Genetics is the study of how genes bring about characteristics, or traits, in living things and how those characteristics are inherited. Genes are specific sequences of nucleotides that code for particular proteins. Through the processes of meiosis and sexual reproduction, genes are transmitted from one generation to the next.
Augustinian monk Gregor Mendel developed the science of genetics. Mendel performed his experiments in the 1860s and 1870s, but the scientific community did not accept his work until early in the twentieth century. Because the principles established by Mendel form the basis for genetics, the science is often referred to as Mendelian genetics. It is also called classical genetics to distinguish it from another branch of biology known as molecular genetics (see Chapter 10).
Mendel believed that factors pass from parents to their offspring, but he did not know of the existence of DNA. Modern scientists accept that genes are composed of segments of DNA molecules that control discrete hereditary characteristics.
Most complex organisms have cells that are diploid. Diploid cells have a double set of chromosomes, one from each parent. For example, human cells have a double set of chromosomes consisting of 23 pairs, or a total of 46 chromosomes. In a diploid cell, there are two genes for each characteristic. In preparation for sexual reproduction, the diploid number of chromosomes is reduced to a haploid number. That is, diploid cells are reduced to cells that have a single set of chromosomes. These haploid cells are gametes, or sex cells, and they are formed through meiosis (see Chapter 8). When gametes come together in sexual reproduction, the diploid condition is reestablished.
The offspring of sexual reproduction obtain one gene of each type from each parent. The different forms of a gene are called alleles. In humans, for instance, there are two alleles for earlobe construction. One allele is for earlobes that are attached, while the other allele is for earlobes that hang free. The type of earlobe a person has is determined by the alleles inherited from the parents.
The set of all genes that specify an organism’s traits is known as the organism’s genome. The genome for a human cell consists of about 20,000 genes. The gene composition of a living organism is its genotype. For a person’s earlobe shape, the genotype may consist of two alleles for attached earlobes, or two alleles for free earlobes, or one allele for attached earlobes and one allele for free earlobes.
The expression of the genes is referred to as the phenotype of a living thing. If a person has attached earlobes, the phenotype is “attached earlobes.” If the person has free earlobes, the phenotype is “free earlobes.” Even though three genotypes for earlobe shape are possible, only two phenotypes (attached earlobes and free earlobes) are possible.
The two paired alleles in an organism’s genotype may be identical, or they may be different. An organism’s condition is said to be homozygous when two identical alleles are present for a particular characteristic. In contrast, the condition is said to be heterozygous when two different alleles are present for a particular characteristic. In a homozygous individual, the alleles express themselves. In a heterozygous individual, the alleles may interact with one another, and in many cases, only one allele is expressed.
When one allele expresses itself and the other does not, the one expressing itself is the dominant allele. The “overshadowed” allele is the recessive allele. In humans, the allele for free earlobes is the dominant allele. If this allele is present with the allele for attached earlobes, the allele for free earlobes expresses itself, and the phenotype of the individual is “free earlobes.” Dominant alleles always express themselves, while recessive alleles express themselves only when two recessive alleles exist together in an individual. Thus, a person having free earlobes can have one dominant allele or two dominant alleles, while a person having attached earlobes must have two recessive alleles.