This is a guest report from a geneticist
For more than two decades, the McGregor DNA project has been building a remarkable picture of shared heritage for Clan Gregor. We see that DNA is passed down through families in predictable but sometimes surprising ways, shaping how we are connected to one another. By understanding how this process works, we can better appreciate what our results are telling us and how they fit into the wider story of our family histories.
This post revisits the biological basis of genetic inheritance and explores it in the context of our DNA project. One thing quickly becomes apparent - understanding genetics is almost like learning a new language. There are many specialised terms and interpreting them is often the first step towards making sense of the bigger picture. Most of these terms will be written in italics when first introduced in this post.
Genetic inheritance
Human eggs carry 22 autosomes and an X chromosome. Interestingly, eggs are the larger gamete containing mitochondria and other molecules to guide the developing embryo. Mitochondria contain their own DNA (mtDNA), which is inherited by both males and females from their mother, but only females pass it on to the next generation.
At fertilisation, male and female gametes combine to form a zygote and through multiple cell divisions during embryonic development, a new individual develops made up of trillions of cells, each containing 46 chromosomes. Males have 44 autosomes and XY sex chromosomes while females have 44 autosomes and XX. All males inherit their Y chromosome paternally and their X maternally. Females inherit one X from each parent.
We don’t inherit an exact copy of our parents’ chromosomes
Along with halving of parental chromosomes, meiosis also involves recombination of chromosomes where DNA from the two copies of each chromosome is shuffled before being passed on. As a result, we don’t inherit an exact copy of our parents’ chromosomes, and they didn’t inherit an exact copy of their parents’ and so on. In just a few generations our DNA can be very different from our ancestors. Because of this, autosomal DNA (chromosomes 1-22) is limited to detecting relationships to approximately six generations, although linking these matches to specific surnames can be difficult without supporting genealogical information.
The X chromosome follows a different inheritance pattern, so X matches can provide additional clues about possible lines of descent, although they do not usually point to a single ancestral line.
The Y chromosome and paternal inheritance
The Y chromosome behaves differently during meiosis.
The majority of the Y chromosome does not recombine. Instead, it is passed from father to son largely intact, forming a continuous line of inheritance over many generations. This unusual stability is what makes Y‑DNA so powerful for tracing paternal ancestry. It also means that any changes that do occur are preserved and can be followed through time.
Mutation: a source of genetic variation
Mutations arise as part of the normal process of DNA replication, although their frequency can be influenced by factors such as age and environmental exposure. Mutations are often thought of as ‘bad’, but in most cases they are biologically neutral with no effect. Only mutations in gamete cells are passed on.
Despite its relative stability, the Y chromosome undergoes mutation events. These mutations are the source of variation along a paternal line. A DNA test does not measure ancestry directly, but the outcome of these mutational processes.
Two main types of mutation are relevant in Y‑DNA analysis:
- single base changes in the DNA sequence, called single nucleotide polymorphisms (SNPs)
- changes in the number of repeated DNA sequences at specific locations, called short tandem repeats (STRs)
These mutations occur at very different rates.
Haplogroups vs Haplotypes
SNPs arise relatively rarely. When they do occur, they tend to be inherited by all descendants of the individual in whom they first appeared. A haplogroup is a branch of the Y chromosome tree defined by shared SNP mutations. In practice, haplogroups are often experienced as large, overarching branches representing lineages that extend far back in time, although modern sequencing can now identify increasingly specific sub-branches within them, sometimes within genealogical timeframes. Haplogroups help answer the question, “Which major branch of the lineage does my paternal line belong to?”
If a haplogroup describes where a lineage sits on the tree, a haplotype describes how that lineage varies within its branches.
With STRs, which mutate more frequently, the number of repeats can increase or decrease from one generation to the next. This produces small, incremental differences between related individuals. A haplotype is a set of linked genetic markers that are inherited together. In Y‑DNA testing, this typically refers to a set of STR marker values (such as DYS393, DYS390, etc.) that together describe the genetic profile of a paternal lineage.
This is how many surname projects were built, by recognising patterns of similarity across STR profiles.
A model, not a record
One important point follows from this.
DNA testing does not reconstruct the past directly. Instead, it models how mutations accumulate over generations and interprets present-day variation in that context. These models are based on observed mutation rates, derived from large numbers of known relationships, such as father/son pairs and documented family lineages. By incorporating these rates, they estimate how likely it is that a given pattern of differences arose over a particular number of generations.
Matches, distances, and predicted relationships are therefore not exact measurements of ancestry, but statistically informed interpretations of biological processes. This does not make them unreliable, but it does place them into context.
From separate systems to integration
Advances in technology: the shift to Y700
New SNPs discovered through testing can define previously unknown branches, so that the structure of the tree is continually being improved. In this sense, modern testing does not simply use the tree, it contributes to building it.
Ongoing surname projects, such as the MacGregor DNA project, illustrate how these principles play out in practice. Initial groupings are often based on similarities in STR markers, suggesting possible shared ancestry. However, as more detailed SNP data become available, particularly through tests such as Y700, some of these apparent relationships are confirmed, while others are shown to be misleading. This reflects the underlying biology - patterns of variation can suggest connections, but only the structure defined by shared mutations can fully resolve them.
Types of DNA testing: strengths and limitations
Different types of DNA tests provide different kinds of information, and each has strengths and limitations.
Autosomal DNA
Identifies relatives across all ancestral lines but within recent generations only and often cannot specify which ancestral line a match comes from without supporting records.
Y-DNA testing
Traces the direct paternal line and is particularly useful for surname studies but only follows paternal ancestry.
mtDNA testing
Traces the direct maternal line, but like Y-DNA represents only one ancestral pathway.
For this reason, DNA results are most powerful when combined with genealogical research rather than interpreted in isolation.
Final thoughts
DNA testing provides powerful insights into shared ancestry, but those insights sit within a biological system shaped by mutation and a statistical framework used to interpret it. By recognising how haplogroups define the broader structure of lineages, and how haplotypes capture variation within them, we can read our results with greater clarity. For a project that has been building evidence over more than 20 years, this perspective allows us to move beyond simply observing patterns to understanding what those patterns mean, and where their limits lie.
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