Topic 3.3: The Chromosomal Basis for Inheritance

Question 1

Explain why the chances of survival are greater for trisomy or monosomy of the sex chromosomes than for autosomes

Answer 1

In normal XX females, one of the X chromosomes becomes inactive and forms a Barr body which indicates that one functional X chromosome is needed in females, just like in males. Extra X chromosomes all become Barr bodies in males, extra X chromosomes also become Barr bodies. Extra Y chromosomes do not seem to interfere with normal development, perhaps because there are so few genes on the Y chromosome.

Question 2

Explain the nondisjunction event that would cause a Turner or Klinefelter syndrome individual

Answer 2

Turner syndrome and Kleinfelter syndrome can be caused by nondisjunction during oogenesis at meiosis I or at meiosis II.

Question 3

Define karyotype

Answer 3

Chromosomes arranged by pairs according to their size, shape, and general appearance in mitotic metaphase.

Question 4

Define nondisjunction

Answer 4

Failure of the homologous chromosomes or sister chromatids to separate during either mitosis or meiosis; produces cells with abnormal chromosome numbers.

Question 5

Define trisomy

Answer 5

Chromosome condition in which a diploid cell has one more chromosome than normal; designated as 2n + 1.

Question 6

Define monosomy

Answer 6

Chromosome condition in which a diploid cell has one less chromosome than normal; designated as 2n −1.

Question 7

Define Barr body

Answer 7

Dark-staining body in the cell nuclei of female mammals that contains a condensed, inactive X chromosome; named after its discoverer, Murray Barr.

Question 8

If a child has type O blood and the mother is type A, then which of the following might be the blood type of the child’s father?
a) A only
b) A or O
c) B only
d) A, B, or O
e) O only

Answer 8

d) A, B, or O

Question 9

A trait in which the heterozygotes have a phenotype that is intermediate between the dominant and recessive phenotypes is an indication of
a) codominance
b) pleiotropy
c) a sex-linked trait
d) incomplete dominance
e) none of the above

Answer 9

a) codominance

Question 10

In the analysis of a genetic disorder, you notice that a phenotypic distribution follows a bell-shaped curve, and that multiple genes are probably involved. This suggests what pattern of inheritance?
a) autosomal dominant
b) autosomal recessive
c) incomplete dominance
d) polygenic inheritance
e) none of the above

Answer 10

d) polygenic inheritance

Question 11

Explain how sex linked patterns of inheritance differ from autosomal inheritance.

Answer 11

Sex linked alleles appear on the X chromosome and more rarely on the Y chromosome. Their inheritance is dependent on the distribution o the X and Y chromosomes in males and females. Recessive alleles on the X chromosome are expressed in males, while they are not heterozygous females.

Question 12

Describe the phenotypic ration that is expected for a cross in which bboth parent have one X-linked recessive allele.

Answer 12

50% of female offspring and 50% of male offspring would express the recessive allele.

Question 13

Summarize the causes of the X-linked recessive disorders discussed in this chapter.

Answer 13

Color blindness: A number of different genetic mechanisms can contribute to X-linked color blindness.
Duchenne muscular dystrophy: caused by the absence of the protein dystrophin which leads to calcium entering the cell, which promotes an enzyme that dissolves muscle fibers.
Fragile X syndrome: An abnormally high number of repeat sequences of CGG on the X chromosome.
Hemophilia: either due to the absence or low level of clotting factor VII or factor IX.

Question 14

Define autosomes

Answer 14

Chromosome pairs that are the same between the sexes; in humans, all but the X and Y chromosomes.

Question 15

Define sex chromosomes

Answer 15

Chromosomes that differ between the sexes; in humans, these represent the X and Y chromosomes.

Question 16

Define sex-linked

Answer 16

Trait controlled by a gene on a sex chromosome; often described as either X-linked or Y-linked.

Question 17

Define X-linked

Answer 17

Allele that is located on an X chromosome; not all X-linked genes code for sexual characteristics.

Question 18

Define color blindness

Answer 18

Inability to detect specific wavelengths of light associated with color; red-green color blindness is the most common type.

Question 19

Define Duchenne muscular dystrophy

Answer 19

Genetic disorder that is characterized by a wasting of muscle tissue; displays an X-linked recessive pattern of inheritance.

Question 20

Define fragile X syndrome

Answer 20

Genetic condition caused by an abnormal number of nucleotide repeats; named after the appearance, and not physical characteristics, of the X chromosome.

Question 21

Define hemophilia

Answer 21

Genetic disorder that is caused by a deficiency of a clotting factor in the blood.

Question 22

An unaffected female marked with an asterisk has an affected father and an unaffected mother and also an affected son who has an unaffected father. Her unaffected brother and sister with unaffected mates have unaffected offspring of both sexes.

Using the letter B for the normal allele, what is the genotype of the lady with the asterisk?
a) x^bx^b
b) x^Bx^b
c) x^Bx^B

Answer 22

b) x^Bx^b