Two-factor cross describes crossing organisms that differ in two traits, signaling that two genes (factors) are being analyzed together. This directly matches the scenario of comparing two differing characteristics and studying how two loci segregate and assort in offspring. A monohybrid cross involves one trait, a back cross is a cross back to a parental type, and a dihybrid cross is the standard term for two-trait crosses (it's the same idea, but the two-factor wording emphasizes the two factors being examined).

In incomplete dominance, neither allele is completely dominant over the other. The heterozygote displays an intermediate phenotype between the two homozygous forms. For example, crossing red-flowered and white-flowered snapdragons gives pink offspring, showing a blend rather than red or white being fully expressed. This differs from a situation where one allele is dominant, which would make the heterozygote look like one of the homozygotes. It also isn’t that the heterozygote shows no phenotype at all—the trait is expressed, just as an intermediate form. Environmental factors alone don’t explain this genetic pattern.

Start codon use centers on how translation begins and which amino acid starts the polypeptide. AUG signals the start in both bacteria and eukaryotes, so translation begins at AUG in either domain. The initiating amino acid differs: in bacteria the initiator tRNA brings formylmethionine, so the first residue is formylmethionine; in eukaryotes the initiator tRNA brings methionine, which is usually not formylated and is often removed later in the process. While bacteria can occasionally use alternative start codons like GUG or UUG, AUG remains the standard start. The other options misstate which codons initiate translation or what amino acid starts the chain (for example, UAA is a stop codon, not a start signal).

Genetics is the science of heredity—the study of how traits are passed from parents to offspring through genes. Traits are carried by genes on chromosomes, copied in DNA, and transmitted through reproductive cells so offspring resemble their parents. This field explores how alleles separate and combine during reproduction, how gene expression shapes traits, and how mutations introduce variation. The other ideas describe related areas—evolution concerns changes in allele frequencies in populations over time, ecosystems study how organisms interact with each other and their environment, and calling genetics merely a “scientific approach” to the field doesn’t define what genetics itself studies.

In a diploid organism, there are two versions of each chromosome—one from each parent. When these two chromosomes carry the same genes in the same order, they are called homologous chromosomes. They may carry different alleles at various gene locations, which is where variation among individuals comes from. This pairing happens because homologs come from different parents, and during meiosis they align and can exchange genetic material, increasing diversity. By contrast, sister chromatids are identical copies of a single chromosome produced during DNA replication, and they stay attached until they separate later in cell division. Alleles are just the different versions of a gene that can reside at the same locus on homologous chromosomes. Chromatids simply refers to the duplicated copies of a chromosome, not to the chromosome from each parent.