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) Wild-type Mutant
Usually most common allele
Designated by “+” symbol
Usually dominant
Wild-type Mutant
+ = 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 Human Y 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 Uncommon in animals
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

51. X-Linked Recessive Inheritance
Recessive allele carried on X chromosome
Males
Recessive allele on X chromosome
Show symptoms
Females
Heterozygous carriers, no symptoms
Homozygous, show symptoms

52. X-Linked Recessive Inheritance

53. Genetic Counseling Identification of parental genotypes
Construction of family pedigrees
Prenatal diagnosis
Allows prospective parents to reach an informed decision about having a child or continuing a pregnancy

54. Genetic Counseling Techniques
Prenatal diagnosis tests cells for mutant alleles or chromosomal alterations
Cells obtained from:
Embryo
Amniotic fluid around embryo (amniocentesis)
Placenta (chorionic villus sampling)
Postnatal genetic screening
Biochemical and molecular tests

55. 13.5 Nontraditional Patterns of Inheritance
Cytoplasmic inheritance follows the pattern of inheritance of mitochondria or chloroplasts
In genomic imprinting, the allele inherited from one of the parents is expressed while the other allele is silent

56. Cytoplasmic Inheritance
Genes carried on DNA in mitochondria or chloroplasts
Cytoplasmic inheritance follows the maternal line
Zygote’s cytoplasm originates from egg cell

57. Cytoplasmic Inheritance
Mutant alleles in organelle DNA
Mendelian inheritance not followed (no segregation by meiosis)
Uniparental inheritance from female

58. Cytoplasmic Inheritance
Inheritance of variegation in Mirabalis

59. Human Mutations in Mitochondrial Genes

60. Genomic Imprinting
Expression of an allele is determined by the parent that contributed it
Only one allele (from either father or mother) is expressed
Other allele is turned off (silenced)
Often, result of methylation of region adjacent to gene responsible for trait

61. Animation: Pedigree diagrams