Punnett Square Examples With Answers

Punnett Square Examples with Answers: Mastering Mendelian Genetics

Understanding Mendelian genetics can feel daunting at first, but with practice, it becomes straightforward. The Punnett square is a fundamental tool used to predict the genotypes and phenotypes of offspring from a cross between two parents. This comprehensive guide provides numerous Punnett square examples with detailed answers, covering various inheritance patterns and complexities, helping you master this crucial concept in biology. We'll explore monohybrid crosses, dihybrid crosses, and even delve into scenarios involving sex-linked traits. By the end, you'll be confidently predicting offspring characteristics!

Understanding the Basics: Genes, Alleles, and Genotypes

Before diving into Punnett squares, let's refresh some key genetic terminology.

• Gene: A segment of DNA that codes for a specific trait, such as eye color or flower color.
• Allele: Different versions of a gene. For example, a gene for flower color might have an allele for purple flowers and an allele for white flowers. Alleles are represented by letters; dominant alleles are usually uppercase (e.g., A), and recessive alleles are lowercase (e.g., a).
• Genotype: The genetic makeup of an organism, representing the combination of alleles it possesses. For example, AA, Aa, and aa are all possible genotypes for a gene with two alleles.
• Phenotype: The observable characteristics of an organism, determined by its genotype. For example, the phenotype might be "purple flowers" or "white flowers".
• Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa). These individuals are also called "true-breeding".
• Heterozygous: Having two different alleles for a particular gene (e.g., Aa). These individuals are also called "hybrids".
• Dominant Allele: An allele that masks the expression of another allele when present. In the case of Aa, the dominant allele 'A' determines the phenotype.
• Recessive Allele: An allele whose expression is masked by a dominant allele. The recessive allele 'a' only manifests its phenotype when present in a homozygous state (aa).

Monohybrid Crosses: One Trait at a Time

Monohybrid crosses involve tracking the inheritance of a single trait. Let's look at a few examples:

Example 1: Flower Color

Let's say we have a homozygous dominant purple-flowered plant (PP) and a homozygous recessive white-flowered plant (pp). What are the genotypes and phenotypes of their offspring (F1 generation)?

• Genotype of F1 generation: 100% Pp (heterozygous)
• Phenotype of F1 generation: 100% Purple flowers (because 'P' is dominant)

Now, let's cross two F1 generation plants (Pp x Pp):

• Genotype of F2 generation: 25% PP, 50% Pp, 25% pp
• Phenotype of F2 generation: 75% Purple flowers, 25% White flowers

Example 2: Seed Shape

Suppose we cross a homozygous dominant round-seeded plant (RR) with a homozygous recessive wrinkled-seeded plant (rr).

• Genotype of F1 generation: 100% Rr
• Phenotype of F1 generation: 100% Round seeds

Crossing two F1 generation plants (Rr x Rr):

• Genotype of F2 generation: 25% RR, 50% Rr, 25% rr
• Phenotype of F2 generation: 75% Round seeds, 25% Wrinkled seeds

These examples demonstrate the classic 3:1 phenotypic ratio often observed in monohybrid crosses involving a single dominant and recessive allele.

Dihybrid Crosses: Tracking Two Traits Simultaneously

Dihybrid crosses track the inheritance of two traits simultaneously. This requires a larger Punnett square (4x4).

Example 3: Seed Shape and Seed Color

Let's consider a plant with round yellow seeds (RRYY) crossed with a plant with wrinkled green seeds (rryy). Assume round (R) is dominant to wrinkled (r), and yellow (Y) is dominant to green (y).

The F1 generation will all be RrYy (round yellow seeds). Crossing two F1 plants (RrYy x RrYy) gives us a more complex Punnett square:

• Genotype of F2 generation: 9 R_Y_ (round yellow), 3 R_yy (round green), 3 rrY_ (wrinkled yellow), 1 rryy (wrinkled green)
• Phenotype of F2 generation: 9 Round Yellow, 3 Round Green, 3 Wrinkled Yellow, 1 Wrinkled Green

This illustrates the classic 9:3:3:1 phenotypic ratio often seen in dihybrid crosses, assuming independent assortment of the genes.

Incomplete Dominance: Blending of Traits

In incomplete dominance, neither allele is completely dominant. The heterozygote shows a blend of the two parental phenotypes.

Example 4: Flower Color (Incomplete Dominance)

Let's say red flowers (RR) and white flowers (WW) exhibit incomplete dominance. The heterozygote (RW) is pink.

• Genotype of F1 generation: 100% RW
• Phenotype of F1 generation: 100% Pink flowers

Crossing two F1 plants (RW x RW):

• Genotype of F2 generation: 25% RR, 50% RW, 25% WW
• Phenotype of F2 generation: 25% Red, 50% Pink, 25% White

Codominance: Both Traits Expressed Simultaneously

In codominance, both alleles are fully expressed in the heterozygote.

Example 5: Coat Color in Cattle

Let's consider cattle with red coats (RR) and white coats (WW). If these alleles are codominant, the heterozygote (RW) will have a roan coat (a mixture of red and white hairs).

• Genotype of F1 generation: 100% RW
• Phenotype of F1 generation: 100% Roan

Crossing two F1 animals (RW x RW):

• Genotype of F2 generation: 25% RR, 50% RW, 25% WW
• Phenotype of F2 generation: 25% Red, 50% Roan, 25% White

Sex-Linked Traits: Traits on Sex Chromosomes

Sex-linked traits are located on the sex chromosomes (X and Y). Since males have only one X chromosome, they express recessive sex-linked traits more frequently than females.

Example 6: Color Blindness

Color blindness is a recessive sex-linked trait carried on the X chromosome. Let's represent the normal allele as X^C and the color-blind allele as X^c.

A female carrier (X^CX^c) mates with a normal male (X^CY).

• Genotype of F1 generation: 25% X^CX^C, 25% X^CY, 25% X^CX^c, 25% X^cY
• Phenotype of F1 generation: 75% Normal Vision (25% Female, 50% Male), 25% Color Blind (Male)

Multiple Alleles: More Than Two Allele Versions

Some genes have more than two alleles. A classic example is the ABO blood group system.

Example 7: ABO Blood Groups

The ABO blood group system has three alleles: I^A, I^B, and i. I^A and I^B are codominant, and both are dominant to i.

Let's consider a cross between a person with blood type A (I^Ai) and a person with blood type B (I^Bi).

• Genotype of F1 generation: 25% I^AI^B, 25% I^Bi, 25% I^Ai, 25% ii
• Phenotype of F1 generation: 25% AB, 25% B, 25% A, 25% O

Conclusion: Practice Makes Perfect

The Punnett square is a powerful tool for predicting the inheritance of traits. While mastering dihybrid crosses and scenarios involving incomplete dominance, codominance, and sex-linked traits may take practice, the fundamental principles remain consistent. The more examples you work through, the more confident you'll become in understanding Mendelian genetics and predicting offspring characteristics. Remember to clearly define your alleles, genotypes, and phenotypes before starting your Punnett square, and carefully analyze the results. With consistent practice, you will confidently navigate the world of genetics.