BIOB 160 Principles of Living Systems · Unit 3: Cell division and inheritance · on Exam 3
3.3 Mendel and the gene idea
Study notes for Mendel and the gene idea in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.
Big ideas
- Mendel crossed true-breeding pea plants (P generation) to get the F₁, then self-crossed them to get the F₂. Alleles are versions of a gene; a dominant allele masks a recessive one.
- Genotype is the allele combination (homozygous AA or aa, heterozygous Aa); phenotype is the trait. A testcross with a homozygous recessive reveals an unknown genotype.
- Law of segregation: the two alleles separate into different gametes. A monohybrid cross Aa × Aa gives a 3:1 phenotype ratio and 1:2:1 genotypes.
- Law of independent assortment: genes on different chromosomes sort independently. A dihybrid cross AaBb × AaBb gives 9:3:3:1. Use the multiplication rule for “and” (independent events) and the addition rule for “or” (mutually exclusive outcomes).
- Beyond Mendel: incomplete dominance (1:2:1 phenotypes, pink snapdragons), codominance (both expressed, AB blood), multiple alleles (ABO), pleiotropy (one gene, many effects), epistasis (one gene masks another, 9:3:4) and polygenic traits (height). Pedigrees trace traits such as cystic fibrosis (recessive) and Huntington’s disease (dominant).
Key formulas
Multiplication rule
$$P(A \text{ and } B) = P(A)\,P(B)$$
Addition rule
$$P(A \text{ or } B) = P(A) + P(B)$$
Worked example
In AaBbCc × AaBbCc (independent genes), what fraction of offspring are aabbcc? What fraction show all three dominant traits?
Show the solution
Each gene gives aa with probability 1/4, so aabbcc = (1/4)³ = 1/64. Each gene gives the dominant phenotype with probability 3/4, so all three dominant = (3/4)³ = 27/64.
Common mistakes
- Using a 16-square Punnett square for three genes. Multiply per-gene probabilities instead.
- Mixing genotype ratios (1:2:1) with phenotype ratios (3:1).
- Calling ABO blood type incomplete dominance. A and B are codominant; ABO also shows multiple alleles.
Exam tipReading: Campbell ch. 14 (pp. 269–280). Break every multi-gene problem into one-gene problems and multiply.
Keep going in StudyCat
StudyCat is free for Montana State students: an endless quizzer with worked explanations, one-question lessons, flashcards, exam prep checklists, the class calendar with every deadline, a grade calculator and a study board for BIOB 160.