Mendelian genetics: crosses, probability and pedigrees

What the exam asks

• Multiple-choice sets: Given a pedigree or experimental breeding data table, calculate the probability of a specific offspring genotype or phenotype, or select the mode of inheritance backed by a specific parent-offspring pair.
• Free response (especially Q1, Q3, Q5, and Q6): State a null hypothesis for a genetic cross, calculate expected phenotypic ratios, explain how chromosome separation in meiosis produces gamete ratios, or analyze a pedigree visual model to justify why a trait cannot be autosomal recessive or X-linked.

Core ideas

Mendel’s laws and fertilization

Mendel’s laws describe the physical movement of chromosomes during meiosis (Topic 5.3):

• Law of Segregation: The two alleles for a heritable character separate during gamete formation (Anaphase I/II) and end up in different haploid ($n$) gametes. Each gamete carries only one allele for each gene.
• Law of Independent Assortment: Genes located on different, non-homologous chromosomes (or very far apart on the same chromosome) assort into gametes independently of one another because homologous pairs orient randomly at the metaphase plate during Metaphase I.
• Fertilization: The random fusion of two haploid ($n$) gametes restores the diploid ($2n$) chromosome number in the zygote and generates novel combinations of alleles, increasing genetic variation.

Genotypes, phenotypes, and standard crosses

A gene is a hereditary unit at a specific chromosomal locus, while an allele is a specific sequence variant of that gene. An organism’s genotype (allele pair: homozygous dominant $AA$, heterozygous $Aa$, or homozygous recessive $aa$) interacts with the environment to determine its observable phenotype.