Genetics Practice Worksheet: Punnett Squares & Inheritance

This comprehensive genetics worksheet provides practice problems covering fundamental genetic concepts, Punnett squares, inheritance patterns, and genetic disorders. Each problem includes detailed solutions and explanations to help you master genetics.

Basic Punnett Square Problems

Problem 1: Monohybrid Cross

Question: In pea plants, tall (T) is dominant to short (t). If a heterozygous tall plant is crossed with a short plant, what are the expected genotypic and phenotypic ratios?

Solution:

• Parent 1: Tt (heterozygous tall)
• Parent 2: tt (short)
• Punnett Square:

     T    t
  t  Tt   tt
  t  Tt   tt
  

• Genotypic ratio: 1 Tt : 1 tt
• Phenotypic ratio: 1 tall : 1 short

Problem 2: Dihybrid Cross

Question: In pea plants, round seeds (R) are dominant to wrinkled (r), and yellow seeds (Y) are dominant to green (y). What are the expected ratios if two heterozygous plants (RrYy) are crossed?

Solution:

• Parent 1: RrYy
• Parent 2: RrYy
• Gametes: RY, Ry, rY, ry
• Punnett Square (4x4):

      RY    Ry    rY    ry
  RY  RRYY  RRYy  RrYY  RrYy
  Ry  RRYy  RRyy  RrYy  Rryy
  rY  RrYY  RrYy  rrYY  rrYy
  ry  RrYy  Rryy  rrYy  rryy
  

• Genotypic ratio: 1 RRYY : 2 RRYy : 1 RRyy : 2 RrYY : 4 RrYy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy
• Phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

Sex-Linked Inheritance

Problem 3: X-Linked Recessive

Question: Color blindness is an X-linked recessive trait. If a carrier female (X^H X^h) marries a normal male (X^H Y), what is the probability of having a color-blind child?

Solution:

• Mother: X^H X^h (carrier)
• Father: X^H Y (normal)
• Punnett Square:

     X^H    Y
  X^H  X^HX^H  X^HY
  X^h  X^HX^h  X^hY
  

• Probability of color-blind child (X^hY): 25%
• Probability of carrier daughter (X^HX^h): 25%

Blood Type Genetics

Problem 4: ABO Blood Types

Question: A person with blood type A (genotype AO) marries a person with blood type B (genotype BO). What are the possible blood types of their children?

Solution:

• Parent 1: AO (blood type A)
• Parent 2: BO (blood type B)
• Punnett Square:

     A    O
  B  AB   BO
  O  AO   OO
  

• Possible blood types: A (AO), B (BO), AB (AB), O (OO)
• Each blood type has 25% probability

Pedigree Analysis

Problem 5: Autosomal Recessive

Question: Analyze the following pedigree for an autosomal recessive trait. What is the genotype of individual III-2?

Solution:

• If the trait is autosomal recessive, affected individuals are homozygous recessive (aa)
• Unaffected individuals can be either AA or Aa
• Individual III-2 is unaffected but has an affected child
• Therefore, III-2 must be heterozygous (Aa)

Genetic Disorders

Problem 6: Cystic Fibrosis

Question: Cystic fibrosis is an autosomal recessive disorder. If both parents are carriers (Cc), what is the probability that their first child will have cystic fibrosis?

Solution:

• Parent 1: Cc (carrier)
• Parent 2: Cc (carrier)
• Punnett Square:

     C    c
  C  CC   Cc
  c  Cc   cc
  

• Probability of affected child (cc): 25%
• Probability of carrier child (Cc): 50%
• Probability of normal child (CC): 25%

Advanced Problems

Problem 7: Incomplete Dominance

Question: In snapdragons, red flowers (RR) and white flowers (rr) show incomplete dominance. Heterozygous plants (Rr) have pink flowers. If two pink plants are crossed, what are the expected ratios?

Solution:

• Parent 1: Rr (pink)
• Parent 2: Rr (pink)
• Punnett Square:

     R    r
  R  RR   Rr
  r  Rr   rr
  

• Genotypic ratio: 1 RR : 2 Rr : 1 rr
• Phenotypic ratio: 1 red : 2 pink : 1 white

Problem 8: Multiple Alleles

Question: In rabbits, coat color is controlled by multiple alleles: C (full color), c^ch (chinchilla), c^h (Himalayan), and c (albino). The dominance hierarchy is C > c^ch > c^h > c. If a full-color rabbit (Cc^ch) is crossed with a Himalayan rabbit (c^h c), what are the expected phenotypes?

Solution:

• Parent 1: Cc^ch (full color)
• Parent 2: c^h c (Himalayan)
• Punnett Square:

     C     c^ch
  c^h  Cc^h   c^ch c^h
  c    Cc     c^ch c
  

• Phenotypes: 50% full color (Cc^h, Cc), 25% chinchilla (c^ch c^h), 25% Himalayan (c^ch c)

Practice Problems

Problem 9: Independent Assortment

Question: In fruit flies, red eyes (R) are dominant to white eyes (r), and normal wings (W) are dominant to vestigial wings (w). If a fly with genotype RrWw is testcrossed, what are the expected phenotypic ratios?

Solution:

• Test cross: RrWw × rrww
• Gametes from RrWw: RW, Rw, rW, rw
• Gametes from rrww: rw only
• Offspring: RrWw, Rrww, rrWw, rrww
• Phenotypic ratio: 1 red normal : 1 red vestigial : 1 white normal : 1 white vestigial

Problem 10: Gene Linkage

Question: Two genes are 20 map units apart. If a heterozygous individual (AaBb) is testcrossed, what percentage of offspring will show parental phenotypes?

Solution:

• Recombination frequency = 20%
• Parental phenotypes = 80%
• Recombinant phenotypes = 20%
• Therefore, 80% of offspring will show parental phenotypes

Key Concepts Review

Mendelian Inheritance

• Law of Segregation: Alleles separate during gamete formation
• Law of Independent Assortment: Genes on different chromosomes assort independently
• Dominance: One allele masks the expression of another

Non-Mendelian Inheritance

• Incomplete Dominance: Heterozygous phenotype is intermediate
• Codominance: Both alleles are expressed equally
• Multiple Alleles: More than two alleles exist for a gene
• Polygenic Inheritance: Multiple genes control one trait

Sex-Linked Inheritance

• X-linked: Genes on X chromosome
• Y-linked: Genes on Y chromosome
• X-inactivation: One X chromosome is inactivated in females

Study Tips

1. Draw Punnett squares for every problem
2. Check your work by ensuring probabilities sum to 100%
3. Understand dominance relationships before solving
4. Practice with different inheritance patterns
5. Use pedigrees to visualize family relationships

Common Mistakes to Avoid

• Forgetting to consider sex-linked inheritance
• Not accounting for incomplete dominance
• Confusing genotype and phenotype ratios
• Ignoring recombination in linked genes
• Forgetting to check for multiple alleles

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Keywords: genetics, Punnett squares, inheritance patterns, genetic disorders, Mendelian genetics, sex-linked inheritance, blood types, pedigree analysis.

Last Updated: July 12, 2025, 05:24 PM +04