BIOL 2040

Which of the following best explains why some inherited genetic disorders do not become apparent until adulthood?

A) The mutation is acquired later in life.

B) The mutation is inactive until fertilization.

C) The harmful effects of the mutation accumulate over time before symptoms appear.

D) The mutation is only inherited from the father.

E) The mutation only affects germ cells.

The harmful effects of the mutation accumulate over time before symptoms appear.

Some inherited diseases persist in human populations despite causing severe illness because

affected individuals often reproduce before symptoms develop.

A family has a disorder in which affected individuals develop symptoms at progressively younger ages with each generation. Which mechanism best explains this pattern?

Genetic Anticipation

Which statement regarding genetic anticipation is incorrect?

A) Disease severity often increases over generations.

B) Symptoms tend to appear earlier in successive generations.

C) DNA repeat expansion can underlie anticipation.

D) Every inherited disorder exhibits anticipation.

E) Repeat expansions may produce toxic proteins.

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Every inherited disorder exhibits anticipation.

A mutation affects a gene located within mitochondrial DNA. Which parent is expected to transmit this mutation to offspring?

Mother, Maternal.

Why is mitochondrial inheritance considered extranuclear inheritance?

Mitochondrial DNA is located outside the nucleus

A muscle biopsy reveals that some mitochondria contain mutant DNA while others contain wild-type DNA within the same cell. This phenomenon is known as

Heteroplasmy

Two siblings inherit the same mitochondrial mutation from their mother. One develops severe neurological symptoms while the other experiences only mild muscle weakness. Which concept best explains this observation?

Heteroplasmy

Which tissues are generally affected first by mitochondrial disorders?

Bone and Skeletal Muscle

During early embryonic development in Drosophila, the phenotype is primarily determined by

The mothers genotype through molecules deposited into the egg, this is maternal effect

A mother has genotype bcd⁻/bcd⁻, while her offspring is bcd⁺/bcd⁻. What phenotype is expected during the earliest stages of embryonic development?

Mutant

Why can an embryo with a mutant bcd genotype still develop a normal anterior-posterior axis during early development?

Maternal gene products are deposited into the egg which help during early development.

Which of the following correctly distinguishes homogametic from heterogametic individuals in humans?
A) Females are heterogametic because they produce X and Y gametes.

B) Males are homogametic because they produce only Y gametes.

C) Females are homogametic because they produce only X-bearing gametes.

D) Both sexes are heterogametic.

E) Gametes contain both X and Y chromosomes.

Females are homogametic because they produce only X-bearing gametes.

A scientist observes that a male produces two genetically distinct classes of sex chromosomes in his gametes. This individual is

Heterogametic

In humans, females are described as the homogametic sex because they

normally produce only X-bearing eggs

Which observation provides the strongest evidence that the presence of the Y chromosome rather than the number of X chromosomes usually initiates male development in humans?
A) Individuals with 45,X are usually phenotypically female.
B) Individuals with 47,XXY are usually phenotypically male.
C) Individuals with 47,XXX may have relatively mild phenotypic effects.
D) Typical females have 46 chromosomes.
E) Both males and females possess autosomes.

Individuals with 47,XXY are usually phenotypically male.

A human karyotype is written as 45,X. Which of the following correctly describes this individual?
A) Turner syndrome resulting from monosomy X
B) Klinefelter syndrome resulting from trisomy X
C) A typical male with a deleted Y chromosome
D) An individual possessing two Barr bodies
E) A typical female with one inactive X chromosome

Turner syndrome resulting from monosomy X

Which of the following correctly compares 47,XXY and 47,XYY individuals?
A) Both are usually phenotypically female because both possess more than one X chromosome.
B) Only the 47,XXY individual possesses a Y chromosome.
C) Both are usually phenotypically male because both possess a Y chromosome containing SRY.
D) Both conditions must result from nondisjunction in the mother.
E) Neither condition can result from meiotic nondisjunction.

Both are usually phenotypically male because both possess a Y chromosome containing SRY.

Nondisjunction of the X and Y chromosomes occurs during meiosis I in a human male. Assuming meiosis II proceeds normally, which sperm types would be produced?

XY-bearing sperm and sperm lacking a sex chromosome

Nondisjunction involving the Y sister chromatids occurs during meiosis II in a human male. Which collection of sperm could result from that single meiotic event?

X, X, YY and O

Which of the following best describes the role of the human SRY gene?

It encodes a regulatory factor that initiates a pathway of male sexual differentiation.

A rare individual has a 46,XX karyotype but develops a male phenotype. What is the most likely explanation?

The SRY region was transferred to an X chromosome through abnormal crossing over. This occurs during prophase I

A researcher introduces the mouse equivalent of the SRY gene into a fertilized XX mouse egg. The resulting mouse develops testes and several male characteristics. What conclusion is best supported?

SRY functions as an important developmental switch that activates downstream genes.

Why can the X and Y chromosomes pair and segregate during male meiosis even though most of their DNA is not homologous?

They share homologous pseudoautosomal regions.

Which statement about the human Y chromosome is correct?

Its pseudoautosomal regions can recombine with corresponding regions of the X chromosome.

Without dosage compensation, which problem would be expected in an XX individual compared with an XY individual?

Many X-linked gene products could be produced at approximately twice the level.

Mammalian dosage compensation is achieved primarily through which mechanism?

Condensing and largely inactivating one X chromosome in each XX somatic cell

What is the expected number of Barr bodies in each of the following karyotypes: 45,X; 47,XXY; and 48,XXXY?

0; 1; 2 (N-1=)

Which statement about a Barr body is most accurate?

It is a condensed and largely inactive X chromosome

According to the Lyon hypothesis, which of the following occurs during early mammalian development?

One X is randomly inactivated, and descendant cells generally maintain the same inactive X.

A female cat is heterozygous for an X-linked coat-colour gene, with one allele producing orange fur and the other producing black fur. Why does she develop distinct orange and black patches?

Different cells randomly inactivated different X chromosomes, and their descendants maintained that pattern.

One colour patch on a calico cat is much larger than most of the others. Which explanation is most reasonable?

An early embryonic cell with a particular inactive X produced many descendant cells in that region.

A researcher grows a colony of cells from one fibroblast taken from an XX individual heterozygous for an X-linked enzyme gene. What pattern is expected among cells in the colony?
A) Every cell randomly chooses a new X chromosome to inactivate.
B) All cells generally maintain the same inactive X as the original fibroblast.
C) Both alleles are expressed equally in every cell.
D) The cells lose their inactive X after several divisions.
E) Only the mutant allele can remain active in cultured cells.

All cells generally maintain the same inactive X as the original fibroblast.

Why can individuals with additional X chromosomes show phenotypic effects even though extra X chromosomes undergo inactivation?

Some X-linked genes escape inactivation and remain transcriptionally active