MCAT Biology : Genes and Chromosomes

Study concepts, example questions & explanations for MCAT Biology

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Genetics

A diploid human cell that is dividing will contain _______ chromosomes. These chromosomes will each consist of _______ chromatids. Fill in the corresponding blanks.

Possible Answers:

46, 2

46, 1

92, 2

23, 1

23, 2

Correct answer:

46, 2

Explanation:

The diploid number is 46 and the haploid number is 23. When cells are dividing, each chromosome is present in duplicate copy. These chromosomes are composed of two chromatids each when they are replicated.

Example Question #1 : Genes And Chromosomes

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

In a normal chromosome 14, what region of the chromosome exists between the p arm and the q arm?

Possible Answers:

Centromere

Telomere

Single nucleotide polymorphism

Exon

Intron

Correct answer:

Centromere

Explanation:

In a normal chromosome, the passage indicates that the p and q arm meet in the center. This central region of the chromosome is known as a centromere.

Example Question #2 : Genes And Chromosomes

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

Untitled

Imagine the child in figure 1 was diagnosed at birth with cystic fibrosis as well as Prader-Willi. Cystic fibrosis is due to a recessive genetic mutation on chromosome 7. Two years later, his parents have another child that has cystic fibrosis, but not Prader-Willi. Which of the following best explains why Prader-Willi and cystic fibrosis are not always inherited together ?

Possible Answers:

Principle of recessivity

Principle of parsimony

Principle of penetrance

Law of independent assortment

Principle of dominance

Correct answer:

Law of independent assortment

Explanation:

The law of independent assortment says that chromosomes, and thus most genes, align independently of each other when being passed from parent to child. In other words, chromosome 7 and chromosome 15 do not directly influence each other's inheritance patterns during meiosis in parental gametes, and can be sent to sperm or eggs in any combination.

Example Question #3 : Genes And Chromosomes

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

Untitled

Based on the information in the passage, which of the following is true of Prader-Willi Syndrome?

I. It must involve a chromosomal deletion on the paternal chromosome 15

II. It must involve normal silencing of maternal chromosome 15

III. It is a sex-linked disorder because it involves chromosome 15

Possible Answers:

I and III

II and III

II only

I only

I, II, and III

Correct answer:

II only

Explanation:

Prader-Willi must involve the silencing of maternal genes on chromosome 15, as well as some disruption of the paternal chromosome. A chromosomal deletion is one example of this kind of disruption, but could also be a nonsense mutation or frameshift mutation that renders the paternal DNA unable to be ultimately translated to protein.

Sex-linked disorders involve the X and Y chromosomes. Prader-Willi is inherited through chromosome 15, and is thus not sex-linked.

Example Question #4 : Genes And Chromosomes

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

Untitled

Chromosome 15 is an autosome. Which of the following is (are) true of all autosomes?

I. They contain histones

II. They determine chromosomal sex

III. They align on the metaphase plate during mitosis

Possible Answers:

I and II

I and III

I only

II and III

I, II, and III

Correct answer:

I and III

Explanation:

Autosomes are the chromosomes that are not sex chromosomes. Any numbered chromosome (1 through 22) is an autosome, while the X and Y chromosomes (the 23rd pair) are the sex chromosomes. Statement II is only true of the X and Y chromosomes. Statements I and III are true of all chromosomes.

Example Question #6 : Genetics

Human chromosomes are divided into two arms, a long q arm and a short p arm.  A karyotype is the organization of a human cell’s total genetic complement.  A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size. 

When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present.  Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome.  Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.

 

In der(14,21), which region of a chromosome might you expect to find in the center of its structure?

Possible Answers:

Telomere

Heavily methylated region

Centromere

Guanine-cytosine rich region

Histone-rich region

Correct answer:

Telomere

Explanation:

Telomeres are present at the ends of chromosomes. If one really understands the passage, one can see that Robertsonian translocation places the ends of chromosomes together to form the middle of the derivative chromosome. We would expect to find telomeres in this region.

Example Question #7 : Genetics

A man with type AB blood marries a woman with type A blood. Which of the following blood types might their sons inherit?

Possible Answers:

Type A or type O

Type A, type B, or type AB

Type AB only

Type A or type AB

Type B or type O

Correct answer:

Type A, type B, or type AB

Explanation:

The genotype of the father is definitely AB. The genotype of the mother is either AA or AO.

Below are the Punnett squares that show that types A, B, and AB are possible. Note that the genotype AO will result in a type A phenotype, while the genotype BO will result in a type B phenotype.

Q3

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: