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Genetics · Chapter 5: Inheritance and Punnett squares

Year 11 Science · About 15 minutes with activities · Turn your sound on

Chapter 5 · Inheritance
Two black parents
A white baby?!
Eggone allele
Spermone allele
Offspringtwo alleles: one from each parent
Dominant: capital letterB = black fur
Recessive: lower caseb = white fur
BBhomozygous dominant · black
Bbheterozygous · black, carries b
bbhomozygous recessive · white
3 (black) : 1 (white)expected ratio
BB or Bb?both have black fur
If Bb: 1 (black) : 1 (white)some white offspring
If BB: 1 (black) : 0 (white)all black offspring

Chapter 5 recap

  • Monohybrid inheritance: one gene. A dominant allele (capital letter) needs one copy to be expressed; a recessive allele (lower case) is expressed only if both alleles are recessive.
  • Genotype: the alleles carried. Phenotype: their physical expression. Homozygous: two of the same allele (BB, bb). Heterozygous: two different alleles (Bb).
  • Punnett squares give expected ratios, e.g. Bb × Bb → 3 (black) : 1 (white). Observed ratios can differ by chance. A test cross with a recessive (bb) individual reveals a hidden b allele.
  • Next: Chapter 6 · Sex, pedigrees and sex-linked traits

How can two black mice have a white baby?

Chapter 5 · About 15 minutes · sound on

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