1096
UNIT 5
Continuity
29
Te complete human
karyotype
(kar
9
e-o-tīp), or diploid chro-
mosomal complement displayed in homologous pairs, is illustrated
in
Figure 29.1
3
. Te diploid
genome
(je
9
nōm), or genetic (DNA)
makeup, represents two sets of genetic instructions—one from the
egg and the other from the sperm.
Gene Pairs (Alleles)
Because chromosomes are paired, it follows that the genes in
them are paired as well. Consequently, each of us receives
two
genes, one from each parent (for the most part), that interact
to dictate each trait. Matched genes, which are at the same
lo-
cus
(location) on homologous chromosomes, are called
alleles
(ah-lēlz
9
) of each other. Alleles may code for the same or for
alternative forms of a given trait. For example, of the alleles that
dictate whether or not you have loose thumb ligaments, one
allele might code for tight ligaments and the other for loose
ligaments (the double-jointed thumb condition). When the two
alleles controlling a trait are the same, a person is said to be
ho-
mozygous
(ho-mo-zi
9
gus) for that trait. When the two alleles
are different, the individual is
heterozygous
(het
0
er-o-zi
9
gus)
for the trait.
Sometimes, one allele masks or suppresses the expression of
its partner. Such an allele is said to be
dominant
, whereas the
allele that is masked is said to be
recessive
. By convention, a
dominant allele is represented by a capital letter (for example,
J
),
However, the urge to understand human inheritance is pow-
erful. Te
Human Genome Project
, which scanned and deter-
mined the human DNA sequence, is enabling geneticists to
manipulate and engineer human genes to examine their expres-
sion. Tis research has tremendous promise for more sophisti-
cated genetic screening, and for drug development to treat or
cure disease. Although we touch upon some of these newer top-
ics, we will concentrate on the principles of heredity discovered
by Mendel more than a century ago.
The Vocabulary of Genetics
Define allele.
Differentiate between genotype and phenotype.
Te nuclei of all human cells except gametes contain the diploid
number of chromosomes (46), consisting of 23 pairs of homolo-
gous chromosomes. Recall that
homologous chromosomes
are
pairs of chromosomes—one from the father (sperm) and one
from the mother (egg)—that look similar and carry genes for
the same traits, but do not necessarily bring about the same
expressions of those traits. ±wo of the 46 chromosomes are
sex
chromosomes
(X and Y), which determine genetic sex (male
5
XY; female
5
XX). Te other 44 are the 22 pairs of
autosomes
that guide the expression of most other traits.
FPO
1
2
3
The slide is viewed
with a microscope, and
the chromosomes are
photographed.
The photograph is entered into a computer, and
the chromosomes are electronically rearranged into
homologous pairs according to size and structure.
The resulting display is the karyotype, which is
examined for chromosome number and structure.
Figure 29.1
Preparing a karyotype.
After lymphocytes are stimulated to divide
and grow in culture for several days, they
are treated with a drug that arrests mitosis in
metaphase, a stage when the chromosomes
are easily identified. The cells are harvested,
treated with a solution that makes their
chromosomes spread out, photographed,
and then subjected to computer analysis
and arrangement of the chromosomes
into homologous pairs. The patterns of
stained bands in the karyotype help to
identify specific chromosomes and parts of
chromosomes.
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