Hybridization is the process by which a single-stranded nucleic acid molecule (DNA or RNA) anneals to its complementary sequence through hydrogen bonds between the bases. This principle of specific base pairing (A with T, G with C) is the foundation for techniques like FISH, Southern blotting, Northern blotting, and microarrays, where a labeled probe is used to detect a target sequence.

Linkage analysis is a method used to map a disease-causing gene to a specific chromosomal region by studying its inheritance pattern alongside known genetic markers in large families. Markers that are physically close to the disease gene on a chromosome tend to be inherited together (i.e., they are 'linked') and will not assort independently. GWAS is a population-based study to find common variants associated with common diseases, not for mapping rare single-gene disorders in families.

Targeted mutation analysis only screens for a specific set of common mutations. A negative result means the person does not carry any of the mutations tested, which significantly lowers their risk. However, it does not rule out the possibility that they could be a carrier of a rare, untested mutation in the same gene. Therefore, a small residual risk of being a carrier always remains after a negative targeted screening test.

A karyotype provides a low-resolution, microscopic view of the chromosomes. It is excellent for detecting aneuploidy (abnormal chromosome number), such as trisomy 18, and large structural changes like major deletions or translocations. However, its resolution is far too low to detect changes within a single gene, such as a point mutation, a small deletion, or an insertion, which require molecular techniques like DNA sequencing.

Huntington disease is a trinucleotide repeat disorder. Polymerase chain reaction (PCR) is used to amplify the region of the HTT gene containing the CAG repeat. The size of the resulting DNA fragment, which is proportional to the number of repeats, can then be accurately determined by fragment analysis (e.g., capillary electrophoresis). This is the standard method for diagnosing such disorders. Karyotyping is used for large chromosomal abnormalities. Western blotting detects proteins, not gene repeats. ELISA detects antigens or antibodies.

DiGeorge syndrome is a classic microdeletion syndrome. The deletion at 22q11.2 is typically too small to be visualized on a standard G-banded karyotype. Fluorescence in situ hybridization (FISH) uses a DNA probe specific to the 22q11.2 region, which will fail to bind if the region is deleted, making it the gold standard for diagnosis. Serum calcium is a biochemical finding, not a genetic test. Y-chromosome analysis is irrelevant.

The karyotype 47,XX,+21 indicates that the child, a female, has 47 chromosomes in total, with an extra copy of chromosome 21. This is the classic karyotype for trisomy 21 (Down syndrome). The most common cause (approximately 95% of cases) is the failure of chromosome 21 to segregate properly during meiosis in a parent (meiotic nondisjunction), leading to a gamete with an extra chromosome 21. Robertsonian translocation would result in 46 chromosomes. Mosaicism would show two cell lines. Uniparental disomy would result in 46 chromosomes.

Allele-specific oligonucleotide (ASO) hybridization uses probes that bind specifically to either the wild-type or the mutant DNA sequence. This makes it an ideal, rapid, and cost-effective method for screening for a known, common point mutation or small deletion like ΔF508. Whole genome sequencing is unnecessarily broad and expensive for this purpose. Northern blotting analyzes RNA, not the DNA mutation itself. Chromosomal microarray detects copy number variants, not small deletions within a gene.

Sanger sequencing, also known as the chain-termination method, relies on the use of dideoxynucleoside triphosphates (ddNTPs). These molecules lack the 3'-hydroxyl group required for the formation of a phosphodiester bond. When a ddNTP is incorporated into the growing DNA strand by DNA polymerase, synthesis is terminated. This creates a set of DNA fragments of different lengths, which allows the sequence to be determined.

The question specifically asks about the presence or absence of a protein. Western blotting is the technique used to separate proteins by size via gel electrophoresis, transfer them to a membrane, and then use a specific antibody (in this case, anti-dystrophin) to detect the protein of interest. Absence of a band would confirm the diagnosis. Southern blot analyzes DNA, Northern blot analyzes RNA, and chromosomal microarray analyzes for DNA copy number variations.