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Genes, Chromosomes, and Human Genetics

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Presentation on theme: "Genes, Chromosomes, and Human Genetics"— Presentation transcript:

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


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