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Chapter 15 - Chromosomes Notes

Chapter 15 - Chromosomes Notes - AP BIOLOGY CH 15 SEX...

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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 daughter cells. • 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 involved) • Trisomic cells have three copies of a particular chromosome type and have 2n + 1 total chromosomes. • 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 chromosome 21. 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 humans. • 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 somatic cells. • 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 females. • 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 retardation. ORGANISMS WITH MORE THAN TWO COMPLETE SETS OF CHROMOSOMES, HAVE UNDERGONE POLYPOIDY. • 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 animals. • 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 structure. • 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 problems. • 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 sister chromatid. • 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 lethal outcome. • 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 chromosome 15. • 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 chloroplasts. • 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. 4 ...
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