(c)1998 Brooks/Cole Publishing Company/ITP 1-800-590-9951
Last Updated August 1998
Chromosomes and Human Genetics (Chapter 12)
Too Young to Be Old
A.Because of a rare defect in an autosomal dominant gene,
persons suffering from Hutchinson-Gilford progeria syndrome
will age a lifetime in the first decade of life.
B.The analysis of human inheritance is a fascinating field of study
which is based on significant discoveries of the past.
1.By 1882, Walther Flemming had observed threadlike
chromosomes in the nuclei of dividing cells.
2.By 1887, August Weismann had suggested that meiosis
halves the number of chromosomes when gametes are
made.
3.By 1900, Mendel's work was finally appreciated, namely
his view that diploid cells have two units for each trait and
the units segregate during gamete formation.
I.The Chromosomal Basis of Inheritance-An Overview
A.Genes and Their Chromosome Locations
1.Genes are units of information about heritable traits.
2.Diploid organisms possess pairs of homologous
chromosomes, which are alike in length, shape, and gene
sequence.
3.Alleles are slightly different molecular forms of the same
gene, which are shuffled during meiosis.
4.Crossing over between homologous chromosomes
results in genetic recombination.
5.A chromosome's structure may change due to deletion,
duplication, inversion, or translocation.
B.Autosomes and Sex Chromosomes
1.Sex chromosomes determine gender.
a.Human females have two X chromosomes.
b.Males have one X and one Y.
2.Most of the chromosomes are of the same quantity and
type in both sexes and are called autosomes (44 in
humans).
C.Karyotype Analysis
1.Chromosomes are visualized in a lab preparation called a
karyotype.
2.Each chromosome has distinct size, length, centromere
location, and banding (staining) patterns.
II.Focus on Science: Preparing a Karyotype Diagram
III.Sex Determination in Humans
A.Each human egg will contain twenty-two autosomes plus one X;
but sperm will carry twenty-two autosomes plus either an X or
a Y.
B.The Y chromosome carries a male-determining gene which leads
to formation of the testes.
C.Absence of the male gene in females results in formation of
ovaries.
D.The X chromosome obviously codes for sexual traits, but it also
carries many genes for nonsexual traits.
IV.Early Questions About Gene Locations
A.Linked Genes-Clues to Inheritance Patterns
1.In his experiments using fruit flies, Thomas Hunt Morgan
confirmed that each gene has a specific location on a
chromosome.
2.Some of the most intriguing linkages are those of X-linked
and Y-linked genes.
3.Now we refer to the large number of linked genes on
specific chromosomes as linkage groups.
B.Crossing Over and Genetic Recombination
1.Linkage is the tendency of genes located on the same
chromosome to be transmitted together in inheritance.
2.Linkage can be disrupted by crossing over.
a.Crossing over is an exchange of parts of
homologous chromosomes.
b.Correlations between specific genes and
cytological markers provides evidence of genetic
recombination.
V.Recombination Patterns and Chromosome Mapping
A.How Close is Close? A Question of Recombination Frequencies
1.Morgan's analysis of fruit fly characteristics convinced him
that certain alleles that are linked on the same
chromosome tend to remain together during meiosis
because they are positioned closer together on the
chromosome.
2.This eventually led to the generalization that the probability
that a cross over will disrupt the linkage of two genes is
proportional to the distance that separates them.
B.Linkage Mapping
1.The careful analysis of recombination patterns in
experimental crosses has resulted in linkage mapping of
gene locations.
2.Linkage maps do not show the actual distances between
genes but rather gives the map distance which can then be
correlated with the physical distance.
VI.Human Genetic Analysis
A.Human genetics is difficult to study.
1.We live under variable conditions in diverse environments.
2.Humans mate by chance and may, or may not, choose to
reproduce.
3.Humans live as long as those who study them.
4.The small family size characteristic of human beings is not
sufficient for meaningful statistical analysis.
B.Constructing Pedigrees
1.A pedigree is a chart that shows genetic connections
among individuals.
2.The analysis of family pedigrees provides data on
inheritance patterns through several generations.
3.Knowledge of probability and Mendelian inheritance
patterns is used in analysis of pedigrees to yield clues to a
trait's genetic basis.
C.Regarding Human Genetic Disorders
1.Genetic abnormality is a term applied to a genetic
condition that is a deviation from the usual, or average,
and is not life-threatening.
2.Genetic disorder is more appropriately used to describe
conditions that cause medical problems.
3.Genetic disease is applied to those instances where a
person's genes increase susceptibility to infection or
weakens the response to it.
VII.Patterns of Autosomal Inheritance
A.Autosomal Recessive Inheritance
1.The characteristics of this condition are:
a.Either parent can carry the recessive allele on an
autosome.
b.Heterozygotes are symptom-free; homozygotes are
affected.
c.Two heterozygous parents have a 50 percent
chance of producing heterozygous children and a
25 percent chance of producing a homozygous
recessive child. When both parents are
homozygous, all children can be affected.
2.Galactosemia (the inability to metabolize lactose) is an
example of autosomal recessive inheritance in which a
single gene mutation prevents manufacture of an enzyme
needed in the conversion pathway. Tay-Sachs is another
example.
B.Autosomal Dominant Inheritance
1.The dominant allele is nearly always expressed and if it
reduces the chance of surviving or reproducing, its
frequency should decrease; mutations, nonreproductive
effects, and postreproductive onset work against this
hypothesis.
2.If one parent is heterozygous and other homozygous
recessive, there is a 50 percent chance that any child will
be heterozygous.
3.Huntington disorder is serious degeneration of the
nervous system with an onset from age 40 onward, by
which time the gene has (usually) been passed to offspring
unknowingly.
4.Achondroplasia (dwarfism) is a benign abnormality
which does not affect persons to the point that
reproduction is impossible so the gene is passed on.
VIII.Patterns of X-Linked Inheritance
A.X-Linked Recessive Inheritance
1.The characteristics of this condition are:
a.The mutated gene occurs only on the X
chromosome.
b.Heterozygous females are phenotypically normal;
males are more often affected because the single
recessive allele (on the X chromosome) is not
masked by a dominant gene.
c.A normal male mated with a female heterozygote
have a 50 percent chance of producing carrier
daughters and a 50 percent chance of producing
affected sons. In the case of a homozygous
recessive female and a normal male, all daughters
will be carriers and all sons affected.
2.A serious X-linked recessive condition is hemophilia A,
(affects 1/7,000 males), which is the inability of the blood
to clot because the genes do not code for the necessary
clotting agent(s).
3.Duchenne muscular dystrophy is a serious failure of the
muscles with an early onset leading to death by the early
twenties.
B.X-Linked Dominant Inheritance
1.The inheritance pattern is similar to that for X-linked
recessive alleles, except it is expressed in heterozygous
females, albeit rarely.
2.In faulty enamel trait the enamel coating of the teeth fails
to develop properly.
C.A Few Qualifications
1.Diagnosis of genetic conditions is complicated and
requires careful analysis of many sources of data by highly
skilled scientists.
2.Keep in mind that affliction with any one condition does
not relegate its possessor to the sidelines of life.
IX.Changes in Chromosome Number
A.Categories and Mechanisms of Change
1.Aneuploidy is a condition in which the gametes or cells of
an affected individual end up with one extra or one less
chromosome than is normal.
2.Polyploidy is the presence of three or more of each type
of chromosome in gametes or cells. It is common in plants
but fatal in humans.
a.A chromosome number can change during mitotic
or meiotic cell division or during the fertilization
process.
b.Tetraploid germ cells can result if cytoplasmic
division does not follow normal DNA replication
and mitosis.
3.Nondisjunction at anaphase I or anaphase II frequently
results in a change in chromosome number.
a.If a gamete with an extra chromosome (n + 1) joins
a normal gamete at fertilization, the diploid cell will
be 2n + 1; this condition is called trisomy.
b.If an abnormal gamete is missing a chromosome,
the zygote will be 2n - 1-monosomy.
B.Changes in the Number of Autosomes
1.Down syndrome results from trisomy 21; 1 in 1,100
liveborns in North America are affected.
2.Most children with Down syndrome show mental
retardation, and 40 percent have heart defects.
3.Down syndrome occurs more frequently in children born
to women over age 35.
C.Changes in the Number of Sex Chromosomes
1.Turner Syndrome
a.Turner syndrome involves females whose cells have
only one X chromosome (designated XO).
b.Affected individuals (1/2,500 to 10,000 girls) are
infertile and have other phenotypic problems such
as premature aging and shorter life expectancy.
c.About 75 percent of the cases are due to
nondisjunction in the father; furthermore, about 98
percent of all XO zygotes spontaneously abort.
2.Klinefelter Syndrome
a.Nondisjunction results in an extra X chromosome
in the cells (XXY) of these affected males (1/500
to 2,000 liveborn males).
b.About 67 percent of these result from
nondisjunction in the mother, 33 percent in the
father.
c.Slight mental retardation, sterility, and body
feminization are symptoms.
3.XYY Condition
a.The extra Y chromosome in these males (1/1,000)
does not affect fertility, but they are taller than
average and are slightly mentally retarded.
b.Erroneous correlations have linked these persons
with predisposition to crime.
X.Changes in Chromosome Structure
A.Deletion
1.A deletion is the loss of a chromosome segment as when
a terminal segment is lost, or when viruses, chemicals, or
irradiation cause breaks in a chromosome region
2.For example, the loss of a portion of chromosome 5
causes a disorder called cri-du-chat with its symptoms of
crying and mental retardation.
B.Duplications
1.Duplication occurs when a gene sequence is in excess of
the normal amount; apparently this is true of chromosome
regions that code for polypeptides of hemoglobin and is
not harmful.
2.In fragile X syndrome there are regions of abnormal
constriction (caused by repeated sequences) in the X
chromosomes of mentally impaired males.
C.Inversion
1.An inversion alters the position and sequence of the genes
so that gene order is reversed.
2.Although the effects of specific inversions are difficult to
determine, it is known that 18 of the human chromosome
pairs are identical to their counterparts in chimpanzees
while the other five pairs differ at inverted (or
translocated) regions.
D.Translocation
1.A translocation occurs when a part of one chromosome is
transferred to a nonhomologous chromosome.
2.An example of this is a form of cancer in which a segment
of chromosome 8 is attached to chromosome 14, or vice
versa.
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