1. Mendel and the Gene Idea
Chapter 14

2. Mendel’s Experiments
Used plants that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
Mendel mated two contrasting, true-breeding varieties, a process called hybridization

3. When Mendel crossed contrasting, true-breeding white and purple flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
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

4. Mendel’s Model
The first concept is that alternative versions of genes account for variations in inherited characters
The second concept is that for each character an organism inherits two alleles, one from each parent
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
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

5. 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

6. 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
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

7. 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

8. 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
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

9. This question confronts a widespread misconception
This question confronts a widespread misconception. Many people think (incorrectly) that the genetically dominant allele will be most frequent and that dominant alleles increase in frequency within populations over time. The Hardy-Weinberg law shows that this is not the case; dominance and frequency are not related. Allele frequency of dominant alleles can be low (dark hair color in northern Europe) or high (dark hair color in southern Europe). Answer b is the only answer that reflects this pattern. Even though the chapter does not address population genetic ideas, it’s a good idea to try to break this misconception early.
Related to textbook page 273.

10. 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

12. 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

13. 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

14. The purpose of this question is to help students figure out gamete types—a step they often rush past. Students need to realize that gametes are haploid and that each gamete contains one, and only one, allele for each of the traits being studied. The presence of the second pea in the question stem is a distractor. Answer a is wrong because it shows gametes with two alleles for flower color and no alleles for seed color. Answer b is wrong because it shows only two possible gamete types. Answer c is right because it shows all four possible gamete types. Answer d is wrong because it shows gametes that are diploid for one trait and containing an allele for only one locus. Answer e is wrong because it shows a gamete diploid for both loci.
Related to textbook pages

15. 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
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
Probability in an F1 monohybrid cross can be determined using the multiplication rule

16. 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

17. 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
What fraction of offspring from this cross would be predicted to exhibit the recessive phenotypes for at least two of the three characters?
Finally, we use the addition rule to add the probabilities for all the different genotypes that fulfill the condition of at least two recessive traits, as shown below.

18. 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
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

19. 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

20. This question is an attempt to help students understand how enzyme action can produce phenotypes. If a person with two defective alleles at a locus in the melanin production pathway is albino, there is no melanin produced. However, a person with only one defective allele is normally pigmented, meaning that the person has a normal amount of melanin. Answer a is correct—one functional allele provides a normal amount of pigment. Answer b is wrong because albinos produce no melanin. Answer c is wrong because it contradicts the statement that heterozygotes are normally pigmented; if this statement were true, heterozygotes would have an intermediate amount of pigment. Answer d is wrong because a heterozygote in a codominance situation will have a different phenotype from both heterozygotes. Answer e is wrong because if heterozygotes have a normal amount of melanin, they will tan (produce more melanin when exposed to uv rays). This question simplifies the inheritance of albinism and its phenotypes. See for more information.
Related to textbook pages

21. This question again tries to connect enzyme action to phenotype
This question again tries to connect enzyme action to phenotype. There is a yellow/red polymorphism in apple skin that appears to be controlled by one locus. Answers a and d are wrong because a heterozygote will produce some red pigmentation, so it cannot be yellow. Answers b and c are possible but there is not enough information given to know which is more likely. Answer e is the best answer because yellowish red or red is possible—yellowish red if a heterozygote produces less pigment than the red homozygote or red if a heterozygote produces as much pigment as a purple homozygote. Students with more sophistication may suggest that the phenotype would be a patchwork of yellow and red cells—red where the purple allele is “turned on” and yellow where the yellow allele is “turned on,” but this is not what happens in the apples. You can read about one gene involved in the pathway at
Related to textbook pages

22. 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

24. This question is designed to reinforce the idea that diploid organisms have two, but only two, copies of alleles for each locus. The existence of four alleles will confuse some students but is not an uncommon situation. To answer this question, students need to figure out gametes produced by the parents (Da or Db for the first parent and Dc or Dd for the second parent). Then using a Punnett square, they will get 25% each of four genotypes, DaDc, DaDd, DbDc, and DbDd. The correct answer is a. All offspring should have two alleles and there cannot be any homozygotes from this set of parents.

25. 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

26. Epistasis
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
For example, in mice 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 C for color and c for no color) determines whether the pigment will be deposited in the hair

28. 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

29. According to thiS model
According to thiS model. three separately mhented genes affect the darkness of skin. The heterozygous IndiVIduals (AaBbCc) represented by the two rectangles at the top of thiS figure each carry three dark-skin alleles (black cirdes. which represent A. B. or C) and three hght-skln alleles (wflite circles, which represent a, b, or C). The Punnett square shO'NS all the pOSSIble genetIC combinations in gametes and in offspnng of a large number of hypothetical matings between these heterozygotes. The results are summanzed by the phenotypic ratios under the Punnett square.

30. Nature and Nurture: The Environmental Impact on Phenotype
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
Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype
Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype
Norms of reaction are generally broadest for polygenic characters

31. Several answers are possible (a, b, and d) because both genes (nature) and environment (nurture) interact to produce the phenotype. However, none of these three answers is “more right” than the others. Genes certainly affect a person’s basal metabolism (although specific genes are not yet identified). Exercise level will also affect a person’s weight. Furthermore, other factors (e.g., total food intake, choice of food types, and even rebellious attitudes towards their parents) can also affect fat levels. After discussion of this question, a faculty member might ask students to write one good answer. One possible, though obvious, answer would be, “Fat levels of the children will depend on the genes they inherit from their parents, their activity levels, and their eating habits.”

32. 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
We can use the multiplication and addition rules to predict the probability of specific phenotypes
Pedigrees can also be used to make predictions about future offspring

35. The correct answer is c. This question begins a series of three questions about Huntington’s disease and seeks to make sure that students understand that the daughter has a 50% probability of inheriting the normal allele (normal phenotype) and a 50% probability of inheriting the Huntington allele (Huntington phenotype). The question will also help students understand the implications of the fact that the Huntington allele is dominant.

36. This is the second of three questions on Huntington’s disease
This is the second of three questions on Huntington’s disease. To choose the correct answer (b), students must realize that the probability of the daughter getting the Huntington allele is (as in the previous question) 50%. If she has the allele, the probability of her son getting the allele from her is also 50%. To calculate the probability of two independent events both occurring, one must multiply the probabilities. Thus, the grandson’s probability of inheriting the allele from his grandfather is (0.5)(0.5) = 0.25 = 25%.

37. This last question in the Huntington’s disease sequence describes a situation that has applied to many families. Part of the tragedy of Huntington’s disease is that it is a gradual degeneration for which there is no cure. However, some of the tragedy of Huntington’s disease is that it is caused by a dominant allele (though the severity of the disease varies among people) and that the onset of the disease often occurs only after a person has had his or her children and even grandchildren. There is no right answer to this question and the question can lead into a discussion of ethics (for example, some people advocate that minors should not be tested and many people argue that the results of genetic testing should not affect insurance rates or employment opportunities). One possible answer that is not offered is to become pregnant, have the fetus tested, and abort an affected fetus. A good site for information is