Unformatted text preview: AP BIOLOGY. CH. 15 SEX CHROMOSOME DISORDER-CHROMOSOMAL MUTATIONS ALTERATIONS OF CHROMOSOME NUMBER CAUSE SOME GENETIC DISORDERS
• Nondisjunction occurs when problems with the meiotic spindle cause errors in
• This may occur if tetrad chromosomes do not separate properly during meiosis I.
• Alternatively, sister chromatids may fail to separate during meiosis II.
• Offspring results from fertilization of a normal gamete with one after nondisjunction
will have an abnormal chromosome number or aneuploidy. (less than an entire set
• Trisomic cells have three copies of a particular chromosome type and have 2n + 1
• Although the frequency of aneuploid zygotes may be quite high in humans,
most of these alterations are so disastrous that the embryos are spontaneously
aborted long before birth.
• These developmental problems result from an imbalance among gene products.
• Certain aneuploid conditions upset the balance less, leading to survival to birth
and beyond. This is likely to have a substantial effect on the organism.
• These individuals have a set of symptoms — a syndrome — characteristic of
the type of aneuploidy. (Trisomy 13: Patau— Trisomy 18: Edwards—
Trisomy 21: Downs)
• One aneuploid condition, Down syndrome, is due to three copies of
It affects one in 700 children born in the United States.
Although chromosome 21 is the smallest human chromosome, it severely
alters an individual’s phenotype in specific ways.
Most cases of Down syndrome result from nondisjunction during gamete
production in one parent.
The frequency of Down syndrome correlates with the age of the mother.
• Monosomic cells have only one copy of a particular chromosome type and have
2n - 1 chromosomes.
• If the organism survives, aneuploidy typically leads to a distinct phenotype.
Estimated that ½ of conceptions may be aneuploid and that 70% of early embryonic
deaths and spontaneous abortions are caused by aneuploidy. 1 AP BIOLOGY. CH. 15 SEX CHROMOSOME DISORDER-CHROMOSOMAL MUTATIONS Nondisjunction of sex chromosomes produces a variety of aneuploid conditions in
• Unlike autosomes, this aneuploidy upsets the genetic balance less severely.
• This may be because the Y chromosome contains relatively few genes.
• Also, extra copies of the X chromosome become inactivated as Barr bodies in
• Klinefelter’s syndrome, an XXY male, occurs once in every 2000 live births.
• Phenotypically male sex organs, but are sterile. There may be feminine
characteristics. No mental retardation.
• Males with an extra Y chromosome (XYY) tend to somewhat taller than average.
• Trisomy X (XXX), which occurs once in every 2000 live births, produces healthy
• Monosomy X or Turner’s syndrome (X0), which occurs once in every 5000 births,
produces phenotypic, but immature females.
• Short, wide chested. Underdeveloped breasts. Narrowing of aorta. No mental
ORGANISMS WITH MORE THAN TWO COMPLETE SETS OF CHROMOSOMES, HAVE
• This may occur when a normal gamete fertilizes another gamete in which there has
been nondisjunction of all its chromosomes.
• The resulting zygote would be triploid (3n).
o Most common form in humans. [69 XXY; 69 XXX]
o Usually the result of dispermy.
o Limited survivability. Enlarged head, syndactyly (fusion of fingers/toes),
malformation of mouth, eyes, genitals.
• Alternatively, if a 2n zygote failed to divide (no cytokinesis) after replicating its
chromosomes, a tetraploid (4n) embryo would result from subsequent successful
cycles of mitosis.
• Polyploidy is relatively common among plants and much less common among
• The spontaneous origin of polyploid individuals plays an important role in the
evolution of plants.
2 AP BIOLOGY. CH. 15 SEX CHROMOSOME DISORDER-CHROMOSOMAL MUTATIONS • Both fishes and amphibians have polyploid species.
• Polyploids are more nearly normal in phenotype than aneuploids.
• One extra or missing chromosome apparently upsets the genetic balance during
development more than does an entire extra set of chromosomes.
ALTERATIONS OF CHROMOSOME STRUCTURE CAUSE SOME GENETIC DISORDERS
• Breakage of a chromosome can lead to four types of changes in chromosome
• A deletion occurs when a chromosome fragment lacking a centromere is lost
during cell division.
• This chromosome will be missing certain genes.
• Deletions, even in a heterozygous state, cause severe physical and mental
• One syndrome, cri du chat, results from a specific deletion in chromosome 5.
These individuals are mentally retarded, have a small head with unusual facial
features, and a cry like the mewing of a distressed cat.
• A duplication occurs when a fragment becomes attached as an extra segment to a
• An inversion occurs when a chromosomal fragment reattaches to the original
chromosome but in the reverse orientation.
• In translocation, a chromosomal fragment joins a nonhomologous chromosome.
• Some translocations are reciprocal, others are not.
• Deletions and duplications are common in meiosis.
• Homologous chromatids may break and rejoin at incorrect places, such that one
chromatid will lose more genes than it receives.
• A diploid embryo that is homozygous for a large deletion or male with a large deletion
to its single X chromosome is usually missing many essential genes and this leads to a
• Duplications and translocations are typically harmful.
• Reciprocal translocation or inversion can alter phenotype because a gene’s expression
is influenced by its location. 3 AP BIOLOGY. CH. 15 SEX CHROMOSOME DISORDER-CHROMOSOMAL MUTATIONS THE PHENOTYPIC EFFECTS OF SOME MAMMALIAN GENES DEPEND ON WHETHER THEY
WERE INHERITED FROM THE MOTHER OR THE FATHER (IMPRINTING)
• For most genes it is a reasonable assumption that a specific allele will have the same
effect regardless of whether it was inherited from the mother or father.
• However, for some traits in mammals, it does depend on which parent passed along
the alleles for those traits.
• Two disorders with different phenotypic effects, Prader-Willi syndrome and
Angelman syndrome, are due to the same cause, a deletion of a specific segment of
• Prader-Willi syndrome is characterized by mental retardation, obesity (due to
compulsive eathing), short stature, and unusually small hands and feet.
• These individuals inherit the abnormal chromosome from their father.
• Individuals with Angelman syndrome exhibit spontaneous laughter, jerky
movements, and other motor and mental symptoms.
• This is inherited from the mother.
EXTRANUCLEAR GENES EXHIBIT A NON-MENDELIAN PATTERN OF INHERITANCE
• Not all of a eukaryote cell’s genes are located in the nucleus.
• Extranuclear genes are found on small circles of DNA in mitochondria (37 genes) and
• These organelles reproduce themselves.
• Their cytoplasmic genes do not display Mendelian inheritance.
• They are not distributed to offspring during meiosis.
• Because a zygote inherits all its mitochondria only from the ovum, all mitochondrial
genes in mammals demonstrate maternal inheritance.
• Several rare human disorders are produced by mutations to mitochondrial DNA.
• These primarily impact ATP supply by producing defects in the electron transport
chain or ATP synthase.
• Tissues that require high energy supplies (for example, the nervous system and
muscles) may suffer energy deprivation from these defects.
• Other mitochondrial mutations may contribute to diabetes, heart disease, and other
diseases of aging.
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