Chapter 6 Genetics Mastering

What is the leading cause of Down syndrome?

The nondisjunctional event that produces Down syndrome occurs more frequently during oogenesis in women older than age 35.

Which of the following statements about the products produced when nondisjunction occurs during meiosis I is true?

Two products of the second meiotic division have both the maternal and paternal chromosomes of a set and the other two products have none for that set.

Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. While there is reduced fertility in both sexes, females have higher fertility than males. Van Dyke et al. (1995; Down Syndrome Research and Practice 3(2):6569) summarize data involving children born of Down syndrome individuals. Given the fact that conceptuses with 48 chromosomes (four #21 chromosomes) are not likely to survive early development, what percentage of surviving offspring would be expected to have Down syndrome if both parents have Down syndrome?

Two-thirds of the surviving offspring would be expected to have Down syndrome.

Although the most frequent forms of Down syndrome are caused by a random error, nondisjunction of chromosome 21, Down syndrome occasionally runs in families. The cause of this form of familial Down syndrome is ________.

a translocation between chromosome 21 and a member of the D chromosome group

Given that a human normally contains 46 chromosomes, give the chromosome number for each of the following conditions: Turner syndrome (female, no Barr bodies) Klinefelter syndrome (male, one Barr body)

45,47

The condition known as cri-du-chat syndrome in humans has a genetic constitution designated as ______

46, 5p-

With respect to the chromosomes involved in the translocation, 14 and 21, what is the total number of different gametes possible for a heterozygous carrier parent to produce?

6

Given that a human normally contains 46 chromosomes, give the chromosome number for each of the following conditions: Triploid Trisomy 13

69,47

What is formed when meiosis occurs in an individual who is heterozygous for an intercalary deletion?

A compensation loop

Which of the following conditions results from a Robertsonian translocation?

Familial Down syndrome

Which of the following statements is TRUE?

Familial Down syndrome is caused by a translocation involving chromosome 21.

Which of the following syndromes is not paired with its causative chromosomal aberration?

Fragile X syndrome - deletion

Which of the following accurately describes a possible meiotic nondisjunction event?

Homologs fail to separate during meiosis I.

Which of the following statements regarding familial Down syndrome is false?

If one parent is a translocation heterozygote for chromosomes 14/21, a child born to this parent is as likely to have Down syndrome as he is to be normal.

What distinguishes paracentric inversions from pericentric inversions?

Inclusion of the centromere in the inversion

Which of the following statements about nondisjunction is false?

It separates maternal from paternal chromatids.

What explanation is generally given for lethality of monosomic individuals?

Monosomy may unmark recessive lethals that are tolerated in heterozygotes carrying the wild-type allele.

The incidence of Down syndrome, also known as trisomy 21, increases with increasing maternal age. Which of the following errors most likely produces this condition?

Nondisjunction during either meiosis I or II in the female gamete

If this trisomic fly is the progeny of a male fly of genotype (ey+ ey−, gw+ gw−) crossed to a female fly with genotype (ey− ey−, gw− gw−), what can you conclude about the events that led to its formation?

Nondisjunction occurred in the male parent at the meiosis I division to produce an (n+1) sperm that fused with a normal (n) egg.

What phenotype would be expected in balanced translocation heterozygotes in the absence of position effects?

Normal, because they have a normal amount of genetic material

Trisomy 21, or Down syndrome, occurs when there is a normal diploid chromosomal complement but one (extra) chromosome 21. Although fertility is reduced in both sexes, females have higher fertility rates than males. Van Dyke et al. (1995; Down Syndrome Research and Practice 3(2): 65-69) summarize data involving children born of Down syndrome individuals. Assume that children are born to a female with Down syndrome and a normal 46-chromosome male. What proportion of the offspring would be expected to have Down syndrome?

One-half of the offspring would be expected to have Down syndrome.

A chromosome without a centromere is _______

acentric

The condition that exists when an organism gains or loses one or more chromosomes but not a complete haploid set is known as ________.

aneuploidy

What is the general term for a condition in which the chromosome number is not a multiple of a complete set?

aneuploidy

Assume that a species has a diploid chromosome number of 24. The term applied to an individual with 25 chromosomes would be ________.

aneuploidy and trisomy

How do deletions and translocations in chromosomes often occur?

chromosomes will break and the sticky ends will rejoin

Which genetic disease is caused by a partial deletion within chromosome 5?

cri du chat syndrome

Having a complete set or sets of chromosomes is called __

euploid

Females with only one X chromosome do not develop; this condition is lethal. t or f

false

In order to create the possibility of generating a trisomy, nondisjunction must occur during meiosis II. t/f

false

What type of chromosomal configuration does the following diagram illustrate?

inversion (paracentric) heterozygote

What is the specific term that describes the loss of a single chromosome (2n - 1)?

monsamy

________ is viewed as a major cause of aneuploidy.

nondisjunction

A child is born with Turner's syndrome and she is red green color blind (recessive X-linked). Her father is red green color blind as well and her mother is homozygous dominant for color sight. What had to happen in order for this child to be born with this chromosomal complement?

nondisjunction in either meiosis I or meiosis II of the mother

An individual with Patau syndrome would be ________.

said to have a trisomy

The chromosomal aberration that causes cri-du-chat syndrome can be referred to as a(n) _______

segmental deletion

A Robertsonian translocation is considered non-reciprocal because _______.

the smaller of the two reciprocal products of translocated chromosomes is lost

Which of the following is not a potential outcome of a gene duplication?

they may lead to translocation cross formation during synapsis

Duplications and deletions can be detected during meiosis by the presence of extrachromosomal loops that do not pair properly with their homolog. t/fk

true

All of the following events occur during normal meiosis except _______.

two haploid gametes fuse to form a diploid cell

What error of meiosis leads to both a duplication and a deletion?

unequal crossing over