Chromosomes are thread-like structures found inside the nucleus of nearly every cell. They serve as tightly packaged carriers of genetic information, consisting of deoxyribonucleic acid (DNA). This DNA contains the complete set of instructions for building and operating a human body. Before reproduction, a specialized cell division process called meiosis creates sex cells that contain only a single set of these genetic packages.
The Maternal Contribution: The Haploid Set
The mother contributes exactly 23 chromosomes to the offspring, contained within her egg cell (ovum). This number represents a single, complete set of genetic instructions. A cell containing only one set of chromosomes is known as a haploid cell.
Most cells in the body are diploid, containing two complete sets. Meiosis reduces the mother’s diploid cells down to the haploid egg, ensuring her contribution is precisely half of the total genetic material needed.
Assembling the Full Set: Autosomes and Homologous Pairs
During fertilization, the 23 chromosomes from the mother combine with 23 from the father, resulting in a total count of 46 chromosomes. This fusion restores the diploid number, creating 23 pairs of chromosomes in the newly formed cell.
Twenty-two of these pairs are autosomes, numbered one through 22. Autosomes contain genes responsible for non-sex-linked traits like eye color, height, and blood type. The maternal contribution includes one chromosome for each of these 22 autosome pairs. These pairs are homologous, meaning the maternal and paternal chromosomes match up and carry genes for the same traits in the same locations.
The Mother’s Role in Determining Biological Sex
The final pair of chromosomes, the 23rd set, are the sex chromosomes responsible for determining biological sex. The mother consistently contributes an X chromosome to this pair. Since a typical female has two X chromosomes (XX), every egg cell she produces carries one X.
The father has one X and one Y chromosome (XY), meaning his sperm can carry either an X or a Y. The father’s contribution of either an X or a Y is the variable factor that determines if the resulting combination will be XX (female) or XY (male).
When the Chromosome Count Is Incorrect
The precise process of meiosis can sometimes fail, leading to an incorrect number of chromosomes contributed by the mother. This failure, known as nondisjunction, occurs when chromosomes do not separate properly during egg cell creation. The resulting egg may contain one chromosome too many or too few, a condition termed aneuploidy.
While nondisjunction can occur in either parent, advanced maternal age correlates with an increased risk of these errors. Examples of aneuploidy include Trisomy 21, where the offspring inherits three copies of chromosome 21 instead of two, leading to Down Syndrome. Other aneuploidies can affect the sex chromosomes, such as Triple X syndrome (XXX) or Klinefelter syndrome (XXY).