Genetics: Principles, Inheritance, and Problem Solving – Study Guide

Principles of Mendelian Genetics

Dihybrid and Monohybrid Crosses

Mendelian genetics explores how traits are inherited from one generation to the next. Crosses between organisms with different genotypes help reveal the patterns of inheritance.

• Dihybrid cross: A cross between individuals heterozygous for two genes (e.g., AaBb x AaBb).

• Monohybrid cross: A cross between individuals heterozygous for a single gene (e.g., Aa x Aa).

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

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

Example: The phenotypic ratio for a dihybrid cross (AaBb x AaBb) with independent assortment is 9:3:3:1.

Phenotypic Ratios and Dominance

Assuming complete dominance, the classic phenotypic ratios for Mendelian crosses are:

• Monohybrid cross (Aa x Aa): 3:1 (dominant:recessive)

• Dihybrid cross (AaBb x AaBb): 9:3:3:1 (two dominant: one dominant/one recessive: one recessive/one dominant: two recessive)

Example: For two genes, the F2 phenotypic ratio is 9:3:3:1 if the genes assort independently.

Genetic Terminology

• Genotype: The genetic constitution of an organism (e.g., AaBb).

• Phenotype: The observable traits of an organism (e.g., purple flowers, tall plants).

• Homozygous: Having two identical alleles for a gene (e.g., AA or aa).

• Heterozygous: Having two different alleles for a gene (e.g., Aa).

Extensions of Mendelian Genetics

Codominance and Multiple Alleles

Some traits do not follow simple dominance. In codominance, both alleles are expressed in the phenotype. Multiple alleles refer to more than two possible alleles for a gene in a population.

• Example: Human ABO blood groups (alleles: IA, IB, i).

• Blood type inheritance: A type O child (ii) can only be born to parents who both carry the i allele.

Sex-Linked Inheritance

Genes located on sex chromosomes (X or Y) show unique inheritance patterns.

• X-linked recessive: More common in males (e.g., hemophilia, color blindness).

• X-linked dominant: Affects both sexes but often more females.

• Y-linked: Only affects males.

Example: An XYY human male has two Y chromosomes and one X chromosome.

Chromosome Behavior and Cell Division

Meiosis and Mitosis

Cell division is essential for growth, development, and reproduction. Mitosis produces identical diploid cells, while meiosis produces haploid gametes.

• Mitosis: 2n → 2n (diploid to diploid)

• Meiosis: 2n → n (diploid to haploid)

• Stages: Prophase, metaphase, anaphase, telophase, cytokinesis

Example: In metaphase I of meiosis, homologous chromosomes align at the metaphase plate; in metaphase II, sister chromatids align.

Chromosome Number and Ploidy

• Diploid (2n): Two sets of chromosomes (e.g., human somatic cells, 2n = 46).

• Haploid (n): One set of chromosomes (e.g., human gametes, n = 23).

Genetic Linkage and Mapping

Linkage and Recombination

Genes located close together on the same chromosome tend to be inherited together (linked). Recombination frequency can be used to estimate the distance between genes.

• Recombination frequency: The percentage of recombinant offspring among the total.

• Map unit (centimorgan, cM): 1% recombination = 1 map unit.

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

Genetic Mapping Problems

Three-point and four-point testcrosses are used to determine gene order and distances.

• Calculate recombination frequencies between pairs of genes.

• Use double crossover data to refine gene order.

Probability in Genetics

Calculating Probabilities

Genetic outcomes can be predicted using probability rules.

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

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

Example: The probability that the first three offspring of a dihybrid cross are all female is $(1/2)^3 = 1/8$.

Pedigree Analysis

Modes of Inheritance

Pedigrees are diagrams that show inheritance patterns in families. They help determine whether a trait is autosomal or sex-linked, dominant or recessive.

• Autosomal dominant: Trait appears in every generation; affected individuals have at least one affected parent.

• Autosomal recessive: Trait can skip generations; affected individuals may have unaffected parents.

• X-linked recessive: More males affected; trait can skip generations.

• X-linked dominant: Both sexes affected; affected fathers pass trait to all daughters.

Bacterial Genetics

Gene Transfer Mechanisms

Bacteria can exchange genetic material through several processes:

• Transformation: Uptake of free DNA from the environment.

• Conjugation: Direct transfer of DNA between bacteria via a pilus.

• Transduction: Transfer of DNA by bacteriophages (viruses).

Example: The F factor in E. coli enables conjugation.

Bacterial Phenotypes and Mutants

• Auxotroph: A mutant that requires a specific nutrient for growth.

• Prototroph: A wild-type strain that can grow on minimal medium.

Molecular Genetics

DNA Structure and Replication

DNA is a double helix composed of nucleotides. Replication is semi-conservative and proceeds in opposite directions on the two strands.

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

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

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

Example: There is no thymine in RNA; uracil replaces it.

Sample Table: Genetic Cross Results

The following table summarizes phenotypic classes observed in a three-gene testcross in tomato plants:

Phenotype	Number Observed
hairless	259
jointless, hairless	49
jointless	391
normal	260
anthocyaninless, jointless, hairless	268
anthocyaninless, hairless	941
anthocyaninless, jointless	259
anthocyaninless	260

Additional info: This table is used to determine gene order and linkage relationships by comparing observed and expected phenotypic ratios.

Key Formulas and Equations

• Chi-square test: $\chi^2 = \sum \frac{(O - E)^2}{E}$, where O = observed, E = expected

• Probability of independent events: $P = p_1 \times p_2 \times ... \times p_n$

• Recombination frequency: $\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}} \times 100\%$