Gregor Mendel discovered basic rules of inheritance by crossing pea plants with different visible traits, such as purple versus white flowers and round versus wrinkled seeds. His experiments showed that traits are passed from parents to offspring in predictable patterns. These patterns matter because they explain why children resemble their parents but are not identical to them.
Mendel's work became the foundation of modern genetics, from family pedigrees to crop breeding and genetic medicine.
Mendel proposed that organisms carry two copies of each gene, one inherited from each parent. During gamete formation, these copies separate so each gamete receives only one allele, which is the law of segregation. When genes are on different chromosomes or far apart on the same chromosome, alleles for different traits assort independently, which is the law of independent assortment.
Punnett squares use these rules to predict the probability of genotypes and phenotypes in offspring.
Understanding Biology: Mendel's Laws of Inheritance
The physical basis of Mendel's patterns is meiosis, the cell division that makes eggs and sperm. Before meiosis, a cell copies its DNA. Each chromosome then has a matching partner, one originally from each parent.
The matching chromosomes line up and move into separate cells. This movement separates the two versions of many genes. A later division separates the copied chromosome parts.
The result is gametes with one chromosome from each pair. Fertilisation joins two gametes at random, restoring pairs of chromosomes in the new organism. This random joining is why a prediction gives chances, not a guarantee for every family.
Dominant does not mean stronger, better, or more common in a population. It only describes what happens when two different alleles are present in a particular genotype. A dominant allele can make enough working protein for a visible trait, while a recessive allele may make less protein or a changed version.
Some genes do not follow complete dominance. In incomplete dominance, the heterozygote has an intermediate appearance, such as red and white flowers producing pink flowers. In codominance, both allele effects are visible, as in the AB blood group.
These cases do not disprove segregation. They change how genotype appears as phenotype.
Independent assortment depends on chromosome behaviour. Different chromosome pairs line up in random orientations during meiosis. This creates many possible combinations in gametes.
Humans have twenty three chromosome pairs, so this process alone can produce over eight million chromosome combinations in one person’s gametes. The rule has an important limit. Genes close together on the same chromosome are linked and tend to travel together.
Crossing over can swap matching DNA sections between chromosome partners during meiosis. A crossover between linked genes can separate them, but it happens less often when the genes are very close. Scientists use these frequencies to estimate where genes lie on chromosomes.
Punnett squares are useful models, but they require careful reading. First identify the alleles and decide how each genotype produces a trait. Then list the possible gametes, giving each one the correct probability.
Multiply probabilities when independent events must occur together. Add probabilities when different outcomes lead to the same result. A square predicts the expected pattern across many offspring.
A small group can differ greatly just by chance, much like several coin tosses need not give equal heads and tails. In real families, many traits such as height, skin colour, and disease risk involve many genes plus environment. Mendelian rules still help students understand single gene cases, carrier status, pedigree patterns, and why genetic predictions are usually probabilities rather than certainties.
Key Facts
- Law of segregation: the two alleles for a gene separate during gamete formation, so each gamete gets one allele.
- Law of independent assortment: alleles of different genes separate independently when the genes are unlinked.
- A monohybrid cross PP x pp produces all Pp offspring in the F1 generation.
- A monohybrid cross Pp x Pp gives a genotype ratio of 1 PP : 2 Pp : 1 pp.
- For complete dominance, Pp x Pp gives a phenotype ratio of 3 dominant : 1 recessive.
- A dihybrid cross AaBb x AaBb gives a phenotype ratio of 9 : 3 : 3 : 1 if the genes assort independently.
Vocabulary
- Allele
- An allele is a version of a gene, such as P for purple flowers or p for white flowers.
- Genotype
- A genotype is the allele combination an organism has for a trait, such as PP, Pp, or pp.
- Phenotype
- A phenotype is the observable trait produced by a genotype, such as purple flowers or white flowers.
- Homozygous
- Homozygous means having two identical alleles for a gene, such as PP or pp.
- Heterozygous
- Heterozygous means having two different alleles for a gene, such as Pp.
Common Mistakes to Avoid
- Treating a dominant allele as more common, which is wrong because dominance describes expression in a heterozygote, not how often an allele appears in a population.
- Writing gametes with two alleles for the same gene, which is wrong because segregation means each gamete receives only one allele from each gene pair.
- Confusing genotype ratio with phenotype ratio, which is wrong because different genotypes can produce the same visible trait when complete dominance occurs.
- Applying independent assortment to linked genes without checking linkage, which is wrong because genes close together on the same chromosome are often inherited together.
Practice Questions
- 1 In pea plants, purple flowers P are dominant to white flowers p. If a PP plant is crossed with a pp plant, what are the genotypes and phenotypes of all F1 offspring?
- 2 Two heterozygous purple-flowered pea plants are crossed: Pp x Pp. Out of 160 offspring, how many would you expect to have white flowers?
- 3 A pea plant heterozygous for seed shape and seed color has genotype RrYy. If the genes assort independently, what are the four possible gamete types and what fraction of gametes should be each type?
- 4 Explain why Mendel's law of segregation predicts a 3:1 phenotype ratio in the F2 generation of a simple dominant-recessive trait, but only when both F1 parents are heterozygous.