Genetics Exam Study Guide: Patterns of Inheritance, Gene Interactions, and Analysis

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Dominant and Recessive Inheritance

Basic Principles of Allelic Expression

Inheritance patterns are determined by the interaction of alleles at a single gene locus. Understanding dominant and recessive alleles is fundamental to classical genetics.

Dominant allele: Expressed in the phenotype when at least one copy is present (heterozygous or homozygous).
Recessive allele: Expressed only when two copies are present (homozygous recessive).
Notation: Dominant allele is often represented by a capital letter (e.g., A), recessive by a lowercase letter (e.g., a).
Homozygous dominant genotype: AA; phenotype shows dominant trait.
Heterozygous genotype: Aa; phenotype shows dominant trait.
Homozygous recessive genotype: aa; phenotype shows recessive trait.

Incomplete Dominance

Intermediate Phenotypes in Heterozygotes

Incomplete dominance occurs when the heterozygote displays a phenotype intermediate between those of the two homozygotes.

Definition: Neither allele is completely dominant; the heterozygote has a blended phenotype.
Difference from complete dominance: Complete dominance shows only the dominant phenotype in heterozygotes, while incomplete dominance shows an intermediate phenotype.
Example: In snapdragons (Antirrhinum majus), red (RR) × white (rr) yields pink (Rr) flowers. F2 ratio is 1 red : 2 pink : 1 white.

Codominance

Simultaneous Expression of Both Alleles

Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits distinctly.

Definition: Both alleles contribute equally and visibly to the phenotype.
Difference from incomplete dominance: In codominance, both traits appear together; in incomplete dominance, the phenotype is intermediate.
Example: Human ABO blood group system: IA and IB alleles are codominant, so genotype IAIB results in AB blood type.

Complementation Tests

Determining Whether Mutations Affect the Same Gene

Complementation tests are used to determine if two mutations producing similar phenotypes are in the same gene or in different genes.

Purpose: To test whether two mutants with similar phenotypes have mutations in the same gene.
Complementation: If F1 offspring have wild-type phenotype, mutations are in different genes.
No complementation: If F1 offspring have mutant phenotype, mutations are in the same gene.
Logic: Complementation occurs because each parent provides a functional copy of the gene the other lacks, restoring the wild-type function.

Epistasis

Gene Interactions Affecting Phenotypic Expression

Epistasis describes a situation where the expression of one gene masks or modifies the expression of another gene.

Definition: One gene's product masks or alters the expression of another gene.
Epistatic gene: The gene whose product masks the other gene's expression.
Difference from dominance: Dominance is within a single gene; epistasis is between two different genes.
Example: Coat color in Labrador retrievers: B (black) and b (brown) alleles, but E (pigment deposition) is epistatic. Modified ratio: 9:3:4 in dihybrid crosses.

Penetrance vs. Expressivity

Variation in Gene Expression Among Individuals

Penetrance and expressivity describe how consistently and to what degree a genotype manifests as a phenotype.

Penetrance: Proportion of individuals with a genotype who express the expected phenotype.
Example: If 80% of individuals with a mutant allele show the trait, penetrance is 80%.
Expressivity: Degree or intensity of phenotype expression among individuals with the same genotype.
Example: Polydactyly: some individuals have extra fingers, others have extra toes.
100% penetrance but variable expressivity: All individuals show the trait, but its severity varies.

Conditional Mutations — Temperature Sensitive

Mutations Expressed Under Specific Environmental Conditions

Conditional mutations are only expressed under certain environmental conditions, such as temperature-sensitive mutations.

Definition: Mutations whose effects are only seen under specific conditions.
Permissive condition: Environment where mutant phenotype is not expressed; protein functions normally.
Restrictive condition: Environment where mutant phenotype is expressed; protein is nonfunctional.
Usefulness: Allows study of essential genes by controlling when the mutation is active.
Example: At permissive temperature, mutant protein is functional; at restrictive temperature, it is nonfunctional.

Chi-Squared (χ²) Test

Statistical Analysis of Genetic Data

The chi-squared test is used to determine whether observed genetic data fit expected ratios.

Purpose: To test if observed data match expected Mendelian ratios.
Null hypothesis (H₀): Observed and expected ratios are the same.
Reject H₀: If p-value < 0.05 (conventional threshold).
Degrees of Freedom (df): where n = number of classes.
Monohybrid cross: 2 classes, df = 1.
Dihybrid cross: 4 classes, df = 3.
Formula:

O: Observed number in each class.
E: Expected number in each class (from Punnett square ratios).
Calculation: Multiply predicted proportion by total observed to get E.
Example: For 1000 offspring, 3:1 ratio: E(dominant) = 750, E(recessive) = 250.

Sample Table: Chi-Squared Calculation

Phenotype Class	Observed (O)	Expected (E)	O−E	(O−E)²	(O−E)²/E
Dominant	792	750	42	1764	2.35
Recessive	208	250	-42	1764	7.06
Total	1000	1000	—	—	—

Using the χ² Table: Find row for df = 1, compare χ² value to p-value.
If p > 0.05: Fail to reject H₀; data fit expected ratio.
If p < 0.05: Reject H₀; data do not fit expected ratio.

Pedigree Analysis

Tracing Inheritance Patterns in Families

Pedigree analysis uses family trees to determine modes of inheritance and carrier status.

Key symbols: Circle = female, square = male, filled = affected, half-filled = carrier, horizontal line = mating.
Autosomal Dominant: Affected individuals in every generation; no carriers; unaffected parents cannot have affected child.
Autosomal Recessive: Can skip generations; carriers present; unaffected parents can have affected child.
X-Linked Dominant: Affected fathers pass trait to all daughters, not sons.
X-Linked Recessive: Carrier mothers can have affected sons; affected males more common.
Verification: Always confirm mode of inheritance with Punnett square analysis.

Probability Calculations in Pedigrees

Carrier probability: Determine possible genotypes based on pedigree.
Multiplication rule: For independent events, multiply probabilities.
Addition rule: For mutually exclusive events, add probabilities.
Example: If both parents are carriers (Aa × Aa), probability that an unaffected child is a carrier = .

Pleiotropy

One Gene, Multiple Phenotypic Effects

Pleiotropy occurs when a single gene affects multiple traits.

Definition: One gene influences several distinct phenotypic traits.
Example: Marfan syndrome (mutation in FBN1 gene) affects connective tissue, leading to effects in eyes, skeleton, and cardiovascular system.
Mechanism: Gene product is expressed in multiple tissues, causing diverse effects.

Essential Genes and Lethality

Genes Required for Survival and Lethal Alleles

Essential genes are required for organism survival; mutations can cause lethality.

Essential gene: Required for viability; loss-of-function leads to death.
Recessive lethal allele: Causes death in homozygous recessive genotype.
Detection: Expected Mendelian ratio is altered; e.g., 2:1 instead of 3:1 in crosses.
Dominant lethal allele: Causes death in heterozygous or homozygous dominant genotypes.
Relation to pleiotropy: Lethal alleles often have pleiotropic effects.
Example: Yellow coat color allele in mice (Agouti gene); homozygous yellow is lethal.

End of Study Guide | Review your notes and textbook for specific examples discussed in class.