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Modern Genetic Analysis Chapter 4. The Inheritance of Genes
Summary

The basis of the transmission of genes through cell and organismal generations is DNA replication. DNA replication precedes both asexual and sexual cell division. During DNA replication the two halves of the double helix separate, and the single strands serve as templates for new polymerization of nucleotides. Hence each daughter DNA molecule is half old and half new; this mechanism of replication is called semiconservative. DNA polymerase catalyzes new polymerization, adding nucleotides only at 3 growing points. Because the two templates are antiparallel, 3 replication on one template is by continuous polymerization (this is the leading strand), but on the other template synthesis is by short bursts in the opposite direction, producing Okazaki fragments, which are later joined together (this chain is the lagging strand).

Asexual cell division is accompanied by a type of nuclear division called mitosis, which parcels out the chromosomes in such a way that the two daughter cells contain exactly the same genes as the original cell. Sexual cell division occurs in specialized diploid cells called meiocytes, which divide twice to produce four haploid cells (which become eggs and sperm in animals and plants). Sexual cell division is accompanied by two nuclear divisions called meiosis, which parcel out the chromo-somes in such a way as to produce products of meiosis with one chromosome set.

Phenotypic ratios in the progeny of crosses are the result of the highly programmed movements of the chromosomes at meiosis. A pair of alleles occupying one position on a pair of homologous chromosomes separate and migrate into different sex cells because homologous chromosomes physically pair and segregate at the first division of meiosis. A heterozygote A/a produces 1/2 A and 1/2 a products of meiosis. If a heterozygote is selfed, a 3:1 ratio of dominant to recessive phenotypes is found in the progeny; if testcrossed to a/a, then the ratio is 1:1.

Genes on the differential region of the X chromo-some can show inheritance patterns in which male and female progeny show different phenotypic ratios.

Human inheritance patterns can be deduced from pedigree analysis. Conversely, knowledge of inheritance patterns can be used to predict risk of a couple's having children with a specific disorder that occurs in the family.

Organellar genes and phenotypes are inherited maternally because virtually all the cytoplasm of progeny is derived from the maternal parent.

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