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9th grade · Science · Unit 8 of 14

Genetic Crosses

Punnett squares, dominance, codominance, incomplete dominance and sex-linked traits.

What your child practices

Practices predicting the outcomes of monohybrid and dihybrid crosses with Punnett squares, and non-Mendelian inheritance: incomplete dominance, codominance, sex-linked traits and multiple alleles.

Alabama Course of Study content standard: BIO.11 · Punnett squares and probability to predict the traits of offspring

How we explain it

Children can open this short lesson before they practice.

  1. Homozygous means two matching alleles (TT, tt); heterozygous means two different (Tt).
  2. A Punnett square predicts offspring. A dihybrid cross of RrYy × RrYy gives 9 : 3 : 3 : 1.
  3. Non-Mendelian traits: incomplete dominance blends, codominance shows both, multiple alleles like ABO blood.

Example: Red and white flowers produce pink offspring. What kind of inheritance is this? Neither allele fully hides the other. The traits blend. Answer: incomplete dominance

Genetic Crosses in Alabama

In the Alabama Course of Study, this is unit 8 of 14 in 9th grade Science, after Carbon & Nitrogen Cycles and before Reproduction.

Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.

Source: Alabama State Department of Education (ALSDE)

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    A sex-linked trait is carried on…

    • only mitochondria
    • every chromosome
    • ribosomes
    • the X chromosome

  2. Question 2

    An organism with two identical alleles (TT or tt) is…

    • homozygous
    • codominant
    • heterozygous
    • hybrid

  3. Question 3

    A roan cow has both red and white hairs. This is…

    • incomplete dominance
    • sex-linkage
    • codominance
    • a mutation

  4. Question 4

    TT × tt. All offspring will be…

    • half TT and half tt
    • Tt
    • TT
    • tt

  5. Question 5

    A carrier mother (XᴮXᵇ) and an unaffected father (XᴮY). What fraction of sons are expected to be color-blind?

    • 1/4
    • 1/2
    • 0
    • all

  6. Question 6

    Tt × Tt parents have 4 children. Must exactly one be short?

    • Yes, the youngest
    • Yes, always
    • No, the ratio is only a probability
    • Only if they are twins

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