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Chapter 15

Chapter 15. Chromosomal basis for inheritance. Mendel Genetics. Mendel published his work in 1866 1900 his work was rediscovered. Parallels between Mendel’s factors & chromosome behavior. Mendel’s Genetics. 1902 Walter Sutton Chromosomal theory of inheritance

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Chapter 15

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  1. Chapter 15 • Chromosomal basis for inheritance

  2. Mendel Genetics • Mendel published his work in 1866 • 1900 his work was rediscovered. • Parallels between Mendel’s factors & chromosome behavior

  3. Mendel’s Genetics • 1902 Walter Sutton • Chromosomal theory of inheritance • Genes are located on chromosomes • Located at specific loci (positions) • Behavior of chromosomes during meiosis account for inheritance patterns

  4. Fig. 15-2 P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) y Y r  R r R Y y Meiosis Fertilization r y Y R Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r r R R Metaphase I y Y y Y 1 1 r r R R Anaphase I Y y Y y Metaphase II r R R r 2 2 y Y y Y y Y Y y Y y y Y Gametes r R r r R r R R 1/4 1/4 1/4 1/4 yr yR YR Yr F2 Generation An F1  F1 cross-fertilization 3 3 : 1 : 3 9 : 3

  5. Fruit fly • Thomas Morgan studied fruit flies • Drosophila melanogaster • Proved chromosomal theory correct • Studied eye color • Red is dominant, white is recessive • Crossed a homozygous dominant female with a homozygous recessive male

  6. Wild type (w+)

  7. Mutant (w)

  8. Fruit fly • F1 offspring were all red eyed • F2 classic 3:1 ratio red:white phenotypes • Showed the alleles segregate • Supported the Chromosomal theory • BUT only males were white eyed • All females were red eyed or wild type

  9. Fig. 15-4 EXPERIMENT P  Generation F1 All offspring had red eyes Generation RESULTS F2 Generation CONCLUSION + P w w X X  Generation X Y + w w Sperm Eggs + + F1 w w + Generation w w + w Sperm Eggs + + w w + F2 w Generation + w w w w + w

  10. Fruit fly • Eye color gene is on the X-chromosomes • Sex-linked genes: • Genes found on the sex chromosomes • X-chromosome has more genes than Y-chromosome • Most sex-linked genes are on the X-chromosome

  11. Human Males • Y chromosome is very condensed • 78 genes • Male characteristics • Sperm production & fertility

  12. Males • SRY is a gene on the Y chromosome • Sex determining region of Y • Present gonads develop into testes • Determines development of male secondary sex characteristics • Not present then individual develops ovaries

  13. Females • X chromosome has 1000 genes • One of the 2 X chromosomes is inactivated • Soon after embryonic development • Choice is random from cell to cell • Female is heterozygous for a trait • Some cells will have one allele • Some cell have the other

  14. Females • Barr body: • Condensed inactive X chromosome • Stains dark

  15. Fig. 15-8 X chromosomes Allele for orange fur Early embryo: Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Inactive X Active X Black fur Orange fur

  16. Sex-linked • Mom passes gene on the X-chromosome to the son • Males have one X-chromosome • Recessive gene is expressed • Recessive alleles on the X are present • No counter alleles on the Y

  17. Sex-linked disorders • Mom passes sex-linked to sons & daughters • Dad passes only to daughters

  18. Sex-linked disorders • Sex-linked genetic defects • Hemophilia • 1/10,000 Caucasian males

  19. Sex-linked disorders • Colored blindness • Red-green blindness • Mostly males • Heterozygous females can have some defects

  20. Sex-linked disorders • Duchenne muscular dystrophy • Almost all cases are male • Child born healthy • Muscles become weakened • Break down of the myelin sheath in nerve stimulating muscles • Wheelchair by 12 years old • Death by 20

  21. Independent assortment

  22. Independent assortment • Dihybrid testcross • 50% phenotypes similar to parents • Parental types • 50% phenotypes not similar to parents • Recombinant types • Indicates unlinked genes • Mendel’s independent assortment

  23. Test cross

  24. Linked genes • Do not assort independently • Genes are inherited together • Genes located on same chromosome • Differs from Mendel’s law of independent assortment

  25. Linked genes • Test cross fruit flies • Wild-type (dihybrid) • Gray bodies and long wings • Mutants (homozygous) • Black bodies and short wings (vestigial) • Results not consistent with genes being on separate chromosomes

  26. Fig. 15-10 Testcross parents Gray body, normal wings (F1 dihybrid) Black body, vestigial wings (double mutant) bvg b+vg+ bvg b vg Replication of chromo- somes Replication of chromo- somes b+vg+ bvg b+vg+ bvg bvg bvg bvg bvg Meiosis I b+vg+ Meiosis I and II b+vg bvg+ bvg Meiosis II Recombinant chromosomes bvg bvg+ b+vg b+vg+ Eggs Testcross offspring 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal bvg b+vg+ bvg b+vg bvg+ bvg bvg bvg bvg Sperm Parental-type offspring Recombinant offspring 391 recombinants Recombination frequency  100 = 17% = 2,300 total offspring

  27. Linked genes • More parental phenotypes • Than if on separate chromosomes • Greater than 50% • Gray body normal wings or black body vestigial • Non-parental phenotype 17% • Gray-vestigial or black-normal wings • Indicating crossing over

  28. Genetic recombination: • New combination of genes • 2 genes that are farther apart tend to cross over more • 2 genes on the same chromosome can show independent assortment • Due to regularly crossing over

  29. Genetic map • Ordered list of gene loci • Linkage map: • Genetic map based on recombination frequencies • Distance between genes in terms of frequency of crossing over • Higher percentage of crossing over the further apart the genes are • Centimorgan (Thomas Hunt Morgan) • A map unit

  30. Fig. 15-12 Mutant phenotypes Short aristae Cinnabar eyes Vestigial wings Brown eyes Black body 0 48.5 57.5 67.0 104.5 Gray body Red eyes Normal wings Red eyes Long aristae (appendages on head) Wild-type phenotypes

  31. Human genetic map • Genetic distance is still proportional to the recombination frequency • Use pedigrees • Newer technology

  32. Alterations in chromosomes • Chromosome number • Chromosome structure • Serious human disorders

  33. Alterations in numbers • Nondisjunction • Failure of homologues or sister chromatids to separate properly • Aneuploidy: • Gain or a loss of chromosomes due to nondisjunction • Abnormal number of chromosomes • Occurs about 5% of the time with humans

  34. Nondisjunction

  35. Meiosis I Nondisjunction Fig. 15-13-3 Meiosis II Nondisjunction Gametes n – 1 n + 1 n – 1 n n n + 1 n + 1 n – 1 Number of chromosomes (b) Nondisjunction of sister chromatids in meiosis II (a) Nondisjunction of homologous chromosomes in meiosis I

  36. Monosomics • Lost a copy of a chromosome (not a sex chromosome) • Usually do not survive • Trisomes: gained a copy of a chromosome • Many do not survive either • 35% rate of aneuploidy (spontaneous abortions)

  37. Polyploidy • More than 2 sets of chromosomes • 3n or 4n • Plants

  38. Fig. 15-14

  39. Alterations in Structure • 1. Deletion: • Missing a section of chromosome • 2. Duplication: • Extra section of chromosome • Attaches to sister or non-sister chromatids

  40. Alterations in Structure • 3. Inversion: • Reverse orientation of section of chromosome • 4. Translocation: • Chromosome fragment joins a nonhomologous chromosome

  41. A B C D E F G H A B C E F G H Deletion (a) A B C D E F G H A B C B C D E F G H Duplication Fig. 15-15 (b) A B C D E F G H A D C B E F G H Inversion (c) A B C D E F G H M N O C D E F G H (d) Reciprocal translocation M N O P Q R A B P Q R

  42. Human disorders • Trisomes • Babies with extra chromosomes can survive • Chromosome 13, 15, 18, 21 and 22 • These are the smallest chromosomes

  43. Trisomy 13

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