phenotype and genotype ratios
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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

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

heterozygous
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!
genotype phenotype ratios
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
heterozygous x homozygous recessive
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.
more interesting ratios
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!
3 1 phenotype ratio
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
1 2 1 genotype ratio
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
possible ratios
Possible Ratios
  • There are only a few ratios you will see in monohybrid crosses:
    • 100%
    • 1:1
    • 3:1
    • 1:2:1
generations
Generations
  • In genetics, the generations of offspring are designated as P (parents), F1 (offspring 1), F2 (offspring 2), etc.
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