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?
Understanding Genetics & Mendelian Inheritance
The physical basis of inheritance is meiosis. Before an egg or sperm cell forms, a cell copies its chromosomes. It then divides twice, leaving each gamete with one chromosome from each pair.
This is why a parent does not pass on both copies of every gene to one child. Which member of a chromosome pair enters a particular gamete is random.
Fertilization adds another layer of chance because any one sperm can combine with any one egg. A genetic cross is therefore a model of many possible fertilization events, not a guarantee for a single child.
Expected ratios become clearer when they are connected to sample size. If a cross has a predicted three out of four chance of one phenotype, four offspring do not have to appear in a perfect three to one pattern. A family might have four offspring with the same phenotype simply through chance.
Larger numbers usually come closer to the predicted ratio. This is the same idea used when scientists breed many plants or analyze large groups of fruit flies.
Probability describes patterns in groups. It cannot predict every individual outcome with certainty.
Independent assortment has an important condition. It works most reliably for genes on different chromosomes, or for genes far apart on the same chromosome. Genes close together on one chromosome are linked.
They tend to travel into the same gamete because the chromosome is passed along as a unit. During meiosis, matching chromosomes can exchange sections in a process called crossing over. This can separate linked genes, but it does not happen equally often for every pair of genes.
Linkage is one reason that real results may not match the familiar dihybrid ratio. Students should treat standard ratios as predictions based on stated assumptions, not as rules that every organism must follow.
Many human traits do not fit a simple dominant or recessive pattern. Some traits involve many genes, such as height or skin color. Environmental conditions can influence how genes are expressed.
Nutrition, temperature, exercise, disease, and exposure to chemicals can affect an organism without changing its DNA sequence. Other cases show incomplete dominance, codominance, multiple alleles, or genes carried on sex chromosomes. Family pedigrees can help trace these patterns across generations, but they cannot always reveal a genotype with complete confidence.
When solving problems, first identify the inheritance model given in the question. Then list possible gametes, keep genotype and phenotype separate, and check whether the assumptions of the model are actually valid.