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Mendelian Genetics

Mendelian Genetics. The laws of probability govern Mendelian inheritance. Mendel’s laws of segregation and independent assortment reflect the rules of probability

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Mendelian Genetics

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  1. Mendelian Genetics

  2. The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities • Probability in a monohybrid cross can be determined using this rule

  3. Rr Rr  Segregation ofalleles into sperm Segregation ofalleles into eggs Sperm r 1/2 1/2 R R R r R R 1/2 1/4 1/4 Eggs r r r R 1/2 r 1/4 1/4

  4. Punnett Square

  5. Monohybrid Cross

  6. The rule of addition • States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

  7. Each box in this dihybrid cross has a 1/16 chance of occurring. Add them up for chances of any phenotype

  8. Dihybrid cross - The traits are: long tail (s), short tail (S), brown hair (B) and white hair (b)

  9. Solving Complex Genetics Problems with the Rules of Probability • We can apply the rules of probability to predict the outcome of crosses involving multiple characters • A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes from such crosses each character first is considered separately and then the individual probabilities are multiplied together

  10. Trihybrid Cross of PpYyRr x Ppyyrr 1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)  1/16 ppyyRr  1/16 ppYyrr 1/41/21/2  2/16 Ppyyrr 1/21/21/2  1/16 1/41/21/2 PPyyrr ppyyrr  1/16 1/41/21/2  6/16 or 3/8 Chance of at least two recessive traits

  11. Summary of Basic Mendelian Genetics • We cannot predict with certainty the genotype or phenotype of any particular seed from the F2 generation of a dihybrid cross, but we can predict the probabilities that it will fit a specific genotype of phenotype. • Mendel’s experiments succeeded because he counted so many offspring and was able to discern this statistical feature of inheritance and had a keen sense of the rules of chance. • Mendel’s laws of independent assortment and segregation explain heritable variation in terms of alternative forms of genes that are passed along according to simple rules of probability.

  12. Extending Mendelian Genetics • The inheritance of characters by a single gene may deviate from simple Mendelian patterns • Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely simple • But we can extend Mendelian principles to patterns of inheritance more complex than Mendel described

  13. The Spectrum of Dominance • Complete dominance occurs when the phenotypes of the heterozygote and dominant homozygote are identical • In incomplete dominance the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

  14. Red and White Snapdragons

  15. P Generation White Red CWCW CRCR Gametes CW CR Incomplete Dominance

  16. P Generation White Red CWCW CRCR Gametes CW CR Incomplete Dominance F1 Generation Pink CRCW 1/2 1/2 CR Gametes CW

  17. P Generation White Red CWCW CRCR Gametes CW CR Incomplete Dominance F1 Generation Pink CRCW 1/2 1/2 CR CW Gametes Sperm F2 Generation 1/2 1/2 CW CR 1/2 CR CRCR CRCW Eggs 1/2 CW CRCW CWCW

  18. The Spectrum of Dominance • In codominance two dominant alleles affect the phenotype in separate, distinguishable ways • The human blood group MN is an example of codominance

  19. MN Blood Groups

  20. The Relation Between Dominance and Phenotype • Dominant and recessive alleles • Do not really “interact” • Lead to synthesis of different proteins that produce a phenotype

  21. Tay-Sachs Disease • Humans with Tay-Sachs disease produce a non-functioning enzyme to metabolize gangliosides (a lipid) which then accumulate in the brain, harming brain cells, and ultimately leading to death. Tay-Sachs most common in Ashkenazic Jews (from Central Europe) • Children with two Tay-Sachs alleles have the disease. • Heterozygotes with one working allele and homozygotes with two working alleles are “normal” at the organismal level, but heterozygotes produce less functional enzymes. • However, both the Tay-Sachs and functional alleles produce equal numbers of enzyme molecules, codominant at the molecular level.

  22. Tay-Sachs Disease

  23. Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • Polydactyly is a dominant trait – Antonio Alfonseca • 399 out of 400 people have 5 digits

  24. Dominance/recessiveness relationships • Range from complete dominance through various degrees of incomplete dominance to codominance • Reflect the mechanisms by which specific alleles are expressed in the phenotype and do not involve the ability of one allele to subdue another at the level of DNA

  25. (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i none Multiple Alleles Carbohydrate B A (b) Blood group genotypes and phenotypes Genotype ii IAIA or IAi IBIB or IBi IAIB Red blood cellappearance Phenotype(blood group) A AB O B

  26. Pleiotropy – gene affects more than one phenotypic trait Sickle-cell Anemia

  27. Pleiotropy - Phenotypic traits affected by sickle-cell anemia • Sickled red-blood cells • Anemia • Heart failure • Brain damage • Spleen damage • Rheumatism • Kidney failure

  28. Coat color in Labrador Retrievers

  29. BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Be BE be bE Epistasis – a gene at one locus alters the phenotypic expression of a gene at another locus Eggs 1/4 BE BbEE BBEe BbEe BBEE 1/4 bE BbEE bbEe bbEE BbEe 1/4 Be BBEe BBee Bbee BbEe 1/4 be BbEe bbEe bbee Bbee : 3 9 : 4

  30. PolygenicTrait, Quantative Characters – Human height in 175 students at Connecticut Agricultural College

  31. PolygenicTrait, Quantative Characters – How human skin color might work AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 6/64 1/64 15/64 Number ofdark-skin alleles: 1 2 3 4 5 0 6

  32. Relationship amongalleles of a single gene Description Example Complete dominanceof one allele Heterozygous phenotype same as that of homo-zygous dominant PP Pp Figure 14.UN03 Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes Incomplete dominanceof either allele CRCR CWCW CRCW Codominance Both phenotypesexpressed inheterozygotes IAIB Multiple alleles In the whole population,some genes have morethan two alleles ABO blood group alleles IA, IB, i One gene is able to affectmultiple phenotypiccharacters Pleiotropy Sickle-cell disease

  33. Relationship amongtwo or more genes Description Example The phenotypicexpression of onegene affects thatof another BbEe Epistasis BbEe BE Be bE be BE Figure 14.UN04 bE Be be 9 : 4 : 3 A single phenotypiccharacter is affectedby two or more genes Polygenic inheritance AaBbCc AaBbCc

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