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Genetics *transmission of traits – heredity *variation *genetics Two main hypotheses on how traits were transmitted: *blending inheritance *particulate inheritance The father of transmission genetics: Gregor Johann Mendel 1822-1884 Mendel tracked heritable characters for three generations *P - parental generation *F1 – first filial generation *F2 – second filial generation -Example: F2 P X Tall Dwarf F1 – all Tall Tall *genes and alleles Mendel’s hypotheses (to explain his results) 1. Alternative versions of genes (alleles) account for variation in inherited characters 2. For each character, an organism inherits two alleles, one from each parent 3. If two alleles differ, one is dominant, the other recessive 4. The two alleles for each character segregate (separate) during gamete production. P: X Tall Dwarf DD dd F1 – all Tall Tall Dd Mendel’s Law of Segregation Punnett Square predicts the results of a genetic cross between individuals of known genotype Tall P: Gamete formation: DD D Dwarf X D dd d d *genotype *Homozygous *phenotype *Heterozygous Mendel’s Law of Independent Assortment *What happened when he looked at two characters? If they segregate together: If they segregate independently: Dihybrid cross- A genetic cross between two individuals involving two characters Punnett square and the law of Example: independent assortment: P1 GW GW GW GW X gw GgWw GgWw GgWw GgWw yellow, round green, wrinkled GGWW ggww gw GgWw GgWw GgWw GgWw gw GgWw GgWw GgWw GgWw F1 All yellow, round GgWw gw GgWw GgWw GgWw GgWw Punnett square and the law of independent assortment: GW F1 F1 X All yellow, round GgWw GgWw F2 9/16 yellow, round gw GGWw GGww GgWw Ggww gW GgWW GgWw ggWW ggWw gw ggWw ggww GgWw Ggww 9:3:3:1 Phenotypic ratio; Genotypic ratio as follows: 1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw 3/16 yellow, wrinkled 1/16 GGww, 2/16 Ggww 3/16 green, round 1/16 ggWw, 2/16 ggWw 1/16 green, wrinkled gW GW GGWW GGWw GgWW GgWw Gw All yellow, round Gw 1/16 ggww Mendelian inheritance is based on probability Example- coin toss *1/2 chance landing heads *Each toss is an independent event *Coin toss, just like the distribution of alleles into gametes *The rule of multiplication – determines the chance that two or more independent events will occur together ½ x ½ = ¼ Sample problem Albinism in humans is inherited as a simple recessive trait. Determine the genotypes of the parents and offspring for the following families. When two alternative genotypes are possible, list both. (A) Two non albino (normal) parents have five children, four normal and one albino. (B) A normal male and an albino female have six children, all normal. 1) establish gene symbols: 2) Establish: genotype AA Aa aa A=normal (not albino) a=albino Move on to part (A): Parents are both phenotypically normal, genotypes could be EITHER AA or Aa, an albino phenotype could only result from an aa genotype. *One a had to come from the mother and one a had to come from the father, so, the parents must be genotypically Aa. A phenotype normal normal albino a Answer to (A): A AA Aa Genotype of the parents = Aa a Aa aa Genotype of normal children = AA or Aa Genotype of albino child = aa 2. (B) A normal male and an albino female have six children, all normal. 1) The female is phenotypically albino; genotype can only be aa 2) The male is phenotypically normal; genotype can be AA or Aa 3) Since all children are 4) BUT male COULD also be Aa ! normal one might assume *IF the father was the male to be AA genotypically Aa, then what is the likelihood A A A a (chance or probability) of this couple having 6 a a Aa aa Aa Aa normal children? a Aa Aa a Aa aa F1: F1: Genotype: all Aa Genotype: ½ Aa, ½ aa or a 1:1 ratio Phenotype: all normal Phenotype: ½ normal, ½ albino or a 1:1 ratio Recall the product law! ½x½x½x½x½x½ = 1/64