AP Biology Campbell Active Reading Guide Chapter 14 – Mendel and the Gene Idea

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1. Explain the concept of blending, and then describe how Mendel’s "particulate" gene hypothesis was different.

Blending – The genetic material of the two parents mix in a manner analogous to the way blue and yellow paints mix to make green. Over many generations, a freely mating population will give rise to a uniform population of individuals. Mendel’s Hypothesis – Parents pass on discrete heritable units, genes, that retain their separate identities in offspring.

2. Explain how using pea plants allowed Mendel to control mating.

The reproductive parts of pea plants are enclosed in a flower, petals hindering cross-pollination, with plants usually self-fertilizing. Mendel removed the immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers. Mendel could thus always be sure of the parentage of new seeds.

3. What is the difference between a character and a trait?

A character is a heritable factor that varies among individuals, eg flower color. Traits are each variant of a character, eg purple or white colored flowers.

4. Define P generation, F₁ generation, and F₂ generation.

P generation – true breeding parents in a genetic cross F₁ generation – offspring of P generation (first filial generation) F₂ generation – offspring of F₁ generation (second filial generation)

5. Explain how Mendel’s simple cross of purple and white flowers did the following:
a) refuted blending
b) determined dominant and recessive characteristics
c) demonstrated the merit of experiments that covered multiple generations

a) If blending was true, the F₁ hybrid from a cross between purple and white flowered pea plants would have had pale purple flowers, but all F₁ offspring had purple flowered plants and the white flower trait appeared in the F₂ generation. b) Mendel reasoned that the heritable factor for white flowers did not disappear in the F₁ plants, but was somehow hidden, or masked, when the purple-flower factor was present. In Mendel’s terminology, purple flower color is a dominant trait, and white flower color is a recessive trait. c) If Mendel had stopped the experiment at F₁ instead of continuing through to F₂, he would not have discovered the law of segregation and the law of independent assortment.

7. In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?

One chromosome is paternal, one is maternal.

8. Mendel’s model consists of four concepts. Describe each concept in the appropriate space below. Indicate which of the concepts can be observed during meiosis by placing an asterisk by the concept.

First concept – Alternative versions of genes cause variation in inherited characters Second concept – For each character, every organism inherits two copies of a gene, one from each parent Third Concept – If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance. Fourth concept (law of segregation) * – the two alleles for each heritable character separate during gamete formation and end up in different gametes.

9. a) What is the F₂ phenotypic and genotypic ratio for a PP x pp cross?
b) Which generation is completely heterozygous?
c) Which generation has both heterozygous and homozygous offspring?

a) phenotype = 3:1 genotype = 1:2:1 b) F₁ generation c) F₂ generation

10. In pea plants, T is the allele for tall plants, while t is the allele for dwarf plants. If you have a tall
plant, demonstrate with a testcross how it could be determined if the plant is homozygous tall or
heterozygous tall.

Breed plant in question with short homozygous, and if F₁ is all tall, then the mystery plant was homozygous, otherwise if 1/2 are tall and 1/2 are short, it was heterozygous.

11. Explain the difference between a monohybrid cross and a dihybrid cross.

Monohybrid cross – cross involving study of only one character Dihybrid cross – cross involving two characters, eg flower color and seed shape

14. Explain Mendel’s Law of Independent Assortment.

Each pair of alleles segregates independently of each other pair of alleles during gamete formation.

16. In probability, what is an independent event?

The outcome of any particular trial is unaffected by what has happened in previous trials.

17. State the multiplication rule and give an original example.

The probability that two or more independent events will occur together in a specific combination is found by multiplying probabilities of each of two events. For example, the probability that two coin tosses will result in heads is 1/2 * 1/2 = 1/4.

18. State the addition rule and give an original example.

The addition rule states that the probability that any two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

19. What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific


20. Explain how incomplete dominance is different from complete dominance, and give an example of
incomplete dominance.

In incomplete dominance, neither allele is completely dominant in the phenotype, and the F₁ generation has a phenotype that is somewhere between the those of the two parental varieties. In complete dominance, the heterozygote and homozygote for the dominant allele are indistinguishable. An example of incomplete dominance is a red parent flower and white parent flower cross to form pink flowered-offspring.

21. Compare and contrast codominance with incomplete dominance.

In codominance two alleles are dominant and affect the phenotype in two different, but equal ways, eg human blood type AB.

22. Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.

Natural selection determines how common an allele is in the gene pool. For example, having six fingers (polydactyly) is dominant to five fingers, but the presence of six fingers is not common in the human gene pool.

23. Explain what is meant when a gene is said to have multiple alleles. Blood groups are an excellent
human example of this.

Most genes exist in more than two allelic forms, for example, ABO blood groups.

26. What is pleiotropy? Explain why this is important in diseases like cystic fibrosis and sickle-cell disease.

Property of a gene that causes it to have multiple phenotypic effects. For example, sickle cell disease and cystic fibrosis have multiple symptoms all due to a single defective gene.

27. Explain epistasis.

The phenotypic effects of a gene at one locus alters the effects of a gene at another locus.

28. Explain why the dihybrid cross detailed in Figure 14.12 in your text has four yellow Labrador retrievers instead of the three that would have been predicted by Mendel’s work.

The E/e gene is epistatic to the B/b gene.

29. Why is height a good example of polygenic inheritance.

Two or more genes have an additive effect on a single character in the phenotype, such as height.

30. Quantitative variation usually indicates _________.

polygenic inheritance

31. Using the terms norm of reaction and multifactorial, explain the potential influence of the
environment on phenotypic expression.

Norms of reaction are broadest for polygenic characters. Both genetic and environment factos contribute to the quantitative nature of those characters.

33. In the pedigree you completed above, explain why you know the genotype of one female in the third
generation, but are unsure of the other.

The presence of a free earlobe could indicate either an FF or Ff genotype, as F is the dominant allele, resulting in free earlobes. The female with the recessive trait can only have one genotype. The female with the dominant trait could be homozygous or heterozygous.

34. Describe what you think is medically important to know about the behavior of recessive alleles.

Thousands of genetic disorders are known to be inherited as simple recessive traits. These disorders range in severity from relatively mild, such as albinism (lack of pigmentation, which results in susceptibility to skin cancers and vision problems) to life-threatening, such as cystic fibrosis.

35. You are expected to have a general knowledge of the pattern of inheritance and the common symptoms of a number of genetic disorders. Provide this information for the disorders listed below.
a) cystic fibrosis
b) sickle-cell disease
c) achondroplasia
d) Huntington’s disease

a) A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated. b) A recessively inherited human blood disorder in which a single nucleotide change in the β-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals. c) A form of dwarfism that occurs in one of every 25,000 people. Heterozygous individuals have the dwarf phenotype. Like the presence of extra fingers or toes, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele. d) A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.

36. Amniocentesis and chorionic villus sampling are the two most widely used methods for testing a fetus
for genetic disorders. Use the unlabeled diagram below to explain the three main steps in
amniocentesis and the two main steps of CVS.

Amniocentesis: 1. A sample of amniotic fluid can be taken starting at the fourteenth to sixteenth week of pregnancy. 2. Biochemical and genetic tests can be performed immediately on the amniotic fluid or later on the cultured cell. 3. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. CVS: 1. A sample of chorionic villus tissue can be taken as early as the eighth to tenth week of pregnancy. 2. Karyotyping and biochemical and genetic tests can be performed on the fetal cells immediately, providing results within a day or so.

37. What are the strengths and weaknesses of each fetal test?

Strength of amniocentesis: In addition to fetal cells, amniotic fluid is also collected. Amniotic fluid can be used to detect additional enzymatic or developmental problems not detectable from the karyotype. Weakness of amniocentesis: Cells must be cultured for several weeks before karyotyping, and the test cannot be performed until the fourteenth to sixteenth week. Strength of CVS: These cells proliferate rapidly enough to allow karyotyping to be carried out immediately, and CVS can be performed as early as the eighth to tenth week. Weakness of CVS: No amniotic fluid is collected with this technique.

38. What are the symptoms of phenylketonuria (PKU)? How is newborn screening used to identify
children with this disorder?

The symptoms of phenylketonuria include an inability to metabolize the amino acid phenylalanine, causing severe mental intellectual disability. Some genetic disorders, including phenylketonuria, can be detected at birth by simple biochemical tests that are now routinely performed in most hospitals in the United States.

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