Genetics Exam Study Guide: Mendelian Inheritance, Linkage, Chromosome Behavior, and Bacterial Genetics

Mendelian Genetics

Types of Genetic Crosses

Genetic crosses are fundamental experiments in genetics used to study inheritance patterns. The type of cross depends on the number of genes and traits being analyzed.

• Monohybrid cross: Involves one gene with two alleles.

• Dihybrid cross: Involves two genes, each with two alleles.

• Testcross: Crossing an individual with a dominant phenotype (but unknown genotype) with a homozygous recessive individual to determine genotype.

• Backcross: Crossing an F1 hybrid with one of its parental genotypes.

• Reciprocal cross: Switching the sexes of the parents to test for sex-linked inheritance.

Example: The cross AaBb x aabb is a testcross for two genes.

Phenotypic Ratios in Mendelian Crosses

Assuming independent assortment and complete dominance, the classic phenotypic ratios for F2 generation are:

• Monohybrid cross: $3:1$ (dominant:recessive)

• Dihybrid cross: $9:3:3:1$ (two genes, four phenotypes)

Example: AaBb x AaBb yields $9:3:3:1$ ratio in F2.

Genotype and Phenotype Calculations

Calculating expected ratios requires understanding of Punnett squares and probability.

• Genotype ratio for AaBb x AaBb: $1:2:1:2:4:2:1:2:1$ (for all possible combinations)

• Phenotype ratio for AaBb x AaBb: $9:3:3:1$

Independent Assortment and Segregation

Mendel's laws describe how alleles segregate and assort independently during gamete formation.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes on different chromosomes assort independently.

Chromosome Behavior and Cell Division

Stages of Mitosis and Meiosis

Cell division is essential for growth and reproduction. Mitosis produces identical cells, while meiosis produces gametes with half the chromosome number.

• Mitosis: Prophase, metaphase, anaphase, telophase; results in two diploid cells.

• Meiosis I: Homologous chromosomes separate; reductional division.

• Meiosis II: Sister chromatids separate; equational division.

Example: Human diploid number is 2n = 46; after meiosis, gametes have n = 23.

Chromosome Number and Ploidy

• Diploid (2n): Two sets of chromosomes.

• Haploid (n): One set of chromosomes (gametes).

Sex Chromosomes and Sex Determination

Sex is determined by the presence of X and Y chromosomes in humans.

• XX: Female

• XY: Male

• XYY, XXY: Variations can result in syndromes (e.g., Klinefelter's, Jacob's syndrome)

Linkage and Recombination

Gene Linkage

Linked genes are located close together on the same chromosome and tend to be inherited together.

• Recombination frequency: Used to estimate genetic distance between genes.

• Map units (centimorgans, cM): 1% recombination = 1 cM.

Example: If two genes are 20 cM apart, 20% of gametes will be recombinant.

Testcrosses and Linkage Analysis

Testcrosses are used to determine linkage and recombination frequencies.

• Parental types: Non-recombinant offspring.

• Recombinant types: Offspring with new allele combinations.

Chi-Square Test for Linkage

The chi-square test is used to compare observed and expected ratios to test hypotheses about linkage.

• Formula: $\\chi^2 = \sum \frac{(O - E)^2}{E}$

• If $\\chi^2$ exceeds the critical value, the hypothesis is rejected.

Probability in Genetics

Calculating Probabilities

Probability is used to predict the likelihood of genetic outcomes.

• Product rule: Probability of independent events occurring together is the product of their individual probabilities.

• Sum rule: Probability of mutually exclusive events is the sum of their probabilities.

Example: Probability that three offspring are all dihybrid females: $(1/8)^3 = 0.002$

Pedigree Analysis

Modes of Inheritance

Pedigrees are used to determine inheritance patterns of traits.

• Autosomal dominant: Trait appears in every generation.

• Autosomal recessive: Trait can skip generations.

• X-linked dominant/recessive: Trait associated with X chromosome; affects males and females differently.

• Holandric: Y-linked inheritance; only males affected.

Bacterial Genetics

Types of Bacterial Strains

• Autotroph: Can synthesize all required nutrients.

• Auxotroph: Requires specific nutrients for growth.

• Prototroph: Wild-type, can grow on minimal medium.

• Pseudotroph: Not a standard term; sometimes used for strains with unusual requirements.

Gene Transfer in Bacteria

Bacteria can exchange genetic material through several processes:

• Transformation: Uptake of free DNA from environment.

• Conjugation: Direct transfer of DNA via cell-to-cell contact, often involving F factor.

• Transduction: Transfer of DNA via bacteriophages.

• Sexduction: Transfer of F factor and associated genes.

• Viral recombination: Exchange of genetic material via viruses.

Conjugation and F Factor

• F+ cells: Possess F factor; can donate it during conjugation.

• F- cells: Lack F factor; can receive it.

• Hfr cells: High frequency recombination; F factor integrated into chromosome.

Interrupted Mating Experiments

Used to map bacterial genes by timing the transfer of genes during conjugation.

• Order of gene transfer: Determined by time at which each gene appears in recipient.

• Example equation: $a \rightarrow b \rightarrow c \rightarrow d$

DNA Structure and Replication

DNA and RNA Structure

• Antiparallel strands: DNA strands run in opposite directions (5' to 3' and 3' to 5').

• Purines: Adenine (A) and guanine (G).

• Pyrimidines: Cytosine (C), thymine (T), and uracil (U in RNA).

• Base pairing: A-T (DNA), A-U (RNA), G-C.

Replication and Transcription

• Replication: DNA is copied before cell division.

• Transcription: DNA is used as a template to synthesize RNA.

HTML Table: Example of Linkage Analysis

The following table summarizes the linkage arrangement of three loci based on observed phenotypes:

Phenotype	Number Observed
Hairless	259
Jointless	49
Anthocyaninless, Jointless, Hairless	268
Normal	260
Anthocyaninless	941
Jointless, Hairless	391
Anthocyaninless, Jointless	259

Main purpose: To determine the linkage arrangement and recombination frequencies among three genes.

Additional info:

• Some questions refer to classic experiments (e.g., Hershey-Chase) and basic molecular biology concepts.

• Probability calculations and chi-square tests are essential for hypothesis testing in genetics.

• Pedigree analysis is used to infer inheritance patterns and distinguish between autosomal and sex-linked traits.