Chapter 7MENDELIAN INHERITANCE—FAMILY RESEMBLANCE:HOW TRAITS ARE INHERITEDLearning Objectives• Explain the concept of a single-gene trait• Understand Mendel’s contributions to the field of genetics• Be able to define the terms: gene, allele, dominant, recessive, homozygous, and heterozygous• Describe the difference between an organism’s genotype and its phenotype• Demonstrate the ability to perform Punnett squares to predict the offspring of parents with particular genotypes• Understand how to use the rules of probability to predict the inheritance of specific traits• Explain how a test-cross can be used to determine the genotype of an organism.• Be able to predict patterns of inheritance based on pedigree analysis• Explain how each of the following genetic “rules” works: incomplete dominance, codominance, multiple alleles, polygenic inheritance, and pleitropy• Understand how the ABO and Rh markers contribute to human blood type• Explain how blood type compatibility is determined• Describe how sex-linked traits are inherited• Understand that phenotypes are a combination of genotypes and the environment• Explain why linked genes do not assort independently

Chapter OutlineI. Why Do Offspring Resemble Their Parents?• In Chapter 5 you learned about DNA and in Chapter 6 you learned about meiosis and the production of gametes. A few points to recall that are pertinent to our discussion of genetics are the following:

o In a diploid cell, there are ________________copies of each chromosome present.

o Each human diploid cell has a total of__________ chromosomes or_________ pairs of chromosomes.

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Chapter 7. Mendelian Inheritance

o A copy of a gene that can be passed from a parent is termed a(n) ____________________________.

A. Single-Gene Traits

• The passing of characteristics from parents to offspring via genes is referred to as

_________________________________________.

• The inheritance of some traits is easy to predict. When a single gene influences a trait (a single-gene trait), will the inheritance of that trait be relatively easy or difficult to predict? Justify your answer.

• Provide an example of a human trait that would not be considered a single-gene trait.

B. Mendel’s Research Design• Gregor Mendel’s work was fundamental to all modern genetic theories. There were three features of Mendel ‘s work that were particularly important to his success. Those were:

1.

2.

3.

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Chapter 7. Mendelian Inheritance 105C. Mendel’s Law of Segregation

• Mendel’s experiments in crossing pea plants led to predictable results. For example, crossing true-breeding purple pea plants to white pea plants always led to purple offspring. This is because purple color is dominant while white color is recessive.

• The concept of dominance is often misunderstood. People tend to think that dominant alleles are more common than recessive alleles or that they are in some way more advantageous than recessive alleles.

o In your own words, describe how a dominant allele is different from a recessive allele.

• For any given trait, an individual receives exactly _________ copy of a gene from each parent via the gamete. This leads to individuals whose diploid cells have exactly ________ copies of each gene.

• When an individual inherits two of the same alleles from both parents, we call this

__________________________ and when an individual inherits two different alleles from both parents,

we call this __________________________

• Briefly summarize Mendel’s law of segregation.

D. Genotypes, Phenotypes, and Punnett Squares• In the following example, there are two possible alleles for the gene that determines whether a person has dimples. Having dimples is dominant and the allele will be represented as D. Not having dimples is recessive and the allele will be represented as d.• For each possible genotype, fill in the corresponding phenotype.

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106 Chapter 7. Mendelian Inheritance

• Suppose your roommate has dimples. Can you be sure of her genotype? Explain your answer.

• Show a Punnett square for the dimples trait between a homozygous recessive parent and a heterozygous parent. Indicate the phenotypic ratio for the offspring.

II. Probability and Chance Play Central Roles in Genetics• There are two reasons that the rules of probability are used in genetics. Those reasons include:

1.

2.

A. Test Crossing• During a test-cross, an organism with a ______________________ phenotype and an unknown

genotype is always bred with a mate that is ______________________________________.

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Chapter 7. Mendelian Inheritance 107o If half of the offspring of the test-cross show the dominant phenotype and half show the recessive phenotype, then the genotype of the unknown parent was___________________________

o If all of the offspring of the test-cross show the dominant phenotype, then the genotype of the unknown parent was________________________

B. Pedigree Analysis• Pedigrees are useful tools for determining the pattern of inheritance for a specific allele. Within a

pedigree, females are represented by _____________________ and males are represented by

____________________. How can you tell which individuals in a pedigree contain a trait of interest?

• If a trait of interest is located on an autosome, the pattern of inheritance will be

___________________ or ___________________• If a trait of interest is located on the X or Y chromosome, the pattern of inheritance will

be __________________________• If a pedigree shows a carrier for a specific trait, what does this mean?

III. The Translation of Genotypes into Phenotypes Is Not a Black Box• Not all genetic traits operate by simple dominance of one allele over a recessive allele. There are other genetic rules that govern the inheritance of specific genetic traits.A. Incomplete Dominance and Codominance• In some cases one allele does not clearly dominate another. In this case, individuals with a

_____________________ genotype express a unique phenotype.• At first glance, incomplete dominance and codominance seem very similar to each other. Explain how they are different.

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108 Chapter 7. Mendelian Inheritance• Roan cows have an interesting coat color. The have blotches of red fur and blotches of white fur. If you were told that their genotype is heterozygous, which rule of inheritance do you think applies to the development of roan coat color?

B. Multiple Alleles and Blood Type• When a single gene has more than two alleles, this is referred to as multiple alleles. Human blood type is an example.

• In the chart below, indicate the four human blood types, the potential genotypes of each, and the antigens that would be present on the red blood cells of an individual with that specific blood type.

• During blood transfusions, compatibilities between donor and recipient must be accounted for. Complete the following chart:

• The universal donor is Type_____ and the universal recipient is Type ________.• Speaking in terms of the immune system, what would happen if a person with type A blood were to receive a type B blood transfusion?

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Chapter 7. Mendelian Inheritance 109

• In addition to the ABO markers, there is another genetic marker that determines whether an

individual’s blood type is positive or negative. This is referred to as the _______________________.

C. Multigene Traits

• Polygenic traits are ones in which a single phenotype is influenced by ________________ genes.

• When alleles from multiple genes influence a single phenotype, this is referred to as

_________________________________________________.

• Provide three examples of human traits that are polygenic:

1.

2.

3.

• In pleitropy, a single gene influences multiple traits. An example of this would be:

D. Sex-Linked Traits

• Traits that are coded for on sex chromosomes have unique patterns of inheritance. Recall that the genotype of a female is ________ and the genotype of a male is __________________________.

• Which gender is more likely to exhibit a sex-linked recessive trait?

• Suppose that a female exhibits red-green color-blindness. What must the genotypes of her parents have been? Explain your answer.

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110 Chapter 7. Mendelian Inheritance

E. Environmental Effects

• It is important to note that traits are not shaped by genotype alone. The ultimate phenotype

expressed is shaped by both the genes inherited in combination with the ______________________.

• Provide at least three examples of human phenotypes that can be shaped by the factors other than genes:

1.

2.

3.

IV. Some Genes are Linked Together

• Mendel’s law of independent assortment can be used to explain how traits are inherited independently of each other.

• In order for independent assortment to occur, the traits must exist on separate

________________________.

• When traits are considered linked, they exist in close proximity on the same chromosome. How can meiosis be used to explain why linked traits do not assort independently of one another?

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Chapter 7. Mendelian Inheritance 111Testing and Applying Your UnderstandingMultiple Choice (For more multiple choice questions, visit www.prep-u.com.)1. Which of the following statements about dominant traits is correct?a) They are observed less frequently than recessive traits.b) They are observed more frequently than recessive traits.c) They are observed one-quarter as frequently as heterozygous traits.d) They increase in frequency over evolutionary time.e) None of the above is correct.

2. In certain plants, red flowers are dominant to white flowers. If a heterozygous plant is crossed with a homozygous red-flowered plant, what is the probability that the offspring will be white-flowered?a) 50%b) 100%c) 25%d) 0%e) It depends on whether the traits are linked.

3. Some genes, such as the human ABO blood groups, have more than two alleles. For thesegenes:a) normal dominance relationships are not possible.b) a greater proportion of the individuals must be heterozygous than homozygous.c) some individuals can be triply heterozygous.d) natural selection cannot alter the allele frequencies.e) individuals can only possess two alleles.

4. A man with the autosomal recessive disorder phenylketonuria (PKU) and a woman without PKU have a son named Peter, who does not have PKU. Peter is curious about whether his mother is a carrier for PKU. Which of the following facts would allow him to know?a) Peter’s sister does not have PKU. b) Peter’s own daughter has PKU. c) Peter’s maternal grandfather has PKU.d) Neither of Peter’s maternal grandparents have PKU.e) Peter submits his own blood sample to a local genotyping lab, which establishes that he is a carrier for PKU.

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112 Chapter 7. Mendelian Inheritance5. Mary, who has type 0 blood, is expecting a child with her husband, who has type B blood. Mary’s husband’s father has type A blood. What is the probability that the child will have type 0 blood?a) 50% b)25%c) 100%d) 75%e) 0%

6. Assuming that a particular disorder is caused by an allele of a single gene. What feature of a pedigree would allow one to conclude that the disorder was caused by a dominant allele?a) Two affected parents have an unaffected child.b) Two unaffected parents have an affected child. c) An affected mother only has affected sons.d) There is no way to tell based on pedigree analysis alone.e) All of the descendants of a particular affected person are also affected.

7. Progeria is a genetic disorder that causes numerous symptoms that resemble premature aging in patients. Progeria is caused by a dominant allele, and it always results in fatality before sexual maturity. Which of the following statements must be true about progeria?a) Both parents of every progeria patient must also be carriers of progeria. b) Progeria must be relatively common.c) At least one parent of every progeria patient must also be a carrier of progeria.d) Progeria must be more common in some geographic areas than in others. e) Every case of progeria must be caused by a de novo mutation.

8. In mice, the allele for brown coat color is completely dominant to the allele for white coat color. Which of the following is true about a true-breeding brown mouse?a) If crossed with a white mouse, all the resulting progeny would be brown.b) If crossed with a white mouse, and two of the resulting progeny mated, the F2 progeny would be 75% brown and 25% white.c) It is homozygous for the brown allele.d) If crossed with another brown mouse, all progeny would be brown.e) All of the above are correct.

9. Is it possible for a woman to have an X-linked recessive trait; if it is, how can this occur?a) No, women cannot have X-linked recessive traits because they are all recessive and women have two X chromosomes.b) Yes, a woman will always have an X-linked recessive trait if her father has the trait. c) Yes, women can have an X-linked recessive trait if her mother is homozygous for the trait.d) Yes, a woman can have an X-linked recessive trait if both her father has the trait and her mother is either homozygous or heterozygous for the trait.e) Both b and c are correct.

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Chapter 7. Mendelian Inheritance 113Short Answer

1. The ability to roll your tongue is dominant (R), whereas the inability to roll your tongue is recessive (r). List the three possible genotypes and the corresponding phenotypes for each.

2. Two parents have the dominant phenotype for tongue rolling yet they produce a child with the recessive phenotype (unable to roll their tongue). What must the genotype of the parents be? What is the chance that they would produce a child with the recessive phenotype (show the Punnett square)?

3. Suppose you have a parent heterozygous for tongue rolling and a parent who is homozygous dominant for tongue rolling. Using the rules of probability, what is the chance that they will have a child who cannot roll his or her tongue? Show your calculation.

4. Round pea seeds are dominant to wrinkled seeds. If a homozygous round seed plant is crossed with a homozygous wrinkled seed plant, all the offspring will be heterozygous. Suppose you cross two of the heterozygous offspring and produce 400 offspring in the next generation. How many of these would have wrinkled seeds? How many would have round seeds?

5. The inheritance of curly hair illustrates incomplete dominance. When a curly haired individual reproduces with a straight-haired one, the children all have wavy hair. What offspring would be produced, in what proportions, when two people with wavy hair reproduce? Write your answer and show a Punnett square.

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114 Chapter 7. Mendelian Inheritance6. Your cousin has three children, all of which are boys. She is convinced that her next pregnancy will produce a daughter because she already has three boys. Based on what you know about probability, is her logic sound? Explain why or why not.

7. A mother is uncertain about the paternity of her newborn baby. Before the DNA analysis is performed, a simple blood test is performed to give some preliminary information. There are two men suspected to be the father in this case so their blood is also collected. Their blood types are as follows:

Mom: Type A Suspected father #1: Type 0Baby: Type B Suspected father #2: Type B

• Which man is the most likely father?

• Write an explanation and show Punnett squares to justify your answer.

• To make the situation more complicated, the baby needs a blood transfusion. The mother and both of the suspected fathers offer to donate.o Which of them (write as many as are appropriate) can donate to the baby?

o What would happen (specifically) if one of the others (an incompatible match) tried to donate to the baby?

8. Your neighbor has planted a bed of pink snapdragons in his yard. As the growing season progresses, he notices that some white and red snapdragons are popping up in what was supposed to be a bed of pink flowers. Explain to him the reason that this is happening.

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