Page 44 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

36 Chapter 2 Mendel’s Principles of Heredity

mucus builds up outside the cells. Thus, CF CF homozy- very recently led to effective treatments for the disease
gotes have no functional CFTR (or not enough of this pro- in patients with particular mutant alleles. For example,
tein) and exhibit cystic fibrosis. Gene therapy—insertion in 2015 the U.S. Food and Drug Administration ap-
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of a normal CF gene into lung cells of patients—has been proved a drug cocktail called Orkambi that helps the
tried to ameliorate the disease’s debilitating symptoms, but particular defective form of CFTR specified by one of
so far without success. these alleles to function properly. Varied approaches to
Despite the failure to date of gene therapy, identifi- the treatment of cystic fibrosis and other inherited dis-
cation of the gene responsible for cystic fibrosis has eases will be discussed later in the book.

essential concepts
• In a vertical pattern of transmission, a trait that appears in • Various kinds of biochemical events may explain
an affected individual also appears in at least one parent, why some disease alleles are dominant. In the case
one of the affected parent’s parents, and so on. If a trait is of Huntington disease, the disease-causing HD allele
rare, a pedigree with a vertical pattern usually indicates specifies an abnormal, deleterious version of the
that the disease-causing allele is dominant. protein produced by the normal, recessive allele.
• In a horizontal pattern of transmission, a trait that appears • Recessive disease alleles, like the CF alleles that
in an affected individual may not appear in any ancestors, cause cystic fibrosis, usually specify either no protein
but it may appear in some of the person’s siblings. A or less-functional versions of the protein that the
pedigree with a horizontal pattern usually indicates a rare normal, dominant allele produces.
recessive disease-causing allele. Affected individuals are
often products of consanguineous mating.

WHAT’S NEXT

Mendel answered the three basic questions about heredity the studies uncovered unanticipated phenotypic ratios, or the
as follows: To What is inherited? he replied, “alleles of results included F 1 and F 2 progeny with novel phenotypes
genes.” To How is it inherited? he responded, “according that resembled those of neither pure-breeding parent.
to the principles of segregation and independent assort- These phenomena could not be explained by Mendel’s
ment.” And to What is the role of chance in heredity? he hypothesis that for each gene, two alternative alleles, one
said, “for each individual, inheritance is determined by completely dominant, the other recessive, determine a sin-
chance, but within a population, this chance operates in a gle trait. We now know that most common traits, including
context of strictly defined probabilities.” skin color, eye color, and height in humans, are determined
Within a decade of the 1900 rediscovery of Mendel’s by interactions between two or more genes. We also know
work, numerous breeding studies had shown that Mendel’s that within a given population, more than two alleles may
laws hold true not only for seven pairs of antagonistic charac- be present for some of those genes. Chapter 3 shows how
teristics in peas, but also for many diverse traits in a wide the genetic analysis of such complex traits, that is, traits
variety of sexually reproducing plant and animal species. produced by complex interactions between genes and be-
Some of these same breeding studies, however, raised a chal- tween genes and the environment, extended rather than
lenge to the new genetics. For certain traits in certain species, contradicted Mendel’s laws of inheritance.

SOLVED PROBLEMS

Solving Genetics Problems like puzzles. Take them in slowly—don’t be overwhelmed
The best way to evaluate and increase your understanding by the whole problem. Identify useful facts given in the
of the material in the chapter is to apply your knowledge in problem, and use the facts to deduce additional informa-
solving genetics problems. Genetics word problems are tion. Use genetic principles and logic to work toward the
DNA: © Design Pics/Bilderbuch RF