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Mendelian Genetics

Mendelian Genetics. Gregor Mendel – Austrian monk – performed extensive genetics experiments with garden peas http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html http://www.quia.com/jfc/65851.html. Gregor Mendel. Important Mendelian Genetics terms:.

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Mendelian Genetics

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  1. Mendelian Genetics • Gregor Mendel – Austrian monk – performed extensive genetics experiments with garden peas • http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html • http://www.quia.com/jfc/65851.html

  2. Gregor Mendel

  3. Important Mendelian Genetics terms: • trait: physical attribute or characteristic displayed by an individual (i.e. seed shape, seed colour, etc.) • allele: forms of a trait (i.e. round or wrinkled allele for the trait seed shape) • dominant allele: allele that is displayed when both dominant and recessive alleles are present (dominant allele masks the recessive allele) • recessive allele: allele that is hidden when both dominant and recessive alleles are present (dominant allele masks the recessive allele), and only expressed when both recessive alleles are present) • genotype: actual gene (allele) combination present (RR or Rr) • phenotype: physical outward appearance shown (round seed or wrinkled seed)

  4. homozygote: an individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene of interest • heterozygote: an individual which contains one of each member of the gene pair; for example the Dd heterozygote • Human Eye color

  5. Multiple Alleles • Multiple Alleles – the existence of several (more than two) alleles for a gene (e.g. ABO blood group)

  6. Incomplete Dominance • shown when a heterozygote has a different phenotype (blending of both homozygotes) than either homozygote (e.g. Snapdragons: 2 alleles - Red – R, White – W • RR (red) x WW (white) produce RW (pink) RED PINK PINK WHITE PINK PINK

  7. Codominance • Similar to incomplete dominance • “co” – together • recessive & dominant traits appear together in the phenotype of hybrid organisms • Ex: roan coat color in cattle (red and white hair on same animal) • 2 allelles - R – red, W – white • 3 different phenotypes – red (RR), white (WW) and roan (RW)

  8. X-linked Inheritance • Sometimes called sex-link inheritance • Males – XY, Females – XX • Some traits are present on the X chromosome • E.g. – hemophilia – X-linked disease (“bleeder disease”) • Normal female – XX, carrier female – XXh • Female with hemophilia – XhXh (only if they have both Xh chromosomes) • Normal male – XY, male with hemophilia – XhY • Color blindness – X-linked

  9. Dihybrid Cross • Monohybrid cross – dealing with only one trait • Dihybrid cross – dealing with two traits (seed shape and seed colour) • (P1)Yellow, round pea plant crossed with wrinkled, green plants produced 100% Yellow, round plants (F1 - YyRr)

  10. F1 generation crossed – YyRr x YyRr • Four possible gametes – YR, yR, Yr, yr

  11. F2 Generation

  12. F2 Generation – phenotypic ratio • 9 yellow, round • 3 green, round • 3 yellow, wrinkled • 1 green, wrinkled

  13. Mendel’s Laws • 3 laws: • Law of dominance – In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype. • Law of segregation - During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.  Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. • Law of independent assortment - Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

  14. Linked Genes • Linked genes - genes that are on the same chromosome and subsequently are inherited together as a package unless crossing-over separates them. • Crossing over - process of sections of homologous chromosomes breaking and reconnecting onto the other homologous chromosome. • Recombination - creation of combinations of alleles in chromosomes not present in either parent.

  15. Linked Genes??

  16. Crossing Over & Recombination

  17. Hardy Weinberg Equation • p + q = 1.0 • p2 + 2pq + q2 = 1.0

  18. Hardy Weinberg Equation • p + q = 1 • p2 + 2pq + q2 = 1 • “Unibrow” is a recessive genetic trait in humans in which it occurs 1 in 30 people. • N – separate eyebrow • n - “unibrow”

  19. q2 = 1 / 30 = 0.033 • q = √q2 = √0.033 = 0.183 = 18.3% of the genes in the population are “unibrow” (nn) • p = 1 – q = 1- 0.183 = 0.817 = 81.7% of the genes in the population are separate eyebrow. • p2 = 0.8172 = 0.668 = 66.8% of the population is NN. • 2pq = 2 (0.817)( 0.183) = 0.298 = 29.8% of the population is Nn (carriers for the “unibrow” gene)

  20. Probability • the study of outcomes of events or occurrences • Probability = # of chances for an event / # of possible outcomes • Probability of a red card = ½ (26/52) • Probability of a red 6 = 1/26 (2/52) • How does this apply to genetics? • Rr - probability of R = ½, probability of r = ½

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