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Fig. 15.11

Fig. 15.11. Linkage and Recombination Recombination Possible to use recombination frequencies to construct genetic map ( linkage map ) of genes on chromosome If two loci are sufficiently far apart, possible to get double crossing over. Fig. 15.12. Sex Chromosomes and Gender

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Fig. 15.11

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  1. Fig. 15.11

  2. Linkage and Recombination • Recombination • Possible to use recombination frequencies to construct genetic map (linkage map) of genes on chromosome • If two loci are sufficiently far apart, possible to get double crossing over

  3. Fig. 15.12

  4. Sex Chromosomes and Gender • Many systems of sex determination • Most animals have sex chromosomes • One gender typically homogametic, the other heterogametic • Humans • Males and females with 22 pairs of autosomes (do not determine gender) • Question:Is female gender in humans determined by presence of two X chromosomes or absence of Y chromosome? • Approach: Examine people with abnormal sex chromosomes

  5. Sex Chromosomes and Gender • Humans • XXY – Klinefelter Syndrome • Nearly normal males with underdeveloped testes • XO – Turner Syndrome • Phenotypically female with underdeveloped ovaries • YO – Embryo doesn’t develop • Conclusions • X chromosome required for development • Genes on Y chromosome determine gender • X and Y chromosome aren’t homologous but have short, homologous pairing regions that permit synapsis during meiosis • Sperm containing X and Y chromosomes produced in equal numbers • More male babies conceived, die before birth, born (1.06:1)

  6. Sex Chromosomes and Gender • Other Species • X-Y system • XX = Female, XY = Male • Used by humans • X-O system • Z-W system • Haplo-diploid system

  7. X-Y system • XX = Female, XY = Male • Used by humans • X-O system (crickets) • Egg carries X • Sperm carries X or nothing • Z-W system (birds, fishes, butterflies, moths) • Egg carries Z or W • Sperm carries Z • Haplo-diploid system (ants) • Females from fertilized ova • Males from unfertilized ova Fig. 15.6

  8. Sex Chromosomes and Gender • Sex-Linked Genes • X chromosome in humans contains many genes required by both males and females • Y chromosome contains fewer genes, mostly related to “maleness” (testicular development, affinity for monster trucks, etc.) • Mutations on X chromosome can lead to genetic disorders (X-linked)

  9. P: Wild type female, red eyes • Mutant male, white eyes • F1: All with red eyes • F2: Females with red eyes • Half of males with white eyes Fig. 15.4

  10. Sex Chromosomes and Gender • Sex-Linked Genes • Females • Inherit one X from mother and one from father • Dominant traits expressed, recessive traits not • Heterozygous – Can be carriers • Males • Inherit all X-linked genes from mother • All X-linked alleles typically expressed • Hemizygous – Can’t be carriers

  11. Fig. 15.7

  12. Sex Chromosomes and Gender • Sex-Linked Genes • Disorders • Color blindness • Most common in men • Duchenne muscular dystrophy • Absence of key muscle protein (dystrophin) • Hemophilia • Absence of protein(s) required for blood clotting

  13. Sex Chromosomes and Gender • X Inactivation • Females have two copies of X chromosome • Fruit flies – Males make single X more active than either female X • Mammals – One X typically inactivated at random in each cell (dosage compensation) • Barr body – Inactivated X, visible during interphase as dark area of highly condensed chromatin • Inactivation incomplete; some genes expressed • Heterozygous female may express traits from each X chromosome in ~50% of cells • Ex: Calico and tortoiseshell cats

  14. Fig. 15.8

  15. Sex Chromosomes and Gender • Sex-Influenced Genes • Some traits inherited autosomally but influenced by gender • Male & female with same genotype, different phenotypes • Ex: Pattern baldness • Proposed that single pair of alleles determines pattern baldness – Dominant in males, recessive in females • B1 = Pattern baldness, B2 = Normal hair growth • B1B1 = Pattern baldness in males & females • B1B2 = Pattern baldness in males, normal hair in females • B2B2 = Normal hair in males & females

  16. Chromosomal Abnormalities • Chromosome Number • Usually due to nondisjunction (chromosomes fail to separate during anaphase of meiosis) • One gamete receives an extra chromosome, the other receives one fewer than normal • Condition = aneuploidy • Nondisjunction during mitosis may lead to clonal cell lines with abnormal chromosome counts • Nondisjunction during meiosis may lead to gametes (and offspring) with abnormal chromosome counts

  17. Fig. 15.13

  18. Chromosomal Abnormalities • Chromosome Number • Possible outcomes • Trisomy – 2n+1 chromosomes in fertilized egg • Monosomy – 2n-1 chromosomes in fertilized egg • Polyploidy – 3n, 4n, 5n, 6n, etc. chromosomes in fertilized egg • Triploidy – 3n chromosomes • Tetraploidy – 4n chromosomes

  19. Chromosomal Abnormalities • Chromosome Number • Possible outcomes • Autosomal aneuploidies highly detrimental and rare • No known autosomal monosomies (100% lethal) • Down’s Syndrome • Trisomy of chromosome 21 • Mental retardation, heart defects, susceptibility to diseases • Affects ca. 1 of every 700 children born in US • Frequency increases with age of mother

  20. Fig. 15.15

  21. Chromosomal Abnormalities • Chromosome Number • Possible outcomes • Sex chromosome aneuploidies less rare, perhaps due to dosage compensation and few genes on Y • Klinefelter Syndrome (XXY) • Phenotypically male but with Barr bodies • Tend to be tall with female-like breasts and reduced testes • May show signs of mental retardation • XYY • Phenotypically male but often very tall • May have severe acne • XXX • Phenotypically normal female • Turner Syndrome (XO) • Phenotypically female with no Barr bodies • Usually with undeveloped reproductive structures

  22. Chromosomal Abnormalities • Chromosome Structure • Often results from breakage of chromosomes and errors in repair

  23. Fig. 15.14

  24. Chromosomal Abnormalities • Chromosome Structure • Deletion • Cri du Chat Syndrome • Deletion of part of short arm of chromosome 5 • Mental retardation, small head, cry like a kitten • Translocation • Down’s syndrome may be caused not by trisomy but by extra material from chromosome 21 attached to other, large chromosome • Reciprocal translocation between chromosomes 9 and 22 can increase likelihood of developing chronic myelogenous leukemia (CML)

  25. Fig. 15.16

  26. Exceptions to Mendelian Inheritance • Genomic Imprinting • Expression of phenotype affected differently by inheritance of allele from mother vs. father • Imprinting may lead to expression of maternal or paternal allele for a particular species and gene

  27. Fig. 15.17

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