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Modern Genetic Analysis
4. The Inheritance of Genes
Key Concepts

DNA replication is a precursor to asexual and sexual cell division.
DNA replicates by unwinding and using the exposed strands as alignment guides for new synthesis; hence, each daughter molecule is half old and half new.
DNA polymerase adds nucleotides at 3 growing points.
At asexual cell division, the accompanying nuclear division (mitosis) results in two genetically identical daughter cells.
Haploid sex cells are produced by two consecutive divisions of a diploid meiocyte, accompanied by two nuclear divisions (meiosis).
Phenotypic ratios in progeny are determined by the movements of chromosome pairs at meiosis.
The members of a pair of heterozygous alleles segregate equally into the sex cells.
X-linked genes can show different phenotypic ratios in male and female progeny.
Organelle genes are inherited maternally.

Down through the ages, one of the biggest mysteries of heredity was the inheritance of type. There are two parts to this mystery. First, the perpetuation of species: people always have babies, and cats always have kittens. Second, the inheritance of variant phenotypes within the species: we all know examples, such as "she has inherited her father's nose" or "red hair runs in the family." What is the mechanism whereby characters and forms of characters (phenotypes) are inherited? In this chapter we study the mechanisms of inheritance at the level of individuals, cells, chromosomes, and genes. The underlying basis for inheritance at all these levels is the replication of DNA. Replication is the process of making replicas or copies. A molecule of DNA can replicate to become two identical "daughter" molecules, and this is how chromosomes can duplicate, how one cell can become two, how a new multicellular organism can arise, how populations grow, and how organisms persist through time. The DNA in every one of us is the end point in a continuous series of replications going back billions of years.

© 1999 by W. H. Freeman and Company. All rights reserved.