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9th grade · Science · Unit 10 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.

Texas Essential Knowledge and Skills (TEKS) teks student expectation: B.8(B) · predicting outcomes of genetic crosses, including non-Mendelian traits

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 Texas

In the Texas Essential Knowledge and Skills (TEKS), this is unit 10 of 14 in 9th grade Science, after DNA, RNA & Proteins and before Evidence & Mechanisms of Evolution.

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: Texas Education Agency (TEA)

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

    Human ABO blood type is controlled by…

    • the environment only
    • one allele
    • multiple alleles (A, B and O)
    • only the Y chromosome

  2. Question 2

    In a dihybrid cross of two RrYy plants, the classic phenotype ratio is…

    • 9 : 3 : 3 : 1
    • 1 : 2 : 1
    • 1 : 1 : 1 : 1
    • 3 : 1

  3. Question 3

    Phenotype means…

    • the DNA bases
    • the alleles
    • the chromosome number
    • the trait that shows

  4. Question 4

    Blood type AB shows that A and B alleles are…

    • incompletely dominant
    • recessive
    • codominant
    • sex-linked

  5. Question 5

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

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

  6. Question 6

    Mendel's law of independent assortment says genes for different traits…

    • blend together
    • are inherited separately
    • are always linked
    • come only from the mother

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