offspring with genotype rr (½, or 50%). Then calculate the chance that yy×Yy parents produce a yy offspring (½, or 50%). Multiply the two probabilities to calculate the probability that both events occur simultaneously: ½×½ = ¼, or 25%. See section 10.4 for more on the product rule.

How to Solve a Genetics Problem: X-Linked Gene

Sample problem: Hemophilia is caused by an X-linked recessive allele. If a man who has hemophilia marries a healthy woman who is not a carrier, what is the chance that their child will have hemophilia?

  1. Write a key. Pick ONE letter to represent the gene in your problem. Use the capital form of your letter to symbolize the dominant allele; use the lowercase letter to symbolize the recessive allele.

    Sample: The dominant allele is H; the recessive allele is h. Because these alleles are on the X chromosome, inheritance will differ between males and females. It is therefore best to designate the chromosomes and alleles together as XH and Xh.

  2. Summarize the problem's information. Make a table listing the phenotypes and genotypes of both parents.

    Sample:

     

    Male

    Female

    Phenotype

    Has hemophilia

    Healthy

    Genotype

    Xh Y

    XH XH

  3. Sketch the parental chromosomes and gametes. Use the genotypes in your table to draw the alleles onto chromosomes. Then draw short arrows to show the chromosomes moving into separate gametes for each parent.

  4. Make a Punnett square. Arrange the gametes you sketched in step #3 along the edges of the square; fill in the genotypes of the offspring.

  5. Calculate the genotypic ratio. Count the number of squares that contain each offspring genotype.

    Sample: 2 XH Xh; 2 XH Y

  6. Calculate the phenotypic ratio. Count the number of squares that correspond to each possible phenotype.

    Sample: 2 female carriers; 2 healthy males

  7. Calculate the probability of each phenotype. Divide each number in step #6 by 4 (the total number of squares) and multiply by 100.

    Sample: 50% probability that a child will be a female carrier; 50% probability that a child will be a healthy male. No child, male or female, will have hemophilia.

Mastering Concepts Questions

10.1

Chromosomes contain tightly packed DNA and associated proteins. DNA are the strands of genetic material that contains genes, sequences of nucleotides that code for amino acid order. Those genes come in varieties called alleles.

Meiosis in the adult organism creates the haploid gamete cells that combine during fertilization to form the diploid zygote cell. That cell undergoes mitosis to make the cells necessary for growth into the adult form.

10.2

Mendel chose pea plants because they are easy to grow, develop quickly, produce many offspring, and have many traits that appear in two alternate forms that are easy to distinguish. It also is easy to hand-pollinate pea plants, so an investigator can control which plants mate with one another.

Dominant alleles appear in a phenotype whenever they are present; recessive alleles contribute to the phenotype only if no dominant alleles are present. An individual is homozygous for a gene if both alleles are identical; in a heterozygous individual, the two alleles for a gene are different. An organism's phenotype is its appearance; the genotype is the alleles an individual possesses. The wild type allele is the most common form of a gene in a population; a mutant allele is different from the norm.

The P generation is the parental or starting generation. F1 and F2 refer to the first and second generations of offspring respectively.

10.3

A monohybrid cross is a mating between two individuals that are both heterozygous for one gene. The genotypic ratio expected in a monohybrid cross is 1:2:1; the phenotypic ratio is 3:1.

Punnett squares show the genotypes of each parent as well as the genotypes of potential offspring. Phenotypic and genotypic ratios of offspring can be predicted from the data in Punnett squares.

A testcross is a mating between a homozygous recessive individual and an individual of unknown genotype. The genotype of the unknown parent can be deduced from the ratio of phenotypes in the F1 generation.

The law of segregation reflects the movement of homologous chromosomes into separate cells during meiosis I.

10.4

In a dihybrid cross, two individuals that are heterozygous for two genes are mated. The phenotypic ratio that is expected is 9:3:3:1.

The law of independent assortment reflects the fact that each homologous pair of chromosomes aligns independently of other chromosome pairs during metaphase I of meiosis.

The product rule allows you to estimate the odds that an offspring will have a certain combination of alleles for multiple genes, by multiplying the probability that each separate event will occur.

10.5

When pairs of genes are linked, they are carried on the same chromosome and are inherited together. Linked genes are often inherited together, whereas non-linked genes are not inherited together.

Recombinant chromatids are chromosomes that have a mixture of maternal and paternal alleles instead of alleles from just a single parent. In contrast, parental chromatids carry the same combinations of alleles that were inherited from the parents. Crossing over has not altered them.

The farther apart genes are on a chromosome, the more frequently they will cross over. By comparison, genes that are close together on a chromosome are less likely to be separated. Analysis of how often the traits appear together helps to establish linkage maps, which show the relative positions of genes on chromosomes.

10.6

Incomplete dominance and codominance produce phenotypes that are intermediate between those produced by homozygous dominant or homozygous recessive individuals.

Pleiotropy occurs when a gene produces multiple phenotypic expressions. Pleiotropy results when the protein encoded by a gene enters several different biochemical pathways or affects more than one body part or process.

Each gene encodes one protein, but many different proteins may interact in a single metabolic pathway. A mutation in a gene encoding any of these proteins may produce a flawed metabolic pathway. In this way, different genotypes can produce the same phenotype (failure of the metabolic pathway to operate properly).

In epistasis, one gene affects the expression of another. The gene interaction may cause some phenotypes to appear to be missing from a population.

10.7

Sex determination is the factor (genetic or environmental) that decides if an organism is male or female.

In human sex determination, the Y chromosome's SRY gene encodes a protein that acts as a master switch. The SRY protein turns on other genes, which direct the undeveloped testes to secrete the male sex hormone testosterone. SRY also turns on a gene encoding a protein that causes embryonic female structures to disassemble. If a functional SRY gene is not present, an embryo will develop as a female.

Each female has a pair of X chromosomes, whereas a male has only one X chromosome. Any trait a male has on its X chromosome will be expressed. Recessive alleles on an X chromosome of a female may be masked by dominant alleles on its homologous X chromosome.

X inactivation happens to one of the two copies of a gene on the homologous X chromosomes. Only females have two copies of the X chromosome.

10.8

Pedigrees track a trait through multiple generations and allow the pattern of transmission and inheritance to be studied. Pedigrees also may help predict the appearance of the trait in future generations.

Autosomal dominant pedigrees show affected individuals in every generation and all affected individuals have at least 1 affected parent. Autosomal recessive conditions show a pedigree in which affected individuals can have normal parents, and the condition often skips generations. X-linked recessive conditions appear in pedigrees where more males are affected than females; also affected males can have normal parents, but affected females must also have an affected father.

10.9

Environment can affect a phenotype in a variety of ways. Temperature can influence gene expression of temperature-sensitive alleles; infectious agents can intensify a genetic disorder; upbringing and nourishment will affect temperament and physical health.

A polygenic trait is one that is controlled by many genes.

10.10

The buffer strip creates a feeding zone for larvae that selects for non-Bt-resistant moths. These moths are likely to mate with the Bt-resistant varieties surviving in the field. This strategy hopes to greatly reduce the likelihood that Bt-resistance will increase significantly in the population.

Researchers knew that if the resistant allele was recessive then matings between moths heterozygous for resistance with moths fully resistant (homozygous recessive) should show a phenotypic ratio in the offspring of approximately 1:1. Experiments revealed that indeed approximately 50% of the offspring thrived while 50% either died or were quite small.

Without the refuge strip resistant moths would only have other resistant moths to mate with and future generations would produce a significant shift in the allele frequencies in the population such that most individuals were homozygous recessive. If this occurs then Bt as a pesticide in corn will no longer be useful.

Write It Out

Both peas and fruit flies are easy to grow, develop rapidly, produce many offspring, and have many traits that appear in two easily distinguishable forms. In addition, it is easy to control genetic crossing in pea plants and fruit flies. Humans cannot be used because they take longer to reach sexual maturity, do not produce an abundance of offspring, and cannot be forced to mate to suit the objectives of an experiment.

Shoes come in all kinds of varieties: sandals, boots, sneakers, but they are paired with their matching shoe which will be the same size, and have straps or laces, rubber treads or uppers all in the same places and of the same materials. Similarly homologous chromosomes are the same length and shape with the same genes in the same places. The sex chromosomes are not homologous however and would be like an adult size 11 sneaker paired with a child's size 3 sandal if the individual were a male. In a female the shoes would be homologous and would match.

The resistance allele is recessive because it was not expressed in the F1 generation but was expressed in some of the plants of the F2 generation.

A recessive allele often encodes a nonfunctional protein. A heterozygous individual has one dominant and one recessive allele, but the recessive allele appears to “hide” because the cell has enough of the normal protein (encoded by the dominant allele), to function properly.

The proteins in normal proto-oncogenes stimulate cell division. Only one copy of the mutant oncogene is necessary to express cancer; this makes the oncogene a dominant version. Tumor suppressor genes promote apoptosis or prevent cell division. Two mutated versions of this gene must be present to express cancer, and so the gene variation is recessive.

The extra chromosomes will provide additional opportunities to mask the expression of a recessive allele.

It would be beneficial because breeders could prevent carriers from mating, thus reducing the incidence of this disease in the dogs.

From his series of monohybrid crosses, Mendel concluded that genes occur in alternative forms (alleles) and that each individual inherits two alleles for each gene. His law of segregation states that two alleles of the same gene separate as they are packaged into gametes. This law reflects meiosis because homologous chromosomes are pulled into separate cells during meiosis I. From his series of dihybrid crosses, Mendel developed the law of independent assortment, which states that during gamete formation, the segregation of the alleles of one gene does not influence the segregation of the alleles for another gene. This law reflects meiosis (as long as the two genes being studied reside on different chromosomes) because the orientation of each homologous pair of chromosomes does not affect the orientation of other homologous pairs during meiosis I.

A testcross is a mating with a homozygous recessive individual. In this case, you would obtain a plant that was homozygous recessive for both alleles. If a plant is homozygous dominant for both genes, all of the offspring will have the dominant phenotype for both traits. If the plant is heterozygous for either gene, about half the offspring will exhibit the recessive phenotype for that trait.

This scenario represents Mendel's principle of independent assortment.

The information reveals that at least some of the genes are located on the same chromosome.

Within each linkage group, dihybrid crosses did not produce the proportions of offspring that Mendel's law of independent assortment predicts. Scientists eventually realized that each linkage group was simply a set of genes transmitted together on the same chromosome. This observation does not disprove Mendel's law of independent assortment, which applies only when genes are located on different chromosomes.

Crossing over separates alleles that occurred together on the same chromatid, so that alleles that were previously linked are no longer transmitted together.

Since the genes are very far apart on the chromosome, they have a high probability of being separated by crossing over.

Incomplete dominance: the heterozygote's phenotype is intermediate between those of the two homozygotes. This goes against the idea that two alleles should produce only two phenotypes, with one allele dominant over the other. Instead of a 3:1 phenotypic ratio, the ratio is 1:2:1.

Codominance: the heterozygote fully expresses two different and equally expressed alleles. This goes against the idea that two alleles should produce only two phenotypes, with one allele dominant over the other. Instead of a 3:1 phenotypic ratio, the ratio is 1:2:1.

Pleiotropy: one gene has multiple phenotypic expressions. Mendel's laws imply that each gene controls only one trait. One allele can therefore change the phenotype in multiple ways.

Epistasis: one gene affects the expression of another gene. Entire classes of phenotypes corresponding to one gene can seem to disappear if the allele of the other gene changes.

If the alleles are labeled A, B, C, and D, the following allele combinations are possible: AA, AB, AC, AD, BB, BC, BD, CC, CD, and DD. Ten phenotypes are possible.

The Y chromosome contains the SRY gene that acts as a switch for other sex determining genes that then activate in the embryo so that it develops as a male and dismantles all female embryonic structures.

The statement is accurate in the sense that all genes on every Y chromosome are expressed (unless a person inherits two Y chromosomes).

The fetus will develop as a female since without the receptors the signal to develop as a male will never be received.

Whereas a female inherits two X chromosomes, a male inherits his single X chromosome from his mother. A male expresses every allele (dominant or recessive) on his X chromosome because he lacks a second allele that could mask the expression of recessive alleles.

In X inactivation, all but one X chromosome is shut off in each cell, a process that happens early in the embryonic development of a mammal. Which X chromosome is inactivated is a random event. This prevents female mammals with two X chromosomes from expressing more X-linked genes than a male.

Because the disorder is severe most males die as a result of inheriting the recessive allele. Females who are heterozygous, however will have the dominant allele inactivated in some cells, leaving the recessive allele to be expressed. The severe effects may not be enough to be lethal since in some cells it is the recessive allele that is inactivated, but will be enough to be severely debilitating.

Cells of a female with an extra X chromosome have an extra Barr body, not just one as in a normal female. Perhaps she is XXX, and so the karyotype should show three and not two X chromosomes.

A female is a mosaic for X-linked genes because the maternal or paternal X chromosome is inactivated at random in each cell.

In cats, the genes encoding black and orange fur are located only on the X chromosome. Calico cats result from the random inactivation of black and orange alleles. Male calico cats are unusual because they would have to be XXY.

The mode of inheritance is autosomal dominant.

If people refuse to supply medical information, it can be impossible to tell who is affected and who is not. Blended families and artificial insemination make it impossible to trace parentage.

Genotype represents what proteins will be produced and how they will interact with each other, but the environment often affects how those proteins will express themselves or when the genes will be activated and inactivated. The combination of all these factors will determine the actual physical expression or phenotype.

Mendelian laws of inheritance rely on the separation of homologous pairs (Law of independent assortment) and alleles within a gene pair (Law of segregation). Both of these separation events are the result of spindle fibers separating chromosomes in the stages of meiosis. Chloroplasts and mitochondria do not undergo meiosis and so their DNA is not subject to the Mendelian laws of inheritance.

Genetics Problems

No calves will inherit citrullinemia; each has a 50% chance of being a carrier.

50% chance for each color.

No. The child faces a 25% chance of inheriting both recessive alleles. The chance that both of those alleles are of the same gene, and lead to a dominant phenotype is much lower.

The largest frequency (< 25%) indicates the two furthest apart genes. The smallest frequency indicates the two closest genes. So the map is _L_K____________J_.

Dilute (Mn). Normal (mm) X All white (MM) = all dilute (Mm) pups

25%. Mm X Mm = 25% normal, 50% dilute, 25% all white

Baby Z, O blood, belongs to couple 2, because an AB parent cannot produce an O child.

Baby X, AB blood, belongs to couple 1, because an O parent cannot produce an AB child.

Baby Y, B blood, therefore belongs to couple 3.

50% change the woman inherits the allele; if she did, her son has a 50% chance of having hemophilia

Pulling it Together

Gametes are haploid cells and nearly all other cells are diploid. Some cells, like red blood cells, lack a nucleus, and are therefore not haploid or diploid.

A genotype describes the genetics and a phenotype describes the outcome of the genetics

If a gene is expressed in the phenotype, then a dominant allele has an affect on the phenotype regardless of the other allele. A recessive allele is expressed in the phenotype only if the other allele is also recessive (and with a similar mutation).