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Biology
Biology
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14–2 Human Chromosomes
14-2 Human Chromosomes
Photo credit: Richard Hutchings/Photo Researchers, Inc.
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Sex-Linked Genes
Sex-Linked Genes
The X chromosome and the Y chromosomes determine sex.
Genes located on these chromosomes are called sex-linked genes.
More than 100 sex-linked genetic disorders have now been mapped to the X chromosome.
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Sex-Linked Genes
X Chromosome
Duchenne muscular dystrophy
The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes.
Melanoma
X-inactivation center
X-linked severe combined immunodeficiency (SCID)
Colorblindness
Hemophilia
Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes.
Y Chromosome
Testis-determining
factor
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Sex-Linked Genes
Why are sex-linked disorders more common in males than in females?
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Sex-Linked Genes
For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes.
Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
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Sex-Linked Genes
Colorblindness
Three human genes associated with color vision are located on the X chromosome.
In males, a defective version of any one of these genes produces colorblindness.
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Sex-Linked Genes
Possible Inheritance of Colorblindness Allele
X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles.
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Sex-Linked Genes
Hemophilia
The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia.
In hemophilia, a protein necessary for normal blood clotting is missing.
Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised.
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Sex-Linked Genes
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle.
It is caused by a defective version of the gene that codes for a muscle protein.
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12. X-Chromosome Inactivation
British geneticist Mary Lyon discovered that in female cells, one X chromosome is randomly switched off.
This chromosome forms a dense region in the nucleus known as a Barr body.
Barr bodies are generally not found in males because their single X chromosome is still active.
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13. Chromosomal Disorders
What problems does nondisjunction cause?
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Nondisjunction occurs when either homologues fail to separate during anaphase I of meiosis, or sister chromatids fail to separate during anaphase II. The result is that one gamete has 2 copies of one chromosome and the other has no copy of that chromosome. (The other chromosomes are distributed normally.)
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If either of these gametes unites with another during fertilization, the result is aneuploidy (abnormal chromosome number) A trisomic cell has one extra chromosome (2n +1) = example: trisomy 21. (Polyploidy refers to the condition of having three homologous chromosomes rather then two) A monosomic cell has one missing chromosome (2n - 1) = usually lethal except for one known in humans: Turner's syndrome (monosomy XO).
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. Monosomy X (Turner's syndrome): 1: live births; the only viable monosomy in humans - women with Turner's have only 45 chromosomes!!! XO individuals are genetically female, however, they do not mature sexually during puberty and are sterile. Short stature and normal intelligence. (98% of these fetuses die before birth)
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Klinefelter syndrome: 47, XXY males. Male sex organs; unusually small testes, sterile. Breast enlargement and other feminine body characteristi cs. Normal intelligence.
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Trisomy X: 47, XXX females. 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female except by karyotype
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The frequency of nondisjunction is quite high in humans, but the results are usually so devastating to the growing zygote that miscarriage occurs very early in the pregnancy. If the individual survives, he or she usually has a set of symptoms - a syndrome - caused by the abnormal dose of each gene product from that chromosome.
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24. Chromosomal Disorders
The most common error in meiosis occurs when homologous chromosomes fail to separate.
This is known as nondisjunction, which means, “not coming apart.”
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25. Chromosomal Disorders
If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.
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26. Chromosomal Disorders
Nondisjunction
Homologous chromosomes fail to separate.
Meiosis I:
Nondisjunction
Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome.
Meiosis II
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27. Chromosomal Disorders
Down Syndrome
If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome.
Down syndrome involves three copies of chromosome 21.
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28. Chromosomal Disorders
Down Syndrome Karyotype
Down syndrome produces mild to severe mental retardation.
It is characterized by:
increased susceptibility to many diseases
higher frequency of some birth defects
This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake
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29. Chromosomal Disorders
Sex Chromosome Disorders
In females, nondisjunction can lead to Turner’s syndrome.
A female with Turner’s syndrome usually inherits only one X chromosome (karyotype 45,X).
Women with Turner’s syndrome are sterile.
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30. Chromosomal Disorders
In males, nondisjunction causes Klinefelter’s syndrome (karyotype 47,XXY).
The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing.
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14–2
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14–2
The average human gene consists of how many base pairs of DNA?
3000
300 20
30,000
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14–2
Which of the following genotypes indicates an individual who is a carrier for colorblindness?
XCX
XCXc
XcY
XCY
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14–2
Colorblindness is much more common in males than in females because
the recessive gene on the male’s single X chromosome is expressed.
genes on the Y chromosome make genes on the X chromosome more active.
females cannot be colorblind.
colorblindness is dominant in males and recessive in females.
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14–2
The presence of a dense region in the nucleus of a cell can be used to determine the
sex of an individual.
blood type of an individual.
chromosome number of an individual.
genotype of an individual.
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14–2
Nondisjunction occurs during
meiosis I.
mitosis.
meiosis II.
between meiosis I and II.
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