4.1 Genes, Karyotypes and Sex Chromosomes

A gene is a DNA sequence whose information contributes to a functional product and characteristic. Its locus is its specific chromosomal position. Homologues carry the same genes at corresponding loci, but their sequences need not be identical. Alternative sequence forms are alleles, often differing by one or a few bases. A common one-base variant is a single-nucleotide polymorphism; many have no phenotypic effect, while some alter coding or regulation or mark a nearby disease-associated region.

An individual's allele combination is part of the genotype. The phenotype is an observable structural, physiological, biochemical or behavioral outcome. Phenotype is not a simple photograph of genotype: gene products interact, many traits involve multiple loci, and environment affects expression. Even so, the distinction lets geneticists infer hidden alleles from family patterns.

A karyotype is the chromosome number and types in a nucleus; a karyogram is the ordered image displaying them. Dividing cells are arrested while chromosomes are condensed, stained for banding, photographed and arranged into homologous pairs by length, centromere position and banding. A karyogram can show sex chromosomes and large abnormalities but not normally a single altered nucleotide.

Nondisjunction is failure of homologues or sister chromatids to separate. It produces gametes with an extra or missing chromosome. Fertilization involving a gamete with an extra chromosome may create trisomy; trisomy 21 causes Down syndrome. A karyogram detects the extra chromosome but cannot predict exact severity. Prenatal sampling also raises questions about procedural risk, consent, uncertainty and the use of results.

Most mammals have XX females and XY males; the remaining chromosomes are autosomes. The sex chromosomes carry genes as well as contributing to sex determination, so their alleles can produce inheritance patterns that differ between XX and XY individuals.

Karyogram preparation requires cells at the correct stage. A blood or fetal-cell sample is cultured, cell division is stimulated, and a spindle inhibitor arrests cells while chromosomes are maximally condensed. Cells are spread, stained and imaged. Software can separate chromosome images, but pairing still depends on several features at once. Two chromosomes of similar length are not necessarily homologues; their centromere positions and band patterns must also correspond.

Aneuploidy means an abnormal number of particular chromosomes, whereas polyploidy changes whole chromosome sets. A trisomic human cell is still described as broadly diploid because it has two complete sets plus one additional chromosome. Nondisjunction in meiosis I places both homologues in one daughter cell; nondisjunction in meiosis II fails to separate sister chromatids. Both can produce aneuploid gametes, but their allele patterns differ and can sometimes reveal which division failed.

Amniocentesis usually obtains fetal cells from amniotic fluid later in pregnancy, while chorionic-villus sampling obtains placental tissue earlier. Both are diagnostic sampling procedures, not karyotyping itself, and both are invasive. Modern screening may estimate risk without directly collecting fetal cells, but a risk estimate and a diagnostic chromosome result are not equivalent. Answers should separate the biological evidence a method provides from decisions made after receiving it.