Transcription is the process of copying the information in a gene’s DNA into an RNA molecule. In a typical eukaryotic cell, this happens in the nucleus because DNA is housed there. The product is messenger RNA (mRNA), which carries the genetic message out of the nucleus to a ribosome. Translation then uses that mRNA as a template to assemble amino acids into a protein, a task carried out at ribosomes in the cytoplasm (often on the rough endoplasmic reticulum as well). So transcription in the nucleus producing mRNA and then translation in the cytoplasm producing the protein is the standard flow of genetic information. Transcription in the cytoplasm would be unusual for most genes, copying RNA into DNA would be reverse transcription, and saying both processes occur in mitochondria ignores the common cellular arrangement in the rest of the cell.

Sex-linked traits are genes located on the X chromosome. In humans, the X carries many important genes, and males have only one X (and a Y). Because there isn’t a second X to mask it, a single allele on the male’s X is expressed in the phenotype. That means if a recessive allele for a trait exists on his single X, he will show the trait, whereas a female would need two copies of that recessive allele to express it. This is why such traits commonly appear more in males. The other options don’t fit the usual idea of a sex-linked trait: a Y-linked trait would affect males, but most sex-linked traits we discuss are on the X; an autosomal trait would affect both sexes roughly equally; and a mitochondrial (maternal) inheritance pattern is separate from sex chromosomes.

The knockout approach works by turning off a gene to see what goes wrong, so researchers can infer what that gene normally does. By disabling the gene’s coding region or key regulatory parts, the organism or cells can’t produce the functional protein, and scientists then look for changes in appearance, development, physiology, or behavior. Comparing a knockout with a normal, unmodified organism reveals the gene’s role. In practice, this can be done by deleting the gene through precise genome editing. Traditional methods used targeted changes in embryonic stem cells to create knockout animals, while modern techniques like CRISPR/Cas9 can introduce mutations that disrupt the gene quickly and efficiently. Some knockouts are designed to be tissue-specific or inducible, so the gene is disabled only in certain tissues or at particular times, which helps when a global knockout would be lethal or confound results. This is different from boosting gene expression by adding extra copies, which would reveal effects of overactivity rather than loss of function, or from sequencing a gene to find mutations or using PCR to amplify DNA, which don’t directly test a gene’s function.

Sex-linked inheritance involves genes on the X chromosome. In humans, males have one X and one Y, while females have two Xs. If a gene on the X is recessive, males will express that trait with just one copy because there’s no second X to mask it. Females would need two copies of the recessive allele to show the trait, so X-linked recessive conditions appear much more often in males. This is the reason we commonly observe these traits in males in families, with examples like red-green color blindness or hemophilia. These traits are not autosomal and aren’t inherited only from the mother: fathers pass their X chromosome to daughters, but not to sons, while mothers pass Xs to both sons and daughters.