Chapter 15: the chromosomal basis of inheritance

Question 1

Define the law of separation

Answer 1

The two alleles for each gene will separate

Question 2

Define the law of independent assortment

Answer 2

alleles of genes on nonhomologous chromosomes assort independently

Question 3

Describe X inactivation in Female mammals

Answer 3

one of the X chromosomes is inactive so double proteins are not expressed

Each allele is randomly activated during embryonic development

“Mosaic” or two different phenotypes

Question 4

Linked genes tend to be inherited together because

Answer 4

they are located near each other on the same chromosome

Question 5

When might Mendel’s law of independent assortment not be true?

Answer 5

chromosomal inheritance, where linked genes can be inherited together because they are near each other on the same chromosome

Question 6

What are recumbent offspring?

Answer 6

offspring that do not look like the parents

Question 7

An inversion will

Answer 7

reverse a segment within a chromosome

Question 8

A translocation will

Answer 8

move a segment from one chromosome to a nonhomologous chromosome

Question 9

A duplication will

Answer 9

repeat a segment of DNA

Question 10

Non disjunction is

Answer 10

incorrect distribution of chromosomes across cells

Question 11

True or false: non disjunction can result in aneuploidy of sex chromosomes

Answer 11

True
note aneuploidy is an incorrect number of sex chromosomes present

Question 12

TorF: Certain cancers are the result of translocation

Answer 12

true

Question 13

What is genomic imprinting

Answer 13

the silencing of certain genes depending on which parent passes them on, as two active copies would be detrimental to the offspring

Question 14

Why do imprinted genes not follow Mendelian pattern of inheritance?

Answer 14

Because imprinting causes offspring to differentiate between maternal and paternal alleles

Question 15

Imprinting is the result of

Answer 15

methylation

Question 16

Why are extranuclear genes inherited maternally?

Answer 16

the zygote’s cytoplasm comes from the egg

Question 17

How can defects in mitochondrial genes affect the cell?

Answer 17

affects ATP production which can lead to diseases that impact the muscles and nervous system

Question 18

15.1.2 Which one of Mendel’s laws describes the inheritance of
alleles for a single character? Which law relates to the inher-
itance of alleles for two characters in a dihybrid cross?

Answer 18

The law of segregation describes the inheritance of alleles for a
single character. The law of independent assortment of alleles describes the
inheritance of alleles for two characters.

Question 19

15.2.2 Neither Tim nor Shonda has Duchenne muscular dystrophy,
but their firstborn son does. What is the probability that a
second child will have the disease? What is the probability if
the second child is a boy? A girl?

Answer 19

1
⁄4 (1
⁄2 chance that the child will
inherit a Y chromosome from the father and be male * 1
⁄2 chance that he will
inherit the X carrying the disease allele from his mother). If the child is a boy,
there is a 1
⁄2 chance he will have the disease; a female would have zero chance
(but 1
⁄2 chance of being a carrier)

Question 20

15.2.3 Consider what you learned about
dominant and recessive alleles in Concept 14.1. If a disorder were
caused by a dominant X-linked allele, how would the inheritance
pattern differ from what we see for recessive X-linked disorders?

Answer 20

In a disorder caused by a dominant allele,
there is no such thing as a “carrier,” since those with the allele have the disorder.
Because the allele is dominant, the females lose any “advantage” in having two
X chromosomes, since one disorder-associated allele is sufficient to result in the
disorder. All fathers who have the dominant allele will pass it along to all their
daughters, who will also have the disorder. A mother who has the allele (and thus
the disorder) will pass it to half of her sons and half of her daughters.

Question 21

15.3.1 When two genes are located on the same chromosome,
what is the physical basis for the production of recombinant
offspring in a testcross between a dihybrid parent and a
double-mutant (recessive) parent?

Answer 21

Crossing over during meiosis I in the heterozygous parent produces some
gametes with recombinant genotypes for the two genes. Offspring with a recom-
binant phenotype arise from fertilization of the recombinant gametes by homo-
zygous recessive gametes from the double-mutant parent.

Question 22

15.3.3 Genes A, B, and C are located on the same chromosome. Testcrosses show that the recombination frequency
between A and B is 28% and that between A and C is 12%.
Can you determine the linear order of these genes? Explain.

Answer 22

No. The order could be A-C-B or
C-A-B. To determine which possibility is correct, you need to know the recombi-
nation frequency between B and C.

Question 23

15.4.1 About 5% of individuals with Down syndrome have a chro-
mosomal translocation in which a third copy of chromo-
some 21 is attached to chromosome 14. If this translocation
occurred in a parent’s gonad, how could it lead to Down
syndrome in a child?

Answer 23

In meiosis, a combined 14-21 chromosome will behave as one chromosome. If a
gamete receives the combined 14-21 chromosome and a normal copy of chromo-
some 21, trisomy 21 will result when this gamete combines with a normal gamete
(with its own chromosome 21) during fertilization.

Question 24

15.4.2 The ABO blood type locus has been mapped on chromosome 9. A father who has type AB blood
and a mother who has type O blood have a child with tri-
somy 9 and type A blood. Using this information, can you
tell in which parent the nondisjunction occurred? Explain
your answer.

Answer 24

No. The child can be either
IA
IA
i or IA
ii. A sperm of genotype IA
IA could result from nondisjunction in the
father during meiosis II, while an egg with the genotype ii could result from non-
disjunction in the mother during either meiosis I or meiosis II.

Question 25

15.4.3 The gene that is activated on the
Philadelphia chromosome codes for an intracellular tyro-
sine kinase. Review the discussion of cell cycle control in
Concept 12.3, and explain how the activation of this gene
could contribute to the development of cancer.

Answer 25

Activation of
this gene could lead to the production of too much of this kinase. If the kinase is
involved in a signaling pathway that triggers cell division, too much of it could trigger unrestricted cell division, which in turn could contribute to the development
of a cancer (in this case, a cancer of one type of white blood cell).

Question 26

15.5.1 Gene dosage—the number of copies of a gene that are
actively being expressed—is important to proper develop-
ment. Identify and describe two processes that establish the
proper dosage of certain genes.

Answer 26

Inactivation of an X chromosome in females and genomic imprinting.
Because of X inactivation, the effective dose of genes on the X chromosome
is the same in males and females. As a result of genomic imprinting, only
one allele of certain genes is phenotypically expressed.

Question 27

15.5.3 Mitochondrial genes are critical to the energy
metabolism of cells, but mitochondrial disorders caused by
mutations in these genes are generally not lethal. Why not?

Answer 27

Each cell contains numerous mitochondria, and in affected
individuals, most cells contain a variable mixture of normal and mutant mito-
chondria. The normal mitochondria carry out enough cellular respiration for
survival. (The situation is similar for chloroplasts.)