Page 76 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

68 Chapter 3 Extensions to Mendel’s Laws

Figure 3.24 Family pedigrees help unravel the genetic • When genes display redundancy for a trait, one dominant
basis of ocular-cutaneous albinism (OCA). (a) An albino and normally functioning allele of either gene is sufficient
Nigerian girl and her sister celebrating the conclusion of the All to generate the normal phenotype.
Africa games. (b) A pedigree following the inheritance of OCA in • Many traits exhibit locus heterogeneity, in which
an inbred family indicates that the trait is recessive. (c) A family in
which two albino parents have nonalbino children demonstrates that homozygosity for mutations at any one of several genes
homozygosity for a recessive allele of either of two genes can can produce the same mutant phenotype.
cause OCA. • Complementation occurs in the progeny of pure-breeding
a: © Radu Sigheti/Reuters parents with the same mutant phenotype if the parents
(a) Ocular-cutaneous albinism (OCA) are homozygous for recessive, nonfunctional alleles of
different genes whose products function in a common
pathway.

3.3 Extensions to Mendel for
Multifactorial Inheritance

learning objectives
1. Discuss the factors that can cause different individuals
with the same genotype to be phenotypically dissimilar.
(b) OCA is recessive 2. Explain how Mendelian genetics is compatible with the
fact that many traits, such as human height and skin
I colors, exhibit continuous variation.

II
The inheritance of many traits appears to be more complex
III
than can be explained by the participation of only one or two
IV genes in patterns compatible with straightforward Mendelian
principles. Of course, one reason for this complexity is that
(c) Complementation for albinism
more than two genes can influence certain traits. But a second
reason is that genes are not the only players: The environment
and chance events can sometimes exert considerable effects
aa BB AA bb on traits that are otherwise genetically determined. In this
Normal section we discuss multifactorial traits—traits determined
Albino by several different genes, or by the interaction of genes with
the environment.

Aa Bb Aa Bb Aa Bb
The Same Genotype Does Not Always
Produce the Same Phenotype

In our discussion of gene interactions so far, we have con-
sidered examples in which a genotype reliably fashions a
essential concepts
particular phenotype. But this is not always what happens.
• Two genes may interact to affect a single trait; these Sometimes a genotype is not expressed at all; that is, even
interactions may be detected by ratios that can be though the genotype is present, the expected phenotype
predicted from Mendelian principles. does not appear. Other times, the trait caused by a genotype
• Retention of the 9:3:3:1 phenotypic ratio usually indicates is expressed to varying degrees or in a variety of ways in
that two genes function in independent pathways and different individuals. Factors that alter the phenotypic ex-
their alleles interact additively. pression of genotype include modifier genes, the environ-
• In epistasis, an allele at one gene can hide traits ment, and chance. These factors complicate the interpretation
otherwise caused by alleles at another gene. of breeding experiments.