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Biology 211

Chapter 14 : The Molecular Basis of Heredity: Mendel and The Gene Idea Chapter 15 : The Chromosomal Basis of Inheritance. Biology 211. You Will Be Able To:. Can you define the terms used in genetic studies? Can you use a Punnett Square for calculating possible inherited traits?

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Biology 211

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  1. Chapter 14: The Molecular Basis of Heredity: Mendel and The Gene Idea Chapter 15: The Chromosomal Basis of Inheritance Biology 211

  2. You Will Be Able To: • Can you define the terms used in genetic studies? • Can you use a Punnett Square for calculating possible inherited traits? • Can you describe Mendel’s basic ideas of how traits are inherited? • Can you explain dominant/recessive, incomplete dominance, co-dominance, sex-linked, multiple alleles, polygenic traits?

  3. Phenotype Genotype Locus Allele Dominant allele Recessive allele Homozygous Heterozygous Terminology Get out your Greek and Latin guidebook because Here we go again!

  4. Mendel’s principles of inheritance: 1. Segregation: • During meiosis, alleles for each locussegregate • How Chromosomes are moved is still being investigated

  5. Mendel’s principles of inheritance: 2. Independent assortment: • Alleles of different • loci are distributed • 3. Recombination:

  6. Seven characters in Mendel’s study of pea plants

  7. One of Mendel’spea crosses: Homozygous tall and Homozygous short are crossed First generation: All tall plants Second generation: ¾ (75%) are tall But ¼ (25%) are short Mendel asked:

  8. Mendel worked on mathematical relationships Mendel knew nothing about chromosomes or cell structures Despite his lack of cellular biology and what we call modern genetics, Mendel drew many amazing conclusions Modern genetics now has explanations for the trends he described

  9. Gene loci = Alleles = Gene lociand theiralleles

  10. Monohybrid cross • Cross between • Differ at • Dihybrid cross • - Differ at • Test cross • - Between individual of unknown genotype • and

  11. Monohybrid cross in guinea pigs One trait followed: • Fur color

  12. Dihybrid cross inguinea pigs: Two traits followed • Hair color • Hair length How many possible combos in offspring?

  13. Why are purebred (homozygous) recessives used as test crosses for unknown genotypes? Test cross inguinea pigs

  14. Product rule predicts combined probability of • Genetic ratios can be expressed as probabilities Sum rule predicts combined probability of

  15. Rulesof probability This applies to segregation of alleles as well as to chances of different combo’s during fertilization

  16. Review Mendel’s Main Concepts: • Independent assortment • -Orientation of homologous chromosomes • on the metaphase plate • Segregation • Results from homologous chromosomes

  17. Chromosomalbasis forsegregation

  18. Linkage • Tendency for a group of genes on same chromosome to be inherited together • Recombination of linked genes • Results from -Creates different genotype than parents before fertilization occurs • By measuring frequency of recombination, • Scientists can construct a

  19. Sections are exchanged Between homologous chromosomes during Crossing-over: More variation! homologous chromosomes separatedin then sister chromatids are separated in

  20. Testcross uses double trait recessive individual 25% of each possible combo = 1:1:1:1 ratio http://www.ucl.ac.uk/~ucbhjow/bmsi/bmsi_8.html

  21. Two-pointtest cross todetect linkagein fruit flies Compare actual results to what you expect to see. Difference due to linkage during crossing over

  22. BbVv = 965 individuals Looks like grey, normal winged parent Bbvv = 944 individuals Looks like black, vestigial winged parent Bbvv = 206 individuals Recombinant variety; NOT like parents bbVv = 185 individuals Recombinant variety; NOT like parents

  23. % crossing over between different loci = total # of recombinant phenotypes divided by total # of individuals x 100: 206 + 185 = 391 965 + 944 + 206 + 185 2,300 391 = 0.17 x 100 = 17% 2,300 This percentage turns into map units = relative distance between loci

  24. map units = relative distance between loci

  25. Genotype: • Cells offemales of many species • contain two • Sex chromosomes: • Males • Y chromosome determines male sex • in most species of mammals

  26. Sex determinationin mammals XX or XY chromosomes

  27. Punnett Squares for Sex-Linked Alleles: “C” represents color vision Color vision allele is missing on Y chromosome X X X X X Y XY

  28. Why is colorblindness rare in girls? X-linkedred-green colorblindness

  29. Female XX reduced effect of 2 alleles becomes comparable to males’ single X-linked allele If female is heterozygous for X-linked trait one of the X chromosomes is inactivated X chromosomes seen in interphase: dark spot on edge of nucleus called Inactivation occurs early in embryonic development Result is after a point, normal mitosis takes over and clones cells that may or may not produce a trait. Dosage compensation

  30. Dosage compensation in female mammals Produces variegation like Calico cats. Colorblindness and hemophilia: rare in females, even with variegation Cells performing normal function mask other cells lacking trait: produce enough pigment in retina for color vision; enough blood clotting factor to prevent hemophilia

  31. Disorders Occur When Too Many or Too Few Sex Chromosomes Occur • Klinefelter’s Syndrome occurs when males receive XXY chromosomes

  32. Turner’s Syndrome Occurs when women receive only one X chromosome • short stature • lack of ovarian development • webbed neck • arms turn out at elbow • low hairline in the back • cardiovascular problems • kidney & thyroid problems • skeletal disorders: • scoliosis or dislocated hips • hearing and ear disturbances

  33. What about when it isn’t a case of strict dominant/recessive allele expression? Incomplete dominance • Heterozygote is • Co-dominance • - Heterozygote

  34. Incompletedominancein four o’clocks Neither allele is dominant. heterozygous shows an in-between blended trait

  35. Co-dominance:Heterozygote simultaneously expresses the phenotypes of both homozygotes First 2 share dominancy, 3rd blood type is recessive

  36. Antigens react to antibodies

  37. Multiple alleles in rabbits Multiple alleles (variations) show a over several different types of alleles for one locus. Sometimes some alleles are co-dominant or incompletely dominant, too. More genetic variation!

  38. Two Gene interaction in chickens 2 loci affecting one trait Opposite is called one gene has e.g. cystic fibrosis gene: thick mucous affects cells in lungs, stomach, and reproductive system.

  39. Epistasis: one gene produces an influence over other genes. ee causes no pigment deposition in the dog’s coat.

  40. Norm of reaction • Range of phenotypic possibilities from a single genotype under different environmental conditions e.g. • Can be modified by factors such as diet • Genes that affect height set “norm of reaction” • (1 foot tall? 12 feet tall? Limitations in a range!) Environment molds phenotype within norm of reaction

  41. Polygenic inheritance in human skin pigmentation Several genes are responsible for a particular trait occurring: Blend of intensity

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