Mendelian Genetics — Drills · Grade 10 Science

Chapter: DNA, Inheritance & Gene Expression

What these drills covers

By the end of Mendelian Genetics you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, Punnett square.

Problem 1

An organism with two different alleles for one gene is
  1. homozygous
  2. heterozygous
  3. haploid
  4. recessive
Answer and explanation

heterozygous

Heterozygous means two different alleles; homozygous means two of the same.

Problem 2

Two heterozygous parents (Tt × Tt) are crossed. What percentage of offspring are expected to show the recessive phenotype?
Answer and explanation

25 %

The expected genotypes are 1 TT : 2 Tt : 1 tt, so one quarter — 25 percent — are tt and show the recessive phenotype.

Problem 3

In the cross Tt × tt, what percentage of offspring are expected to be heterozygous?
Answer and explanation

50 %

Half the offspring receive T from the heterozygous parent and t from the homozygous recessive parent, giving Tt.

Problem 4

A test cross with a homozygous recessive individual is used to determine
  1. whether a trait is inherited
  2. the genotype of an individual showing the dominant phenotype
  3. the number of chromosomes
  4. whether a mutation occurred
Answer and explanation

the genotype of an individual showing the dominant phenotype

Crossing with a homozygous recessive reveals the unknown genotype: any recessive offspring shows that the tested parent carried a recessive allele.

Problem 5

A Tt × Tt cross produces 160 offspring. How many are expected to show the dominant phenotype?
Answer and explanation

120

Three quarters are expected to show the dominant phenotype: 0.75 × 160 = 120.

Problem 6

Two parents who both show a dominant phenotype have a child showing the recessive phenotype. Give both parents' genotypes and explain your reasoning.
Answer and explanation

Both parents must be heterozygous. The child showing the recessive phenotype must be homozygous recessive, which requires one recessive allele from each parent. Since both parents show the dominant phenotype, each must also carry a dominant allele, so each is heterozygous. The reasoning runs backwards from the child's genotype to what the parents must have contributed.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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