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Chapter 24: Patterns of Chromosome Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 24-1 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. 24-2 Normal male karyotype Fig. 24.1 2 copies of 22 autosomal chromosomes 2 copies of 1 sex chromosome 24-3 Down syndrome karyotype Fig. 24.1 (Trisomy 21) Total of 47 chromosomes 24-4 Changes in Chromosome Number Nondisjunction occurs when: 1.) both members of a homologous pair go into the same daughter cell or 2.) when sister chromatids fail to separate and both daughter chromosomes go into the same gamete. The result is a trisomy or a monosomy. 24-5 Nondisjunction in meiosis I Fig. 24.2 trisomy monosomy 24-6 Nondisjunction in meiosis II Fig. 24.2 monosomy trisomy 24-7 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. 24-8 Down Syndrome Down syndrome is caused by trisomy 21, a result of nondisjunction. Nondisjunction risk increases after age 40. 24-9 Changes in Sex Chromosome Number The presence/absence of a Y chromosome determines maleness. An abnormal number of sex chromosomes is the result of inheriting too many or too few X or Y chromosomes. 24-10 Turner syndrome: females with one X chromosome; XO. Klinefelter syndrome: males that have two or more X chromosomes and a Y chromosome. A Poly-X female has more than two X chromosomes and extra Barr bodies. Jacobs syndrome males are XYY. 24-11 Changes in Chromosome Structure A mutation is a permanent genetic change. Radiation, organic chemicals, or even viruses may cause chromosomes to break, leading to mutations. A change in chromosome structure is a chromosome mutation. 24-12 Chromosomal mutations include: 1.) deletion 2.) duplication 3.) inversion 4.) translocation 24-13 Deletions Deletions occur when part of the chromosome is lost. An individual with 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 may result. Fig. 24.5 24-14 Duplications Duplication: a chromosome segment is repeated in the same chromosome. Fig. 24.6 24-15 Translocation Translocation is exchange of chromosomal segments between two, nonhomologous chromosomes (Ex: part of chromosome 2 is swapped with part of chromosome 7). Fig. 24.7 24-16 Inversion Inversion involves a segment of a chromosome being turned 180 degrees; the reverse sequence of alleles can alter gene activity. See Fig. 24.8 A A B C D B F E E D F C NORMAL INVERSION 24-17 Sex-Linked Traits Traits controlled by genes on the X or Y chromosomes are called sex-linked (i.e., on sex chromosomes). 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. 24-18 XX = female XY = male X Y X XX XY X XX XY Y Chromosome determines sex of offspring 24-19 Comparison of the sex chromosomes Male X Female Y X X Alternate alleles available. Alternate alleles available. No alternate alleles available. 24-20 X-Linked Alleles The key for an X-linked problem shows the allele attached to the X as in: XB = normal vision Xb = color blindness. Females with the genotype XBXb 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. XbXb and XbY are both colorblind. 24-21 Cross involving an X-linked allele A woman is not color blind but her father was color blind. She has a child with a man that is not color blind. Could her children be color blind? Woman XB Xb Man XB Y XB Color blind Not Color blind Y 1 XB XB XB XB Y Xb XB Xb Xb Y 3 24-22 X-Linked Disorders In pedigree charts that show the inheritance pattern for X-linked recessive disorders, more males than females have the trait. X-linked recessive disorders include redgreen color blindness, muscular dystrophy, and hemophilia. 24-23 X-linked recessive pedigree chart (Color Blindness) 24-24 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 redgreen color blindness than females. About 8% of Caucasian men have redgreen color blindness. 24-25 Other X-linked Disorders Muscular dystrophy is characterized by the wasting of muscles. Hemophilia blood does not clot normally. Fragile X syndrome one of the most common forms of mental retardation. -CGG Repeats 24-26