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MENDELIAN GENETICS

MENDELIAN GENETICS. Genotype. Acytivity of other genes. Environment. Phenotype. Pisum sativum. Allele R. Allele r. Anther Stamen (male reproductive organ) m Stigma

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MENDELIAN GENETICS

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  1. MENDELIAN GENETICS

  2. Genotype Acytivity of other genes Environment Phenotype

  3. Pisum sativum

  4. Allele R Allele r

  5. Anther Stamen (male reproductive organ) m Stigma Pistil (female reproductive organ) f Stylus Ovary Autofecondation  Stamen produces pollen  deposed on pistil (fecundation) Organism (Pisum Sativum)

  6. Terminology Gametes = a mature reproductive cell specialized for sexual fusion. A gamete is haploid and fuses to a gamete of the other sex. Cross = mating of two individuals with fusion of gametes Zygote = cell resulting from the fusion of two gametes

  7. Parental generation = P First generation = F1 Second generation = F2 Crossing F2 x F2 = F3 F3 x F3 = F4 and so on Crossing F2 x F1 = backcross

  8. Mendel initially crossed pure lines differing for a single trait (monohibrids) Es. P = smooth (m) x rough (f) F1 = smooth Reciprocal cross P = Smooth (f) x rough (m) F1 = smooth

  9. Interpretation: The F1 trait was identical to the one of the two P, and NOT a mixture of the two. Principle of F1 uniformity F1 x F1  F2 (smooth and rough, 3:1 ratio) Same results with ALL other 6 traits considered

  10. Mendel concluded the principle of DOMINANCE: Crosses of plants differing in a single trait with two phenotypes result in hybrids in which only one phenotype, termed dominant, is manifest.

  11. The data

  12. General conclusions 1) Results of crosses very similar 2) F1 always similar to one of the parental lines 3) F2 show the trait “disappeared” in F1 Mendel concluded that alternative phenotypes were due to “factors” (which now call genes) Mendel hypothesized that such factors would exist in alternative forms (alleles) each controlling a character carattere (smooth or rough etch)

  13. Mendel also hypothesized that: • Every parental pure line (P) has two characters 2) Since F2 shows both, F1 must have the one on display and the one not shown. The allele masking one of the character in F1 is termed dominant. The allele masked in F1 is termed recessive.

  14. Individuals with the SAME couple of alleles are called homozygotes Individuals with different alleles are called heterozygotes Dominant traits are on display in homozygosity and heterozygosity Recessive traits are on display only in homozygotes.

  15. First principle of Mendel Principle of segregation: Each diployd individual has 2 alleles segregating during gametes formation, each with an allele. The principle of dominance establishes that when two alleles are present, only the dominant allele is manifest in the phenotype.

  16. Albinism= recessive aa Normal phenotype= AA o Aa • What is the probability that normal heterozygous individuals generate an albino son/daughter? • Three albinos? • One albino and two with a normal phenotype?

  17. DIHYBRIDS CROSS

  18. Mendel crossed pure lines with two distinct phenotypic traits He showed that the segregation of one trait does not depend upon the presence of the other: second principle of independent segregation. We now know that genes positioned on different chromosomes behave independently upon the individual segregation of the chromosome in gametes.

  19. Second PRINCIPLE 0f Mendel Independent segregation of traits: The “factors” controlling different traits are distributed in an independent way one from the others.

  20. Test cross

  21. Crosses with pure lines of three traits

  22. Number of Alleles couples (traits) Number of genotypic classes Number of phenotypic classes 1 2 3 2 4 9 3 8 27 4 16 81 n 2n 3n

  23. Seed smooth-yellow X Seed rough-green Ss Yy ss yy 2 2 Χ = ∑ (o-a) a df= n-1

  24. Dominant hereditary diseases: Brachydactily

  25. Recessive hereditary diseases Mutations inducing a loss-of-function. Heterozygous (carriers) are normal, una riduzione del prodotto proteico del 50% viene tollerato se il rimanente 50% è sufficiente per una funzione normale Sickle Cell anemia

  26. 1. INCOMPLETE DOMINANCE The phenotype of heterozygotesis intermediate betweenhomozygotes

  27. 2. CODOMINANCE The heterozygote shows the phenotypes of bothhomozygotes (dominant and recessive)

  28. Blood groups Phenotype Genotipo 0 i/i A IA/IA o IA/i B IB/IB o IB/i AB IA/IB 3 allelicvariants: IA; IB (dominant); i(recessive)

  29. The ABO locus codes for a glycosyltransferase

  30. Phenotype Genotype M LM /LM N LN/LN MN LM/LN Two allelic variants: LM ; LN (codominant) Phenotype Genotype Rh+ D/D o D/d Rh- d/d Two allelic variants: D (Dominant); d

  31. Fetal Erythroblastosis Father Rh+ DD Mother Rh- dd Offspring Rh+ Dd Antibody production against D Possible anemia of the offspring

  32. Interactions between genes Epistasis: Form of interaction between genes, in which the activity of one gene masks the phenotypic contribution of a second gene. The 9:3:3:1 ratio is modified in a dyhybrid cross The gene masking the expression of the second gene is calledepistatic, the masked gene ipostatic.

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