This cheat sheet covers advanced Mendelian and non-Mendelian genetics for high school biology. It helps students connect inheritance patterns to probability, Punnett squares, pedigrees, and chromosome behavior. Students need these tools to predict offspring traits, interpret genetic crosses, and explain why real inheritance can differ from simple dominant and recessive patterns.
Core ideas include segregation, independent assortment, genotype and phenotype ratios, and probability rules such as product and sum. Advanced topics include incomplete dominance, codominance, multiple alleles, sex-linked inheritance, linked genes, recombination frequency, and polygenic traits. The most important skill is choosing the correct inheritance model before calculating ratios or interpreting data.
Key Facts
- 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 assort independently if the genes are on different chromosomes or far apart on the same chromosome.
- The product rule says that the probability of two independent events both occurring is P(A and B) = P(A) x P(B).
- The sum rule says that the probability of either of two mutually exclusive events occurring is P(A or B) = P(A) + P(B).
- A monohybrid cross of two heterozygotes with complete dominance gives a genotype ratio of 1 AA : 2 Aa : 1 aa and a phenotype ratio of 3 dominant : 1 recessive.
- A dihybrid cross of two double heterozygotes with independent assortment gives a phenotype ratio of 9 : 3 : 3 : 1.
- Recombination frequency is calculated as recombination frequency = recombinant offspring / total offspring x 100%, and 1% recombination equals 1 map unit.
- Linked genes are inherited together more often than expected by independent assortment because they are located close together on the same chromosome.
Vocabulary
- Genotype
- The allele combination an organism has for one or more genes.
- Phenotype
- The observable traits or characteristics produced by a genotype and influenced by the environment.
- Incomplete dominance
- An inheritance pattern in which the heterozygote has an intermediate phenotype between the two homozygotes.
- Codominance
- An inheritance pattern in which both alleles in a heterozygote are fully expressed.
- Linked genes
- Genes located on the same chromosome that tend to be inherited together unless crossing over separates them.
- Recombination frequency
- The percentage of offspring with new allele combinations caused by crossing over between linked genes.
Common Mistakes to Avoid
- Assuming every cross gives a 3:1 ratio is wrong because 3:1 applies only to a monohybrid heterozygote cross with complete dominance.
- Treating linked genes as independently assorting is wrong because genes close together on the same chromosome are inherited together more often than chance predicts.
- Confusing incomplete dominance with codominance is wrong because incomplete dominance blends phenotypes, while codominance shows both phenotypes fully.
- Using phenotype to determine genotype without enough evidence is wrong because dominant-looking individuals can be homozygous dominant or heterozygous.
- Forgetting that males have only one X chromosome is wrong in sex-linked problems because a single recessive allele on the X chromosome can be expressed in XY individuals.
Practice Questions
- 1 In pea plants, tall is dominant to short. Cross Tt x Tt. What are the expected genotype ratio and phenotype ratio?
- 2 In a dihybrid cross AaBb x AaBb with independent assortment, what fraction of offspring are expected to show both recessive phenotypes?
- 3 A testcross produces 84 parental offspring and 16 recombinant offspring. What is the recombination frequency, and how many map units apart are the genes?
- 4 A red flower crossed with a white flower produces all pink offspring. Explain which inheritance pattern is most likely and why complete dominance does not fit.