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Phenotype and Genotype Ratios

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 . Is the blue parent heterozygous or homozygous?

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Phenotype and Genotype Ratios

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  1. 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 . Is the blue parent heterozygous or homozygous? Draw a Punnett square of the cross. Phenotype and Genotype Ratios

  2. Heterozygous! • 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!

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

  4. Heterozygous x Homozygous recessive • 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.

  5. More Interesting Ratios • 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!

  6. 3:1 Phenotype Ratio • 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

  7. 1:2:1 Genotype Ratio • 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

  8. Possible Ratios • There are only a few ratios you will see in monohybrid crosses: • 100% • 1:1 • 3:1 • 1:2:1

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

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