Mendelian Monohybrid Cross Guide

BiologyBeginnerReading time: 3 min

Overview

Why do brown-eyed parents sometimes have a blue-eyed child? Why do tall pea plants produce short offspring? In the mid-19th century, Gregor Mendel unveiled the mysteries of heredity through his work with pea plants. This simulation models a monohybrid cross, helping you understand how traits segregate and the statistical laws behind genetic randomness.

Background

In 1865, Austrian monk Gregor Mendel published his findings on pea plant hybridization. In an era before the discovery of chromosomes or DNA, he used rigorous mathematical analysis to deduce the fundamental laws of biological inheritance. He discovered that traits are determined by pairs of 'hereditary factors' that segregate during gamete formation. This discovery, known as Mendel's First Law or the Law of Segregation, marked the birth of modern genetics.

Key Concepts

Allele

One of a pair of genes located at the same position on homologous chromosomes that control a specific trait. Commonly denoted as $A$ (dominant) and $a$ (recessive).

Genotype

The genetic makeup of an individual organism. Includes homozygotes ( $AA$ , $aa$ ) and heterozygotes ( $Aa$ ).

Phenotype

The observable physical traits of an organism. In complete dominance, both $AA$ and $Aa$ express the same dominant phenotype.

Law of Segregation

During gamete formation, the two alleles of a gene pair segregate from each other so that each gamete carries only one allele for each gene.

Formulas & Derivation

Heterozygous Self-Cross Ratio ( $Aa \times Aa$ )

\text{Genotype} = 1AA : 2Aa : 1aa, \quad \text{Phenotype} = 3 \text{ Dom} : 1 \text{ Rec}

This is the most classic ratio in Mendelian experiments. The ratio of dominant to recessive traits approaches

3:1

Experiment Steps

1
Set Parents
Select the genotypes for Parent 1 and Parent 2 in the controls. Try setting the classic ' $Aa \times Aa$ ' model.
2
Observe Gamete Segregation
View the central diagram to observe how each parent produces $A$ and $a$ gametes. What is the probability of each type of gamete?
3
Construct Punnett Square
The square shows all possible outcomes of random gamete combination ( $2 \times 2 = 4$ combinations). What is the probability of each outcome?
4
Large-Scale Simulation
Set the offspring count to $N=100$ and click 'Run Simulation'. Observe the ratio of dominant to recessive phenotypes—does it exactly match the theoretical expectation? Try increasing $N$ to $2000$ and observe the trend. (Hint: The larger the sample size, the more ____ the pattern.)

Learning Outcomes

Master the definitions of allele, genotype, and phenotype
Deeply understand the mechanism of the Law of Segregation (gamete segregation and random fertilization)
Proficiently use Punnett squares to predict genotype and phenotype distributions in offspring
Recognize the relationship between genetic randomness and the Law of Large Numbers

Real-world Applications

Genetic Counseling: Predicting the risk of recessive genetic disorders (e.g., Albinism) based on family history.
Plant and Animal Breeding: Concentrating desirable traits (e.g., disease resistance, high yield) into a single variety through selective crossing.
Clinical Diagnosis: Inferring an individual's carrier status using pedigrees to support precision medicine.
DNA Paternity Testing: Forensic science and personal genetic testing services use Mendelian inheritance principles to determine biological relationships by comparing alleles.
Gene Editing (CRISPR): Modern gene editing technologies are built upon Mendelian inheritance principles, enabling precise modification of specific genes.

Common Misconceptions

Misconception

Dominant traits are always the most common in a population.

Correct

Incorrect. Dominance only means the allele masks the expression of the recessive one. The prevalence of a trait depends on natural selection and allele frequency, not dominance itself. For example, polydactyly is dominant but rare.

Misconception

The experiment is a failure if the offspring ratio is not exactly

3:1

Correct

Incorrect. Inheritance is inherently probabilistic. Smaller sample sizes lead to larger random deviations. Theoretical ratios are approached only with large sample sizes (Law of Large Numbers).

Ready to start?

Now that you understand the basics, start the interactive experiment!

Start Experiment Start Quiz