Genetics Exam Study Guide: Mendelian Laws, Chromosome Behavior, and Gene Pathways

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Mendelian Genetics and Transmission

Mendel’s First and Second Laws

Mendel’s laws form the foundation of classical genetics, describing how traits are inherited from one generation to the next.

Law of Segregation (1st Law): Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.
Law of Independent Assortment (2nd Law): Genes for different traits assort independently of one another during gamete formation, provided they are not linked.
Example: In a dihybrid cross (AaBb x AaBb), the alleles for A and B segregate independently, producing a 9:3:3:1 phenotypic ratio.

Punnett Squares and Branch Diagrams

Punnett squares and branch diagrams are tools used to predict the outcome of genetic crosses.

Punnett Square: A grid that shows all possible combinations of alleles from parental gametes.
Branch Diagram: A stepwise method to calculate probabilities for multi-gene crosses.
Example: A monohybrid cross (Aa x Aa) yields a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.

Probability Calculations

Probability is used to predict the likelihood of genotypes and phenotypes in offspring.

Genotype Ratio: The proportion of different genotypes expected from a cross.
Phenotype Ratio: The proportion of observable traits expected.
Formula: for independent events.
Example: Probability of getting AaBb from AaBb x AaBb is .

Pedigree Analysis

Pedigrees are diagrams that show inheritance patterns across generations.

Modes of Inheritance (MOI): Autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive.
Key Points: Autosomal traits affect both sexes equally; X-linked traits often show sex bias.
Example: X-linked recessive traits often skip generations and are more common in males.

Cell Division and Chromosome Heredity

Mitosis

Mitosis is the process by which somatic cells divide, producing two genetically identical daughter cells.

Phases: Prophase, metaphase, anaphase, telophase.
Outcome: Maintains diploid chromosome number.

Meiosis I and II

Meiosis produces gametes with half the chromosome number of the parent cell.

Meiosis I: Homologous chromosomes separate; cells become haploid.
Meiosis II: Sister chromatids separate; four haploid cells result.
Diploid vs. Haploid: Diploid (2n) cells have two sets of chromosomes; haploid (n) cells have one set.

Extensions to Mendelian Genetics

Incomplete Dominance vs. Codominance

These are non-Mendelian inheritance patterns where the phenotype does not follow simple dominance.

Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents).
Codominance: Both alleles are fully expressed (e.g., AB blood type).

Epistasis vs. Pleiotropy

Epistasis: One gene masks the effect of another gene.
Pleiotropy: One gene affects multiple traits.
Example: In Labrador retrievers, one gene determines pigment color, another gene determines pigment deposition (epistasis).

Allelic vs. Genetic Heterogeneity

Allelic Heterogeneity: Different mutations in the same gene cause similar phenotypes.
Genetic Heterogeneity: Mutations in different genes cause similar phenotypes.

Penetrance vs. Expressivity

Penetrance: The proportion of individuals with a genotype who express the phenotype.
Expressivity: The degree to which a phenotype is expressed.

Sex-linked, Sex-influenced, and Sex-limited Traits

Sex-linked: Traits associated with genes on sex chromosomes.
Sex-influenced: Expression differs between sexes (e.g., pattern baldness).
Sex-limited: Trait only appears in one sex (e.g., milk production in cows).

Sex Determination Mechanisms

Protenor vs. Lygaeus Systems

Different organisms use different mechanisms for sex determination.

Protenor: Sex determined by presence or absence of a chromosome (e.g., XX female, XO male).
Lygaeus: Sex determined by distinct sex chromosomes (e.g., XX female, XY male).

Sex Determination in Mammals, Birds, and Fruit Flies

Mammals: XX female, XY male.
Birds: ZW female, ZZ male.
Fruit Flies: Ratio of X chromosomes to autosomes determines sex.

Gene Pathways

Drawing and Interpreting Gene Pathways

Gene pathways illustrate the sequence of gene interactions leading to a phenotype.

Key Points: Pathways can show epistasis, complementation, and metabolic steps.
Example: A pathway for pigment synthesis may involve multiple genes, each encoding an enzyme.

Chromosome Abnormalities and Non-disjunction

Outcomes of Non-disjunction

Non-disjunction is the failure of chromosomes to separate properly during cell division.

Mitosis: Can lead to mosaicism.
Meiosis I: Homologous chromosomes fail to separate; results in gametes with abnormal chromosome numbers.
Meiosis II: Sister chromatids fail to separate; also produces abnormal gametes.

Chromosomal Abnormalities

Euploid: Normal chromosome number.
Aneuploidy: Abnormal number of chromosomes (e.g., trisomy, monosomy).
Polyploidy: More than two sets of chromosomes.
Deletions: Loss of chromosome segment.
Duplications: Repeat of chromosome segment.
Inversions: Segment of chromosome is reversed; can form inversion loops during meiosis.
Translocations: Reciprocal (exchange between non-homologous chromosomes) vs. Robertsonian (fusion of two acrocentric chromosomes).

Table: Types of Chromosomal Abnormalities

Type	Description	Example
Euploid	Normal chromosome number	Human: 46 chromosomes
Aneuploidy	Abnormal chromosome number	Trisomy 21 (Down syndrome)
Polyploidy	Extra sets of chromosomes	Triploid (3n) plants
Deletion	Loss of segment	Cri du chat syndrome
Duplication	Repeat of segment	Charcot-Marie-Tooth disease
Inversion	Segment reversed	Inversion loop formation
Reciprocal Translocation	Exchange between chromosomes	Philadelphia chromosome
Robertsonian Translocation	Fusion of chromosomes	Translocation Down syndrome

Complementation Analysis

Principles of Complementation

Complementation analysis is used to determine whether mutations causing similar phenotypes are in the same gene or different genes.

Key Point: If two mutants produce a wild-type phenotype when crossed, mutations are in different genes (complementation).
Example: Two white-flowered plants produce purple flowers when crossed, indicating mutations in different genes.

Additional info: Academic context was added to expand brief points and ensure completeness for exam preparation.