Law of Independent Assortment

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Cytogenetics
MD 3Y1-6
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Law of Independent Assortment

  1. 1. Law of Independent Assortment ● Mendel's 2nd Law ● “Inheritance Law” ● formulated after his 1st principle (law of segregation) - alleles for a trait separate when gametes are formed - allele pairs randomly unite at fertilization
  2. 2. ● What would happen if he studied plants that differed in 2 traits? ● Would they be transmitted to the offspring together, or ● Would one trait be transmitted independently of the other? ● Performed dihybrid crosses (a cross b/w 2 parents that differ by 2 pairs of alleles; ex. GGYY x ggyy) in plants that were true-breeding for 2 traits ● Hybridization experiments done b/w 1856-1863 using garden peas (Pisum sativum)
  3. 3. ● First experiment: cross pollinated pure bred round, yellow seed producing pea with pure bred wrinkled, round seed producing pea. ● Cross pollinated a plant w/ green pod colour (G) & yellow seed colour (Y) and a plant w/ yellow pod colour (g) & green seed colour (y) ● Dominant: green pod colour (GG) yellow seed colour (YY) ● Recessive: yellow pod colour (gg) green seed colour (yy) ● Resulting offspring/F1 generation: all heterozygous for green pod colour and yellow seeds (GgYy)
  4. 4. ● F1 plants: – Genotype: GgYy – Phenotype: green pod colour & yellow seed colour ● both dominant traits
  5. 5. ● Allowed all of the F1 plants to self-pollinate and referred to the resulting offspring as “F2 generation”
  6. 6. ● 9:3:3:1 phenotypic ratio ● F2 plants: 9 green pods & yellow seeds 3 green pods & green seeds 3 yellow pods & yellow seeds 1 yellow pod & green seeds ● 9 different genotypes ● 4 different phenotypes
  7. 7. ● Mendel performed similar experiments focusing on other traits including: pod colour & pod shape flower position & stem length ● phenotypic ratio of 9:3:3:1 present in each ● formulated his second principle, Law of Independent Assortment from these findings: allele pairs assort/separate independently during the formation of gametes ● Biological selection for a particular gene in the gene pair for one trait to be passed to offspring has no relations with the selection of the gene for any other trait – true for genes that are not linked to each other
  8. 8. ● Independent assortment takes place during meiotic metaphase I ● Produces a gamete with a mixture of the organism's chromosomes ● Physical basis of independent assortment: - random orientation of each bivalent chromosome along the metaphase plate with respect to the other bivalent chromosomes ● Independent assortment + crossing over = increased genetic diversity by producing novel genetic combinations
  9. 9. ● Resulting chromosomes are randomly sorted from all possible combinations of maternal & paternal chromosomes ● Gametes are “assorted independently” b/c they end up w/ a random mix instead of receiving a pre-defined “set” from either parent ● Any possible combinations of gametes formed from maternal & paternal chromosomes occur w/ equal frequency ● no. of possibilities for human gametes, w/ 23 pairs of chromosomes is 223 or 8,388,608 possible combinations ● This contributes to the genetic variability of progeny (offspring)
  10. 10. ● Mendelian trait: trait controlled by a single locus in an inheritance pattern ● Mutation in a single gene can cause a disease that is inherited according to Mendel's laws ● Over 4000 human diseases caused by single gene defects ● Disorders can be passed onto future generation in several ways: Autosomal dominance Autosomal recessive X-linked dominant X-linked recessive Y-linked
  11. 11. ● X-linked dominant – Caused by utations in genes on X- chromosome – Males & females both affected w/ males usually affected more severely – Sons of man w/ disorder will be unaffected since they receive father's chromosome; daughters will inherit condition – Woman w/ disorder has 50% chance of having an affected fetus – Some conditions like Rett syndrome, incontinentia pigmenti type II & Aicardi syndrome are often fatal in males in utero or shortly after birth, so are more predominantly seen in females
  12. 12. ● X-linked recessive – Disorders also caused by mutations in genes on X chromosomes – Males more frequently affected than females – Sons of man w/ X-linked recessive disorder will not be affected; daughters will carry one copy of mutated gene – Woman who is a carrier has 50% chance of having sons who are affected; 50% chance of having daughters who carry one copy of mutated gene
  13. 13. ● X-linked recessive (cont.,) – Serious diseases: hemophilia A Duchenne muscular dystrophy Lesch-Nyhan syndrome – Less serious conditions: male pattern baldness red-green colour blindness – May manifest in females due to skewed X-inactivation/ monosomy X (Turner syndrome)
  14. 14. ● Y-linked – Disorders caused by mutations on Y- chromosome – Since males inherit a Y chromosome from their fathers, every son of an affected father will be affected – Since females only inherit X chromosomes, female offspring are never affected – Since Y chromosomes are small & contain very few genes, only a few Y- linked disorders occur – Symptoms usually include infertility (w/c may be circumvented w/ help of fertility treatments)
  15. 15. Disorder Prevalence (approximate) X-linked Duchenne muscular dystrophy 1 in 7000 Hemophilia 1 in 10,000 Sample Problem A man w/ hemophilia marries a homozygous normal woman. Predict the genotypes and phenotypes of their children. X-linked recessive trait: Hemophilia XH X chromosome w/ normal dominant allele Xh X chromosome w/ recessive hemophilia allele
  16. 16. Step 1. Determine the genotypes of the parents. Hemophiliac male Xh Y Normal female XH XH (Xh Y ) x (XH XH) Step 2. Determine the gamete genotypes produced by each parent. (Xh Y) --> Xh , Y; --> XH Step 3. Set up a Punnet square using the gamete genotypes. Xh Y XH
  17. 17. Step 4. Combine the gamete genotypes of one parent with those of the other parent to show all possible offspring genotypes. Step 5. State the genotype and phenotype ratios of the offspring: genotype: 1 (XH Xh): 1 (XH Y) phenotype: 1 normal female : 1 normal male Xh Y XH XH Xh XH Y
  18. 18. ● Modifier Genes – Instead of masking the effects of another gene, a gene can modify the expression of the second gene – ex. coat colour in mice B: black coat b: brown coat D: full coat - modifier gene d: dilute (faded) coat - modifier gene Sample Problem: Using a Punnet square, show the results of mating 2 mice that are heterozygous for both traits. Give the phenotypic ratio.
  19. 19. ● Modifier Genes – Instead of masking the effects of another gene, a gene can modify the expression of another gene – ex. coat colour in mice B: black coat b: brown coat D: full coat - modifier gene d: dilute (faded) coat - modifier gene Sample Problem: Using a Punnet square, show the results of mating 2 mice that are heterozygous for both traits. Give the phenotypic ratio.
  20. 20. BbDd x BbDd = BD, Bd, bD, bd; BD, Bd, bD, bd 9: full black coat (BD) 3: dilute black coat (B_dd) 3: full brown coat (bbD_) 1: dilute brown coat (bbdd) BD Bd bD bd BD BDBD BDBd BDbD BDbd Bd BdBD BdBd BdbD Bdbd bD bDBD bDBd bDbD bDbd bd bdBD bdBd bdbD bdbd
  21. 21. ● Post Mendelian Genetics: 1. Co- Dominance – 2 alleles are expressed at the same time Eg. horses RR: red WW: white RR x WW = RW (all are roan: red & white) 2. Incomplete Dominance – Strict dominance/recessiveness does not apply – Heterozygous individuals have an intermediate phenotype – Eg. Snapdragons R: red R': white
  22. 22. Sample Problem: On planet XY, a plant exists where a red flower allele (P) is incompletely dominant to the colour blue (P'). Another pair of incompletely dominant genes control pigment production where the allele (I) gives no colour (white) and the allele (I') gives full colour. Give the phenotypic ratio of the offspring whose parents are heterozygous for both traits. Use a Punnet square. PP'II' x PP'II' = PI, PI', P'I, P'I' ; PI, PI', P'I, P'I'
  23. 23. PPII': red faded – 2 PPI'I': red full – 1 P'PII': purple faded – 4 P'PI'I': purple full - 2 P'P'I'I: blue faded – 2 P'P'I'I': blue full – 1 PPII: (red) white – 1 white flowers = 4 PP'II: (purple) white – 2 P'P'II: (blue) white – 1 PI PI' P'I P'I' PI PIPI PIPI' PIP'I PIP'I' PI' PI'PI PI'PI' PI'P'I PI'P'I' P'I P'PII P'IPI' P'IP'I P'IP'I' P'I' P'I'PI P'I'PI' P'I'P'I P'I'P'I'

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