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Chromosomes and Human Genetics

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 .

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Chromosomes and Human Genetics

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  1. Chromosomes andHuman Genetics Chapter 21

  2. 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

  3. Genes • Units of information about heritable traits • In eukaryotes, distributed among chromosomes • Each has a particular locus • Location on a chromosome

  4. 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

  5. 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

  6. Sex Chromosomes • Discovered in late 1800s • Humans • XX is female, XY is male • Human X and Y chromosomes function as homologues during meiosis

  7. 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

  8. Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY)

  9. X X X Y Y X X X XX XX XY XY Sex Determination eggs sperm Female germ cell Male germ cell Sex-chromosome combinations possible in new individual

  10. 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

  11. 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

  12. appearance of structures that will give rise to external genitalia appearance of “uncommitted” duct system of embryo at 7 weeks Effect of YChromosome 7 weeks Y present Y absent Y present Y absent testes ovaries 10 weeks ovary testis birth approaching

  13. 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”

  14. Barr Body • Condensed X chromosome • Visible in micrographs • May be either the maternal or the paternal X chromosome

  15. 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

  16. A A a B B b A a B b a b Full Linkage AB ab Parents: x F1 offspring: All AaBb meiosis, gamete formation 50%AB 50%ab With no crossovers, half of the gametes have one parental genotype and half have the other

  17. A a a c c C A C Incomplete Linkage AC ac x Parents: F1 offspring All AaCc meiosis, gamete formation Unequal ratios of four types of gametes: a a A A C c C c Most gametes have parental genotypes A smaller number have recombinant genotypes

  18. 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

  19. I II III IV V *Gene not expressed in this carrier. Pedigree for Polydactyly female male 5,5 6,6 * 5,5 6,6 6,6 5,5 6,6 5,5 6 7 5,5 6,6 5,5 6,6 5,5 6,6 5,5 6,6 5,6 6,7 12 6,6 6,6

  20. 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

  21. 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

  22. Autosomal-Recessive Inheritance Patterns • If parents are both heterozygous, child will have a 25% chance of being affected

  23. Examples of Recessive Traits • Cystic fibrosis • Phenylketonuria (PKU) • Tay-Sachs disease • Many other genetic disorders

  24. Autosomal- Dominant Inheritance Trait typically appears in every generation

  25. Examples of Dominant Traits • Huntington disorder • Achondroplasia • Familial hypercholesterolemia

  26. X-Linked Recessive Inheritance • Males show disorder more than females • Son cannot inherit disorder from his father

  27. Examples of X-Linked Traits • X-linked recessive • Red/green color blindness • Hemophilia A • Duchenne muscular dystrophy (DMD) • X-linked dominant • Faulty enamel trait

  28. 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

  29. Duplication • Gene sequence that is repeated several to hundreds of times • Duplications occur in normal chromosomes • May have adaptive advantage

  30. Duplication normal chromosome one segment repeated three repeats

  31. Inversion A linear stretch of DNA is reversed within the chromosome

  32. 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

  33. Translocation chromosome nonhomologous chromosome reciprocal translocation

  34. Deletion • Loss of some segment of a chromosome • Most are lethal or cause serious disorder

  35. Aneuploidy • Individuals have one extra or less chromosome • (2n + 1 or 2n - 1) • Major cause of human reproductive failure • Most human miscarriages are aneuploids

  36. 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

  37. Nondisjunction n + 1 n + 1 n - 1 n - 1 chromosome alignments at metaphase I nondisjunction at anaphase I alignments at metaphase II anaphase II

  38. 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

  39. 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

  40. 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

  41. 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

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