1 / 52

Chromosomes and Human Genetics

Chromosomes and Human Genetics. Chapter 11. Genes . Units of information about heritable traits In eukaryotes, distributed among chromosomes Each has a particular locus Location on a chromosome. Homologous Chromosomes.

Download Presentation

Chromosomes and Human Genetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.


Presentation Transcript

  1. Chromosomes andHuman Genetics Chapter 11

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

  3. 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

  4. Homologous Chromosomes

  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 • Mammals, fruit flies • XX is female, XY is male • In other groups XX is male, XY female • Human X and Y chromosomes function as homologues during meiosis

  7. 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

  8. 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

  9. 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

  10. The X Chromosome • Carries more than 2,062 genes • Most genes deal with nonsexual traits • Genes on X chromosome can be expressed in both males and females

  11. Karyotype Preparation - Stopping the Cycle • Cultured cells are arrested at metaphase by adding colchicine • This is when cells are most condensed and easiest to identify

  12. 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

  13. Human Karyotype

  14. Linkage Groups • Genes on one type of chromosome • Fruit flies • 4 homologous chromosomes • 4 linkage groups • Indian corn • 10 homologous chromosomes • 10 linkage groups

  15. 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

  16. 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

  17. Crossover Frequency Proportional to the distance that separates genes Crossing over will disrupt linkage between A and B more often than C and D

  18. Human Genetic Analysis • Geneticists often gather information from several generations to increase the numbers for analysis • If a trait follows a simple Mendelian inheritance pattern they can be confident about predicting the probability of its showing up again

  19. 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

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

  21. Genetic Disorder • 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

  22. 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

  23. Autosomal Dominant Inheritance • Trait typically appears in every generation

  24. Achondroplasia • Autosomal dominant allele • In homozygous form usually leads to stillbirth • Heterozygotes display a type of dwarfism • Have short arms and legs relative to other body parts

  25. Huntington Disorder • Autosomal dominant allele • Causes involuntary movements, nervous system deterioration, death • Symptoms don’t usually show up until person is past age 30 • People often pass allele on before they know they have it

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

  27. Galactosemia • Caused by autosomal recessive allele • Gene specifies a mutant enzyme in the pathway that breaks down lactose enzyme 1 enzyme 2 enzyme 3 GALACTOSE-1- PHOSOPHATE GALACTOSE-1- PHOSOPHATE LACTOSE GALACTOSE +glucose intermediate in glycolysis

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

  29. Examples of X-Linked Traits • Color blindness • Inability to distinguish among some of all colors • Hemophilia • Blood-clotting disorder • 1/7,000 males has allele for hemophilia A • Was common in European royal families

  30. Hutchinson-Guilford Progeria • Mutation causes accelerated aging • No evidence of it running in families • Appears to be dominant • Seems to arise as spontaneous mutation • Usually causes death in early teens

  31. Duplication • Gene sequence that is repeated several to hundreds of times • Duplications occur in normal chromosomes • May have adaptive advantage • Useful mutations may occur in copy

  32. Duplication

  33. Inversion A linear stretch of DNA is reversed within the chromosome segments G, H, I become inverted

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

  35. 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

  36. Translocation

  37. Philadelphia Chromosome • First abnormal chromosome to be associated with a cancer • Associated with a chronic leukemia • Overproduction of white blood cells

  38. A Reciprocal Translocation 1 2 Chromosome 9 and chromosome 22 exchanged pieces 6 13 15 19 20

  39. An Altered Gene • When the reciprocal translocation occurred, a gene at the end of chromosome 9 fused with a gene from chromosome 22 • This hybrid gene encodes an abnormal protein that stimulates uncontrolled division of white blood cells

  40. Does Chromosome Structure Evolve? • Alterations in the structure of chromosomes generally are not good and tend to be selected against • Over evolutionary time, however, many alterations with neutral effects became built into the DNA of all species

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

  42. Polyploidy • Individuals have three or more of each type of chromosome (3n, 4n) • Common in flowering plants • Lethal for humans • 99% die before birth • Newborns die soon after birth

  43. Nondisjunction n + 1 n + 1 n - 1 chromosome alignments at metaphase I n - 1 nondisjunction at anaphase I alignments at metaphase II anaphase II chromosome number in gametes

  44. 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

  45. 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

  46. 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

  47. 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

  48. Genetic Screening • Large-scale screening programs detect affected persons • Newborns in United States routinely tested for PKU • Early detection allows dietary intervention and prevents brain impairment

  49. Phenotypic Treatments • Symptoms of many genetic disorders can be minimized or suppressed by • Dietary controls • Adjustments to environmental conditions • Surgery or hormonal treatments

  50. Prenatal Diagnosis • Amniocentesis • Chorionic villus sampling • Fetoscopy • All methods have some risks

More Related