Patterns of Inheritance- Practice Worksheet and Answers
What You Need to Know About Patterns of Inheritance
Genetics isn't optional in biology—it shows up everywhere. From Punnett squares to pedigrees, understanding how traits pass from parents to offspring is foundational. If you're struggling with inheritance patterns, a practice worksheet isn't just helpful. It's necessary.
This guide gives you real practice questions, detailed answers, and explanations that actually make sense. No vague definitions. No recycled textbook fluff.
Key Patterns of Inheritance You Must Know
Most students only learn Mendel's three laws and stop there. That's a mistake. Real genetics is messier.
Mendelian Patterns
Autosomal Dominant: One copy of the allele causes the trait. If a parent has it, roughly half their offspring will too.
Autosomal Recessive: Need two copies to show the trait. Parents can be carriers without knowing it.
X-linked Dominant/Recessive: Genes on the X chromosome. Males are more frequently affected because they only have one X.
Non-Mendelian Patterns
Incomplete Dominance: Neither allele is fully dominant. Red flower Ă— White flower = Pink flower.
Codominance: Both alleles show. Think ABO blood types—A and B both express.
Multiple Alleles: More than two allele options exist. ABO blood groups have three alleles.
Polygenic Traits: Multiple genes control one trait. Height, skin color, eye color. These don't follow simple Punnett square logic.
Comparison of Inheritance Patterns
| Pattern | Dominance | Phenotype Ratio (F2) | Example |
|---|---|---|---|
| Autosomal Dominant | Complete | 3:1 (affected:unaffected) | Huntington's disease |
| Autosomal Recessive | Complete | 1:2:1 (homo-dom:hetero:homo-rec) | Cystic fibrosis |
| Incomplete Dominance | None | 1:2:1 (all distinct) | Snapdragon color |
| Codominance | Both express | 1:2:1 (all distinct) | ABO blood type |
| X-linked Recessive | Linked to X | Males affected more | Hemophilia |
Practice Worksheet: Questions
Try these questions before checking the answers. Work through each one—you'll learn more from your mistakes than from reading perfect explanations.
Question 1: Autosomal Recessive Inheritance
In humans, attached earlobes (a) are recessive to free earlobes (A). A man with attached earlobes marries a woman who is heterozygous for this trait.
a) What is the man's genotype?
b) What is the woman's genotype?
c) Draw a Punnett square.
d) What percentage of their children will have attached earlobes?
Question 2: Incomplete Dominance
In snapdragons, red flowers (R) crossed with white flowers (R') produce pink-flowered offspring. Cross two pink-flowered plants.
a) What are the genotypes of the parent plants?
b) What phenotypic ratio is expected in the offspring?
c) What genotypic ratio is expected?
Question 3: X-linked Recessive
Color blindness is X-linked recessive (b). A color-blind father has children with a carrier mother.
a) What is the father's genotype?
b) What is the mother's genotype?
c) What percentage of sons will be color-blind?
d) What percentage of daughters will be carriers?
Question 4: Codominance and Multiple Alleles
ABO blood types demonstrate both codominance and multiple alleles. Alleles: IA, IB, i.
Cross a Type A heterozygous mother (IAi) with a Type B heterozygous father (IBi). List all possible offspring genotypes and phenotypes.
Question 5: Dihybrid Cross
In peas, yellow seeds (Y) are dominant to green (y), and round shape (R) is dominant to wrinkled (r). Cross YyRr Ă— YyRr.
a) How many boxes in your Punnett square?
b) What fraction will be homozygous dominant for both traits?
c) What fraction will show the recessive phenotype for both traits?
Answers and Explanations
Answer 1: Autosomal Recessive
a) The man has attached earlobes, which is recessive. His genotype is aa.
b) The woman is heterozygous and has free earlobes. Her genotype is Aa.
c) Punnett square:
| a | a | |
| A | Aa | Aa |
| a | aa | aa |
d) 50% of children will have attached earlobes (aa).
Answer 2: Incomplete Dominance
a) Pink flowers result from R Ă— R'. Both parents must be Rr'.
b) Phenotypic ratio: 1 Red : 2 Pink : 1 White
c) Genotypic ratio: 1 RR : 2 Rr' : 1 R'R'
Note: In incomplete dominance, the heterozygous phenotype is distinct. This isn't blending at the genetic level—it's just that neither allele dominates completely.
Answer 3: X-linked Recessive
a) Father is color-blind: XbY
b) Mother is a carrier: XBXb
c) 50% of sons will be color-blind (XbY)
d) 50% of daughters will be carriers (XBXb)
This is why X-linked recessive conditions hit males harder. Males only need one copy of the recessive allele to show the trait. Females need two.
Answer 4: Codominance and Multiple Alleles
Cross: IAi Ă— IBi
| IA | i | |
| IB | IAIB (Type AB) | IBi (Type B) |
| i | IAi (Type A) | ii (Type O) |
Possible offspring:
- IAIB — Type AB (both antigens present)
- IAi — Type A
- IBi — Type B
- ii — Type O
Each has a 1/4 probability. IA and IB are codominant—they both express fully when together.
Answer 5: Dihybrid Cross
a) 16 boxes (4Ă—4). Each parent produces four gamete types: YR, Yr, yR, yr.
b) Homozygous dominant for both: YYRR = 1/16
c) Recessive phenotype for both (yyrr): 1/16
The classic 9:3:3:1 ratio applies here:
- 9 Yellow, Round (at least one Y and one R)
- 3 Yellow, Wrinkled (YYrr or Yyrr)
- 3 Green, Round (yyRR or yyRr)
- 1 Green, Wrinkled (yyrr)
How to Use This Worksheet Effectively
Don't just read the answers and move on. Here's what actually works:
- Try first. Attempt every question before looking at solutions. Struggle is part of learning.
- Check your work. If you got it wrong, figure out why before reading the explanation.
- Redraw the Punnett squares. Hand-drawing reinforces the process better than reading it.
- Change the variables. Once you solve one cross, try swapping the genotypes. Can you still get the right answer?
- Time yourself. If you can't complete these in under 10 minutes, you need more practice.
Common Mistakes Students Make
Forgetting that X-linked traits affect males and females differently. Males are XY, so one recessive allele shows the phenotype. Females need two. This trips up even good students.
Confusing genotype ratios with phenotype ratios. In a monohybrid cross of heterozygotes (Aa Ă— Aa), the genotypic ratio is 1:2:1. The phenotypic ratio is 3:1. These are not the same thing.
Misidentifying the inheritance pattern. Incomplete dominance gives three phenotypes. Codominance gives three phenotypes too—but the heterozygous phenotype looks different. Know the difference.
Skipping the hard problems. Dihybrid crosses look intimidating but they're just two monohybrid crosses combined. Break them down.
When You're Ready for More
If you've mastered these questions, move on to:
- Pedigree analysis problems
- Linked genes and recombination frequencies
- Epistasis (one gene masks another)
- Population genetics (Hardy-Weinberg equilibrium)
Genetics gets harder. But the foundation here—knowing your alleles, building Punnett squares correctly, and distinguishing genotypes from phenotypes—will carry you through.