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Chromosomes and Human Genetics Chapter 21 Cystic Fibrosis • Caused by mutations in CFTR gene • Defective channel protein in membrane of cells in exocrine glands • Glands secrete abnormally thick, gluey mucus • Interferes with breathing, pancreatic function Genes • Units of information about heritable traits • In eukaryotes, distributed among chromosomes • Each has a particular locus – Location on a chromosome Homologous Chromosomes • Homologous autosomes are identical in length, size, shape, and gene sequence • Sex chromosomes are nonidentical but still homologous • Homologous chromosomes interact, then segregate from one another during meiosis Alleles • Different molecular forms of a gene • Arise through mutation • Diploid cell has a pair of alleles at each locus • Alleles on homologous chromosomes may be same or different Sex Chromosomes • Discovered in late 1800s • Humans – XX is female, XY is male • Human X and Y chromosomes function as homologues during meiosis Karyotype Preparation • Arrested cells are broken open • Metaphase chromosomes are fixed and stained • Chromosomes are photographed through microscope • Photograph of chromosomes is cut up and arranged to form karyotype diagram Human Karyotype 1 2 3 4 13 14 15 16 5 17 6 7 8 9 18 19 20 21 10 22 11 12 XX (or XY) Sex Determination eggs sperm X Y X X Female germ cell Male germ cell X X X XX XX Y XY XY Sex-chromosome combinations possible in new individual The X Chromosome • Carries more than 2,300 genes • Most genes deal with nonsexual traits • Genes on X chromosome can be expressed in both males and females The Y Chromosome • Fewer than two dozen genes identified • One is the master gene for male sex determination – SRY gene (sex-determining region of Y) • SRY present, testes form • SRY absent, ovaries form Effect of Y Chromosome appearance of structures that will give rise to external genitalia appearance of “uncommitted” duct system of embryo at 7 weeks 7 weeks Y present Y absent Y present Y absent testes ovaries 10 weeks ovary birth approaching testis X Chromosome Inactivation • Mammalian females have two X chromosomes per cell • One X is inactivated in each cell • Inactivation is random • Female is a “mosaic” Barr Body • Condensed X chromosome • Visible in micrographs • May be either the maternal or the paternal X chromosome Linkage • Genes on the same chromosome are “linked” • Crossing over can rearrange linked genes • Farther apart two genes are on chromosome, the more they are to be rearranged by crossing over A B C D Full Linkage Parents: B AB A ab x B b A F1 offspring: a b a All AaBb meiosis, gamete formation 50% AB B A b a 50% ab With no crossovers, half of the gametes have one parental genotype and half have the other Incomplete Linkage AC A C Parents: ac c x A a a c C F1 offspring All AaCc meiosis, gamete formation Unequal ratios of four types of gametes: C A c a Most gametes have parental genotypes c A a C A smaller number have recombinant genotypes Pedigree • Chart that shows genetic connections among individuals • Standardized symbols • Knowledge of probability and Mendelian patterns used to suggest basis of a trait • Conclusions most accurate when drawn from large number of pedigrees Pedigree for Polydactyly female I male II 5,5 6,6 * III IV 5,5 6,6 6 6,6 5,5 6,6 5,5 7 5,5 6,6 5,5 6,6 5,5 6,6 5,5 6,6 5,6 6,7 12 V *Gene not expressed in this carrier. 6,6 6,6 Genetic Abnormality • A rare, uncommon version of a trait • Polydactyly – Unusual number of toes or fingers – Does not cause any health problems – View of trait as disfiguring is subjective Genetic Disorder • Inherited conditions that cause mild to severe medical problems • Why don’t they disappear? – Mutation introduces new rare alleles – In heterozygotes, harmful allele is masked, so it can still be passed on to offspring Autosomal-Recessive Inheritance Patterns • If parents are both heterozygous, child will have a 25% chance of being affected Examples of Recessive Traits • Cystic fibrosis • Phenylketonuria (PKU) • Tay-Sachs disease • Many other genetic disorders AutosomalDominant Inheritance Trait typically appears in every generation Examples of Dominant Traits • Huntington disorder • Achondroplasia • Familial hypercholesterolemia X-Linked Recessive Inheritance • Males show disorder more than females • Son cannot inherit disorder from his father Examples of X-Linked Traits • X-linked recessive – Red/green color blindness – Hemophilia A – Duchenne muscular dystrophy (DMD) • X-linked dominant – Faulty enamel trait Sex-Influenced Traits • Appear more frequently in one sex than in the other, or phenotypes differ with sex • Genes are carried on autosomes • Pattern baldness Duplication • Gene sequence that is repeated several to hundreds of times • Duplications occur in normal chromosomes • May have adaptive advantage Duplication normal chromosome one segment repeated three repeats Inversion A linear stretch of DNA is reversed within the chromosome Translocation • A piece of one chromosome becomes attached to another nonhomologous chromosome • Most are reciprocal • Philadelphia chromosome arose from a reciprocal translocation between chromosomes 9 and 22 Translocation chromosome nonhomologous chromosome reciprocal translocation Deletion • Loss of some segment of a chromosome • Most are lethal or cause serious disorder Aneuploidy • Individuals have one extra or less chromosome • (2n + 1 or 2n - 1) • Major cause of human reproductive failure • Most human miscarriages are aneuploids Polyploidy • Individuals have three or more of each type of chromosome (3n, 4n) • Lethal for humans – 99% die before birth – Newborns die soon after birth Nondisjunction n+1 n+1 n-1 chromosome alignments at metaphase I n-1 nondisjunction at anaphase I alignments at metaphase II anaphase II Down Syndrome • Trisomy of chromosome 21 • Mental impairment and a variety of additional defects • Can be detected before birth • Risk of Down syndrome increases dramatically in mothers over age 35 Turner Syndrome • Inheritance of only one X (XO) • 98% spontaneously aborted • Survivors are short, infertile females – No functional ovaries – Secondary sexual traits reduced – May be treated with hormones, surgery Klinefelter Syndrome • XXY condition • Results mainly from nondisjunction in mother (67%) • Phenotype is tall males – Sterile or nearly so – Feminized traits (sparse facial hair, somewhat enlarged breasts) – Treated with testosterone injections XYY Condition • Taller-than-average males • Most otherwise phenotypically normal • Some mentally impaired • Once thought to be predisposed to criminal behavior, but studies now discredit