How are genes inherited?

During conception, one of each pair of chromosomes is passed on from the mother and the father.


23 pairs
of chromosomes
XY
males
XX
females

Chromosomes are passed from parents to offspring via sperm and eggs. The specific kind of chromosome that contains a gene determines how that gene is inherited. There are three major kinds of chromosomes: autosomes, sex chromosomes and mitochondrial.

Most chromosomes are present in two copies in both men and women and are called autosomes. These chromosomes are numbered from 1 to 22. While most cells in our body have two copies of each autosome, sperm and eggs carry only one copy of each autosome. When a sperm fertilizes an egg, the embryo now contains two copies of each autosome, one from the father and one from the mother. Consequently, each person has two copies (alleles) of every gene carried on an autosome: one inherited from their father and one from their mother.

The X and Y chromosomes are the “sex chromosomes”. Women have two copies of the X chromosome, one from their father and one from their mother. Men have one X chromosome, from their mother, and one Y chromosome, from their father. Because of this, a man will pass his copy of the X chromosome—and the genes it contains—only to his daughters and his copy of the Y chromosome only to his sons. Women have two alleles of every gene on the X chromosome, one inherited from their father and one from their mother. Men have a single allele of each gene on the X chromosome, inherited from their mother, and a single allele of each gene on the Y chromosome, from their father.

Mitochondrial chromosomes are inherited solely from the mother. Men inherit their mother’s mitochondrial genes but do not pass them to their offspring.

References

Alberts, B., et al (2008). Molecular biology of the cell. New York, NY: Garland Science; Hartl, D.L. and Jones, E.W. (2005). Genetics: analysis of genes and genomes. Sudbury, MA: Jones and Bartlett Publishers.