MCAT Biology : Biology
Study concepts, example questions & explanations for MCAT Biology
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Example Questions
Example Question #1041 : Biology
Seven thousand years ago, a species of oryx indigenous to the Arabian Peninsula was separated when an earthquake caused an insurmountable barrier to form between different geological segments of the population. A recent population genetics study showed that the two populations were no longer able to successfully interbreed.
What type of speciation event does this information indicate?
Parapatric speciation
None of these
Allopatric speciation
Artificial speciation
Peripatric speciation
Allopatric speciation
Allopatric speciation occurs when a physical barrier, or in some cases emigration of subpopulations of a species, prevents interspecies mingling that eventually leads to the inability to interbreed. Sympatric speciation describes a process by which new species form while in the same geographic location. Parapatric and peripatric speciation are subcategories of sympatric speciation.
Example Question #1062 : Mcat Biological Sciences
Passage:
A population of 1200 flamingos exists on an island in the tropics. The flamingos had previously been studied for many years due to their fascinating foraging habits. Many flamingos within this population hunt their prey out in the open, without any attempts to disguise themselves, which some feel predisposes them to being seen by their prey and evaded, or even attacked, sooner than flamingos who more stealthily hunt their prey. Still, so many of these flamingos continue to live viably and reproduce highly successfully, so it has puzzled scientists for years that this is an "evolutionarily successful" strategy. Of the 1200 flamingos initially present, 800 had pink feathers and 400 had white feathers. A tragic hurricane then struck the island, killing all but 10 of the flamingos. 8 of these 10 had white feathers. Years later, scientists again studied the flamings on the island and found a population of 600 flamingos. Of the 600 flamingos, 560 had white feathers and 40 had pink feathers.
Which of the following evolutionary principles best describes the situation present in the passage above?
Genetic Drift
Bottlenecking
Spontaneous Mutation
Selective Mating
Genetic Engineering
Bottlenecking
The situation described is a classic example of the evolutionary principle of "bottlenecking," making this the best answer choice. Bottlenecking describes the phenomenon in which the genetic diversity of a population changes suddenly, often due to a natural disaster, which then results in future generations appearing more genetically and phenotypically similar to one another than in the pre-disaster generations. In this instance, pink flmaginos were initially more common than white flamingos, but both were fairly prevalent. They then encounter a hurricane that kills all but a few flamingos, which were almost entirely white-feathered, which led to all ensuing generations being predominantly white-feathered.
This does not represent "selective mating," "spontaneous mutation," or "genetic engineering," as white-feathered flamingos became most prevalent due to a natural disaster, not white flamingos selectively seeking to mate with other white flamingos, not due to a single mutation, and not due to scientists artificially manipulating the genes within the flamingos.
This does not represent "genetic drift," because the changes in phentype prevalence were not due to random chance fluctuations, they were due to an explainable event, a natural disaster that resulted in an initially low number of white flamingos remaining and becoming most predominant.
Example Question #13 : Evolution
Inbreeding reduces the fitness of a population. This is the result of which increased genetic effect of inbreeding?
Expression of deleterious recessive traits
Rate of spontaneous mutation
Levels of aggression
Genetic diversity
Expression of deleterious recessive traits
Inbreeding increases the expression of recessive traits due to more heterozygous carriers mating with each other. As the same individuals mate, the chance of a homozygous recessive child increases. This is the same as estimating the likelihood of a single healthy child from two carrier parents (0.75) versus eight healthy children from two carrier parents (0.10).
Inbreeding decreases genetic diversity, rather than increasing it. The rate of spontaneous mutation is not impacted by this type of breeding. There is no reason to infer increased levels of aggression.
Example Question #1 : Understanding Hardy Weinberg Equilibrium
Which is not a necessary condition for the Hardy-Weinberg equation to be true?
No net migration of individuals into or out of the population
No natural selection
No mutations in the gene pool
Small population
Random mating
Small population
For the Hardy-Weinberg equation to be true, the population in question must be very large. This ensures that coincidental occurrences do not drastically alter allelic frequencies.
Example Question #2 : Other Evolution Principles
In a population that is in Hardy-Weinberg equilibrium, the frequency of homozygous dominant individuals is 0.36. What is the percentage of homozygous recessive individuals in the population?
The two equations pertaining to Hardy-Weinberg equilibrium are:
In this second equation, each term refers to the frequency of a given genotype. 
From the question, we know that:
We now know the dominant allele frequency. Using the other Hardy-Weinberg equation, we can find the recessive allele frequency:
Returning to our genotype frequency terms, we can use this recessive allele frequency to find the homozygous recessive frequency:
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.
46, 1
92, 2
23, 1
46, 2
23, 2
46, 2
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 : Cell Biology, Molecular Biology, And 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.
A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:
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?
Telomere
Single nucleotide polymorphism
Exon
Intron
Centromere
Centromere
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 : Cell Biology, Molecular Biology, And Genetics
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.
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 ?
Principle of dominance
Law of independent assortment
Principle of penetrance
Principle of recessivity
Principle of parsimony
Law of independent assortment
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 #1 : 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.
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
II and III
I only
I and III
II only
I, II, and III
II only
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 : Cell Biology, Molecular Biology, And Genetics
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.
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
I only
I and II
I, II, and III
I and III
II and III
I and III
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.
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