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

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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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