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Flashcards · Biology · Genetic Inheritance

Single Gene Crosses and Punnett Squares

16 cards

  1. How many alleles does each gamete carry?

    One allele of each gene.

  2. What is a Punnett square?

    A diagram showing all the possible combinations of gametes in a cross.

  3. What gametes does a parent with the genotype Bb make?

    B and b.

  4. What gametes does a parent with the genotype bb make?

    Only b.

  5. What are the offspring genotypes in Bb x Bb?

    1 BB : 2 Bb : 1 bb.

  6. What is the phenotype ratio in Bb x Bb?

    3 dominant to 1 recessive.

  7. What is the phenotype ratio in Bb x bb?

    1 dominant to 1 recessive.

  8. What is the chance of a recessive offspring from Bb x Bb?

    1 in 4, or 25%.

  9. What is the chance of a recessive offspring from Bb x bb?

    1 in 2, or 50%.

  10. Which allele do you write first in a heterozygous genotype?

    The dominant allele, so Bb.

  11. Does a 3 : 1 ratio mean that exactly 3 in every 4 offspring show the dominant phenotype?

    No. It is the expected ratio over many offspring and each fertilisation is a chance event.

  12. A couple have a child with a recessive disorder. What is the chance that the next child has it too?

    The same as before. Each fertilisation is independent.

  13. If two black mice have a brown offspring, what are the parents' genotypes? (Higher tier)

    Both are Bb.

  14. How do you find the probability of two independent events both happening? (Higher tier)

    Multiply their probabilities.

  15. What is the chance that two offspring from Bb x Bb are both brown? (Higher tier)

    \(\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}\)

  16. Complete the Punnett square for two heterozygous black mice (Bb x Bb) and give the genotype and phenotype ratios. A completed Punnett square for Bb x Bb giving three black offspring to one brown.

    The squares are BB, Bb, Bb and bb. The genotype ratio is 1 BB : 2 Bb : 1 bb. The phenotype ratio is 3 black to 1 brown, so a 25% chance of brown fur.