Genetics and Evolution for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about genetics and evolution, plus practice questions and answers
(Note: This guide is part of our MCAT Biology series.)
Table of Contents
Part 1: Introduction to genetics and evolution
Part 2: Mendelian genetics
a) Chromosomes
b) Laws of Mendelian genetics
c) Dominance and recessivity
d) Punnett squares
Part 3: Non-Mendelian genetics
a) Sex-linked genetics
b) Exceptions to complete dominance
c) Genetic linkage
d) Extranuclear inheritance
Part 4: Evolution
a) Natural selection
b) Genetic drift
c) Hardy-Weinberg equilibrium
Part 5: High-yield terms
Part 6: Passage-based questions and answers
Part 7: Standalone questions and answers
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Part 1: Introduction to genetics and evolution
How do our cells know what to do and what to become? Genes play a large role in our development and function and dictate a large portion of what happens in our bodies. Thus, it is important to understand genetics and how differences in individuals with different genetic makeups can drive larger population changes.
In this guide, we will give you a broad overview of the topics that you must know when it comes to genetics and evolution on the MCAT. In addition to understanding the concepts, you will also need to quickly apply them when thinking about genetic relationships at both the individual and population levels. Thus, it is important that you practice with materials involving pedigrees, Punnett squares, and Hardy-Weinberg concepts.
Throughout the guide, several key terms will be emphasized in bold. At the end of the guide, there are also several MCAT-style questions that can be used to test your knowledge.
Let’s begin!
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Part 2: Mendelian genetics
a) Chromosomes
In most cells, genetic information is stored as DNA packaged in the form of chromosomes. (To learn more about the genetic molecules, be sure to refer to our guide on DNA.)
Prokaryotes are organisms without any membrane-bound organelles. Prokaryotic cells include archaea and bacteria. Within these organisms, the DNA molecule is usually circular and unprotected within the cytoplasm.
Eukaryotes are organisms with membrane-bound organelles. These include animals, plants, fungi, and protists—thus including humans as well. Eukaryotic cells contain linear chromosomes of DNA scaffolded in proteins. This DNA is stored in a cell’s nucleus, and each chromosome consists of one single DNA molecule.
Figure: Prokaryotic and eukaryotic chromosomes.
DNA that is transcriptionally active, or relaxed, is referred to as euchromatin. DNA that is transcriptionally inactive, or condensed, is referred to as heterochromatin. In a mature eukaryotic cell, chromosomes are formed by pairs of chromatids that are joined at a centromere. Each chromatid has a long and a short arm. Telomeres are located at the end of each chromatid and are regions of high G-C content. These regions are used as buffer regions to prevent the shortening of the DNA molecule during replication and thus shorten with each round of cell replication. When a number of cell divisions known as the Hayflick limit are reached, the cell can no longer safely replicate and will undergo cell death.
Figure: Structure of chromosomes.
Karyotypes visualize the chromosomes in an organism’s cell. Karyotypes are usually performed in a laboratory, where chromosomes are usually stained under the microscope: sometimes with fluorescent dyes to differentiate the different regions of the chromosome. Photographs of an individual’s condensed chromosomes are taken and digitally manipulated to arrange individual chromosomes by decreasing size.
Figure: Steps of karyotyping.
Karyotyping is useful in detecting chromosomal anomalies that can cause diseases. Chromosomal abnormalities include aneuploidy (the absence of a chromosome or the presence of an extra one), or structural abnormalities (changes in one or more individual chromosomes). Examples of aneuploidy include trisomy 21, also known as Down syndrome, in which an extra copy of chromosome #21 is inherited by an offspring. Structural abnormalities can occur in a wide range of varieties, including the following:
Deletion: in which a segment of a chromosome is missing
Duplication: in which a segment of a chromosome is repeated on the same arm
Inversion: in which two regions on the same chromosome are swapped
Substitution: in which a region on one chromosome is inserted into the arm of another chromosome
Translocation: in which the terminal regions of two different chromosomes are swapped
Additional changes in DNA, also known as a mutation, can occur on a smaller scale. Mutations generally arise as a result of the presence of mutagens in the environment.
A point mutation is a result of the change of a single nucleotide within the DNA sequence. The nucleotide (A, C, T, or G) may be swapped out for another nucleotide at the same location.
A frameshift mutation is a result of either a deleted or inserted set of nucleotides that alter the reading frame of the sequence. (Recall that three-letter codons are translated by the ribosome into amino acids; changing the location of the frame can alter the translated amino acids.)
A mispairing mutation results from the separation and failed re-attachment of two strands of DNA. This mutation results in the mispairing of individual DNA nucleotides: for instance, A failing to bind with T or C failing to bind with G.
The same locus or gene can also give rise to multiple different phenotypes. For instance, a single gene encoding eye color can encode one of multiple different eye colors: blue, brown, green, and so forth. Such a mutation is referred to as a polymorphism.
Humans normally have 23 pairs of chromosomes in each cell: each parent contributes one set of 23 chromosomes. Thus, a normal human karyotype should contain 46 chromosomes. The human genome is composed of 23 pairs of homologous chromosomes, implying that each paternal and maternal chromosome contains the same sets of genes. Humans and most other mammals are diploid organisms, in which ploidy refers to the number of sets of chromosomes found in a cell.
A minor exception to this rule is the 23rd pair of chromosomes, known as the sex chromosomes. These chromosomes determine the genetic sex of an individual: females have 2 X chromosomes (karyotype XX), while a male has an X chromosome and a smaller Y chromosome (karyotype XY). The other 44 chromosomes are known as autosomes and are numbered from pairs 1 to 22.
Figure: An example of a human karyotype.
If all humans have the same set of chromosomes that carry the same sets of genes, how do our individual differences arise? One of the most important sources of variation lies in the concept of alleles, or gene variants. While members of the same species have the same genes, these genes can vary in DNA sequence between individual members and even between the two copies within a diploid cell. Multiple alleles can exist for a single gene, each with potentially different characteristics. Each human cell carries at least 2 alleles of each gene, and these can either be identical or unique.
While all alleles for a gene are found at the same position, or locus (plural: loci) on a chromosome, variations in gene sequences confer different characteristics for gene activity. It is this complex interplay between the different genetic profiles of alleles, or genotypes, that leads to the different physical characteristics, or phenotypes, of different individuals.
b) Laws of Mendelian genetics
Gregor Mendel, a 19th century monk, conducted many important studies on the genetic inheritance of traits using pea plants. Mendel proposed a number of laws on the heredity of traits, which have since been refined and confirmed by modern discovery. Several of Mendel’s laws dictate the events that occur during meiosis, a key step during sexual reproduction in which haploid gametes are formed.
The law of segregation states that the two allelic copies of genes in a diploid organism are equally segregated into gametes, such that the two gametes that are formed both receive one of the pairs of alleles. Since diploid organisms carry two copies of all genes, this applies to all genes present during meiosis I: alleles are equally distributed to daughter gametes from a dividing mother cell.
The law of independent assortment states that the separation of the alleles of one gene occurs separately and uniquely from the separation of alleles of another gene. Thus, different genes pass on alleles to gametes without affecting the assortment of other genes. The independent assortment of homologous chromosomes prior to anaphase allows alleles to be differently distributed into their respective daughter cells, creating variation within the gametes themselves.
Figure: The Law of Segregation and Law of Independent Assortment.
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