Genetics and Evolutions on the MCAT

Table of Contents

Genes are key stakeholders in determining what happens in our bodies. Genetic variation and evolutionary change are responsible for telling our cells what to do and what to become. Thus, it is important to understand how genetic variation results in large population changes.

This guide will introduce genetics and evolution, key terms and definitions, and things you should remember as you prepare for the MCAT

Let’s get started!

Genetics and evolutions on the MCAT: What You Need to Know?

Genetics and evolution are covered in the Biology section of the MCAT

Introductory biology accounts for 65% of the Biological and Biochemical Foundations of Living Systems section (Bio/Biochem) and 5% of the Psychological, Social, and Biological Foundations of Behavior section (Psych/Soc)

It’s hard to predict the exact number of questions about genetics and evolution that will appear on the MCAT. However, you can expect it to appear in both the Bio/Biochem and Psych/Soc sections.

Important Sub-Topics – Genetics and Evolutions

The theory of evolution, is one of the key players in modern biological theory. It is based on the idea that living things can be traced back to other preexisting types and that the differences are due to minor modifications that occurred over many generations. Genetics is an essential part of the study of evolution; this field of study looks at the inheritance of characteristics from parents to their offspring.

Genetics and evolution can be complicated, so let’s break them down into important sub-topics.

1. Fundamentals & Patterns of Dominance

Genetics can be thought of as the relationship between genes and the physical characteristics of organisms. Here are some main terms you should be familiar with:




Can refer to the genetic constitution of an organism or an individual’s allelic combination


Observable physical characteristics that are coded by the genotype


Describes a phenotype which only requires one allele in order to be expressed


Describes a phenotype which requires more than one allele in order to be expressed


Refers to when the two alleles for a gene are the same


Refers to when the two alleles for a gene are different

Now phenotypes and genotypes have three main types of patterns of dominance:

1. Complete dominance

In complete dominance, a dominant allele can “mask” the recessive allele as only one dominant allele is needed to express that phenotype. There are 3 possible cases shown below using “A” as the dominant allele and “a” as the recessive allele:

  • Homozygous Dominant: AA → “A” Phenotype Expressed
  • Heterozygous Dominant: Aa → “A” Phenotype Expressed
  • Homozygous Recessive: aa → “a” Phenotype Expressed

2. Codominance

This occurs when there is more than 1 dominant allele! In other words, 2 or more     gene phenotypes can be expressed simultaneously!

This is best exemplified through a heterozygous genotype because there will be 2 different alleles in the allelic combination!

3. Incomplete dominance

Occurs when the heterozygous genotype results in an intermediate between the phenotypes coded by the alleles!

A good mnemonic/trick to remember this is to think an “INtermediate” phenotype occurs for “INcomplete” dominance. 

These dominance patterns and fundamental terms can also be applied to a population level through these two terms:




Refers to the percentage or portion of the population that expresses a phenotype given a specific genotype 


Refers to the wide variability of phenotypes that can emerge from a single genotype.

For more in-depth content review on fundamentals and patterns of dominance in genetics, check out these detailed lesson notes created by top MCAT scorers. 

2. Mendelian Genetics: Mendel’s Laws

Mendel’s laws allow us to understand the basics of genetics and inheritance, there are three Mendelian laws outlined in the table below:



Law of Dominance

When two alleles of an inherited pair is heterozygous, then, the allele that is expressed is dominant whereas the allele that is not expressed is recessive

Law of Segregation

During gamete formation, the alleles of a gene will segregate into separate gametes (seen in anaphase I of meiosis)

Law of Independent Assortment

The acquisition of one allele from a gene does not affect what allele is received from another gene. This law allows for equal distribution of gamete formation, no gamete is favored

For more in-depth content review on Mendel’s laws, check out these detailed lesson notes created by top MCAT scorers. 

3. Mendelian Genetics: Disease Inheritance

Disease inheritance can be recessive or dominant and can be further subcategorized into whether they occur on autosomal or sex chromosomes. Disease inheritance can follow many patterns. However, you’ll only be tested on complete dominance for disease inheritance for the MCAT.

If the inheritance pattern is recessive, two recessive alleles are required for the disease phenotype to be expressed. The only exception is for X-linked recessive diseases in males, where only one recessive allele is required due to them being hemizygous.

If the inheritance pattern is dominant, only one dominant allele is required to express the disease phenotype. 

For more in-depth content review on disease inheritance, check out these detailed lesson notes created by top MCAT scorers. 

4. Linked Genetics

The one exception to Mendel’s law is gene linkage. Specifically, this is an exception to independent assortment.

Linked genes refer to the genes on the same chromosome that are located in close proximity to one another. As such, they are more likely to be “paired together” during crossing over and be together in the same gamete, violating Mendel’s independent assortment law.

For more in-depth content review on linked genetics, check out these detailed lesson notes created by top MCAT scorers. 

5. Sex-Linked Genetics

Sex chromosomes are one of the 23 pairs of chromosomes within a human cell; as the name implies, they determine the sex of an organism. They can either be XX (female) or XY (male).

For a male (XY) combination, this is the only case where the chromosomes are not homologous because they do not contain the same genes.

Sex chromosomes are the location of sex-linked genes and traits.  For the scope of the MCAT, you will only be tested on genes and traits located on the X chromosomes, termed X-linked genes and traits. The most common expression pattern you will be tested on is complete dominance, which follows the same rules as autosomal chromosomes! 

However, the male (XY) combination is a special case because males only have one X chromosome and hence only one copy of a gene, termed hemizygous. In this case, they are an exception to complete dominance, as males only need one recessive allele on the X chromosome to express the recessive phenotype. 

For more in-depth content review on sex-linked genetics, check out these detailed lesson notes created by top MCAT scorers. 

6. Punnett Squares

Punnett squares are used to determine the probability and ratio of genotypes and phenotypes of offspring if two parents mate. These punnett squares can either be mono- or di-hybrid crosses.

A monohybrid cross is the simplest cross, with only one trait/gene of interest; a dihybrid cross is just an extension of two traits/genes. When setting up a punnett square, the letter(s) that line the outside on the top and left are the possible gametes that can be formed from the paternal & maternal parents. The letter combinations on the inside represent the possible genotypes of the resulting zygote. 

For more in-depth content review on punnett squares with examples and for more tips, check out these detailed lesson notes created by top MCAT scorers. 

Key Terms and Definitions – Genetics and Evolutions

Here are some of the more important key terms and definitions to remember for this general guide to Genetics and Evolution!




A branch of biology concerned with the study of genes, genetic variation, and heredity in organisms.


Stored and packaged DNA.


One who can pass on the recessive allele to their offspring but does not express the recessive phenotype themselves.

Natural Selection

The process through which individuals with traits that favor their survival and reproduction outcompete individuals less fitted to the living environment.

Genetic Drift

Random, spontaneous allele frequency change that is most prominent when a population faces no selective pressures

Hardy-Weinberg equilibrium

A theoretical state in which no allele frequencies change

Additional FAQs – Genetics and Evolutions on the MCAT

Is there a lot of genetics on the MCAT?

Genetics are included in introductory biology, and since this comprises 65% of the biology and biochemistry section, along with 5% of the chemistry/physics section, genetics are heavily covered on the MCAT! The topic of genetics is extremely vast, but some important concepts include chromosomes, patterns of inheritance, sex-linked genes, genotype, phenotype, the law of independent assortment etc.

Is Evolution Important on the MCAT?

Evolution is definitely an important topic on the MCAT. Evolution is covered in introductory biology which comprises the majority of the biology/biochemistry section and 5% of the chemistry/physics section. Due to introductory biology being such a large part of the biology and biochemistry section, it would be a good idea to understand evolution. Important concepts to cover according to AAMC include – natural selection (fitness concept and selection by differential reproduction) and speciation (polymorphism, adaptations, inbreeding, bottlenecks).

Is Molecular Genetics on the MCAT?

Molecular Genetics is heavily covered in the Biology/Biochemistry section of the MCAT, so it would be a good idea to brush up on these concepts! Similar to evolution and genetics, molecular genetics falls under introductory genetics which is covered in both the biology/biochemistry and chemistry/physics sections of the MCAT. Important concepts to review according to AAMC include – DNA replication, repair of DNA, genetic code, transcription, translation, mutations etc.

What is Autosomal Dominant – MCAT?

Autosomal dominant inheritance is a way in which a genetic trait or condition can be passed down from parent to child. Autosomal dominance happens when one copy of a mutated (changed) gene from one parent can cause the genetic condition. A child who has a parent with the mutated gene has a 50% chance of inheriting that mutated gene.

Additional Reading Links – Study Notes for Genetics and Evolutions on the MCAT

For more in-depth content review about genetics and evolutions on the MCAT, check out these detailed lesson notes created by top MCAT scorers!

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