BiologyPredicting Inheritance With Mendel's Laws
Lumi stands at a sunlit greenhouse bench covered with pea plants bearing purple and white flowers, holding a magnifying glass over a cross-pollination diagram, carefully sketching a Punnett square grid in a field notebook while dried seed pods and colored allele tokens are arranged nearby.
- Explain how the Law of Segregation ensures each gamete carries exactly one allele for each gene.
- Predict offspring genotype and phenotype ratios from monohybrid crosses using a Punnett square.
- Compare homozygous and heterozygous genotypes and distinguish genotype from phenotype.
- Apply the Law of Independent Assortment to construct and interpret a dihybrid Punnett square.
- Calculate the probability of a specific offspring phenotype from a given parental cross.
Key terms
- Law of Segregation
- The principle that paired alleles separate so each gamete carries one allele per gene
- Law of Independent Assortment
- The principle that genes on different chromosomes sort into gametes independently
- Homozygous
- Having two identical alleles for a gene, such as BB or bb
- Heterozygous
- Having two different alleles for a gene, such as Bb
- Punnett square
- A grid tool predicting offspring genotype and phenotype ratios from parental gametes
Segregation and Dominance
The Law of Segregation states that an organism's two alleles for a gene separate during gamete formation, so each gamete receives just one. A heterozygote Bb therefore produces half B and half b gametes. Dominance is not blending: a dominant allele fully suppresses the recessive allele in a heterozygote, so BB and Bb both show the dominant phenotype while only bb reveals the recessive one. This complete masking is what produces the characteristic 3 to 1 ratio.
Predicting With Punnett Squares
A Punnett square organizes the recombination of alleles at fertilization. For a monohybrid cross of two heterozygotes, Bb by Bb, the grid yields a 1 BB to 2 Bb to 1 bb genotype ratio and a 3 dominant to 1 recessive phenotype ratio. Placing one parent's gametes across the top and the other's down the side and combining them in each cell makes the probability of each outcome explicit and countable rather than guessed.
Independent Assortment and Probability
The Law of Independent Assortment extends prediction to two genes on different chromosomes. Because each homologous pair orients randomly at metaphase I, alleles for separate genes combine in all possible ways, so a dihybrid cross yields the classic 9 to 3 to 3 to 1 ratio. For independent genes you can also use the product rule: multiply individual probabilities, as in the chance of a double recessive being one-quarter times one-quarter, which equals one-sixteenth.
Worked examples
Cross two heterozygotes Aa by Aa. Find the phenotype ratio.
- Each parent produces gametes that are half A and half a by the Law of Segregation.
- Fill the Punnett square: combining gametes gives genotypes 1 AA, 2 Aa, and 1 aa.
- Apply dominance: AA and Aa both show the dominant phenotype, so 3 offspring are dominant for every 1 recessive.
Answer: 3 dominant : 1 recessive.
In YyRr by YyRr, find the fraction of green wrinkled (yyrr) offspring.
- Treat the genes independently since they assort independently.
- Find each single-gene probability: P(yy) is one-quarter and P(rr) is one-quarter.
- Multiply by the product rule: one-quarter times one-quarter equals one-sixteenth.
Answer: 1/16.
Activity
Complete both stages: first build a monohybrid Punnett square for Tt × Tt by sorting genotype tiles into the correct cells, then extend your prediction to a dihybrid cross by placing TtRr × TtRr gamete tokens into the 4×4 grid and calculating the probability of the double-recessive phenotype.
Practice
For a Bb by bb cross, predict the genotype and phenotype ratios of the offspring.
Use the product rule to find the probability of an aabb offspring from AaBb by AaBb.
Common mistakes to avoid
- Heterozygotes show a blended phenotypeA dominant allele completely masks the recessive one, so a heterozygote shows the full dominant phenotype, not a blend.
- Lowercase alleles are strongerLowercase denotes the recessive allele, which is masked whenever a dominant uppercase allele is present.
Check your understanding
In a monohybrid cross between two heterozygous parents (Aa × Aa), what is the expected phenotype ratio of the offspring?
A student claims that an organism with genotype Aa must show a blended phenotype — half dominant and half recessive — because it has one of each allele. What is wrong with this reasoning?
Two genes, seed color (Y = yellow dominant over y = green) and seed shape (R = round dominant over r = wrinkled), assort independently. What fraction of offspring from a YyRr × YyRr cross is expected to have green, wrinkled seeds?
Which of the following best explains WHY alleles for two genes on different chromosome pairs assort independently during meiosis?
Recap
Mendel's Law of Segregation gives each gamete one allele per gene, and dominance lets a dominant allele fully mask a recessive one, producing the 3 to 1 monohybrid ratio. Independent assortment and the product rule extend prediction to dihybrid crosses, yielding the 9 to 3 to 3 to 1 ratio and precise offspring probabilities.
Reflect
Why does multiplying probabilities work for genes on different chromosomes but not for linked genes?