CHROMOSOMES AND GENES
Each of the cells of an individual (except mature red
blood cells and egg and sperm cells) contains 46 chro-
mosomes, the diploid number. These chromosomes
are in 23 pairs called homologous pairs. One mem-
ber of each pair has come from the egg and is called
maternal, the other member has come from the sperm
and is called paternal. The autosomes are the chromo-
some pairs designated 1 to 22. The sex chromosomes
form the remaining pair. In women these are desig-
nated XX and in men XY.
Chromosomes are made of DNA and protein; the
DNA is the hereditary material. You may wish to refer
to Chapter 3 to review DNA structure. The sequence
of bases in the DNA of chromosomes is the genetic
code for proteins, structural proteins as well as
enzymes. The DNA code for one protein is called a
gene. For example, a specific region of the DNA of
chromosome 11 is the code for the beta chain of
hemoglobin. Because an individual has two of chro-
mosome 11, he or she will have two genes for this pro-
tein, a maternal gene inherited from the mother and a
paternal gene inherited from the father. This is true
for virtually all of the 20,000 to 25,000 genes estimated
to be found in our chromosomes. In our genetic
makeup, each of us has two genes for each protein.
GENOTYPE AND PHENOTYPE
For each gene of a pair, there may be two or more pos-
sibilities for its “expression,” that is, how it will appear
or how it will affect the individual. These possibilities
are called alleles. A person, therefore, may be said to
have two alleles for each protein or trait; the alleles
may be the same or may be different.
If the two alleles are the same, the person is said to
be homozygousfor the trait. If the two alleles are dif-
ferent, the person is said to be heterozygousfor the
trait.
The genotypeis the actual genetic makeup, that is,
the alleles present. The phenotypeis the appearance,
or how the alleles are expressed. When a gene has two
or more alleles, one allele may be dominantover the
other, which is called recessive. For a person who is
heterozygous for a trait, the dominant allele (or gene)
is the one that will appear in the phenotype. The
recessive allele (or gene) is hidden but is not lost, and
may be passed to children. For a recessive trait to be
expressed in the phenotype, the person must be
homozygous recessive, that is, have two recessive al-
leles (genes) for the trait.
An example will be helpful here to put all this
together and is illustrated in Fig. 21–7. When doing
genetics problems, a Punnett squareis used to show
the possible combinations of genes in the egg and
sperm for a particular set of parents and their children.
Remember that an egg or sperm has only 23 chromo-
somes and, therefore, has only one gene for each trait.
In this example, the inheritance of eye color has
been simplified. Although eye color is determined by
many pairs of genes, with many possible phenotypes,
one pair is considered the principal pair, with brown
eyes dominant over blue eyes. A dominant gene is usu-
ally represented by a capital letter, and the correspon-
ding recessive gene is represented by the same letter,
but lowercase. The parents in Fig. 21–7 are both het-
erozygous for eye color. Their genotype consists of a
gene for brown eyes and a gene for blue eyes, but their
phenotype is brown eyes.
Each egg produced by the mother has a 50%
chance of containing the gene for brown eyes, or an
equal 50% chance of containing the gene for blue eyes.
Similarly, each sperm produced by the father has a
50% chance of containing the gene for brown eyes and
a 50% chance of containing the gene for blue eyes.
Now look at the boxes of the Punnett square; these
represent the possibilities for the genetic makeup of
each child. For eye color there are three possibilities:
a 25% (one of four) chance for homozygous brown
eyes, a 50% (two of four) chance for heterozygous
brown eyes, and a 25% (one of four) chance for
homozygous blue eyes. Notice that BB and Bb have
the same phenotype (brown eyes) despite their differ-
ent genotypes, and that the phenotype of blue eyes is
possible only with the genotype bb. Can brown-eyed
parents have a blue-eyed child? Yes; if each parent is
heterozygous for brown eyes, each child has a 25%
chance of inheriting blue eyes. Could these parents
have four children with blue eyes? What is the proba-
bility or chance of this happening? The answers to
these questions will be found in Box 21–7: Solution to
Genetics Question.INHERITANCE: DOMINANT–RECESSIVE
The inheritance of eye color just described is an exam-
ple of a trait determined by a pair of alleles, one
of which may dominate the other. Another example
is sickle-cell anemia, which was discussed in ChapterHuman Development and Genetics 487