ANSWERS TO REVIEW QUESTIONS – CHAPTER 09
1.
What particular aspects of Mendel’s experiments enabled him to eliminate phenotypic
variation due to factors other than those directly concerned with the characters he was
studying? (p. 188)
Mendel selected pure-breeding varieties that were genetically homogeneous. Mendel studied seven
phenotypic traits of pea plants, each with only two alternative forms (i.e. yellow or green seed coats).
2.
Distinguish between monohybrid and dihybrid crosses and indicate how Mendel interpreted
the results of each type of cross. (pp. 188–196)
Monohybrid cross—A cross involving organisms that are heterozygous at a single locus.
Dihybrid cross—A cross involving organisms that are heterozygous at two different loci.
Mendel discovered that the alleles of the different genes were shuffled with respect to one another and
that particular ratios of different phenotypes were consistently observed. From this Mendel derived the
‘Principle of Independent Assortment’ which states: ‘Alleles of a gene controlling one trait assort into
gametes independently of another gene controlling a different trait.’
3. The key terms ‘gene’, ‘locus’ and ‘allele’ are often used incorrectly. State clearly the
meaning of each term and contrast one term with another by composing a single sentence in
which all three terms are used in their correct context. (Table 9.1)
Gene—Discrete hereditary factors that determine traits.
Locus—The position of a gene on a chromosome; a locus may be occupied by any alleles of a gene.
Allele—One of two or more forms of a gene located in the same position on homologous
chromosomes.
The gene for human hair colour is found at the same locus in people who have the alleles for blonde
hair as people who have the alleles for brown hair.
4.
Use diagrams to show how a study of chromosomes during meiosis provided a physical
explanation for Mendel’s laws. Explain why mitosis does not explain Mendel’s laws. (pp.
198–202)
Refer to Figures 9.8, 9.9 and 9.10 and Table 9.4. Mitosis is the process of cell replication, whereby
autosomes separate and produce two identical daughter cells. Mitosis does not explain how cells from
two separate individuals (parents) recombine to produce offspring.
5.
Use hypothetical human pedigrees to illustrate the similarities and differences between the
patterns of inheritance of autosomal and X-linked traits.
Refer to Figures 9.8, 9.11.
6.
Explain the differences between:
(a) penetrance and expressivity (p. 205)
Penetrance is the proportion of individuals of a particular genotype that show a phenotypic effect,
whereas expressivity is the degree to which an allele is expressed phenotypically in an individual.
(b) dominance and epistasis (p. 206)
Dominance occurs when a homozygous phenotype, such as yellow colour in seeds, appears in a
heterozygous (Yy) organism, whereas epistasis is the masking of the phenotype of one gene by the
phenotype of a different gene.
7.
‘The phenomenon of X-chromosome inactivation has evolved as a dosage compensation
mechanism for X-linked genes.’ Briefly discuss this statement. (pp. 206–207)
Females have twice the number of X chromosomes as males. X-chromosome inactivation is necessary
to ensure that X-linked genes are expressed in the same level in both males and females. For example,
in female humans one of the two X chromosomes is inactivated.
8.
Show how the genetic disease sickle cell anaemia illustrates that dominance and
recessiveness are not properties of the alleles involved (i.e. it is incorrect to say that the
mutant allele for sickle cell anaemia is recessive) but are properties of the phenotypes. (Table
9.3)
Sickle cell anaemia is not discussed in detail in the chapter. Instructors may wish to refer students to
the description at http://www.nhlbi.nih.gov/health/dci/Diseases/Sca/SCA_WhatIs.html. Individuals
with sickle cell anaemia have homozygous recessive alleles for the disease, whereas heterozygous
carriers are phenotypically normal.
9.
Mendel’s laws of inheritance have required modification and refinement in several
important respects. List the changes that have become necessary and briefly summarise the
evidence on which these changes were based. (pp. 198–206)
Mendel’s studies required extension to genetics, in particular with reference to sex linkage, linkage and
recombination, and epigenetic regulation.
Evidence for changes based on sex linkage: T. H. Morgan found that when red-eyed female drosophila
melanogaster (vinegar fly) were crossed with a white-eyed male, only red-eyed F1 progeny result.
These results suggested that the red-eyed type allele was dominant over the white-eyed type. However,
when male and female F1 progeny were crossed, the F2 progeny produced females with red eyes,
whereas half of the males had red eyes and the other half had white eyes. This does not obey Mendel’s
rules. Rather, the behaviour of sex chromosomes at meiosis explains the inheritance of sex-linked
genes.
Evidence for changes based on linkage and recombination: Mendel’s Principle of Independent
Assortment isn’t always robust. Refer to Figure 8.11 and Table 8.3. Linkage is the tendency of two or
more genes on the same chromosome to be inherited together.
Evidence for changes based on epigenetic regulation: Epigenetic regulation is the modification of
activity of a gene that is independent of genotype and inherited through cell divisions. Examples
include X-chromosome inactivation (e.g. in human females, deactivation of one of the X chromosomes,
refer to pages 206–207) and imprinting (e.g. Angelman syndrome and Prader-Willi syndrome in
humans, refer to pages 207–208).
10. A mother with the ABO blood group phenotype A has a child of phenotype O. List the ABO
blood groups that would be possible for the father of the child and those that would be
extremely unlikely. (pp. 193–195)
Mother
Father
Possible offspring
Impossible offspring
A
A
A, O
B, AB
A
B
A, B, AB, O
All types possible
A
AB
A, B, AB
O
A
O
A, O
B, AB