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Chapter 28
Modern
Mendelian
Genetics
(I) Gene-Chromosome Theory
• Genes exist in a linear
fashion on
chromosomes
• Two genes associated
with a specific
characteristic are
known as alleles and
are located on
homologous
chromosomes
Gene Linkage
• Genes for different traits are located
on the same chromosome pair, and
are said to be linked
• Therefore they are usually inherited
together
Crossing-Over
• The chromatids of homologous chromosomes
often twist around each other, break, exchange
segments and rejoin. Crossing-over is a source of
genetic variation in sexual reproduction
(II) Sex-Linked Traits
1. Each human cell contains 46 chromosomes
22 pairs of autosomes
1 pair of sex chromosomes
a) sex chromosomes are designated as
“X” and “Y”
XX= female
XY= male
2. The sex of a human is genetically
determined at fertilization when
a sperm cell containing either
the X or Y chromosome unites
with an egg cell containing
the X chromosome
3. Sex-Linkage
a) Morgan’s work with Drosophila demonstrated that
genes for certain traits are located on the X chromosome
and do not appear on the Y chromosome
b) genes found on the X chromosome are said to be
sex-linked genes
c) recessive traits that are sex-linked occur more
frequently in males than in females. For the recessive
trait to show in a female, she must be homozygous
recessive. The gene must be present on both of her X
chromosomes ( X` X`)
d) for the recessive trait to show in males, the recessive
gene on the X chromosome will be expressed since it
`
does not appear on the Y chromosome ( X Y)
Human Disorders associated with
Sex-Linked Genes
1. Hemophilia- disease in which the blood does
not clot properly
2. Colorblindness- inability to see certain colors,
most commonly red and green
* Both of these disorders are more common in
males than in females because a female will
not show the disorder as long as she has one
normal gene. Females who are heterozygous
for a sex-linked trait are said to be carriers
for that trait
Sex-Linked Traits
XH
Xh
XH
XH XH
XH XH
Y
XH Y
Xh Y
How many offspring have hemophilia?
Sex-Linked Traits
XH
Xh
XH
XH XH
XH XH
Y
XH Y
Xh Y
How many offspring have hemophilia? 1
Sex-Linked Traits
XH
Xh
Xh
XH Xh
Xh Xh
Y
XH Y
Xh Y
How many offspring have hemophilia?
How many offspring are carriers?
Sex-Linked Traits
XH
Xh
Xh
H
h
X X
Xh Xh
Y
XH Y
Xh Y
How many offspring have hemophilia? 2
How many offspring are carriers? 1
Sex-Linked Traits
XH
Xh
Xh
XH Xh
Xh Xh
Y
XH Y
Xh Y
How many offspring have hemophilia?
How many offspring are carriers?
Human Pedigree
(III)
Mutations
• Changes in genetic material are called mutations
• If a mutation occurs in the sex cell, it may be
transmitted to the offspring
• Mutations occurring in body cells may be passed
on to new cells of the individual due to mitosis,
but will not be transmitted to the offspring by
sexual reproduction
Ex: cancer
• Mutations can be classified as chromosomal
alterations or gene mutations
Chromosomal Alterations
• Is a change in the number of
chromosomes or in the structure of the
chromosome
• The effects of chromosomal alteration are
often visible in the phenotype of an
organism because many genes are usually
involved
• An example of chromosomal alteration is
nondisjunction
Nondisjunction
• One or more pairs of
homologous chromosomes
fail to separate.
• This results in gametes
with more (or less) than
their normal haploid
chromosome number
If these gametes are involved in fertilization, the
resulting zygote may have more (or less) than the
normal diploid chromosome number
Examples:
1. Down’s Syndrome- results from the possession
of an extra chromosome. This is due to the
nondisjunction of chromosome #21 in one of the
parents. Will result in mental retardation or
physical abnormalities
2. Polyploidy- sometimes all 22 chromosomal pairs
fail to separate. The resulting 2n gamete fuses
with the normal n gamete, producing a 3n zygote.
This is common in plants but rare in humans
Gene Mutations
• Involves a random change in the
chemical nature of the genetic
material (DNA)
• Some gene mutations, like albinism
are obvious, while others are not
noticeable
A woman with Abinism
Mutations
Mutations
Mutations
Polydactyl mutation
(IV) Mutagenic Agents
• Causes mutations
1. Radiation- ultraviolet light, x-rays,
radioactive substances
2. Chemicals- asbestos fibers
(V) Human Genetic Disorders
1. Phenylketonuria (PKU)
• A disorder in which the body
cannot synthesize an enzyme
necessary for the normal
metabolism of phenylalanine
• Results in mental retardation
2.
Sickle-cell Anemia
• A gene mutation that results in the
production of abnormal
hemoglobin molecules and
abnormal red blood cells
• African descent
3.
Tay-Sachs
• Deterioration of the nervous system
due to the accumulation of fatty
material as a result of the inability
to synthesize a specific enzyme
• Jewish people of Central Europe
descent
(VI) Types of Genetic Disorder
Detection Techniques
1.
Screening
• Chemical analysis of body
fluids such as blood and urine
• Detection of PKU and
Tay-Sachs
2.
Karyotyping
• The preparation of an
enlarged photograph of
chromosomes
2.
Karyotyping
3.
Amniocentesis
• Removal of amniotic fluid for
chemical and/or cellular
analysis
• Detection of sickle-cell anemia
3.
Amniocentesis