Heredity
Heredity is the passing of traits from parents to offspring through genes carried on chromosomes in the sperm and egg.
Definition
Heredity is the passing of traits from parents to offspring, explaining why children often resemble their parents or grandparents, and in sexual reproduction each parent contributes one allele for every gene through its gametes, so an offspring receives half its genes from each parent. Gregor Mendel worked out the basic rules of heredity in the 1860s by crossing pea plants and counting offspring, establishing that traits are inherited as separate units rather than blended together, which is why a hidden trait can reappear in a grandchild. Formally, heredity is the transmission of genetic information across generations through DNA replication and the segregation of chromosomes in meiosis: Mendel's law of segregation states that the two alleles at a locus separate into different gametes, and his law of independent assortment holds only for loci on different chromosomes or far apart on the same one, since linked genes are inherited together in proportion to their recombination distance; non-Mendelian patterns include sex linkage, maternal mitochondrial inheritance, epigenetic marks, and polygenic traits.
Example
Mendel crossed true-breeding tall and short pea plants and found all the offspring tall, yet crossing those offspring together brought the short trait back in about $1$ in $4$ of the next generation. Genes $10$ map units apart in a cross of $AaBb \times aabb$ yield about $45$ percent $AB$, $45$ percent $ab$, and $5$ percent each of the recombinant classes rather than the $25$ percent each predicted by independent assortment, and mitochondrial DNA passes only from mother to child, so mitochondrial disease reaches every child of an affected woman but none of an affected man's.
Key Insight
The chromosome theory of inheritance linked Mendel's abstract factors to physical structures visible under a microscope, since alleles segregate because homologous chromosomes separate during meiosis I, the moment genetics and cell biology became a single science.