Do Now: In a species of fish, the allele for blue scales (B) is dominant to the allele for white scales (b). A blue fish mates with a white fist and produces the following offspring: 88 blue fish and 92 white fish. 1. Is the blue parent heterozygous or homozygous? 2. Draw a Punnett square of the cross.

 Since blue (B) is dominant to white (b), the blue fish might be BB or Bb.  If the blue fish was homozygous dominant, all of the offspring would be blue! BB bBb b Bb b bb bBbbb

 Remember those definitions:  Genotype = what alleles an organism has (Aa, etc.)  Phenotype = the characteristic (green seeds, etc.)  A ratio is a relationship between numbers

 Consider the fish cross:  Since 50% of the offspring will be Bb and 50% will be bb (2 squares:2 squares), the genotype ratio will be 1:1 Bb:bb  Since all of the fish are blue, there’s not really a phenotype ratio for this one – just 100% blue. Bb bBbbb bBbbb

 When Mendel crossed two heterozygous purple flowered pea plants, he noticed a 3:1 phenotype ratio. In other words, there were 3 times as many purple flowered plants as white flowered plants.  He immediately knew the plants had 2 alleles!

 Notice that 3 of the squares represent purple-flowered plants, and 1 represents white flowered plants.  CAUTION: RANDOM BEHAVIOR  In the real world, these ratios might not be perfect, because of chance. You might flip a coin 10 times and get heads 6 and tails 4…  Just remember: 305:98 is just as valid as 300:100 Pp PPPPp p pp

 The previous cross produces 3 different genotypes: PP, Pp, and pp.  Thus, our genotype ratio must also have 3 terms  1:2:1 PP:Pp:pp Pp PPPPp p pp

 There are only a few ratios you will see in monohybrid crosses:  100%  1:1  3:1  1:2:1

 In genetics, the generations of offspring are designated as P (parents), F1 (offspring 1), F2 (offspring 2), etc.