24-1 Chapter 24: Patterns of Chromosome Inheritance
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- 24-1 Chapter 24: Patterns of Chromosome Inheritance
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- 24-2 Viewing the Chromosomes A karyotype is a display of chromosomes paired according to their size, location of the centromere, and staining patterns. A karyotype reveals abnormalities in chromosome number or structure. Humans have 23 pairs of chromosomes; 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males are XY.
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- 24-3 Amniocentesis Amniocentesis uses a needle to extract amniotic fluid from the uterus of a pregnant woman from the 14 th to 17 th week of pregnancy. Up to 400 chromosome and biochemical problems can be detected by culturing fetal cells that are in the amniotic fluid. There is a slight risk of spontaneous abortion with this procedure.
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- 24-4 Amniocentesis
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- 24-5 Chorionic Villi Sampling Chorionic villi sampling (CVS) uses a thin suction tube to sample chorionic cells from the placenta as early as the fifth week of pregnancy. The cells do not have to be cultured, and karyotyping can be done immediately. CVS carries a slightly greater risk of spontaneous abortion but can be performed earlier than amniocentesis.
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- 24-6 Chorionic villi sampling
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- 24-7 Karyotyping Sampled fetal cells are stimulated to divide in culture medium and another chemical stops division during metaphase when chromosomes are highly condensed. The stained cells are photographed and can be paired based on stained cross- bands, and size and shape.
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- 24-8 Human karyotype preparation
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- 24-9 Normal male karyotype
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- 24-10 Down syndrome karyotype
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- 24-11 Changes in Chromosome Number Nondisjunction occurs during meiosis I when the members of a homologous pair both go into the same daughter cell or during meiosis II when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete. The result is a trisomy or a monosomy.
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- 24-12 Nondisjunction in meiosis I
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- 24-13 Nondisjunction in meiosis II
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- 24-14 Normal development depends on the presence of two of each kind of chromosome, but an extra chromosome is tolerated more than a missing chromosome. The Barr body is an inactive X chromosome and is seen whenever more than one X chromosome is present (i.e., XX, XXY, XXX). Cells of females function with a single chromosome just as those of males do.
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- 24-15 Down Syndrome Down syndrome is caused by trisomy 21, three copies of chromosome 21 as a result of nondisjunction. Symptoms include mental retardation, short stature, eyelid fold, flatter face, a palm creases, and stubby fingers, among others. Nondisjunction usually occurred in producing the mothers egg and risk increases at age 40.
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- 24-16 Abnormal autosomal chromosome number
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- 24-17 The genes causing Down syndrome are located on the bottom third of chromosome 21. In particular, the Gart gene leads to a high level of purines, which contribute to mental impairment but may allow future preventive treatment.
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- 24-18 Changes in Sex Chromosome Number The presence of a Y chromosome determines maleness. The SRY gene on the short arm of the Y produces a testis-determining factor that begins the development of a male; otherwise an embryo develops as a female. An abnormal number of sex chromosomes is the result of inheriting to many or too few X or Y chromosomes.
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- 24-19 Turner Syndrome Individuals with Turner syndrome are females that have only one X chromosome; therefore they are XO. They are short, with a broad chest, and webbed neck. They do not undergo puberty or menstruate, and there is a lack of breast development. Intelligence is normal and individuals can lead normal lives.
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- 24-20 Klinefelter Syndrome Individuals with Klinefelter syndrome are males that have two or more X chromosomes in addition to a Y chromosome. The Y chromosome drives development as a male but gonads are underdeveloped and breasts develop. Klinefelter males are usually slow to learn but are not mentally retarded.
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- 24-21 Abnormal sex chromosome number
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- 24-22 Poly-X Females A poly-X female has more than two X chromosomes and extra Barr bodies in the nucleus. An XXX female has a normal phenotype except there may be menstrual difficulties, but she is fertile; her children usually have normal karyotypes. Females with XXXX are usually tall and severely retarded; they may menstruate normally.
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- 24-23 Jacobs Syndrome Jacobs syndrome males are XYY which can only result from nondisjuction during spermatogenesis. They tend to be tall, have persistent acne, and have speech and reading problems. At one time it was suggested that XYY males were unusually aggressive, but this was found not to be true.
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- 24-24 Changes in Chromosome Structure A mutation is a permanent genetic change. A change in chromosome structure is a chromosome mutation. Radiation, organic chemicals, or even viruses may cause chromosomes to break, leading to mutations. Chromosomal mutations include inversion, translocation, deletion, and duplication.
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- 24-25 Deletions and Duplications Deletions occur when a single break causes a lost end piece, or two breaks result in a loss in the interior. An individual who inherits a normal chromosome from one parent and a chromosome with a deletion from the other parent no longer has a pair of alleles for each trait, and a syndrome can result.
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- 24-26 In Williams syndrome, chromosome 7 loses an end piece and children have a pixie look and the skin ages prematurely from lack of the gene that governs elastin production. An end piece of chromosome 5 produces cri du chat syndrome where larynx is abnormal and the infants cry is like that of a cat, the head is small, and there are facial abnormalities.
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- 24-27 Deletion
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- 24-28 Duplication results in a chromosome segment being repeated in the same chromosome or in a nonhomologous chromosome, producing extra alleles for a trait. An inverted duplication in chromosome 15 causes inv dup 15 syndrome with poor muscle tone, mental retardation, and related symptoms.
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- 24-29 Duplication
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- 24-30 Translocation Translocation is exchange of chromosomal segments between two, nonhomologous chromosomes. In a small percent of cases, a translocation between chromosomes 21 and 14 causes Down syndrome. The tendency for this particular translocation can run in the family of either the mother or father of affected individuals.
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- 24-31 Alagille syndrome results from a deletion of chromosome 20 or a translocation that disrupts an allele on chromosome 20. The symptoms for Alagille syndrome range from mild to severe, so people may not be aware they have the syndrome.
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- 24-32 Translocation
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- 24-33 Inversion Inversion involves a segment of a chromosome being turned 180 degrees; the reverse sequence of alleles can alter gene activity. Crossing-over between inverted and normal chromosomes can cause recombinant chromosomes due to the inverted chromosome needing to form a loop to align.
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- 24-34 Inversion
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- 24-35 Sex-Linked Traits Traits controlled by genes on the X or Y chromosomes are sex-linked although most are unrelated to gender. An allele on the X chromosome that is in the region where the Y chromosome has no alleles will express even if recessive; it is termed X-linked. A female would have to have two recessive genes to express the trait; a male would only need one.
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- 24-36 X-Linked Alleles The key for an X-linked problem shows the allele attached to the X as in: X B = normal vision X b = color blindness. Females with the genotype X B X b are carriers because they appear to be normal but each son has a 50% chance of being color blind depending on which allele the son receives. X b X b and X b Y are both colorblind.
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- 24-37 Cross involving an X-linked allele
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- 24-38 X-Linked Disorders In pedigree charts that show the inheritance pattern for X-linked recessive disorders, more males than females have the trait because recessive alleles on the X chromosome are expressed in males. A grandfather passes an X-linked recessive disorder to a grandson through a carrier daughter. X-linked recessive disorders include red- green color blindness, muscular dystrophy, and hemophilia.
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- 24-39 X-linked recessive pedigree chart
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- 24-40 Color Blindness Three types of cones are in the retina detecting red, green, or blue. Genes for blue cones are autosomal; those for red and green cones are on the X chromosome. Males are much more likely to have red- green color blindness than fema
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