Mendelian Terminology

In Mendelian genetics, a fairly small set of technical terms is used again and again. This chapter focuses on those terms and how they fit together, without re‑telling Mendel’s experiments or the laws themselves.

Basic hereditary units and their locations

Gene

A gene is a section of DNA that usually carries the information for one functional product (often a protein, sometimes an RNA). In Mendelian genetics, you can think of a gene as a “hereditary unit” that influences a particular trait, such as seed color in peas.

Locus (plural: loci)

A locus is the fixed position of a gene on a chromosome.

You can imagine a chromosome as a long string and each gene has its specific “address” on that string.
The same gene (e.g., “seed color gene”) occurs at the same locus on homologous chromosomes.

Allele

An allele is a specific version (variant) of a gene.

Example: A gene for flower color might have an allele for purple (A) and an allele for white (a).
Different alleles can produce different forms of the same trait.

Genetic makeup vs. observable trait

Genotype

The genotype is the genetic constitution of an organism (or of a particular locus or set of loci).

It is usually written using letters that represent alleles.

Example: AA, Aa, aa for a single gene with two alleles.

The genotype is invisible directly; we infer it from crosses, pedigrees, or molecular tests.

When focusing on one specific gene, we speak of the single‑gene genotype at that locus; when many genes are considered together, we may speak of the overall genotype or genome, depending on context.

Phenotype

The phenotype is the observable form of a trait (or set of traits) in an organism.

Example phenotypes:

Purple vs. white flowers
Tall vs. short plants
Blood group A, B, AB, or O in humans

The phenotype is influenced by both genotype and environment.

In Mendelian problems, we often assume that the environment is not masking the effect of the genotype, so that genotype–phenotype relationships are clear.

Zygosity: allele combinations at a locus

Homozygous

An organism is homozygous for a gene if both alleles at that locus are the same.

Examples:

AA – homozygous for the dominant allele
aa – homozygous for the recessive allele

A homozygote is an individual that is homozygous at the locus under consideration.

Heterozygous

An organism is heterozygous if it carries two different alleles at a locus.

Example:

Aa – one dominant and one recessive allele

A heterozygote is an individual with two different alleles at that locus.

Hemizygous

Hemizygous means that there is only one copy of a gene (one allele) present, instead of a pair.

Important example: many genes on the X chromosome in human males:

Genotype for a certain X‑linked gene in a male might be written as X^A Y (only one allele A present, no second copy on the Y).

Hemizygosity often appears for sex‑linked genes in organisms with XY sex determination.

Dominance relationships between alleles

Dominant

An allele is dominant if its effect on the phenotype is seen when only one copy is present (in a heterozygote).

If A is dominant over a:

AA → dominant phenotype
Aa → dominant phenotype
aa → recessive phenotype (dominant allele absent)

In symbolic notation, dominant alleles are often written with capital letters (e.g., A).

Recessive

An allele is recessive if its effect on the phenotype is “masked” in a heterozygote and only appears when both alleles are recessive (homozygous recessive).

With A dominant and a recessive:

Only aa shows the recessive phenotype.

Recessive alleles are typically written with lowercase letters (e.g., a).

Complete dominance

Complete dominance is a special case where the phenotype of the heterozygote (Aa) is indistinguishable from the phenotype of the homozygous dominant (AA).

Expression pattern:

AA: dominant phenotype
Aa: dominant phenotype (exactly the same)
aa: recessive phenotype

This is the pattern in many of Mendel’s classic pea traits.

Incomplete dominance

In incomplete dominance (sometimes called partial dominance), the phenotype of the heterozygote is intermediate between the two homozygotes.

Example pattern:

AA: red flowers
aa: white flowers
Aa: pink flowers (intermediate)

The genotypic ratio and phenotypic ratio in a typical Aa × Aa cross become the same (1:2:1), because each genotype gives a distinct phenotype.

Codominance

In codominance, both alleles in a heterozygote are fully and separately expressed in the phenotype.

The classic example is human ABO blood group (at the I locus), where alleles I^A and I^B are codominant:

I^A I^A → blood group A
I^B I^B → blood group B
I^A I^B → blood group AB (both traits expressed)

Here, the heterozygote shows characteristics of both homozygotes simultaneously, not a blend.

Generations and types of crosses

Parent generation and offspring generations

Mendel used specific symbols for generations in his breeding experiments:

P generation (P₁): the parental generation used to start a cross. Typically, these are true‑breeding (homozygous) lines.
F₁ generation: the first filial generation; offspring of the P generation.
F₂ generation: the second filial generation; offspring of a cross between F₁ individuals (or self‑fertilization of F₁ plants).

The index “F” stands for “filial” (Latin filius = son, filia = daughter).

Monohybrid cross

A monohybrid cross considers just one gene (one trait) at a time.

Example: Crossing AA (purple) × aa (white) and tracking only flower color.
F₁ individuals are monohybrids: heterozygous at one locus (Aa).

Monohybrid crosses are used to illustrate Mendel’s first law (segregation).

Dihybrid cross

A dihybrid cross follows two genes (two traits) at the same time.

Example: Seed color (Y = yellow, y = green) and seed shape (R = round, r = wrinkled).
A dihybrid is heterozygous at both loci: YyRr.

A typical dihybrid cross is YyRr × YyRr and illustrates Mendel’s second law (independent assortment), assuming the genes are not linked.

Test cross (backcross)

A test cross (often a special type of backcross) is used to determine the genotype of an individual that shows the dominant phenotype.

Principle:

Cross the individual of unknown genotype (A?) with a homozygous recessive individual (aa).
Observe the phenotypes of the offspring:

If all offspring show the dominant phenotype → the tested parent is likely AA.
If a mix of dominant and recessive phenotypes appears → the tested parent is Aa.

A backcross more generally means crossing an F₁ individual back to one of the P generation genotypes; if the P genotype is homozygous recessive, the backcross is also a test cross.

Ratios and probability concepts in Mendelian crosses

Genotypic ratio

The genotypic ratio describes the relative frequencies of different genotypes in the offspring.

Example: For a monohybrid cross Aa × Aa:

Genotypes: AA, Aa, aa
Genotypic ratio: $1\,\text{AA} : 2\,\text{Aa} : 1\,\text{aa}$

Phenotypic ratio

The phenotypic ratio describes the relative frequencies of the different phenotypes in the offspring.

With complete dominance (A dominant over a), the same cross Aa × Aa yields:

Phenotypes: dominant (from AA and Aa), recessive (from aa)
Phenotypic ratio: $3\ \text{dominant} : 1\ \text{recessive}$

These standard ratios can change when the dominance relationships are incomplete or codominant, or when more genes are involved.

Law of segregation and independent assortment (terminological view)

Without re‑explaining the laws in detail:

Segregation: the separation of allele pairs into different gametes.

Each gamete gets only one allele from each pair (one per locus).

Independent assortment: the concept that allele pairs at different loci assort independently into gametes, if the genes are on different chromosomes (or far apart on the same chromosome).

These terms explain why certain genotypic and phenotypic ratios appear in monohybrid and dihybrid crosses.

Extensions and special terms

Multiple alleles

When more than two alleles exist for a gene in a population, we speak of multiple alleles.

Each individual still carries only two alleles (in diploid organisms), but the population as a whole has more possibilities.
Example: ABO blood group system with three common alleles: I^A, I^B, i.

Pleiotropy

Pleiotropy is the situation in which one gene influences multiple distinct traits.

A single gene mutation may cause effects in many organs or systems, not just one visible trait.

Epistasis

Epistasis describes interactions where one gene masks or modifies the effect of another gene at a different locus.

The epistatic gene can hide or change the phenotype expected from the other gene.

While these phenomena affect how Mendelian ratios appear in practice, the terms themselves describe specific gene–gene relationships.

Notation conventions

In Mendelian problems, several conventions are common:

Dominant/recessive pairs:

Dominant allele: capital letter (e.g., A).
Recessive allele: same letter, lowercase (a).

Different genes: different letters (e.g., A/a for one gene, B/b for another).
Sex‑linked genes:

Often written with superscripts on the sex chromosomes, e.g., X^A X^a, X^a Y.

These notations help quickly communicate genotypes and are used in Punnett squares and pedigree diagrams.

This terminology forms the basic “language” of Mendelian genetics and will be used throughout discussions of Mendel’s laws, genetic crosses, and inheritance patterns.

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