Human Genetics for the Molecular Lab

Why a molecular technologist learns genetics

The chemistry module established that DNA varies from one copy to the next, and that a sequence variant may or may not affect how a gene works. This lesson adds the vocabulary needed to say whose DNA is varying, how a variant is passed on, and what a result therefore means. A genotyping or sequencing result is a string of bases; turning that string into a clinically useful statement requires the language of human genetics. Two ideas in particular shape how a molecular result is interpreted and reported: the zygosity of a variant — how many copies are present — and the inheritance pattern of the gene involved ¹.

This lesson builds on the mutations and variation lesson and does not re-teach the classes of variant. Instead it organizes those variants into the framework of genes, alleles, and inheritance that the genetics and applications courses depend on.

Genes, loci, and alleles

A gene is a defined stretch of DNA that the cell reads as a unit, typically to specify one protein or functional RNA ². The fixed physical address of that gene on a chromosome is its locus (plural loci): the locus is the location, the gene is the content at that location.

Humans carry two copies of most chromosomes — one inherited from each parent — so most loci are present in two copies. The alternative versions of a gene that can occupy a locus are its alleles. Where the chemistry module spoke of a variant as any difference from a reference sequence, genetics speaks of alleles as the specific sequence states a locus can take. A variant, in other words, is what makes one allele differ from another.

              homologous chromosome pair
   maternal  ----------[ gene X locus ]----------
   paternal  ----------[ gene X locus ]----------
                              |
                  the two alleles at this locus
                  may be identical or different

Genotype, phenotype, and zygosity

The genotype is the pair of alleles an individual carries at a locus (or, more broadly, the individual’s genetic makeup). The phenotype is the observable trait or clinical state that results. Genotype is what the laboratory measures; phenotype is what the clinic sees, and the two do not map one to one ¹.

Zygosity describes the relationship between the two alleles at a locus:

• Homozygous — the two alleles are identical (both the reference state, or both the same variant).
• Heterozygous — the two alleles differ (commonly one reference and one variant).
• Hemizygous — only one allele is present, so the question of a second copy does not arise. This is the normal situation for most genes on the single X chromosome in genetically male individuals, and it also occurs when the partner copy of a region is deleted ².

Zygosity is not a bookkeeping detail. Whether a variant is found in one copy or two can change its clinical meaning entirely, and a molecular report routinely states zygosity alongside the variant itself.

Dominant and recessive alleles

When the two alleles at a locus differ, the phenotype reveals how they interact. An allele is dominant if a single copy is sufficient to produce its effect; it is recessive if its effect appears only when both copies carry it — that is, only in the homozygous state ¹.

These terms describe alleles in relation to a phenotype, not a fixed property of a sequence. The same chemical change can behave as dominant for one trait and recessive for another, and dominance speaks to whether an effect appears, not to how common or how severe it is.

A Punnett square lays out the allele combinations expected in offspring. Writing the variant allele as a and the reference allele as A, a cross between two heterozygous carriers gives:

                 parent 2
               A         a
           +-------+-------+
   p   A   |  AA   |  Aa   |
   a       +-------+-------+
   r   a   |  Aa   |  aa   |
   1       +-------+-------+

   AA = homozygous reference
   Aa = heterozygous (carrier)
   aa = homozygous variant

For a recessive trait, only the aa quarter shows the phenotype; the Aa individuals are unaffected carriers. For a dominant trait, both Aa and aa would be affected.

Mendelian inheritance patterns

Single-gene conditions tend to follow one of a few classic patterns, distinguished by which chromosome carries the gene and whether the responsible allele is dominant or recessive ¹.

• Autosomal dominant — the gene is on a non-sex chromosome (autosome) and one variant allele is enough to cause the condition. An affected parent transmits it to about half of offspring, and the condition typically appears in every generation.
• Autosomal recessive — the gene is on an autosome and the condition appears only in homozygotes. Two unaffected heterozygous carriers can have an affected child; the trait may skip generations and is more likely when parents are related.
• X-linked — the gene is on the X chromosome. Because genetically male individuals are hemizygous for X, a single recessive variant there is sufficient to cause the condition in them, while heterozygous female individuals are usually unaffected carriers. Fathers cannot pass an X-linked allele to sons.

A pedigree diagrams these patterns across a family, using squares for males, circles for females, and filled symbols for affected individuals. The shape of the pedigree often suggests the pattern before any gene is named.

   autosomal recessive (carriers unaffected)

      ( )---[ ]          both parents are carriers
       |     |
    +--+-----+--+
    |     |     |
   ( )   [#]   ( )       one affected child (filled);
                         siblings unaffected or carriers

Recognizing the pattern matters in the lab because it predicts which genotypes are plausible, which family members may need testing, and how a given result should be phrased.

Carrier status

A carrier is a heterozygous individual who harbors a recessive variant allele but does not show the condition, because the second allele is functional. Carriers are central to recessive genetics: they are unaffected yet can transmit the variant, and two carriers can produce an affected child even though neither parent is affected ¹. For X-linked recessive conditions, female heterozygotes are the typical carriers. Identifying carrier status is a common reason a molecular test is ordered, and a “carrier” result must be reported in a way that conveys it is a heterozygous, unaffected state rather than a diagnosis.

Penetrance and expressivity

Genotype does not rigidly dictate phenotype, and two concepts capture the slack between them ¹.

Penetrance is the proportion of individuals with a given genotype who actually show any associated phenotype. When penetrance is incomplete, some people who carry a disease-associated genotype never develop the condition — so an unaffected relative may still carry, and transmit, the same variant.

Expressivity is how strongly or in what form the phenotype appears among those who do show it. Variable expressivity means the same genotype produces mild disease in one person and severe disease in another.

Both ideas warn against reading a genotype as a verdict. They are introduced here at the concept level; the applications courses revisit them for specific disorders.

Polymorphism, pathogenic variant, and allele frequency

The mutations lesson drew the line between a harmless polymorphism and a pathogenic variant, and tied it to allele frequency — how common an allele is in a population. Genetics puts that distinction to work at the level of alleles: a high-frequency allele is part of normal human diversity, while a rare allele that tracks with disease in families is a candidate for pathogenicity ¹. Population allele frequencies are therefore a routine input to interpreting a result, because they help separate an incidental common variant from one worth acting on. Frequency is a filter, not a proof — commonness does not guarantee safety, nor rarity harm — but it is where interpretation begins.

SNPs and short tandem repeats

Two especially useful classes of common variation deserve naming because so much laboratory work rests on them.

A single-nucleotide polymorphism (SNP) is a single-base position that varies commonly among individuals. SNPs are the most abundant form of human variation, and because each one is a simple two- or few-state marker at a known locus, they are ideal for genotyping ¹.

A short tandem repeat (STR) is a short sequence motif repeated head-to-tail, in which the number of repeats varies between individuals. Because many different repeat counts circulate in the population, an STR locus is highly informative: the combination of repeat lengths across several STR loci is, in practice, nearly unique to an individual ². (The chemistry module noted that excessive repeat expansion can also cause disease; here the point is the ordinary length variation that makes STRs useful markers.)

Together, SNPs and STRs are the molecular basis of identity and parentage testing, which the applications course develops as a distinct topic. They also serve as the markers behind many linkage and association analyses ³.

Germline versus somatic, revisited for inheritance

The chemistry module distinguished germline variants — present in essentially every cell and heritable — from somatic variants, which arise in a single cell during life and are not passed on. Genetics is fundamentally the study of germline variation: inheritance patterns, zygosity, carrier status, and pedigrees all concern alleles transmitted through the germline ².

Acquired disease sits on the other side of this line. Variation that accumulates in body cells over a lifetime is somatic and does not follow Mendelian inheritance; it is the province of the oncology applications course, which treats acquired, tumor-specific change as its own topic. Holding the two apart is essential at the bench: an inherited-disease test asks what alleles a person was born with, whereas an acquired-disease test compares a tissue against the person’s own normal genome ¹.

Why this matters at the bench

Zygosity and inheritance pattern are not academic labels; they govern what a result means and how it is reported. The same variant detected in one copy versus two can imply carrier status or affected status depending on the inheritance pattern of the gene, and a report that omits zygosity or the relevant pattern can be read wrongly. The genetics framework built here — genes and alleles, genotype and phenotype, dominance, Mendelian patterns and pedigrees, carriers, penetrance and expressivity, allele frequency, and the germline/somatic divide — is the interpretive layer that sits between a raw sequence and a clinical statement. The applications courses apply it to specific inherited disorders, to identity and parentage testing, and to acquired disease in cancer.