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Genes, Chromosomes, and Human Genetics

Genes, Chromosomes, and Human Genetics. Chapter 13. Why It Matters. Progeria. 13.1 Genetic Linkage and Recombination. The principles of linkage and recombination were determined with Drosophila Recombination frequency can be used to map chromosomes

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Genes, Chromosomes, and Human Genetics

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  1. Genes, Chromosomes, and Human Genetics Chapter 13

  2. Why It Matters • Progeria

  3. 13.1 Genetic Linkage and Recombination • The principles of linkage and recombination were determined with Drosophila • Recombination frequency can be used to map chromosomes • Widely separated linked genes assort independently

  4. Chromosomes • Genes • Sequences of nucleotides in DNA • Arranged linearly in chromosomes

  5. Linked Genes • Genes carried on the same chromosome • Linked during transmission from parent to offspring • Inherited like single genes • Recombination can break linkage

  6. Drosophila melanogaster • Fruit fly • Model organism for animal genetics • Compared to Mendel’s peas • Used to test linkage and recombination

  7. Gene Symbolism • Normal alleles (wild-type) • Usually most common allele • Designated by “+” symbol • Usually dominant Wild-typeMutant + = red eyes pr = purple + = normal wings vg = vestigial wings

  8. Genetic Recombination • Alleles linked on same chromosome exchange segments between homologous chromosomes • Exchanges occur while homologous chromosomes pair during prophase I of meiosis

  9. Evidence for Gene Linkage

  10. Recombination Frequency • Amount of recombination between two genes reflects the distance between them • The greater the distance, the greater the recombination frequency • Greater chance of crossover between genes

  11. Recombination Frequency

  12. Chromosome Maps • Recombination frequencies used to determine relative locations on a chromosome • Linkage map for genes a, b, and c: • 1 map unit = 1% recombination = 1 centimorgan

  13. Map: Drosophila Chromosome 2

  14. Recombination Occurs Often • Widely separated linked genes often recombine • Seem to assort independently • Detected by testing linkage to genes between them

  15. 13.2 Sex-Linked Genes • In both humans and fruit flies, females are XX, males are XY • Human sex determination depends on the Y chromosome

  16. 13.2 (cont.) • Sex-linked genes were first discovered in Drosophila • Sex-linked genes in humans are inherited as they are in Drosophila • Inactivation of one X chromosome evens out gene effects in mammalian females

  17. Sex Chromosomes • Sex chromosomes determine gender • X and Y chromosomes in many species • XX: female • XY: male • Other chromosomes are called autosomes

  18. Sex Determination in Humans

  19. Human Sex Chromosomes • Human X chromosome • Large (2,350 genes) • Many X-linked genes are nonsexual traits • Human Y chromosome • Small (few genes) • Very few match genes on X chromosome • Contains SRY gene • Regulates expression of genes that trigger male development

  20. Sex Linkage • Female (XX): 2 copies of X-linked alleles • Male (XY): 1 copy of X-linked alleles • Only males have Y-linked alleles

  21. Sex Linkage • Males have only one X chromosome • One copy of a recessive allele results in expression of the trait • Females have two X chromosomes • Heterozygote: recessive allele hidden (carrier) • Homozygote recessive: trait expressed

  22. Eye Color Phenotypes in Drosophila • Normal wild-type: red eye color • Mutant: white eye color

  23. Evidence for Sex-Linked Genes

  24. Animation: Morgan’s reciprocal crosses

  25. Human Sex-Linked Genes • Pedigree chart show genotypes and phenotypes in a family’s past generations • X-linked recessive traits more common in males • Red-green color blindness • Hemophilia: defective blood clotting protein

  26. Inheritance of Hemophilia • In descendents of Queen Victoria of England

  27. X Inactivation (1) • Dosage compensation • In female mammals, inactivation of one X chromosome makes the dosage of X-linked genes the same as males • Occurs during embryonic development

  28. X Inactivation (2) • Random inactivation of either X chromosome • Same X chromosome inactivated in all descendents of a cell • Results in patches of cells with different active X chromosomes

  29. Calico Cats • Heterozygote female (no male calico cats)

  30. Barr Body • Tightly coiled condensed X chromosome • Attached to side of nucleus • Copied during mitosis but always remains inactive

  31. 13.3 Chromosomal Alterations That Affect Inheritance • Most common chromosomal alterations: deletions, duplications, translocations, and inversions • Number of entire chromosomes may also change

  32. Chromosomal Alterations (1) • Deletion: broken segment lost from chromosome • Duplication: broken segment inserted into homologous chromosome

  33. Chromosomal Alterations (2) • Translocation: broken segment attached to nonhomologous chromosome • Inversion: broken segment reattached in reversed orientation

  34. Nondisjunction (1) • Failure of homologous pair separation during Meiosis I

  35. Nondisjunction (2) • Failure of chromatid separation during Meiosis II

  36. Changes in Chromosome Number • Euploids • Normal number of chromosomes • Aneuploids • Extra or missing chromosomes • Polyploids • Extra sets of chromosomes (triploids, tetraploids) • Spindle fails during mitosis

  37. Aneuploids • Abnormalities usually prevent embryo development • Exception in humans is Down syndrome • Three copies of chromosome 21 (trisomy 21) • Physical and learning difficulties • Frequency of nondisjunction increases as women age

  38. Down Syndrome

  39. Aneuploidy of Sex Chromosomes

  40. Aneuploidy of Sex Chromosomes

  41. Animation: Karyotype preparation

  42. Polyploids • Common in plants • Polyploids often hardier and more successful • Source of variability in plant evolution • Uncommon in animals • Usually has lethal effects during embryonic development

  43. 13.4 Human Genetics and Genetic Counseling • In autosomal recessive inheritance, heterozygotes are carriers and homozygous recessives are affected by the trait • In autosomal dominant inheritance, only homozygous recessives are unaffected

  44. 13.4 (cont.) • Males are more likely to be affected by X-linked recessive traits • Human genetic disorders can be predicted, and many can be treated

  45. Modes of Inheritance • Autosomal recessive inheritance • Autosomal dominant inheritance • X-linked recessive inheritance

  46. Autosomal Recessive Inheritance • Males or females carry a recessive allele on an autosome • Heterozygote • Carrier • No symptoms • Homozygote recessive • Shows symptoms of trait

  47. Autosomal Recessive Inheritance

  48. Cystic Fibrosis

  49. Autosomal Dominant Inheritance • Dominant gene is carried on an autosome • Homozygote dominant or heterozygote • Show symptoms of the trait • Homozygote recessive • Normal

  50. Autosomal Dominant Inheritance

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