Genetics is the study of heredity, the means by which offspring inherit physical traits from their parents. Although most offspring produced by sexual reproduction resemble their parents, there is always some variation. Heredity is a complex process in which many different characteristics are transmitted to the offspring, of which only certain ones will show themselves in their physical appearance.

Single-factor inheritance can be seen in the inheritance of coat color in rats. When a black rat with the dominant genotype BB mates with a brown rat of the recessive genotype bb, the resulting offspring will all be genotype Bb and have black coats. But when rats of genotype Bb mate, three out of four of their offspring will have black coats, and only one will have a brown coat. Of the black rats, only one will be homozygous for a black coat (BB), as will the brown rat for its coat color (bb). The rest will be heterozygous (Bb).

Chromosomes, genes, and alleles

Genes are the carriers of hereditary information. Every individual contains millions of genes, each of which controls one or more characteristics in that individual. Each gene is is made up of a pair of alternate forms, called alleles, and each pair of alleles controls a single characteristic in a normal body cell.

Genes lie on the chromosomes, which are located in the nucleus of a celi. The chromosomes occur in homologous, or matched, pairs—one originally from the mother and one from the father. A sperm or egg cell only carries one of each pair of alleles. This is because during meiosis (cell division) in sex cells, homologous pairs of chromosomes are separated so that each new cell contains only one of a pair of chromosomes and, therefore, one of a pair of alleles. When sperm and egg unite, the new individual inherits one allele for each gene from each parent. Genes on the same chromosome form a linkage group and are usually transmitted together to the offspring. Chromosomes vary in size and shape, their dimensions corresponding to the number of genes—the larger the chromosome, the more genes it contains.

When a gene’s two alleles are identical, the gene is said to be homozygous; when they are not, the gene is heterozygous. A heterozygous gene contains one dominant and one recessive allele. Regardless of whether the alleles are homozygous or heterozygous, the dominant gene of a pair of alleles always expresses itself. Recessive genes express themselves only when they are homozygous.
For example, in human beings, the allele for brown eyes (B) is dominant and the allele for blue eyes (b) is recessive. Say, for example, a man who carries a homozygous gene for brown eyes (BB) has children with a woman who has blue eyes (bb). All the father’s sperm cells would carry the B allele, and all the mother’s egg cells would carry the b allele. As a result, all their offspring would carry the Bb eye-color gene, and because B is dominant, all would have brown eyes. The offspring of two parents with the Bb eye-color gene could either be BB, Bb, bB (all brown-eyed), or bb (blue-eyed). So brown eyes can be produced by either the BB or Bb genotypes. In such simple cases of inheritance, the likelihood of a dominant characteristic occurring is 3:1.

But heredity is not always so simple. Inheritance is not simply a matter of sorting out traits, in which the dominant trait always expresses itself. If this were so, there would only be blue-eyed and brown-eyed people. Instead, many physical characteristics are determined by a number of genes. As a result, characteristics of both parents can mix to produce in the offspring an intermediate result. For example, a parent with blue eyes and a parent with brown eyes may produce a child with green eyes.

When heredity involves the transmission of a pair of characteristics, each of a pair of alleles combines with either member of another pair. This is known as a dihybrid cross. For example, in the fruit fly Drosophila melanogaster, if the long wing (WW) and the short wing (ww) characteristics are paired with a broad abdomen (BB) and a narrow one (bb), with one parent being WWBB and the other wwbb, and if 16 offspring were produced, only 1 in 16 would be the same genotype as the parent with recessive genes. But 9 in 16 would bear the dominant phenotypic characteristics—long wings and a wide abdomen, 3 in 16 would carry long wings and a narrow abdomen, and 3 in 16 would have short wings and a wide abdomen.

The chromosomes that determine an individual’s sex are medium-sized. In humans and the fruit fly, the female sex chromosomes are known as X chromosomes and are given the genotype XX. Of a pair of male chromosomes, one is rod-shaped, and the other—the Y chromosome-crooked; this pair is given the geno-type XY. In birds, the female chromosomes are XY and the males XX. In some insects, the female chromosomes are XX, but the males have no Y chromosomes, so their genotype is XO.

During a special stage in meiosis called chiasmata, the alleles leave their chromosome and change places with the corresponding chromosome of the homologous pair. This process remits in new combinations of characteristics, known as recombinants, which increase the genetic variation found among individuals.

Some diseases and disorders, such as albinism, in which the skin, hair, and eyes have no pigment, are hereditary. Albinism is not fatal, but some hereditary diseases, such as cystic fibrosis, are. This disease, carried by homozygous recessive alleles, causes the glands to secrete excessive amounts of abnormally thick mucus, and death as a result of intestinal or lung problems usually occurs before adulthood.

Albinism is a hereditary disorder that manifests itself in an absence of pigments in the body. It is carried by recessive alleles and therefore occurs infrequently. The disorder is not lethal, although it makes life more difficult for victims. For example, lack of protective pigment in the eyes makes them extremely sensitive to light and lack of pigment in the skin increases the danger of sunburn.

The structure of a gene

Genes are made up of a chemical called deoxyribonucleic acid (DNA). The DNA molecule is a double helix, resembling a twisted ladder. Its long parallel strands are constructed from alternating sugar and phosphate molecules, with cross-bridges formed from special groupings called bases. There are four types of bases: adenine, cytosine, guanine, and thymine, represented by the initials A, C, G, and T. The bases are arranged in specific pairs: adenine always pairs with thymine, and guanine with cytosine. Hereditary information is coded in the sequence of these bases along the DNA molecule. The code is an arrangement of 20 amino acids, the basic components of proteins, which form the structure of living organisms and all the enzymes that control their metabolism.