Genetic Crosses Practice- Test Your Knowledge

What Are Genetic Crosses?

Genetic crosses are breeding experiments that predict offspring traits based on parent genotypes. You take the genes from two parents, combine them following Mendelian rules, and see what comes out.

This isn't rocket science, but students consistently mess it up because they rush through the setup. Slow down. Follow the steps. You'll get it.

Key Terms You Need to Know First

Don't skip this section. If you're confused about genotype vs. phenotype, you're going to fail every problem.

• Allele — A version of a gene. You get one from each parent.
• Genotype — The genetic makeup. The letters you write down (like TT or Tt).
• Phenotype — What you actually see. The physical trait.
• Homozygous — Both alleles are the same (AA or aa). Also called "purebred."
• Heterozygous — Alleles are different (Aa). Also called "hybrid."
• Dominant allele — Shows up if it's present. Written with a capital letter.
• Recessive allele — Only shows up if there's no dominant allele. Written with lowercase.

Monohybrid vs. Dihybrid Crosses

Monohybrid Cross

One trait. That's it. You're tracking one gene across two parents.

Example: Flower color in peas. Purple (P) is dominant over white (p).

Dihybrid Cross

Two traits. You're tracking two genes simultaneously.

Example: Seed color AND seed shape in peas. This gets messy fast because you have 16 possible genotype combinations instead of 4.

Most teachers hit you with monohybrid first. Dihybrid comes later and punishes people who didn't master the basics.

How to Solve Any Genetic Cross Problem

Follow these steps in order. Every time. No exceptions.

Step 1: Identify the Alleles

Figure out which trait is dominant and which is recessive. Assign letters.

Dominant = capital letter. Recessive = lowercase.

Example: Tall (T) is dominant over short (t) in pea plants.

Step 2: Write Parent Genotypes

Convert what you know about the parents into letters.

Homozygous dominant = TT

Heterozygous = Tt

Homozygous recessive = tt

Step 3: Set Up the Punnett Square

For monohybrid crosses, you need a 4-box square. One parent's alleles go on top. The other parent's alleles go down the side.

For dihybrid crosses, you need a 16-box square. Each parent contributes two different alleles.

Step 4: Fill in the Boxes

Match the column allele with the row allele. Write the result in each box.

Step 5: Calculate the Ratios

Count your phenotypes or genotypes. Express as a ratio.

Example: 3 purple : 1 white (phenotypic ratio)

Practice Problem #1: Monohybrid Cross

Problem: In humans, attached earlobes (E) are dominant over free earlobes (e). Cross a heterozygous parent with a homozygous recessive parent.

Step 1: Parent 1 is heterozygous = Ee. Parent 2 is homozygous recessive = ee.

Step 2: Set up your Punnett square.

The top row gets alleles from Parent 1: E and e.

The side column gets alleles from Parent 2: e and e.

Step 3: Fill in the boxes.

Your answer: 50% attached earlobes, 50% free earlobes.

Practice Problem #2: Dihybrid Cross

Problem: In peas, round seeds (R) are dominant over wrinkled (r), and yellow seeds (Y) are dominant over green (y). Cross two heterozygous plants (RrYy × RrYy).

This is where students panic. Don't. Just work methodically.

Step 1: Determine gametes using the FOIL method:

• RY
• Ry
• rY
• ry

Step 2: Create a 16-box Punnett square. Put one set of gametes across the top, the other down the side.

Step 3: Fill and count.

Classic dihybrid ratio: 9:3:3:1

This ratio only shows up when you cross two heterozygous parents for both traits. Memorize it.

Common Mistakes That Kill Your Score

• Confusing genotype with phenotype. If you write "Tt" when the question asks for phenotypes, you lose points. Read carefully.
• Screwing up the Punnett square setup. Put one parent's alleles on top, the other on the side. Don't mix them up.
• Forgetting that dominant doesn't mean "more common." Dominant alleles can be rare in a population.
• Misidentifying heterozygous vs. homozygous. Heterozygous always has one capital and one lowercase. Homozygous has either both capitals or both lowercase.
• Rushing the gamete combinations. In dihybrid crosses, take your time listing all four gamete types before filling the square.

Test-Taking Tips

When you're staring at a genetic cross problem during a test:

1. Write the key information first. Dominant allele, recessive allele, what the question is asking for.

2. State the parental genotypes clearly. Before you touch the Punnett square.

3. Show your work. Teachers give partial credit. A wrong answer with correct setup gets more points than a right answer with no work.

4. Check your ratios. They should add up to 4 (monohybrid) or 16 (dihybrid).

Quick Reference Table

Final Word

Genetic crosses follow rules. The rules don't change. Once you understand how to set up a Punnett square and count the results, you can solve any problem they throw at you.

Practice the basics until they're automatic. Most errors come from rushing, not from not understanding the material.