Page 79 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.3 Extensions to Mendel for Multifactorial Inheritance 71

The effects of random events on penetrance Figure 3.27 Continuous traits in humans. (a) Women
and expressivity runners at the start of a 5th Avenue mile race in New York City
demonstrate that height is a trait showing continuous variation.
Whether a carrier of the retinoblastoma mutation described (b) The skin color of most F 1 offspring is usually between the
earlier develops the phenotype, and whether the cancer af- parental extremes, while the F 2 generation exhibits a broader
fects one or both eyes, depend on additional genetic events distribution of continuous variation.
that occur at random. To produce retinoblastoma, these a: © Rudi Von Briel/PhotoEdit
events must alter the second allele of the gene in specific
body cells. Examples of random events that can trigger the
onset of the disease include cosmic rays (to which humans
are constantly exposed) that alter the genetic material in
retinal cells or mistakes made during cell division in the
retina. Chance events provide the second hit—a mutation
in the second copy of the retinoblastoma gene—necessary
to turn a normal retinal cell into a cancerous one. The phe-
notype of retinoblastoma thus results from a specific heri-
table mutation in a specific gene, but the incomplete
penetrance and variable expressivity of the disease depend
on random genetic events that affect the other allele in
certain cells. The relationship between genotype and
phenotype as it applies to cancer will be discussed fully (a)
in Chapter 20.
By contributing to incomplete penetrance and variable Northern European whites African blacks
expressivity, modifier genes, the environment, and chance P
give rise to phenotypic variation. The probability of pene-
trance and the level of expressivity cannot be derived from Children of mixed marriages
the original Mendelian principles of segregation and inde- F
pendent assortment; they are established empirically by 1
observation and counting. Mating of F 1 individuals
F 2
Mendelian Principles Can Also Explain Amount of dark pigment in skin
Continuous Variation (b)
In Mendel’s experiments, height in pea plants was deter-
mined by two segregating alleles of one gene (in the
wild, plant height is determined by many genes, but in descent, for example, often seem to be a blend of their
Mendel’s inbred populations, the alleles of all but one of parents’ skin colors. Progeny of these F 1 individuals pro-
these genes were invariant). The phenotypes that resulted duce offspring displaying a wide range of skin pigmenta-
from these alternative alleles were clear-cut, either short tion; a few may be as light as the original Northern
or tall, and pea plant height was therefore known as a European parent, a few as dark as the original African
discontinuous trait (or discrete trait). In contrast, be- parent, but most will fall in a range between the two
cause human populations are not inbred, height in people (Fig. 3.27b). For such reasons, early human geneticists
is determined by alleles of many different genes whose were slow to accept Mendelian analysis. Because they were
interaction with each other and the environment produces working with outbred populations (populations in which
continuous phenotypic variation; height in humans is individuals differ in alleles of many genes), these scientists
thus an example of a continuous trait (or quantitative found very few examples of either-or Mendelian traits in
trait). Within human populations, individual heights normal, healthy people.
vary over a range of values that when charted on a graph By 1930, however, studies of corn and tobacco demon-
produce a bell curve (Fig. 3.27a). In fact, many human strated conclusively that it is possible to provide a Mende-
traits, including height, weight, and skin color, show lian explanation of continuous variation by simply
continuous variation, rather than the clear-cut alterna- increasing the number of genes contributing to a pheno-
tives analyzed by Mendel. type. The more genes, the more phenotypic classes, and the
Continuous traits often appear to blend and un- more classes, the more the variation appears continuous.
blend. Think for a moment of skin color. Children of mar- As a hypothetical example, consider a series of genes
riages between people of African and Northern European (A, B, C, . . .) all affecting the height of pole beans. For