new human being. The zygote has two sets of chromosomes (46), the appropriate
number for the human species. Thus, in the process of fertilization, the human genetic
makeup is reconstituted.
b. The existence of separate male and female sexes provides an important
advantage. Each individual is the product of a new combination of human genetic
material. Thus, there is always the potential for improvement in the human species.
a. Genotype/Phenotype. The genotype is the actual genetic makeup of an
individual. The phenotype is the physical and functional makeup of an individual as
determined both by the genotype and the environment.
b. Dominant/Recessive. Consider a gene in one set of chromo-somes and
the corresponding gene in the other set. If one of the genes alone can produce a
characteristic of the phenotype, the gene is said to be dominant. If both genes must be
the same to produce a characteristic of the phenotype, then the genes are recessive. In
a situation where one of the pair is dominant and the other is recessive, the dominant
gene determines the ultimate characteristic.
c. Homozygous/Heterozygous. Again, consider a gene in one set of
chromosomes and the corresponding gene in the other set. If the two genes are the
same, we say that the individual is homozygous for that trait. If the two genes are
different, we say that the individual is heterozygous for that trait.
d. Fraternal/Identical. In multiple births, two or more of the newborn may or
may not resemble each other closely. They may resemble each other in sex (gender)
and other physical and functional traits.
(1) If two of the individuals are different, they are called fraternal twins.
(2) If they closely resemble each other, they are called identical twins.
Identical twins are believed to originate in a common zygote, which separates into two
entities at a very early stage. Thus, identical twins have the same genetic makeup.
However, one is often right-oriented and the other left-oriented.
a. The Monohybrid Crossing (Figure 14-2). Again, consider a gene in one
set of chromosomes and the corresponding gene in the other set. This involves two
genes of a single inherited element. Assume that each parent has one dominant gene
(A) and one recessive gene (a), a heterozygous condition (Aa). Thus, 50% of the
gametes from each parent will carry the dominant gene (A), and 50% of the gametes
will carry the recessive gene (a). The potential crossings of the genes are AA, Aa, Aa,