Page 38 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

30 Chapter 2 Mendel’s Principles of Heredity

recessive y allele does not. Homozygous YY or heterozy- 2.3 Mendelian Inheritance
gous Yy peas are yellow because they each have enough Sgr
to break down all the chlorophyll. Homozygous yy peas in Humans
stay green because they lack the Sgr enzyme, and the chlo-
rophyll remains.
Two general principles emerge from these molecular learning objectives
discoveries. First, a specific gene determines a specific 1. Analyze human pedigrees to determine whether a
protein (in these cases an enzyme). The activity of the genetic disease exhibits recessive or dominant
protein may affect the phenotype of the pea plant in any inheritance.
number of ways, depending on the biochemical pathway 2. Explain why Huntington disease is inherited as a
in which it functions. Second, a pattern can be seen in dominant allele while cystic fibrosis is caused by a
both of these examples: The dominant allele determines a recessive allele.
normally functioning protein, while the recessive allele
does not specify a functional protein. You will see in
Chapter 3 that, although it is certainly not always the case, Although many human traits clearly run in families, most
the molecular explanation described here is the most do not show a simple Mendelian pattern of inheritance.
common reason why one allele is dominant to another Suppose, for example, that you have brown eyes, but both
(recessive) allele. Genes likely to be those Mendel de- your parents’ eyes appear to be blue. Because blue is nor-
scribed for stem length and flower color have also been mally considered recessive to brown, does this mean that
identified recently. In both cases, the dominant allele en- you are adopted or that your father isn’t really your father?
codes a normally functioning protein, and the recessive Not necessarily, because eye color is influenced by more
allele specifies either no protein or a less functional ver- than one gene.
sion of the normal protein. Like eye color, most common and obvious human phe-
notypes arise from the interaction of many genes. In contrast,
single-gene traits in people usually involve an abnormality

essential concepts that is disabling or life-threatening. Examples are the pro-
gressive neurological damage of Huntington disease and the
• Discrete units called genes control the appearance of clogged lungs and potential respiratory failure of cystic fi-
inherited traits; genes come in alternative forms called brosis. A defective allele of a single gene gives rise to Hun-
alleles. tington disease; defective alleles of a different gene are
• A sexually reproducing organism’s body cells contain two responsible for cystic fibrosis. Table 2.1 lists some of the
alleles for every gene. These alleles may be the same (in roughly 6000 such single-gene, or Mendelian, traits known
a homozygote) or different (in a heterozygote). in humans as of 2016. As you will see, the allele that causes
• Genotype refers to the alleles an individual possesses; Huntington disease is dominant and the normal (nondisease)
phenotype refers to the traits the individual exhibits. allele of this gene is recessive. The opposite is true for cystic
• The dominant allele controls the phenotype of a trait in fibrosis—the disease-causing allele is recessive and the nor-
heterozygotes; the other allele in the heterozygote is mal (nondisease) allele is dominant.
recessive. In monohybrid crosses, the dominant and
recessive phenotypes will appear in the progeny in a ratio
of 3:1. Pedigrees Aid the Study of Hereditary
• Alleles segregate during the formation of gametes, which Traits in Human Families
thus contain only one allele of each gene. Male and
female gametes unite at random at fertilization. These two Determining a genetic defect’s pattern of transmission is
processes correspond to Mendel’s law of segregation. not always an easy task because people make slippery ge-
• The segregation of alleles of any one gene is netic subjects. Their generation time is long, and the fami-
independent of the segregation of the alleles of other lies they produce are relatively small, which makes
genes. This principle is Mendel’s law of independent statistical analysis difficult. Humans do not base their
assortment. According to this law, crosses between Aa choice of mates on purely genetic considerations. Thus, no
Bb dihybrids will generate progeny with a phenotypic pure-breeding lines exist and no controlled matings are
ratio of 9 (A– B–) : 3 (A– bb) : 3 (aa B–) : 1 (aa bb). possible. Furthermore, people rarely produce a true F 2 gen-
• Most often, the dominant allele of a gene specifies a eration (like the one in which Mendel observed the 3:1 ra-
functional product (a protein), while the recessive allele tios from which he derived his rules) because brothers and
determines either a less functional or nonfunctional sisters almost never mate.
version of the protein, or no protein at all. Geneticists circumvent these difficulties by working
with a large number of families or with several generations