Genetics

Question 1

Define:

gene

Answer 1

A gene is a nucleic acid sequence that determines some trait of an organism. In eukaryotes, genes are composed of DNA and located on chromosomes.

Many aspects of inheritance were discovered by Gregor Mendel, though the term “gene” had not yet been coined. Instead, Mendel called genes “factors.”

Question 2

Define:

phenotype

Answer 2

A phenotype refers to a physical or observable characteristic of an organism, determined by its genotype.

For example, if a pea plant is homozygous for the Y allele and is yellow in color, “yellow” would be its phenotype.

Question 3

Define:

genotype

Answer 3

A genotype refers to the actual set of alleles possessed by an organism.

For example, if a pea plant is homozygous for the Y allele and is yellow in color, “YY” would be its genotype.

Question 4

What term describes the position of a particular gene on a chromosome?

Answer 4

The chromosomal position of a gene is called a locus (plural: loci).

Each locus falls at the same relative position in a species. This allows the construction of genetic maps.

Question 5

Define:

allele

Answer 5

An allele is a variation of a specific gene, usually denoted by a single letter. Humans always have two alleles at each genetic locus.

For example, the B and b alleles might code for brown and blue eye color, respectively.

Question 6

Define:

polymorphism

Answer 6

Polymorphism occurs when a single species displays discrete phenotypic forms, such as Mendel’s green and yellow peas.

At the DNA level, polymorphisms refer to minor variations in the sequence of a gene that occur relatively frequently in a population.

Question 7

In dogs, the C locus determines the extent to which melanin is expressed. Possible alleles at this locus include C (full expression), C^b (gray fur), and c (albino), among others. What inheritance pattern does this trait exemplify?

Answer 7

This is an example of multiple allelism, in which three or more alleles exist for each a particular trait. Note that a normal individual still cannot possess more than two alleles per locus.

Question 8

Explain the difference between a homozygous and a heterozygous genotype.

Answer 8

A homozygous genotype includes two copies of the same allele, while a heterozygous individual possesses two different alleles.

Let’s say that a plant species has two alleles that determine height, T (tall) and t (short). TT and tt individuals would be homozygous while Tt organisms would be heterozygous.

Question 9

What is the term “wild type” used to signify?

Answer 9

Wild type refers to the allele or phenotype that naturally predominates in a population.

On the MCAT, experiments with bacteria often compare a wild type strain with one or more sets of mutants. Wild type bacteria exist as they would in nature, while mutants either lack a normal function or gain an abnormal one.

Question 10

Define:

complete dominance

Answer 10

Complete dominance is the simplest inheritance pattern tested on the MCAT. One allele is dominant, meaning that it determines the phenotype whenever present. The other allele is recessive and only affects the phenotype when the dominant allele is not present.

Mendel observed complete dominance in his experiments with pea plants.

Question 11

In a certain population, the R allele codes for red color and the r allele codes for white color. If 100% of individuals with an Rr genotype appear red, how would we describe the r allele?

Answer 11

The r allele must be recessive.

In complete dominance, heterozygotes always display the dominant phenotype. In this example, that allele would be R. The recessive allele is completely “masked” in such cases.

Question 12

A female child is born with cystic fibrosis, but neither her father nor her mother suffers from the disease. What pattern of inheritance must cystic fibrosis display?

Answer 12

Cystic fibrosis must be autosomally recessive. With any mode of inheritance, at least one parent must have the disease allele for the child to suffer from the disease. Since neither parent has cystic fibrosis, another allele must have the ability to mask its action.

How do we know this disorder is not sex-linked? The child suffering from CF is female, meaning that she would need both X chromosomes to carry the CF allele. Since her father is healthy, he must possess a normal X chromosome, which he would pass down to his daughter.

Question 13

Explain codominance and give an example.

Answer 13

Codominance is an inheritance pattern in which two alleles contribute equally to an individual’s phenotype.

On the MCAT, the most common example of codominance occurs in blood typing, where three alleles (A, B, and O) exist. Both A and B are dominant over O, and both will be expressed simultaneously in an AB individual.

Question 14

A woman is heterozygous for blood type but displays the type A phenotype. If this woman has a child with an AB man, what is the probability that this child will carry two dominant alleles with regard to blood type?

Answer 14

The probability is 50%.

First, note that the A and B alleles are both dominant over O. Heterozygous individuals with type A blood have a genotype of AO, with A being dominant over O. Therefore, the woman in this question must have an AO genotype. The cross described in this question is pictured below. The child has a 50% chance of being either AA or AB (two dominant alleles), leaving a 50% probability of either AO or BO (one dominant and one recessive allele).

Question 15

Explain incomplete dominance and give an example.

Answer 15

Incomplete dominance is an inheritance pattern in which the heterozygous phenotype is a blend of the two homozygous traits.

The most familiar example of incomplete dominance involves flower color. In this pattern, if RR individuals display red coloring and rr individuals are white in color, then Rr flowers would be expected to be pink.

Question 16

The gene for height in a wild horse species exhibits incomplete dominance, with tall, medium, and short phenotypes. When a tall horse is crossed with the offspring of a tall and a short horse, what potential phenotypes can result?

Answer 16

Offspring from this cross can display either tall or medium phenotypes.

Use T to denote the “tall” allele and t to denote the “short” one. The tall parent must have a genotype of TT. Though the genotype of the second parent is less obvious, it results from the cross of a tall (TT) and a short (tt) horse. The second parent, then, must have a Tt genotype. Crossing a TT and a Tt horse can only result in two phenotypes: the same as those of the parents, tall (TT) and medium (Tt).

Question 17

Define:

genetic leakage

Answer 17

Leakage occurs when genes from one species travel into the gene pool of another.

Question 18

The dominant allele K codes for a kidney disease, but 20% of individuals with the K allele have normal kidney function and no other symptoms. What term in genetics is most relevant to this example?

Answer 18

Penetrance refers to the percentage of individuals who express the phenotype expected from their genotype. Since 20% of people in this example possess the disease phenotype but do not have the disease, the K allele is not completely penetrant.

Unless mentioned otherwise, it can be assumed that all alleles on the MCAT show complete, or 100%, penetrance.

Question 19

The dominant allele M codes for a heart malformation. All individuals with the M allele have the disorder, but some die immediately after birth while others experience few symptoms. What term in genetics is most relevant to this example?

Answer 19

Expressivity refers to the extent to which an individual’s phenotype is affected by their genotype, and is often a range. Since those with the M allele can range from very sick to fairly healthy, this allele shows variable expressivity.

Expressivity is more nuanced than penetrance. While penetrance simply describes whether someone has a trait at all, expressivity describes the range between the most and least extreme examples of the trait.

Question 20

A certain recessive immune disorder exhibits reduced penetrance. If a man with the disorder has a son with a female carrier, what is the probability that this child will suffer from the disease?

Answer 20

Not enough information is given.

While the description of the parents is sufficient to predict the child’s genotype, this disease does not display full, or 100%, penetrance. Therefore, even a child with two recessive alleles might not suffer from the disease at all. To find the answer, it is necessary to know how penetrant this disease is, which is never mentioned.

Question 21

Name Mendel’s two laws.

Answer 21

• the Law of Segregation (1^st Law)
• the Law of Independent Assortment (2^nd Law)

Question 22

What term refers to the set of all genes present in a population?

Answer 22

The gene pool

A diverse gene pool allows a species to more easily adapt to changes in its environment. This quality is referred to as genetic variation.

Question 23

Explain Mendel’s Law of Segregation.

Answer 23

An organism carries two alleles for each trait, but these alleles “segregate” during the formation of gametes. Thus, a parent organism will only pass one allele per trait to its progeny.

Though Mendel did not know this at the time, segregation of alleles occurs during anaphase of meiosis I.

Question 24

Explain Mendel’s Law of Independent Assortment.

Answer 24

Mendel hypothesized that the inheritance of one trait will be unaffected by another. In other words, alleles at different loci assort independently.

This law only holds true when genes are not located on the same chromosome.

Question 25

What is a test cross used to determine?

Answer 25

A test cross is used to find the genotype of an individual with a dominant phenotype. The unknown organism is crossed with a homozygous recessive individual. If any of the offspring are recessive, the unknown parent must be heterozygous.

A test cross would be unnecessary for an organism with a recessive phenotype. Such individuals can only have one genotype: homozygous recessive.

Question 26

Label the diagram below with the P, F₁, and F₂ generations.

Answer 26

P refers to the parent generation, shown in the first cross. F₁ is the generation made up of their offspring, while F₂ is the generation that results when F₁ individuals are crossed.

Question 27

Describe the proper setup of a Punnett square for a single-trait cross.

Answer 27

1. Draw a large square divided into four smaller quadrants.
2. Along the top of the Punnett square, write the first letter of the first parent’s genotype above the left-hand column. Write the second letter above the right-hand column.
3. Along the left side of the square, do the same for the other parent’s genotype, now with one letter corresponding to each row.
4. Fill in the smaller boxes with the corresponding letters - one from the top of the box, one from the left side.
5. Each quadrant now contains a potential genotype for the offspring.

Question 28

A condition that causes dwarfism displays incomplete dominance. BB individuals grow to normal height, Bb individuals have short limbs, and those with the bb genotype do not survive long after birth. Using a Punnett square, determine the probability that two parents with dwarfism will have a bb baby as their first child.

Answer 28

The probability is 25%.

Each parent has dwarfism, meaning that they are both heterozygous (Bb). The Punnett square below shows that a cross of two heterozygous individuals has a 1/4 chance of producing homozygous recessive offspring.

Question 29

What individuals are involved in a dihybrid cross?

Answer 29

A dihybrid cross involves two separate traits. Specifically, both parents must be dihybrids, or heterozygous for both traits being observed.

Often, a dihybrid cross is preceded by crossing two strains that are pure-breeding (homozygous) for different traits. For example, a cross of AABB and aabb parents will yield 100% AaBb offspring, which are dihybrids and can be further crossed.

Question 30

What does the ratio “3:1” signify?

Answer 30

Assuming complete dominance, 3:1 reflects the ratio of dominant to recessive phenotypes in the offspring of a monohybrid cross.

For example, consider a cross between two heterozygous (Rr) organisms. Of the offspring, ¼ will be RR and ½ will be Rr, combining for a ¾ chance of exhibiting the dominant phenotype. Only the ¼ with the rr genotype will display the recessive phenotype.

Question 31

What does the ratio “9:3:3:1” signify?

Answer 31

Assuming complete dominance, 9:3:3:1 reflects the ratio of phenotypes obtained in a dihybrid cross.

Of every 16 offspring, 9 will display both dominant phenotypes. 3 will display one (Trait A) but not the other (Trait B), while an additional 3 will display Trait B but not Trait A. 1 individual of 16 will exhibit both recessive phenotypes.

Question 32

An organism with the genotype GgAA is crossed with a ggAa partner. What fraction of their offspring will have a genotype that matches one of the parents?

Answer 32

50%

As this Punnett square shows, the offspring have a 50% chance of having either a GgAA or ggAa genotype.

Question 33

In his famous experiments, Mendel did not consider many complex methods of inheritance. Which of Mendel’s laws is broken in cases that involve linked genes?

Answer 33

Linked genes violate Mendel’s Law of Independent Assortment.

Mendel hypothesized that the inheritance of one trait was unaffected by that of another, or that alleles “assorted independently.” However, genes are inherited as parts of chromosomes, not as individual alleles. If two genes are linked, or located on the same chromosome, they will likely assort into gametes together.

Question 34

Which process of genetic recombination occurs during prophase I of meiosis?

Answer 34

Crossing over, or the trading of DNA segments between homologous chromosomes, occurs when these chromosomes are paired together in prophase I.

Question 35

Explain the significance of single vs. double crossovers during meiosis.

Answer 35

A single crossover involves one double-stranded break and the exchange of one piece of genetic material.

A double crossover requires two breaks to occur, often resulting in a piece of material crossing back over to its original position.

Question 36

What is the maximum possible recombination frequency between two linked genes?

Answer 36

50%

Even unlinked genes, or those located on different chromosomes, will only form recombinant gametes 50% of the time. Linked genes, even those located very far apart on the same chromosome, cannot exceed this frequency.

Question 37

How many chromosomes do human cells contain?

Answer 37

Human cells contain 46 chromosomes.

Human chromosomes are organized into 23 homologous pairs, the members of which code for the same traits but are genetically different. Corresponding alleles are located on homologous chromosomes.

Question 38

What is the difference between a sex chromosome and an autosome?

Answer 38

Sex chromosomes determine whether an individual is male or female; human cells contain a single pair for a total of two chromosomes. The remaining 22 pairs, or 44 chromosomes, are classified as autosomes.

Unlike autosomal pairs, the sex chromosomes X and Y are not fully homologous.

Question 39

What is the genotype of a male with regard to sex chromosomes?

Answer 39

XY

Since only males possess Y chromosomes, each man inherits his Y from his father. Therefore, his X chromosome invariably comes from his mother.

Question 40

What is the genotype of a female with regard to sex chromosomes?

Answer 40

XX

Females inherit an X chromosome from each parent.

Question 41

Which of the two sex chromosomes contains more genetic information?

Answer 41

The X chromosome is much larger and contains many more genes than the Y chromosome.

The most notable gene on the Y chromosome, SRY, is integral in the determination of male sexual development.

Question 42

Which sex chromosome is more commonly associated with sex-linked genes?

Answer 42

Since it carries much more genetic material, the X chromosome is generally associated with sex-linked disorders.

In fact, virtually all sex-linked traits on the MCAT will be X-linked.

Question 43

Which gender is more likely to suffer from X-linked disorders?

Answer 43

Males, who possess only one X chromosome, show a higher incidence of X-linked disorders. Since females have two X chromosomes, they are able to mask a single disease-carrying allele with a normal one.

Note that males cannot inherit X-linked disorders from their fathers, since men always donate their Y chromosome to their sons.

Question 44

A man’s mother and his father’s mother both suffer from colorblindness, an X-linked condition. What is the percent chance that the man is colorblind as well?

Answer 44

100%

Since a male acquires his Y chromosome from his father, his inheritance of X-linked disorders depends on his mother alone. If this man’s mother suffers from colorblindness, then both of her X chromosomes code for the disease and the man is guaranteed to have it as well.

Question 45

What is extranuclear inheritance, and with which eukaryotic organelle is it generally associated?

Answer 45

Extracellular inheritance refers to the transfer of DNA that is not located in the nucleus. In eukaryotes, this is associated with mitochondria, which contain their own genes.

Mitochondrial genes are inherited through the maternal line.

Question 46

What is a mutation, and how does it arise?

Answer 46

A mutation is an alteration in DNA sequence. Mutations can arise spontaneously (often from errors in DNA replication) or be induced by mutagenic substances.

For the MCAT, remember that mutations never arise in response to selective pressure. For example, a resistant strain of bacteria did not mutate to become resistant; the mutation arose spontaneously and persisted due to its evolutionary advantage.

Question 47

A certain mRNA sequence reads 5’-AUGCCAUGU-3’. In this context, what term describes AUG, CCA, and UGU?

Answer 47

Those three sequences are codons. A codon is a set of three nucleotides that corresponds to a particular amino acid.

Question 48

In the replication of a DNA strand, an adenine base is included instead of a cytosine. The rest of the strand is replicated normally. What type of mutation occurred?

Answer 48

Since one base was replaced with another, a substitution mutation occurred.

Such a mutation can also be described as a point mutation, since only a single base was altered.

Question 49

Name two types of mutation that lead to a frameshift.

Answer 49

A frameshift mutation can be caused by:

• Insertion, or the addition of one or more nucleotides
• Deletion, or the removal of one or more nucleotides

Note that the insertion or deletion of a multiple of three nucleotides would not result in a frameshift.

Question 50

Which is more likely to be harmful: a base substitution or a frameshift mutation?

Answer 50

A frameshift mutation is much more likely to adversely affect the organism.

While a single-base substitution only changes at most a single codon, a frameshift causes the entire reading frame to be moved. This alters many or even all of the amino acids after the aberration, resulting in a drastically different or nonfunctional protein.

Question 51

What is the result of a missense mutation?

Answer 51

A missense mutation replaces one amino acid with another.

For example, the codon AGU codes for the placement of a serine residue, while AGG codes for the placement of arginine. A simple substitution of G for U, then, can result in a missense mutation.

Question 52

What is the result of a nonsense mutation?

Answer 52

A nonsense mutation transforms an amino acid into a stop codon, resulting in a shortened and often nonfunctional protein.

The three stop codons are UGA, UAA, and UAG. The start codon is AUG, which corresponds to methionine.

Question 53

Which is more likely to be harmful: a missense or a nonsense mutation?

Answer 53

A nonsense mutation is much more likely to adversely affect the organism.

While a missense mutation only changes a single amino acid to another, a nonsense mutation causes the entire process of translation to be prematurely stopped. This results in a shorter and often functionally useless protein.

Question 54

With regard to mutations, what is an inversion?

Answer 54

An inversion is a chromosomal mutation in which a nucleotide sequence is completely reversed.

Question 55

With regard to DNA, what is a translocation?

Answer 55

A translocation is a chromosomal mutation in which a nucleotide sequence moves to another position, either within the same or between different chromosomes.

Question 56

A student argues that a species without mutations would be evolutionarily favorable, since its genetic code would be free of errors. Describe the major flaw in this student’s reasoning.

Answer 56

Mutations provide genetic variation, which is evolutionarily vital.

In bacteria, for example, mutations can result in the survival advantage of antibiotic resistance. Without these mutations, the entire species would be equally susceptible to attack.

Question 57

Define:

degeneracy

Answer 57

Degeneracy refers to the property of the genetic code that allows multiple codons to code for the same amino acid.

Specifically, there are 64 possible codons and only 20 relevant amino acids. Codons for the same amino acid often differ at the position of their third base.

Question 58

Some changes in DNA sequence, known as silent mutations, do not affect the organisms in which they occur. Describe two situations that could lead to a silent mutation.

Answer 58

A silent mutation could be caused by:

• A mutation in an intron, or noncoding, sequence
• A mutation that replaces one degenerate codon with another

Question 59

A mutagen is a chemical or other substance that can induce changes in DNA. In what related category are many mutagens included?

Answer 59

Mutagens are often classified as carcinogens, or cancer-causing substances.

Question 60

What is genetic drift?

Answer 60

Genetic drift describes changes in allele frequencies that result from random events.

Often, genetic drift involves situations where part of the population experiences some disaster or becomes isolated from the rest. For example, consider a rat population where allele freqencies for coat color are fairly even. If a flood happens to kill most of the white rats, the resulting decrease in the “white” allele is a result of genetic drift.

Question 61

Name three events or mechanisms that increase a population’s genetic diversity.

Answer 61

Many mechanisms serve to increase genetic diversity. Three of the most prevalent are:

• Mutation
• Migration between populations
• Sexual reproduction, especially homologous recombination (crossing over)

Question 62

In a hypothetical situation, allele frequencies in a population remain unchanged over multiple generations. Though this condition does not occur naturally, it is often used as the basis for mathematical models. What term describes this situation?

Answer 62

This situation is called Hardy-Weinberg equilibrium.

Question 63

Name the five prerequisites for Hardy-Weinberg equilibrium.

Answer 63

Hardy-Weinberg equilibrium requires five conditions:

1. Large population size
2. No mutation
3. No inward or outward migration
4. No natural selection
5. Random mating

Question 64

A very large, mutation-free population of finches is entirely isolated within their geographic area. Males tend to grow bright feathers to attract potential mates. What condition(s) of Hardy-Weinberg equilibrium does this population violate?

Answer 64

This population violates the condition of random mating.

A large population size, lack of mutation, and lack of migration all fit Hardy-Weinberg criteria. However, if mates are chosen based on certain characteristics, mating is not random and equilibrium is violated.

Question 65

What two equations can be used when a population is assumed to be in Hardy-Weinberg equilibrium?

Answer 65

The main Hardy-Weinberg equation is p² + 2pq + q² = 1. Since this statement assumes that only two alleles are present, and these alleles are represented by “p” and “q,” p + q = 1 is also relevant.

For these equations to hold true, the proportion of each allele must be described in decimal form. For example, if the A allele comprises 36% of the gene pool, its frequency would be 0.36.

Question 66

In the Hardy-Weinberg equation, what variable or expression refers to the frequency of the dominant allele in the gene pool?

Answer 66

p

When dealing with Hardy-Weinberg, remember that alleles are denoted by single letters. Conventionally, the dominant allele is “p” and the recessive allele is “q.”

Question 67

In the Hardy-Weinberg equation, what variable or expression refers to the frequency of the heterozygous genotype in the gene pool?

Answer 67

2pq

Remember that genotypes in the Hardy-Weinberg equation are denoted by expressions, not single letters. p² represents the proportion of homozygous dominant individuals, while q² represents the frequency of the homozygous recessive genotype.

Question 68

A and a are the only two alleles that exist for a certain trait. In a stable population, 9% of individuals have the aa genotype. What percentage of individuals are heterozygous?

Answer 68

42%

The frequency of the homozygous recessive genotype, q², is 0.09. From this information, it can be found that q = 0.3. Since p + q = 1, p must be equal to 0.7. The heterozygote frequency, 2pq, is thus (2)(0.7)(0.3) or 0.42.