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Mendelian Genetics Explained

Before anyone knew what a gene was, an Austrian monk named Gregor Mendel figured out the mathematical rules by which traits are inherited. His work with pea plants in the 1860s forms the foundation of modern genetics.

Mendel's Experiments

Between 1856 and 1863, Gregor Mendel cultivated and crossed approximately 29,000 pea plants (Pisum sativum) in the garden of his monastery in Brno. He chose peas because they breed quickly, exist in clearly distinguishable varieties, and can be self-fertilised or cross-fertilised under experimental control. Mendel tracked seven traits, each of which occurred in two distinct forms: seed colour (yellow or green), seed shape (round or wrinkled), pod colour (green or yellow), pod shape (inflated or constricted), flower colour (purple or white), flower position (axial or terminal), and plant height (tall or short).

After allowing plants to self-fertilise for several generations to obtain true-breeding lines (lines that always produced the same trait), Mendel crossed them and recorded the results across thousands of offspring. His meticulous record-keeping and application of statistical ratios to biological data was revolutionary — no one had applied mathematics to heredity before.

Key Vocabulary

• Gene: a unit of hereditary information that influences a specific trait (Mendel called these "factors").
• Allele: an alternative version of a gene. For seed colour, there is a yellow allele and a green allele.
• Dominant allele: the allele whose trait is expressed when at least one copy is present. Conventionally written with a capital letter (e.g., Y for yellow).
• Recessive allele: the allele whose trait is only expressed when two copies are present. Conventionally written with a lower-case letter (e.g., y for green).
• Genotype: the specific allele combination an organism carries (e.g., YY, Yy, or yy).
• Phenotype: the observable physical expression of the genotype (yellow seeds or green seeds).
• Homozygous: having two identical alleles (YY or yy).
• Heterozygous: having two different alleles (Yy).

The Law of Segregation

Mendel's first law states: each organism carries two alleles for each trait; these alleles separate during gamete formation so each gamete carries only one allele; the two alleles in offspring — one from each parent — are combined at random.

When Mendel crossed true-breeding yellow-seeded plants (YY) with true-breeding green-seeded plants (yy), all offspring (F₁ generation) had yellow seeds — genotype Yy. Yellow was dominant. When he allowed the F₁ plants to self-fertilise, the F₂ generation showed a ratio of approximately 3 yellow : 1 green. The green trait had not been destroyed in F₁ — it had been masked, and it reappeared when two recessive alleles were combined.

The Punnett Square

A Punnett square is a grid used to predict the probability of each genotype in offspring. For a cross between two Yy parents:

• Parent 1 gametes: Y and y (written across the top)
• Parent 2 gametes: Y and y (written down the side)
• Filling in the grid: YY, Yy, Yy, yy
• Genotype ratio: 1 YY : 2 Yy : 1 yy
• Phenotype ratio: 3 yellow (YY + 2 Yy) : 1 green (yy)

This is the 3:1 ratio Mendel observed. The Punnett square confirms it: 75% of offspring will show the dominant phenotype (yellow) and 25% will show the recessive phenotype (green).

The Law of Independent Assortment

Mendel's second law states: alleles for different traits assort independently of each other during gamete formation — provided the genes are on different chromosomes. This means knowing which allele for seed colour is inherited tells you nothing about which allele for seed shape will be inherited.

In a dihybrid cross — crossing plants heterozygous for both traits (YyRr × YyRr, where R = round, r = wrinkled) — Mendel predicted and observed a 9:3:3:1 phenotype ratio among offspring: 9 yellow-round : 3 yellow-wrinkled : 3 green-round : 1 green-wrinkled. This ratio falls directly from the two 3:1 ratios multiplied together, confirming independent assortment.

We now know this law has an important exception: genes located on the same chromosome tend to be inherited together (genetic linkage) rather than independently, especially if they are close together. Independent assortment applies fully only to genes on different chromosomes or far apart on the same chromosome.

Beyond Simple Dominance

Mendel's pea traits were convenient because each showed complete dominance — one allele fully masked the other. Many traits do not follow this simple pattern:

• Incomplete dominance: the heterozygous phenotype is intermediate. Crossing a red snapdragon (R^RR^R) with a white snapdragon (R^WR^W) produces pink heterozygotes (R^RR^W).
• Codominance: both alleles are fully expressed simultaneously. The AB blood type (genotype I^AI^B) displays both A and B antigens on red blood cells.
• Multiple alleles: a gene may have more than two alleles in the population. The ABO blood group system has three alleles (I^A, I^B, i), giving four possible blood types.
• Polygenic inheritance: traits like height, skin colour, and intelligence are influenced by many genes simultaneously, producing continuous variation rather than discrete categories.

Test Cross

A test cross is used to determine whether an organism showing a dominant phenotype is homozygous dominant (YY) or heterozygous (Yy) — which look identical. Cross the unknown plant with a homozygous recessive (yy). If all offspring show the dominant phenotype, the unknown was YY. If approximately half show the dominant and half the recessive phenotype, the unknown was Yy.

Summary

Mendel's two laws — segregation and independent assortment — describe how alleles are inherited. Dominant alleles mask recessive alleles in heterozygotes. Punnett squares predict offspring genotype and phenotype probabilities. The 3:1 ratio in monohybrid F₂ crosses and the 9:3:3:1 ratio in dihybrid crosses are hallmarks of Mendelian inheritance. Extensions to Mendel's laws — incomplete dominance, codominance, multiple alleles, polygenic traits, and linkage — show that his core framework applies broadly, even where the patterns are more complex than pea-plant traits.