For inheritance of simple genetic traits, the two inherited copies of a gene determine the phenotype (the observable characteristic) for that trait. When genes for a particular trait exist in two or more different forms that may differ among individuals and populations, they are called alleles. For example, brown and blue eye colors are due to different alleles for eye color. For every gene, the offspring receives two alleles, one from each parent. The combination of inherited alleles is the genotype of the organism, and its expression—the observable characteristic—is its phenotype. Figure 2.18 is a graphic example of phenotype.
For many traits the phenotype is a result of an interaction between the genotype and the environment. Some of the most readily apparent traits in humans, such as height, weight, and skin color, result from interactions between genetic and environmental factors. In addition, there are complex phenotypes that involve multiple gene-encoded proteins; the alleles of these particular genes are influenced by other factors, either genetic or environmental. So while the presence of certain genes indicates susceptibility or likelihood to develop a certain trait, it does not guarantee expression of the trait.
For a specific trait, some alleles may be dominant while others are recessive. The phenotype of a dominant allele is always expressed, while the phenotype of a recessive allele is expressed only when both alleles are recessive. Recessive genes continue to pass from generation to generation, but they are only expressed in individuals that
SOURCE: "Recessive," in Talking Glossary of Genetic Terms, U.S. Department of Health and Human Services, National Institutes of Health, National Human Genome Research Institute, http://www.genome.gov/Pages/Hyperion//DIR/VIP/Glossary/Illustration/recessive.shtml (accessed February 3, 2005)
do not inherit a copy of the dominant gene for the specific trait. Figure 2.19 shows the inheritance of a recessive trait, in this example a recessive mutation in mice that produces an albino offspring from black parents.
There also are some instances, known as incomplete dominance, when one allele is not completely dominant over the other, and the resulting phenotype is a blend of both traits. Skin color in humans is an example of a trait often governed by incomplete dominance, with offspring appearing to be a blend of the skin tones of each parent. Furthermore, some traits are determined by a combination of several genes (multigenic or polygenic), and the resulting phenotype is determined by the final combination of alleles of all the genes that govern the particular trait.
Some multigenic traits are governed by many genes, each contributing equally to the expression of the trait. In such instances a defect in a single gene pair may not have a significant impact on expression of the trait. Other multigenic traits are predominantly directed by one major gene pair and only mildly influenced by the effects of other gene pairs. For these traits, the impact of a defective gene pair depends on whether it is the major pair governing expression of the trait or one of the minor pairs influencing its expression.
A range of other factors enters into whether a trait will be evidenced and the extent to which it is expressed. For example, different individuals may express a trait with different levels of severity. This phenomenon is known as variable expressivity.
Determining Genetic Probabilities of Inheritance
Conventionally, geneticists use uppercase letters to represent dominant alleles and lowercase letters to stand for recessive alleles. An organism with a pair of identical alleles for a trait is described as a homozygote, or homozygous for that particular trait. When organisms are homozygous for a dominant trait, all uppercase letters symbolize the trait, while those that are homozygous for a recessive trait are represented by all lowercase letters. A heterozygote is an organism with different alleles for a trait, one donated from each parent, and when one is dominant and the other is recessive the trait is shown using a combination of uppercase and lowercase letters.
Although the combination of alleles is a random event, it is possible to predict the probability that an offspring will have the same or a different phenotype from its parents when the genotypes with respect to the specific trait of both parents and the phenotype associated with each possible combination of alleles are known. The formula used to determine genetic possibilities was developed by English geneticist R. C. Punnett (1875–1967). The "Punnett square" is a grid configuration that
SOURCE: "Punnett Square," created by the staff of Information Plus, July 2003
depicts genotype and phenotype. In Punnett squares the genotypes of parents are represented with four letters. There are two alleles for each trait. Genotypes of haploid gametes are represented with two letters. Gametes will contain one allele for each trait in every possible combination, and all possible fertilizations are calculated. Punnett squares are used to compute the cross of a single gene or two genes and their alleles, but they become extremely complicated when used to predict the offspring of more than two alleles.
To construct a Punnett square, the alleles in the gametes of one parent are placed along the top, and the alleles in the gametes of the other parent are placed along the left side of the grid. The number of characteristics considered determines the size of the Punnett square. Monohybrid crosses, or crosses looking at one trait, have a two-by-two structure, with four possible genotypes resulting from the cross. A dihybrid cross looks at the possibilities from looking at two traits and has a four-by-four structure, giving sixteen different genotypes. Figure 2.20 is a simple Punnett square that shows one parent homozygous for the AA allele and another that is heterozygous (Aa), with the four possible offspring: 50% AA and 50% Aa. This is an example of offspring that show just one phenotype even though two genotypes are present. Figure 2.21 is a Punnett square that shows the predicted patterns of recessive inheritance. When both parents are carriers of the trait and dominant inheritance,
Punnett square with predicted patterns of recessive inheritance. Argosy Publishing, Thomson Gale.
the offspring is likely to arise from an affected parent and a normal parent.
Mitochondria are the organelles involved in cellular metabolism and energy production and conversion. Mitochondria are inherited exclusively from the mother and contain their own DNA, known as mtDNA, some of which multiplies during the organism's growth and development and as such are more susceptible to mutation (changes in DNA sequence). They also acquire it more quickly than other DNA. The fact that mitochondria contain their own DNA has prompted scientists to speculate that they originally existed as independent one-celled organisms that over time developed interdependent relationships with more complex, eukaryotic cells.
SOURCE: "Mitochondrial Inheritance Passes Only through the Mother," in Genetics, vol. 2, E–I, Macmillan Reference USA, Gale Group, 2002
Mitochondrial inheritance looks very much like Mendelian inheritance (or genetic inheritance as described by Gregor Mendel's laws), with two important exceptions. First, all maternal offspring are usually affected, whereas even in autosomal dominant disorders (those not related to the sex genes) only 50% of offspring are expected to be affected. (See Figure 2.22.) Second, mitochondrially inherited traits are never passed through the male parent. Males are as likely to be affected as females, but their offspring are not at risk. In other words, when there is a mutation in a mitochondrial gene, it is passed from a mother to all of her children; sons will not pass it on, but daughters will pass it on to all of their children.
Mutations in mtDNA have been linked to the development of disease in humans. The first human disease that was associated with a mutation in mtDNA is called Leber's hereditary optic neuropathy, a painless loss of vision that afflicts persons between the ages of twelve and thirty. Many diseases linked to mtDNA affect the nervous system, heart or skeletal muscles, liver, or kidneys—sites of energy usage.