Summary
Highlights
Heredity is the passing of genetic traits from parents to offspring. Historically, ideas about heredity, such as Aristotle's concept of a mixture of parental traits, were largely incorrect. Aristotle believed the father provided the life force and the mother the building blocks, and that traits blended.
Gregor Mendel, an Austrian monk, revolutionized the study of heredity in the mid-1800s. Through experiments with pea plants, he discovered that inheritance follows specific patterns and developed a framework for understanding how traits are passed across generations.
DNA is organized into chromosomes, with humans having 23 pairs. A gene is a section of DNA on a chromosome that determines a trait. Most physical traits are polygenic (influenced by multiple genes), while some traits can be influenced by a single gene (pleiotropic). Mendelian traits are those determined by a single gene, like pea flower color or earwax consistency.
Each gene can have different versions called alleles. For example, the earwax gene has alleles for wet or dry earwax, differing by a single amino acid. Most body cells (somatic cells) are diploid, containing two sets of chromosomes, one from each parent. Sex cells (gametes: sperm and egg) are haploid, carrying only one set of chromosomes, which combine during fertilization to form a diploid organism.
Mendel's discoveries included the concept of dominant and recessive alleles. A dominant allele can mask the expression of a recessive allele. For earwax, the wet allele (W) is dominant, and the dry allele (w) is recessive. This means if you inherit both a wet and dry allele, you will have wet earwax.
Genotype refers to a person's genetic makeup for a trait. If both alleles are the same (e.g., WW or ww), the genotype is homozygous. If they are different (e.g., Ww), it is heterozygous. Phenotype is the physical expression of the trait. Punnett Squares, invented by Reginald C. Punnett, are used to predict the probabilities of offspring genotypes and phenotypes based on parental genotypes.
The earwax gene is an example of pleiotropy, where a single gene affects multiple traits. The same gene that determines earwax consistency also influences body odor because earwax and armpit sweat are produced by similar glands.
Sex-linked inheritance involves traits determined by genes on sex chromosomes (X and Y). Women have two X chromosomes (XX), and men have one X and one Y chromosome (XY). Because men only have one X chromosome, recessive alleles on their X chromosome are often expressed even if they only have one copy, as there is no corresponding dominant allele on the Y to counteract it. Baldness is a common example, caused by a recessive allele on the X chromosome, making it more prevalent in men.