Our genetic makeup is contained within structures called chromosomes, which are essentially tightly packaged bundles of deoxyribonucleic acid, or DNA. Segments of this DNA are known as genes, acting as specific instructions for building and maintaining the body. The general rule for human inheritance is a close to perfect 50/50 split, where a child receives half of their chromosomes from each biological parent. However, this simple fraction is complicated by different chromosome types and a unique exception found outside the cell nucleus.
The Core Inheritance Rule of Autosomes
The nucleus of every human cell contains 46 chromosomes, arranged in 23 pairs. Twenty-two of these pairs are known as autosomes, which are the non-sex chromosomes numbered 1 through 22. These autosomes contain the majority of the genes that determine physical traits and bodily functions. Every person inherits one complete set of 22 autosomes from their biological mother and one complete set of 22 autosomes from their biological father.
This process establishes a truly equal baseline for the inheritance of nuclear DNA. The full set of 44 autosomes in each cell is composed of 22 chromosomes from the egg and 22 chromosomes from the sperm. Since the autosomes from each parent are paired with a homologous chromosome from the other parent, the genetic contribution is precisely balanced. This 50/50 split of autosomal chromosomes represents the largest quantitative portion of the human genome.
The chromosomes from each parent are considered haploid sets (single copies), which combine to form the diploid set of 46 chromosomes in the offspring. While the number of chromosomes is exactly half from each parent, the specific combination of gene variations, or alleles, on these chromosomes is unique. Because of this pairing, the inheritance of traits governed by these autosomes is fundamentally symmetrical between the parents.
The Specific Contribution of Sex Chromosomes
The 23rd pair of chromosomes, the sex chromosomes, are the exception to the rule of perfectly matched pairs in males. A female inherits two X chromosomes (XX), while a male inherits one X and one Y chromosome (XY). The mother always contributes an X chromosome to the offspring through the egg cell. The father determines the biological sex by contributing either an X or a Y chromosome through the sperm.
If the sperm carries an X chromosome, the offspring will be female (XX), and the inheritance of the sex chromosomes will be an equal X from each parent. If the sperm carries a Y chromosome, the offspring will be male (XY). When the offspring is male, the genetic contribution is not equal in terms of raw gene count. The X chromosome is much larger and contains approximately 900 protein-coding genes.
In contrast, the Y chromosome is significantly smaller, containing a much lower number of genes, typically estimated between 70 and 200. Therefore, a male inherits a greater number of genes from the mother (via the X chromosome) than from the father (via the Y chromosome).
The Maternal Exception: Mitochondrial DNA
A complete picture of genetic inheritance must include a small but distinct set of DNA found outside the cell nucleus. This is mitochondrial DNA (mtDNA), located within the mitochondria, which are the cell’s energy-producing organelles. Unlike the nuclear DNA, which is inherited from both parents, mtDNA is an almost exclusively maternal inheritance.
Mitochondria are present in both the egg and the sperm, but the egg cell contains vastly more mitochondria than the sperm. During fertilization, the small number of mitochondria present in the sperm’s tail are typically tagged for destruction and eliminated by the fertilized egg. This mechanism, known as paternal mitochondria elimination, ensures that the child’s mitochondria and mtDNA come solely from the mother.
The tiny, circular mitochondrial genome contains 37 genes that are important for energy production. Though this represents less than 0.1% of the total genetic material, it provides a clear instance of 100% maternal inheritance. The exclusion of the father’s mitochondrial DNA is a biological process, and its failure has been linked to potential developmental issues in the embryo.