140 ■ CHAPTER 08 Chromosomes and Human Genetics
GENETICS
chromosomes. If the two alleles at a locus are
different, the cell is heterozygous for the gene. If
the two alleles at a locus are identical, the cell is
homozygous for the gene (Figure 8.7).
The sex chromosomes, however, are different.
Roughly 1,240 of the estimated 20,000 human
genes are found on the X and Y chromosomes.
Approximately 1,180 of those 1,240 genes aresex-linked inheritance—an inherited mutation
on the X or Y chromosome. But where exactly
was the mutation located?
The physical location of a gene on a chro-
mosome is called its locus (plural “loci”).
Because a gene can occur in different versions,
or alleles, a diploid cell can have two different
alleles at a given locus on a pair of homologousPeople with Down syndrome are born
with an extra copy of chromosome 21.People with Turner syndrome are born
with one less X chromosome.Two copies of
each chromosomeDeletion, in which
a segment breaks
off and is lost from
the chromosome.Inversion, in which
a segment breaks
off and is
reattached in
reverse order.Translocation, in
which a segment
breaks off one
chromosome and
becomes attached
to a different,
nonhomologous
chromosome.Duplication, in
which a
chromosome
becomes longer
after acquiring an
extra copy of one
of its chromosome
segments.People with cri du chat
syndrome are born with a
deletion on chromosome 5.NormalTrisomyMonosomyChange in chromosome numberDeletionInversionChange in chromosome structureTranslocationDuplicationDuplicationNonhomologous
chromosomesFigure 8.6
Chromosomal abnormalities can cause serious genetic disorders
Any increase or decrease in the number of chromosomes almost invariably results in spontaneous abortion of the fetus, which
is estimated to occur in up to 20 percent of all pregnancies. Down syndrome and a missing or additional sex chromosome are
exceptions. Changes in chromosome structure may have relatively minor or more severe effects, depending on the size and
location of the change.
Q1: Why are changes in chromosome number almost always more severe than changes in chromosome structure?Q2: In which part of meiosis would you predict that chromosomal abnormalities are produced? (Refer back to Chapter 6 if
necessary.)Q3: Create a mnemonic to help remember the four kinds of structural changes (for example, Doctors Improve Treatment Daily).