Grade 12 Monohybrid Cross Worksheet

Monohybrid Crosses: Mendelian Genetics

Read each question carefully and answer to the best of your ability. Show all your work for Punnett squares and probability calculations.

Monohybrid Crosses: Understanding Single-Trait Inheritance

A monohybrid cross involves the inheritance of a single trait, where two true-breeding parents differing in a single characteristic are crossed. This forms the basis of Mendelian genetics. We use Punnett squares to predict the genotypes and phenotypes of offspring.

1. An organism that is   for a trait has two identical alleles for that gene.

2. The observable physical properties of an organism are referred to as its  .

3. A   allele will always express its trait if present.

4. The genetic makeup of an organism, or the combination of alleles, is called its  .

5. In pea plants, tallness (T) is dominant over shortness (t). Cross a homozygous tall pea plant with a homozygous short pea plant. Complete the Punnett square and answer the questions below.

a) What are the genotypes of the parents?

b) What is the genotypic ratio of the F1 offspring?

c) What is the phenotypic ratio of the F1 offspring?

6. If you cross two heterozygous parents (Tt x Tt) for the tallness trait (T = tall, t = short), what is the probability of having a short offspring?

7. In guinea pigs, black fur (B) is dominant over white fur (b). If a heterozygous black guinea pig is crossed with a white guinea pig, what percentage of their offspring would you expect to have black fur? Justify your answer with a Punnett square.

8. A test cross is used to determine the genotype of an individual displaying the recessive phenotype.