Chapter 8: Inheritance, Genes, and Chromosomes
0. Application
1. Annette's maternal grandfather died of Huntington's disease when he was
53 and Annette was 8. Huntington's is a rare autosomal dominant
degenerative neural disorder that usually hits people in their late forties,
early fifties. Annette loved her grandfather very much and was deeply
affected by the painful progression of his disease. She wants to get
pregnant but does not want to risk giving birth to a child that will suffer as
much as her grandfather. If she tested positive for Huntington's, what is
the probability of her first child to develop Huntington's? Of her second
child? Of her mother? (Note that since Huntington’s is so rare, you can
assume that neither grandmother, nor father or partner have the
Huntington’s mutation).
2. Why are some conditions, such as hemophilia and red-green color
blindness, more common in males than in females?
3. Antibiotic resistance is a global health problem that will most probably
become even more severe. The mutation rate, however, is low
(e.g.,according to Denamur and Matic (2006) in one E. coli strain (K-12) it
has been estimated to be 2-8 x 10 -4 per genome and replication). And of
all those mutations, usually not even 1/1000 provides an adaptive
advantage. How then, have bacteria been able to evolve multiple antibiotic
resistance so rapidly?
I. The big picture
4. We saw in the last chapter that chromosomes come in pairs, one inherited
from our mothers, one from our fathers. Our parents, in turn, have
inherited their pairs of homologous chromosomes from their parents, so
the chromosome they pass on to us is either our grandfathers’ or our
grandmothers’. The process that creates random combinations of
chromosomes accounts for the variation of offspring and results from
_____________ on the metaphase plane in meiosis I. We also learned
that homologous chromosomes contain the same genes in the same
locations, but that the DNA sequence on the homologs might differ,
leading to different versions of the gene called________. In this chapter
we will study what happens when those alleles encode different
information. Keep in mind that Mendel did not know about meiosis or the
physical basis of inheritance (mitosis and meiosis were discovered in the
1880s).
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II. Gregor Mendel’s Questions
5. What was known at the time (when did Mendel start working with peas?)
about cells, chromosomes, mitosis, meiosis, and DNA?
6. How does Mendel’s particulate mechanism differ from the blending
theory of inheritance?
7. What questions led Mendel to his monohybrid crosses?
8. What made pea plants a good choice for genetic experiments? Note:
many of the reasons apply to all model organisms used in biology.
9. Define the following terms: true-breeding, hybridization, P generation,
F1 generation, and F2 generation (Fig. 8.1 and 8.2).
10. List and explain the four components of Mendel’s hypothesis that led him
to deduce the law of segregation.
11. Use a Punnett square (p. 147) to predict the results of a monohybrid
cross, stating the phenotypic and genotypic ratios of the F2 generation
(Fig. 8.2).
12. Distinguish between the following pairs of terms: gene and allele;
character and trait; heterozygous and homozygous; genotype and
phenotype.
Note: There are a few common misconceptions relating to the concept of
dominant phenotypes.
a. When talking about phenotypes, the terms dominant and recessive are
often used. Note that this is not a dominance on the genetic level (one allele
somehow "knocking out" its homolog)); but it's about the particular expression
of one allele versus another. Let's say that allele 1 codes for red pigment,
while allele b codes for a colorless substance (e.g., a mutated pigment). Both
substances are made and deposited in the petals of a flower. What you'll see
is the red pigment, not the "no pigment" - that's why red is dominant (see also
the Tay Sachs example below).
b. Whether a phenotype is dominant or not has nothing to do with its frequency,
as you can infer from the example above. If a trait does not impair
reproductive success, its frequency is usually mostly determined by the time
elapsed since it evolved (consider the rare but dominant human traits of
Huntington's disease and polydactyly).
13. Purple in pea petals is considered wild-type (ancestral) and is dominant
over the mutant phenotype of white. However, having six digits as in
polydactyly is considered the mutation, and it is dominant over the wildtype five digits. Explain what makes the ancestral purple dominant and
ancestral GLI3 (the gene responsible for some types of polydactyly)
recessive. Distinguish between loss of function and gain of function
mutations and give examples. Hypothesize which is likely to be inherited
in a recessive, and which in a dominant manner.
14. Which question does a testcross answer? How is it set up (Fig. 8.4)?
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15. What question did Mendel try to answer with his dihybrid crosses (8.5)?
16. State Mendel’s law of independent assortment. How does your
knowledge of meiosis a) support and b) contradict this law (Fig. 8.6)?
In Lab
17. Use a Punnett square to predict the results of a dihybrid cross and state
the phenotypic and genotypic ratios of the F2 generation (Fig. 14.8).
18. Use the rule of multiplication to calculate the probability that a particular
F2 individual will be homozygous recessive or dominant.
19. Given a Mendelian cross, use the rule of addition to calculate the
probability that a particular F2 individual will be heterozygous.
20. Use the laws of probability to predict, from a trihybrid cross between two
individuals that are heterozygous for all three traits, what expected
proportion of the offspring would be: (a) homozygous dominant for the
three traits (b)heterozygous for all three traits and (c) homozygous
recessive for two specific traits and heterozygous for the third
21. Given a simple family pedigree, deduce the genotypes for some of the
family members (Fig.8.8).
III. Alleles and genes interact to produce phenotypes
22. Review the four components of Mendel’s hypothesis about the
segregation of alleles (see question 8) and explain which of the patterns of
inheritance in this section extends which of the four components.
23. Explain how phenotypic expression of the heterozygote differs with
complete dominance, incomplete dominance, and codominance (Fig.
8.10 and 8.11).
24. Define and give examples of hybrid vigor (p. 154) and epistasis (Fig.
8.12).
25. Describe a simple model for polygenic inheritance and explain why most
polygenic characters are described in quantitative terms.
26. Describe how environmental conditions can influence the phenotypic
expression of a character.
27. Distinguish between expressivity and penetrance.
IV. Genes are carried on chromosomes
28. Explain why Drosophila melanogaster is a good experimental organism
for genetic studies (note that Latin species’ names are italicized).
29.
30. Morgan bred female D. melanogaster with gray bodies and normal wings
with black male flies with vestigial wings. What are the genotype
frequencies expected from Mendel’s law of independent assortment in the
F1 generation? How did Morgan explain the discrepancy (Fig. 8.13)?
31. Explain how crossing over results in genetic recombination (Fig. 8.14)
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32. How does linkage affect inheritance? Distinguish between parental and
recombinant phenotypes (Fig. 8.15) and explain what recombination
frequency depends upon.
33. Morgan also bred white-eyed male fruit flies with red-eyed females. All F1
offspring had red eyes, but In the F2 generation, he found a ratio of 50%
red eyed females, 25% red eyed males and 25% white eyed males. What
conclusion did he draw from these data (Figure 8.16)?
34. Why are X-linked recessive disorders more common in males than in
females (in lab).
35. How does the inheritance pattern change for a X-linked dominant
condition such as faulty enamel trait or for a Y-linked disorder? Why is Xlinked recessive inheritance often used synonymously with sex-linked (in
lab)?
36. Distinguish between linked genes and sex-linked genes (Fig. 8.15-8.16).
37. Explain why Mendel did not find linkage between seed color and flower
color, despite the fact that these genes are on the same chromosome.
38. Explain why extranuclear (or cyotoplamic) genes are not inherited in a
Mendelian fashion.
V. Prokaryotes can exchange genetic material
39. Explain how bacterial conjugation can lead to genetic recombination
(Fig. 8.19).
40. Genetic recombination due to bacterial conjugation leads to mutations.
Why do these mutations have a much higher probability of having
adaptive advantages than spontaneous mutations?
41. Name two categories of plasmids.
42. Explain how genes can be transferred by plasmids from one bacterium to
another (Fig. 8.20).
Essay question
Discuss alleles. Explain how they arise and how the interactions of their
products lead to different inheritance pattern. Discuss those patterns.
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