A main feature of inheritance is predictability: given certain conditions, the likelihood of the occurrence or recurrence of a specific trait is remarkably predictable. The units of inheritance are the genes, and the pattern of single-gene expression can often be predicted using Mendel’s laws of genetic transmission. Techniques and discoveries since Gregor Mendel’s original work was published in 1865 have led to some modification of the original laws.
Mendel discovered the basic pattern of inheritance by conducting carefully planned experiments with simple garden peas. Experimenting with several phenotypic traits in peas, Mendel proposed that inherited traits are transmitted from parents to offspring by means of independently inherited factors—now known as genes—and that these factors are transmitted as recessive and dominant traits. Mendel labeled dominant factors (his round peas) “A” and recessive factors (his wrinkled peas) “a.” Geneticists continue to use capital letters to designate dominant traits and lowercase letters to identify recessive traits. The possible combinations that can occur with transmission of single-gene dominant and recessive traits can be described by constructing a figure called a Punnett square using capital and lowercase letters (Fig. 6.11).
The observable traits of single-gene inheritance are inherited by the offspring from the parents. During maturation, the primordial germ cells (i.e., sperm and ovum) of both parents undergo meiosis, or reduction division, in which the number of chromosomes is divided in half (from 46 to 23). At this time, the two alleles from a gene locus separate so that each germ cell receives only one allele from each pair (i.e., Mendel’s first law). According to Mendel’s second law, the alleles from the different gene loci segregate independently and recombine randomly in the zygote. People in whom the two alleles of a given pair are the same (AA or aa) are called homozygotes. Heterozygotes have different alleles (Aa) at a gene locus. A recessive trait is one expressed only in a homozygous pairing; a dominant trait is one expressed in either a homozygous or a heterozygous pairing. All people with a dominant allele (depending on the penetrance of the genes) manifest that trait. A carrier is a person who is heterozygous for a recessive trait and does not manifest the trait. For example, the genes for blond hair are recessive and those for brown hair are dominant. Therefore, only people with a genotype having two alleles for blond hair would be blond; people with either one or two brown alleles would have brown hair.