Genetics & Mendelian Inheritance Cheat Sheet

A printable reference covering alleles, genotypes, Punnett squares, Mendel’s laws, monohybrid ratios, dihybrid ratios, and inheritance patterns for grades 10-11.

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Genetics and Mendelian inheritance explain how traits pass from parents to offspring through genes and alleles. This cheat sheet helps students organize the most important patterns, symbols, and ratios used in Grade 10 to 11 biology. It is especially useful for solving Punnett square problems, predicting offspring genotypes, and recognizing inheritance patterns. Clear genotype notation helps connect the mathematics of probability to real biological traits. The core ideas include dominant and recessive alleles, homozygous and heterozygous genotypes, and Mendel’s laws of segregation and independent assortment. A monohybrid cross usually gives a 3:1 phenotypic ratio when two heterozygotes are crossed. A dihybrid cross between two double heterozygotes often gives a 9:3:3:1 phenotypic ratio when genes assort independently. Probability rules such as product rule and sum rule help solve genetic crosses without drawing every box.

Key Facts

A dominant allele is written with an uppercase letter, such as A, and can mask a recessive allele in a heterozygous genotype.
A recessive allele is written with a lowercase letter, such as a, and is expressed only when the genotype is homozygous recessive, such as aa.
In a monohybrid cross Aa x Aa, the genotype ratio is 1 AA : 2 Aa : 1 aa and the phenotype ratio is 3 dominant : 1 recessive.
Mendel’s law of segregation states that the two alleles for a gene separate during gamete formation, so each gamete receives one allele.
Mendel’s law of independent assortment states that alleles of different genes separate independently during gamete formation if the genes are unlinked.
In a dihybrid cross AaBb x AaBb with independent assortment, the expected phenotypic ratio is 9 both dominant : 3 first dominant only : 3 second dominant only : 1 both recessive.
The product rule states that the probability of two independent events both happening is P(A and B) = P(A) x P(B).
A test cross uses an organism with a dominant phenotype and a homozygous recessive organism, such as A_ x aa, to determine the unknown genotype.

Vocabulary

Allele: An allele is a different version of a gene, such as A or a, that can affect a trait.
Genotype: A genotype is the allele combination an organism has for a trait, such as AA, Aa, or aa.
Phenotype: A phenotype is the observable trait or characteristic produced by a genotype and the environment.
Homozygous: Homozygous means having two identical alleles for a gene, such as AA or aa.
Heterozygous: Heterozygous means having two different alleles for a gene, such as Aa.
Punnett Square: A Punnett square is a grid used to predict possible offspring genotypes from parental gametes.

Common Mistakes to Avoid

Confusing genotype with phenotype is wrong because genotype is the allele combination, while phenotype is the visible or measurable trait.
Writing recessive alleles as uppercase letters is wrong because standard notation uses uppercase letters for dominant alleles and lowercase letters for recessive alleles.
Assuming a dominant phenotype always means homozygous dominant is wrong because both AA and Aa can show the dominant trait.
Forgetting to separate alleles into gametes is wrong because each gamete receives only one allele from each gene pair.
Using the 9:3:3:1 ratio for every two-trait cross is wrong because that ratio applies only when both parents are heterozygous for both genes and the genes assort independently.

Practice Questions

1 In a monohybrid cross Tt x Tt, what are the expected genotype ratio and phenotype ratio if T is dominant over t?
2 A plant with a dominant phenotype is crossed with a homozygous recessive plant, and half the offspring show the recessive phenotype. What is the genotype of the dominant parent?
3 In the cross AaBb x AaBb, assuming independent assortment, what fraction of offspring are expected to show both recessive traits?
4 Why can two parents who both show a dominant trait have a child with the recessive phenotype?