How do you calculate a Punnett square?

To calculate a Punnett square, place one parent's alleles along the top of the grid and the other parent's alleles down the left side. Combine the corresponding top and left alleles in each cell. By counting the genotypes in the grid, you can calculate the probabilities for the offspring's genetic makeup.

What is the phenotypic ratio of a dihybrid cross?

For two heterozygous parents (e.g., AaBb x AaBb), the classic dihybrid phenotypic ratio is 9:3:3:1. This represents 9 parts dominant for both traits, 3 parts dominant-recessive, 3 parts recessive-dominant, and 1 part recessive-recessive.

What is the difference between genotype and phenotype?

The genotype is the genetic makeup of an organism, represented by allele letters (e.g., BB, Bb, bb). The phenotype is the observable physical trait expressed by the organism (e.g., brown eyes or blue eyes).

Punnett Square Calculator - Monohybrid & Dihybrid Genetic Crosses

The Science of Mendelian Genetics: Punnett Square Analysis

Explore the principles of genetic inheritance. This guide explains alleles, monohybrid and dihybrid crosses, genotype-phenotype relationships, and Mendelian laws.

Monohybrid Cross

Tracks a single gene locus (2x2 grid), demonstrating the classical 3:1 phenotypic ratio in heterozygous parent crosses.

Dihybrid Cross

Tracks two independent genes (4x4 grid), illustrating the law of independent assortment and the 9:3:3:1 phenotypic ratio.

Dominance Laws

Under standard Mendelian dominance, a dominant allele masks the expression of a recessive allele in heterozygous individuals.

Introduction to Mendelian Genetics

Modern genetics began with the work of Gregor Mendel, an Austrian monk who conducted hybridization experiments with pea plants (Pisum sativum) in the 1850s and 1860s. Mendel tracked several distinct traits, such as flower color (purple vs. white) and seed shape (round vs. wrinkled), and discovered the basic laws of heredity.

Mendel established that traits are inherited as discrete units (now called genes) rather than through a blending of parental characteristics. Each organism carries two copies of a gene for a given trait—one inherited from each parent. These different versions of a gene are known as **alleles**.

Key Genetic Terms:

Locus: The specific location of a gene on a chromosome.
Genotype: The genetic makeup of an organism (e.g., BB, Bb, bb).
Phenotype: The observable physical trait expressed by the organism (e.g., brown eyes vs. blue eyes).
Homozygous: Having two identical alleles for a gene (e.g., BB or bb).
Heterozygous: Having two different alleles for a gene (e.g., Bb).

Monohybrid vs. Dihybrid Crosses

Punnett squares are visual grids used to predict the genotypic and phenotypic ratios of offspring from a genetic cross. The complexity of the grid depends on the number of genes being tracked:

Monohybrid Cross (1 Gene)

A monohybrid cross tracks a single gene locus with two alleles. The classic example is crossing two heterozygous parents (Bb × Bb). This cross yields a 2x2 grid with four possible outcomes:

• 1 BB (Homozygous Dominant) - 25%

• 2 Bb (Heterozygous) - 50%

• 1 bb (Homozygous Recessive) - 25%

This results in a genotypic ratio of 1:2:1 and a phenotypic ratio of 3:1 (75% dominant trait, 25% recessive trait).

Dihybrid Cross (2 Genes)

A dihybrid cross tracks two separate genes located on different chromosomes. For example, crossing two double heterozygotes (BbTt × BbTt) involves generating four possible gametes from each parent: BT, Bt, bT, and bt.

The resulting 4x4 grid produces 16 combinations. The classic phenotypic ratio for a dihybrid cross of two heterozygotes is 9:3:3:1:

• 9/16: Dominant Gene 1 & Dominant Gene 2

• 3/16: Dominant Gene 1 & Recessive Gene 2

• 3/16: Recessive Gene 1 & Dominant Gene 2

• 1/16: Recessive Gene 1 & Recessive Gene 2

Step-by-Step: How to Construct a Punnett Square

To manually construct and solve a genetic cross, follow this step-by-step process:

1
Identify Parent Genotypes: Write down the genetic makeup of both parents. For example, a heterozygous mother (Bb) and a homozygous recessive father (bb).
2
Determine Parental Gametes: Separate the alleles of each parent to find the possible gametes they can pass to offspring. The mother produces B and b gametes; the father produces b and b gametes.
3
Set Up the Grid: Draw a table. For one gene, draw a 2x2 grid. Write one parent's gametes along the top row and the other parent's gametes down the left column.
4
Combine Alleles: Fill in each cell of the grid by combining the allele from the corresponding column header with the allele from the row header. By convention, write capital (dominant) letters first.
5
Calculate Ratios: Count the occurrences of each genotype and phenotype to determine their relative frequencies and percentages.

Complex Inheritance: Non-Mendelian Patterns

While Mendel's laws apply to many traits, inheritance patterns are often more complex. Key non-Mendelian genetic patterns include:

Incomplete Dominance

In incomplete dominance, the heterozygous genotype expresses a phenotype that is an intermediate blend of the two homozygous phenotypes. A classic example is the snapdragon flower: crossing a red flower (RR) with a white flower (rr) produces pink flowers (Rr).

Codominance

In codominance, both alleles are fully expressed in heterozygous individuals, rather than blending. In human ABO blood groups, an individual with one A allele and one B allele has blood type AB, expressing both antigen markers on their red blood cells.

Sex-Linked Inheritance

Sex-linked traits are determined by genes located on the sex chromosomes (usually the X chromosome). Recessive sex-linked conditions, such as red-green colorblindness and hemophilia, are expressed more frequently in males because they carry only one X chromosome (XY), leaving them without a second allele to potentially mask a recessive trait.

Polygenic Inheritance

Polygenic inheritance occurs when a single physical trait is controlled by the combined effect of multiple independent genes. Human traits such as skin color, eye color, and height exhibit continuous variation because they are determined by the cumulative contribution of several genetic loci.

Punnett Square Calculator

What is a Punnett Square?

Genetic Punnett Square Calculator