MCAT Biology : Cell Biology, Molecular Biology, and Genetics
Study concepts, example questions & explanations for MCAT Biology
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Example Questions
Example Question #31 : Genetics
Pea plants have two independently assorted genes that code for seed shape (round or wrinkled) and seed color (yellow or green), respectively. A researcher crosses two pea plants and observes that all F1 offspring have the same phenotype: round shape and yellow seeds. He then performs a test cross with an F1 offspring and observes four different phenotypes in a 1:1:1:1 ratio. Based on this information the researcher concludes the genotypes of the parents.
Which of the following is true regarding a recessive allele?
In a heterozygous offspring both the dominant and recessive alleles are expressed, but dominant allele is expressed more frequently
In a heterozygous offspring the dominant allele is expressed and the recessive allele is suppressed
In a heterozygous offspring the dominant and the recessive alleles are on the same chromosome
In a homozygous recessive offspring both recessive alleles are on the same chromosome
In a heterozygous offspring the dominant allele is expressed and the recessive allele is suppressed
A Mendelian gene generally has two types of alleles: dominant and recessive. An individual always contains two sets of chromosomes (homologous chromosomes) that contain the gene. If both chromosomes contain a dominant allele, then the dominant trait is expressed. If both contain the recessive allele, then the recessive trait is expressed.
If an individual carries both alleles (a heterozygous individual), then only the dominant trait is observed. This occurs because the recessive allele is silenced and is not expressed in the presence of a dominant allele. If the recessive allele is expressed in conjunction with the dominant allele, then it is called incomplete dominance. In normal heterozygous genes, however, the recessive allele is not expressed.
Each chromosome carries only one copy of each gene, and cannot accommodate two alleles for a single trait.
Example Question #32 : Genetics
Pea plants have two independently assorted genes that code for seed shape (round or wrinkled) and seed color (yellow or green), respectively. A researcher crosses two pea plants and observes that all F1 offspring have the same phenotype: round shape and yellow seeds. He then performs a test cross with an F1 offspring and observes four different phenotypes in a 1:1:1:1 ratio. Based on this information the researcher concludes the genotypes of the parents.
Which of the following is the correct pairing of recessive alleles?
Wrinkled and yellow
Wrinkled and green
Round and green
Round and yellow
Wrinkled and green
The passage states that the F1 generation only had round, yellow seeds. Test crossing an F1 offspring lead to an equal ratio of four different phenotypes. When you test cross, you are crossing the F1 offspring with a homozygous recessive individual; therefore, the test cross individual had recessive seed shape and seed color.
???? x aabb
If round and yellow were recessive, then the F2 offspring would all be round and yellow (because you would cross round/yellow with round/yellow).
aabb x aabb (all offspring round and yellow)
If only round was recessive then you wouldn’t get any wrinkled seeds in F2 generation; similarly, if only yellow was recessive then you wouldn’t get any green seeds.
Aabb x aabb (all offspring have one recessive trait, bb)
If round and yellow are both dominant, then means that wrinkled and green are recessive. If the F1 offspring is heterozygous for both traits, then we can see the observed ratios from the test cross.
AaBb x aabb
1 AaBb (round/yellow), 1 aaBb (wrinkled/yellow), 1 Aabb (round/green), 1 aabb (wrinkled/green)
We can conclude that green and wrinkled must be recessive to yellow and round.
Example Question #31 : Genetics
Consider a plant with the following characteristics.
Round leaves (R) are dominant to pointed leaves (r).
White flowers (W) are dominant to pink flowers (w).
Plants heterozygous for both traits are crossed.
What is the probability of obtaining a plant with pointed leaves and pink flowers?
The scenario described is a dihybrid cross. Draw a Punnett Square for the scenario, and identify the frequency of obtaining the genotype (wwrr), which is the only genotype that will display both recessive traits.
You should be familiar with the ratios represented in a dihybrid cross: 9:3:3:1. There will be nine inidividuals with both dominant traits (pointed leaves and white flowers), three individuals dominant for one trait (round leaves and pink flowers), three individuals dominant for the other trait (pointed leaves and white flowers), and only one individual recessive for both traits (pointed leaves and pink flowers).
Example Question #1102 : Mcat Biological Sciences
Consider a plant with the following characteristics.
Round leaves (R) are dominant to pointed leaves (r).
White flowers (W) are dominant to pink flowers (w).
Plants heterozygous for both traits are crossed.
What is the probability of obtaining a plant with pointed leaves and white flowers?
The genotype to obtain pointed leaves is rr, while the genotypes to obtain white flowers are WW or Ww. Determine the frequence of obtaining WWrr or Wwrr using a Punnett Square.
You should be familiar with the ratios represented in a dihybrid cross: 9:3:3:1. There will be nine inidividuals with both dominant traits (pointed leaves and white flowers), three individuals dominant for one trait (round leaves and pink flowers), three individuals dominant for the other trait (pointed leaves and white flowers), and only one individual recessive for both traits (pointed leaves and pink flowers).
Example Question #33 : Genetics
Assume that a trait is expressed according to the mechanism of complete dominance. Each gene contributes one of two possible alleles. Which of the following statements is true?
The genotype is the expressed trait of the individual
If two heterozygous organisms mate, 75% of the progeny will display the dominant phenotype
There are three possible phenotypes the organism can express
If two heterozygous organisms mate, 25% of the progeny will be homozygous
If two heterozygous organisms mate, 75% of the progeny will display the dominant phenotype
If a trait is expressed by complete dominance, the dominant allele will be expressed over the recessive allele. The only time that the recessive trait is expressed is when the organism has two recessive alleles. The genotype is the genetic makeup of an organism, while the phenotype is the physical expression of a trait.
Since there are only two alleles for the trait, there are two distinct phenotypes that the organism can express. A Punnett square reveals that two heterozygous organisms will yield a progeny with 75% displaying the dominant phenotype. 50% of the progeny will be heterozygous, and the other half will be homozygous.
Aa x Aa
Child 1: AA (homozygous, dominant)
Child 2: Aa (heterozygous, dominant)
Child 3: Aa (heterozygous, dominant)
Child 4: aa (homozygous, recessive)
Example Question #31 : Genetics
A condition is known to be X-linked dominant. An affected man and unaffected woman have a son. What is the likelihood that the son will be affected?
The son will be affected
There is a one in two chance that the son will be affected
There is a one in four chance the son will be affected
There is a three in four chance that the son will be affected
The son cannot be affected
The son cannot be affected
Males carry an X-chromosome and a Y-chromosome, while females carry two X-chromosomes. A son, by necessity, must inherit a Y-chromosome from the father and an X-chromosome from the mother. In this case, the affected father cannot possibly transfer his allele to the son because the condition is linked to the X-chromosome. We also know that the trait is X-linked dominant and that the mother is unaffected; since the trait is dominant, she cannot possible be a carrier, and must have two copies of the wild type allele. As a result, the son must inherit a Y-chromosome from the father and a wild type X-chromosome from the mother. The son cannot possibly be affected.
Example Question #31 : Cell Biology, Molecular Biology, And Genetics
A certain type of plant is only tall when it has a heterozygous genotype. If two heterozygous plants are crossed, what is the probability their offspring will also be tall?
0%
75%
100%
25%
50%
50%
The cross between two heterozygous plants (Tt and Tt) will result in 25% TT, 50% Tt and 25% tt.
Example Question #32 : Cell Biology, Molecular Biology, And Genetics
Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear.
Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:
PrPC + PrPRes → PrPRes + PrPRes
The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.
This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:
Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:
Rabbit 1 and 2: injected with normal saline on days 1 and 2
The above trials serve as controls.
Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2
The above trials use unmodified brain matter.
Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2
The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.
Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2
The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.
Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2
The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.
Some patients have a predisposition to developing neurodegenerative disease due to genetic mutations in the PrPC gene. If a father knows that he has the mutation, but never develops the disease, and the mother knows she is not a carrier, what is the probability that their son will develop the disease? Assume that this disease follows simple Mendelian inheritance patterns.
50%
100%
75%
0%
25%
25%
This is a simple Mendelian recessive trait. It must be recessive if the father is a carrier but never affected.
Example Question #22 : Mendel And Inheritance Patterns
In peas, the gene for yellow color (C) is dominant to the gene for green color (c). To determine the genotype of an unknown yellow pea, what kind of pea should you cross with it?
It is impossible to determine
Homozygous dominant
Another unknown green pea
Homozygous recessive (cc) or known heterozygous (Cc)
Any genotype
Homozygous recessive (cc) or known heterozygous (Cc)
We are unsure if our yellow pea is homozygous dominant (CC) or heterozygous (Cc). Crossing it with a homozygous recessive (cc) green pea will yield only yellow peas if it is homozygous dominant, or a mix of green and yellow if it is heterozygous. We could also cross it with a known heterozygote, as we would see the same pattern as if we had crossed with a homozygous recessive.
Example Question #31 : Mendel And Inheritance Patterns
A mother is homozygous for blood type A and a father has blood type AB. Which of the following could describe their child's blood type?
Type A
Type O
Type A or type AB
Type B
Type A or type AB
We are told that the mother is homozygous for blood type A (AA) and the father has blood type AB (AB). Using a simple punnett squre we can find that 50% of the offspring would be blood type A and 50% would be blood type AB.
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