Classical Genetics

Question 1

Who is considered to be “the father of modern genetics,” and which organism did he famously study?

Answer 1

Gregor Mendel, who studied pea plants.

Mendel, pictured here, was a monk who conducted heredity experiments in the 19th century - before the term “gene” had even been coined. Heredity that follows the patterns that Mendel discovered is still termed “Mendelian genetics” today.

Question 2

Define:

genetics

Answer 2

The study of heredity and inherited traits.

On the AP Biology exam, you will likely face questions related to both classical genetics (Punnett squares, dominance and recessivity, etc.) and molecular genetics (DNA replication, nucleotides, etc.).

Question 3

Define:

gene

Answer 3

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 4

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

Answer 4

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

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:

phenotype

Answer 6

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 7

Define:

genotype

Answer 7

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 8

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

Answer 8

Refers to the allele or phenotype that naturally predominates in a population.

For example, experiments involving 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 9

Explain the difference between a homozygous and a heterozygous genotype.

Answer 9

• A homozygous genotype includes two copies of the same allele.
• 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 10

Define:

complete dominance

Answer 10

It is the simplest inheritance pattern. 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

For a particular trait, the dominant allele is denoted as A, while the recessive allele is denoted as a. With regard to this trait, what must be the genotype of a heterozygous individual?

Answer 12

Aa

Heterozygous individuals have one dominant and one recessive allele. Note that a homozygous dominant individual would have a genotype of AA, while a homozygous recessive individual would have a genotype of aa.

Question 13

Name Mendel’s two laws.

Answer 13

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

Mendel also proposed a third law, the Law of Dominance, which explained that dominant alleles will always be displayed, while recessive alleles will be “hidden” when a dominant allele is also present. However, the Law of Dominance is currently categorized as a principle, not technically a law.

Question 14

Explain Mendel’s Law of Segregation.

Answer 14

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 15

Explain Mendel’s Law of Independent Assortment.

Answer 15

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 16

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

Answer 16

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 17

How is a monohybrid cross typically conducted?

Answer 17

It is a genetic cross that studies a single trait. In a typical procedure, an individual who is homozygous dominant for the trait is crossed with a homozygous recessive individual. Their offspring will then be heterozygotes, or “hybrids,” and can be crossed further.

For example, imagine that a parental (P) cross takes place between an RR and an rr organism. All offspring will have an Rr genotype, and crossing these offspring further will yield a 3:1 dominant-to-recessive ratio of phenotypes according to Mendelian prediction.

Question 18

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

Answer 18

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 19

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 19

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 20

What individuals are involved in a dihybrid cross?

Answer 20

This 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 21

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 21

50%

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

Question 22

What is a test cross used to determine?

Answer 22

It 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 23

What does the ratio “3:1” signify?

Answer 23

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 24

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

Answer 24

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 25

Do all inherited traits follow Mendel’s two laws?

Answer 25

No, many inherited traits do not follow Mendel’s laws.

Phenomena that can cause traits to deviate from Mendel’s laws include gene linkage, polygenic inheritance, and extranuclear inheritance. Such traits fall under the general umbrella of “non-Mendelian inheritance.”

Question 26

Explain incomplete dominance and give an example.

Answer 26

It 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 27

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 27

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 28

Explain codominance and give an example.

Answer 28

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

On the AP Biology exam, 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 29

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 29

The probability is 50%.

Heterozygous individuals with type A blood have a genotype of AO, with A being dominant over O. 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 30

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 30

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 31

Define:

polygenic trait

Answer 31

It is a phenotypic trait that depends on the interaction of more than one gene.

On the AP Biology exam, height is a common example of a polygenic trait. Height is determined by multiple genes, which explains why the heights of adult humans vary along a broad spectrum, as opposed to having only two or three possible values.

Question 32

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 32

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 33

If two genes are said to be linked, where must they be located?

Answer 33

On the same chromosome.

It is important to remember that unlike the traits that Mendel studied, alleles for these genes do not assort independently. Instead, since they are on the same chromosome, they are highly likely to be sorted into gametes together. As a result, linked genes do not follow classic Mendelian ratios.

Question 34

For linked genes, how does recombination frequency relate to the distance that separates the genes on the chromosome?

Answer 34

Recombination frequency increases as distance apart increases. In other words, these quantities are directly related.

Recombination frequency denotes the percent of gametes that differ in genotype from the gametes produced in the parental cross, one generation before. With linked genes, recombination frequency is directly related to the chance of a crossing-over event occurring between the linked genes. The farther the genes are apart, the more likely it is that crossing-over will separate them.

Question 35

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

Answer 35

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 36

Define:

epistasis

Answer 36

It is observed when the phenotypic expression of one gene can be “masked” or otherwise regulated by a separate gene.

For example, imagine that fur color in squirrels is determined by two genes, A and B. Gene A determines how dark the fur will be, while gene B codes for the presence of any pigment at all. Even if Gene A codes for dark fur, if gene B codes for a lack of pigment, the squirrel’s fur will be white. In short, the phenotype dictated by gene A has been “masked” by gene B.

Question 37

Define:

pleiotropy

Answer 37

It is observed when a single gene has multiple distinct phenotypic effects.

For example, Marfan syndrome is a condition marked by tall height, flexible joints, and a predisposition to serious heart problems. Marfan syndrome is actually caused by mutations in a single gene, so that gene must simultaneously impact height, flexibility, and heart function or anatomy. Thus, Marfan syndrome exemplifies pleiotropy.

Question 38

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 38

Penetrance refers to the percentage of individuals who express the phenotype expected from their genotype. Since many 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 AP Biology exam show complete, or 100%, penetrance.

Question 39

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 39

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 40

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 40

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 41

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

Answer 41

• 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 42

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

Answer 42

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 43

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

Answer 43

XX

Females inherit an X chromosome from each parent.

Question 44

Which of the two sex chromosomes contains more genetic information?

Answer 44

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 45

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

Answer 45

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 AP Biology exam will be X-linked.

Question 46

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

Answer 46

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 47

Define:

a carrier

Answer 47

In genetics, this is an individual who has one allele for a recessive trait (often a disease) but does not express disease symptoms themselves.

A carrier is typically heterozygous, meaning that her disease-causing allele is “masked” by a dominant, non-disease-causing allele. For example, imagine that a female carries one allele for colorblindness, which is found on the X chromosome. If her other X chromosome carries an allele for healthy vision, she will not be colorblind herself, but she can pass the allele for colorblindness down to her offspring.

Question 48

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 48

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 49

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 49

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 50

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

Answer 50

It 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 51

What characteristic of mitochondrial inheritance deviates from Mendel’s laws?

Answer 51

Mitochondria are only passed down through the maternal line.

Though sperm contain paternal mitochondria, they are quickly destroyed by the egg after fertilization.

Question 52

A woman has a muscle disease and passes it down to her two sons and three daughters. Her brother has the same disease, but none of his children do. Where is the gene for this condition likely located?

Answer 52

The gene is likely mitochondrial.

Unlike nuclear genes, mitochondrial genes are inherited only through the maternal line. A mother can pass the gene on to both sons and daughters.

Question 53

Define:

polymorphism

Answer 53

This 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 54

In humans, the X chromosome is much larger and carries many more genes than the Y chromosome. However, the level of overall gene expression between male and female humans is roughly equal. What process explains this phenomenon?

Answer 54

X-inactivation

Since female humans have two X chromosomes and males have only one, we would expect females to experience significantly more gene expression than males. Instead, one X chromosome in each of a female’s cells is packaged as dense heterochromatin, which is not transcriptionally active. This effectively silences the “extra” X chromosome.

Question 55

Define:

Barr bodies

Answer 55

These are X chromosomes that have been silenced due to X-inactivation. These structures appear as dark, dense spots in the nuclei of cells.

Note that X-inactivation typically occurs only in females, as it is needed to compensate for their possession of one more X chromosome than males.