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Chromosomal Basis of Inheritance: Mendel's Genetics and the Chromosomal Theory

This chapter discusses the chromosomal basis for inheritance and how Mendel's work on genetics supports the chromosomal theory. It covers topics such as the behavior of chromosomes during meiosis, the laws of segregation and independent assortment, and sex-linked genes and disorders.

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Chromosomal Basis of Inheritance: Mendel's Genetics and the Chromosomal Theory

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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. Figure 15.1

  4. 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

  5. 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

  6. 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

  7. Wild type (w+)

  8. Mutant (w)

  9. 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

  10. 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

  11. 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

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

  13. 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

  14. 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

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

  16. 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

  17. 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

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

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

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

  21. 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

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

  23. Independent assortment

  24. 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

  25. Gametes from yellow-rounddihybrid parent (YyRr) Figure 15.UN02 yr yR YR Yr Gametes from testcrosshomozygousrecessiveparent (yyrr) yr YyRr yyrr Yyrr yyRr Parental-typeoffspring Recombinantoffspring

  26. 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

  27. Experiment P Generation(homozygous) Wild type (graybody, normal wings) Double mutant(black body, vestigial wings) bbvgvg b+b+vg+vg+ F1 dihybrid testcross Figure 15.9 Homozygousrecessive (blackbody, vestigial wings) Wild-type F1 dihybrid(gray body, normal wings) bbvgvg b+bvg+vg Testcrossoffspring Eggs bvg b+vg bvg+ b+vg+ Gray-vestigial Wild type(gray-normal) Black-normal Black-vestigial bvg Sperm b+bvg+vg bbvgvg b+bvgvg bbvg+vg PREDICTED RATIOS Genes on differentchromosomes: 1 : 1 : 1 : 1 Genes on the samechromosome: 1 : 1 : 0 : 0 Results 965 : 944 : 206 : 185

  28. 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

  29. bvg b+vg+ F1 dihybrid femaleand homozygous recessive malein testcross Figure 15.UN01 bvg bvg b+vg+ bvg Most offspring or bvg bvg

  30. 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

  31. 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

  32. 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

  33. Results Figure 15.11 Recombinationfrequencies 9% 9.5% Chromosome 17% cn vg b

  34. Mutant phenotypes Shortaristae Marooneyes Down-curvedwings Vestigialwings Cinnabareyes Blackbody Brown eyes Figure 15.12 48.5 67.0 57.5 16.5 104.5 0 75.5 Longaristae(appendageson head) Redeyes Graybody Normalwings Redeyes Redeyes Normalwings Wild-type phenotypes

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

  36. 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

  37. Nondisjunction

  38. 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

  39. 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

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

  41. Fig. 15-14

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

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

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