Study Plans


Knowledge Base


Biological and Biochemical Foundations of Living Systems

Mendelian Concepts

The concepts of Mendelian genetics are based on observations made in the 19th century of traits (e.g. color, shape) that were expressed in pea plants across generations as a result of cross and self-pollination. Consistent ratios of different versions of these traits (e.g. purple/white, wrinkled/smooth) suggested a system of genetic inheritance that balanced determinants for different types.

Phenotype and genotype

An organism's phenotype is an observable characteristic, the expression of a specific trait, while its genotype is a reference to the inherited factors, the underlying genes the organism has.


A gene is the genetic material responsible for a particular trait.


Genes have been identified as having specific positions on the chromosome. This specific location on the chromosome is called a locus and is the same on both inherited chromosomes carrying a specific gene.

Allele: single and multiple

A specific version of a gene is called an allele. A gene may have a single allele, meaning there is only one version of the gene, which would be sensible for a gene that encodes for a critical enzyme. Alternatively, a gene may have multiple alleles or different versions that could be present in an individual depending on the versions in the parental generation, such as the case of blood type.

Homozygosity and heterozygosity

An individual, inheriting a set of chromosomes from each parent (23 + 23 = 46 in humans), therefore inherits two alleles for every gene. Homozygosity is the conditioning of inheriting the same allele on both copies of a gene. Heterozygosity is the condition of inheriting two different alleles for a gene. An individual can then be designated a homozygote or heterozygote in terms of a specific gene or trait. A homozygote will only pass on one particular allele, whereas a heterozygote presents a probability of passing one or the other to the next generation.


The most common form of an allele is called the wild-type. Alternate, less common forms may be referred to as mutants.


Recessiveness is a characteristic of phenotypic expression for certain traits where one allele has a masking effect over the other. In these cases, the phenotypically stronger allele is referred to as dominant while the other allele, which is masked, is referred to as recessive. In a genotype expression these are represented as a capital letter or lower case letter, respectively. Thus dominant traits (phenotypes) may have an underlying genotype that is either homozygous (AA) or heterozygous (Aa), but recessive traits always have a homozygous genotype (aa).

Complete dominance

In the case of complete dominance, in an individual with a heterozygous genotype, the presence of a dominant allele altogether determines the phenotype, blocking the effect of a recessive allele. For traits determined by complete dominance, both alleles must be recessive for the recessive phenotype to be expressed.


Co-dominance is the phenotypic expression where both of two different alleles have an effect. An AB blood type is an example where neither the A-allele or B-allele dominates the other, but they both have a phenotypic presence. These alleles are written in a genotype as two capital letters with superscripts to differentiate them.

Incomplete dominance, leakage, penetrance, expressivity

The case of incomplete dominance produces an alternate phenotype different from that created by either of two different alleles on their own (e.g. a heterozygote with one allele for red flowers and another allele for white flowers producing a phenotype of pink flowers). This form of inheritance has a great range. The terms penetrance and expressivity try to capture the nature and magnitude for a given genotype-phenotype relationship. Penetrance measures the proportion of individuals with a given allele that display its associated phenotype (e.g. cases of complete dominance have 100% penetrance). Expressivity quantifies the proportion of individuals of a given genotype that will express a specified phenotype (i.e. the same genotype may result in different phenotypes dependent on other factors). Leakage applies to the introduction of new alleles from an outside source, such as a different species or cases like paternal mitochondrial DNA being passed to offspring.

Hybridization: viability

Some species can cross reproduce in hybridization. Such offspring, when viable, may share or display completely new traits from each of the parent species. Hybrids are also likely to be infertile due to pairing issues between chromosomes. For example, a mule is the offspring between a female horse and a male donkey, and is infertile (1/2*64 + 1/2*62 = 63 chromosomes).

Gene pool

A gene pool refers to all of the alleles for a gene across a population. It is a description of genetic variation. From data on a gene pool, one can determine specific allele frequencies and track microevolution within the population. Increased variation allows for more adaptability of the population to environmental changes.

Learn more about planning and tracking your MCAT prep! Tour →

Get updates!