Patterns of Inheritance — Chapter 7 Summary & Study Notes

🧬 Overview

These notes summarize how genetic traits are inherited, from Mendel’s pea experiments to modern views of complex traits. Focus on the difference between genotype and phenotype, Mendel’s laws, how to predict offspring using Punnett squares, and how multiple genes and the environment shape real-world traits.

🔑 Key terms

• Gene: a stretch of DNA that influences a trait.
• Allele: a variant form of a gene.
• Genotype: the combination of alleles an individual carries for a trait.
• Phenotype: the observable expression of a trait.
• Dominant allele: an allele that masks the effect of another when paired (often written as an uppercase letter, e.g., B).
• Recessive allele: an allele masked by a dominant allele (written lowercase, e.g., b).
• Homozygous: two identical alleles (BB or bb).
• Heterozygous: two different alleles (Bb).

🌱 Gregor Mendel and the foundation of genetics

Gregor Mendel performed controlled crosses of pea plants and discovered consistent patterns of inheritance. His methods—true-breeding parents, counting offspring, and predicting ratios—established the empirical basis for later genetic theory. Mendel’s work led to two fundamental laws that explain how alleles are transmitted through gametes.

⚖️ Law of Segregation (Mendel’s first law)

The law of segregation states that the two alleles for a gene separate during meiosis I so each gamete receives one allele. When gametes fuse at fertilization, offspring regain two alleles. This law lets you predict single-gene inheritance: cross two heterozygotes (Bb × Bb) and expect genotype ratios 1:2:1 and phenotype ratios (dominant:recessive) 3:1 when dominance is complete.

🔀 Law of Independent Assortment (Mendel’s second law)

The law of independent assortment states alleles of different genes segregate into gametes independently, provided the genes are on different chromosomes or far apart on the same chromosome. This underlies dihybrid crosses (two-trait crosses) and explains why combinations of traits can appear in new assortments.

☑️ Punnett squares and probability

• A Punnett square is a grid used to list possible allele combinations from parental gametes and predict offspring genotypes and phenotypes.
• Inheritance outcomes are probabilistic: each fertilization event is independent. For two heterozygous parents (Bb × Bb), chance of expressing the dominant phenotype is 3/4; chance of a specific genotype (BB, Bb, or bb) follows the Punnett-square counts.
• For independent events (like coin tosses), multiply probabilities to get joint probabilities (e.g., probability of two independent events both occurring = product of each event’s probability).

🔬 Chromosome theory of inheritance

Mendel’s laws have a cellular basis: chromosomes come in homologous pairs, carry alleles, and are shuffled and separated during meiosis. Random fusion of sperm and egg produces genetically unique offspring. The theory links observable ratios to physical movement of chromosomes.

🧩 Types of dominance

• Complete dominance: heterozygote shows the dominant phenotype (Bb looks like BB).
• Incomplete dominance: heterozygote shows an intermediate phenotype between the two homozygotes (e.g., red × white → pink).
• Codominance: both alleles are visible in the heterozygote (e.g., blood type AB, or spots and background color expressed together).

🧠 Complex patterns beyond simple Mendelian inheritance

Many traits do not follow single-gene Mendelian rules. Important patterns include:

• Pleiotropy: a single gene affects multiple phenotypic traits (one gene, many effects). Example: one gene influencing skull shape and limb bones in dogs.
• Polygenic traits: a single trait is governed by multiple genes (many genes, one trait). Examples: human height, skin color, running speed.
• Multiple alleles: a gene may have more than two alleles in a population, but each individual carries at most two (example: ABO blood group with alleles A, B, O).
• Epistasis: alleles of one gene modify or mask the effects of alleles at a different gene (genes interact; example: coat color in Labrador retrievers where one gene controls pigment and another controls pigment deposition).

🌡️ Environmental effects on phenotype

When the environment affects trait expression, genotype alone cannot reliably predict phenotype. External and internal conditions like temperature, light, or CO₂ levels can change how genes are expressed. Example: Siamese cat coat color is temperature-sensitive; darker fur appears on cooler body regions.

🐶 Case study: dog genetics (Lark and Ostrander)

Researchers like Gordon Lark and Elaine Ostrander applied genetic tools to dogs to map genes for size, skull shape, fur color, leg length, and disease susceptibility. Dogs are valuable models because many genetic disorders occur in both dogs and humans (e.g., certain cancers, Addison’s disease). Ostrander’s work produced the first dog genome map and identified genes linked to diseases shared with humans.

🏥 Complex diseases and genetics

Most chronic human diseases (heart disease, diabetes, many cancers, Alzheimer’s) are complex traits caused by multiple genes interacting with each other and the environment. Genetic tests can identify risk alleles, enabling prevention strategies and personalized medicine tailored to specific genetic variants.

✅ Summary and practical takeaways

• Distinguish genotype (alleles) from phenotype (observable trait).
• Use the law of segregation for single-gene crosses and independent assortment for two-gene crosses (when genes assort independently).
• Predict outcomes with Punnett squares and probability rules; remember events are independent.
• Expect many real traits to be influenced by incomplete dominance, codominance, pleiotropy, polygenic inheritance, epistasis, and environmental factors.
• Modern genetics connects Mendel’s principles to chromosomes and allows mapping of disease-related genes in humans and model species like dogs.