Dihybrid Crosses and the 9:3:3:1 Ratio · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Crossing two genes at once

Science · Genetics & Evolution · ages 17-18
Name ______________________   Date ____________

  1. A parent is RrYy. Which four gamete types can it make?

    • RY, Ry, rY and ry
    • RR, Rr, YY and Yy
    • RY and ry only

  2. What is the Mendelian dihybrid cross?

    • A cross of clones with identical genotypes
    • A two-gene cross of double heterozygotes
    • A cross of one gene with identical recessive parents

  3. What does independent assortment mean?

    • Alleles of different genes sort independently
    • Sister chromatids never separate in meiosis
    • All offspring of a cross are genetically identical

  4. In a 9:3:3:1 split, what does the 9 stand for?

    • Offspring showing both recessive traits
    • Offspring showing both dominant traits
    • Offspring that failed to germinate

  5. A full dihybrid grid is 4 boxes by 4 boxes. How many boxes does it hold?

    Answer: ______________

  6. At which stage does independent assortment physically happen?

    • Metaphase I, as homologous pairs line up at random
    • Cytokinesis, as the cytoplasm finally splits
    • Fertilisation, as two gametes fuse

  7. A class counts peas and lands near but not on 9:3:3:1. What should they conclude?

    • The ratio is exact law and someone miscounted
    • Only one gene must have been involved
    • The ratio is a prediction and chance wobbles real counts

  8. Two genes sit close together on one chromosome. What happens to the ratio?

    • The 9:3:3:1 pattern breaks, as alleles travel together
    • The ratio stays 9:3:3:1 because dominance is unchanged
    • All offspring show both recessive traits
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Crossing two genes at once W1-mt_9IlVo04Dbt-s1

  1. RY, Ry, rY and ry · Each gamete takes one allele of each gene, giving four equal combos.
  2. A two-gene cross of double heterozygotes · Di means two: two genes tracked at once, both parents heterozygous for each.
  3. Alleles of different genes sort independently · One gene does not influence which allele another gene sends to a gamete.
  4. Offspring showing both dominant traits · Nine of the sixteen boxes combine a dominant allele of each gene.
  5. 16 · Four rows of four boxes give 4 times 4, which is 16 boxes.
  6. Metaphase I, as homologous pairs line up at random · Each pair chooses its facing independently, shuffling allele combos into gametes.
  7. The ratio is a prediction and chance wobbles real counts · 9:3:3:1 predicts the pattern. Real batches scatter around it by chance.
  8. The 9:3:3:1 pattern breaks, as alleles travel together · Linked genes do not assort independently, so the classic split fails.
Worksheet · LightMySky