4.2 Sex Linkage, Pedigrees and Genetic Conditions

An X-linked locus lies on X. An XY individual is hemizygous for most such loci because Y lacks a corresponding allele, so a recessive X-linked allele is expressed. An XX heterozygote may be an unaffected carrier under complete dominance. Write alleles as superscripts on X; do not place the allele on Y unless the gene is genuinely Y-linked.

Fathers give X to every daughter and Y to every son, so X-linked alleles do not pass father to son. A male cannot be an unaffected carrier of a fully penetrant X-linked recessive allele. Hemophilia and red–green color blindness illustrate this inheritance pattern.

Pedigrees use squares for males, circles for females, filled symbols for affected individuals and lines for partnerships and descent. Begin with phenotypes that fix genotype. Unaffected parents with an affected child support recessive inheritance. X-linked recessive conditions often affect more males, lack father-to-son transmission, and require an affected daughter's father to be affected.

Conditional probability changes the denominator. In $Aa\times Aa$, an unaffected child's possible weighted outcomes are AA, Aa and Aa; aa has been excluded. The probability that this unaffected child is a carrier is therefore $2/3$, not $1/2$. Reduced penetrance, new mutation and small family size can obscure simple pedigree patterns.

Cystic fibrosis, beta thalassemia, sickle-cell disease and phenylketonuria are commonly modeled as autosomal recessive; Huntington disease is autosomal dominant. The labels describe inheritance, not molecular mechanism. Variants may change amino-acid sequence, expression, folding or protein abundance.

PKU usually results from recessive variants affecting phenylalanine hydroxylase, reducing conversion of phenylalanine toward tyrosine. Accumulated phenylalanine can harm the developing nervous system. Newborn screening followed by a controlled low-phenylalanine diet can prevent much damage: treatment changes phenotype without repairing the inherited alleles.

Genetic testing may disclose disease, carrier status or future risk and may imply information about relatives. Ethical interpretation addresses consent, privacy, possible discrimination, uncertain penetrance, treatment options and the right to know or not know. A genotype can constrain probability without deciding a person's future.

For a carrier mother $X^HX^h$ and unaffected father $X^HY$, daughters receive the father's normal X and cannot be affected under the simple model, although half are expected to be carriers. Sons receive Y from the father, so half are expected to inherit $X^h$ from the mother and be affected. Saying there is a '50% risk' without naming the denominator is ambiguous: it is 50% among sons but 25% among all children when sex is equally likely.

Pedigree evidence must be tested against every connection. A dominant autosomal trait generally appears in successive generations, affects both sexes and allows father-to-son transmission. A recessive autosomal trait can skip generations and cluster among siblings. X-linked dominant inheritance has no father-to-son transmission but sends an affected father's allele to every daughter. These are probabilistic patterns; a small family may omit an expected class by chance.

Cystic fibrosis variants reduce chloride transport, changing water movement and producing unusually viscous secretions. Beta thalassemia variants reduce synthesis of beta-globin chains. The sickle-cell allele changes one amino acid in beta-globin and promotes hemoglobin polymerization under low oxygen. These examples connect a DNA change to protein behavior, cell physiology and organism symptoms; inheritance notation alone is not a mechanistic explanation.

Huntington disease illustrates a different challenge. An expanded CAG repeat in one HTT allele produces a dominant late-onset condition with high but age-dependent penetrance. A person may reproduce before symptoms develop, so the allele can remain in a population despite serious effects. Predictive testing can clarify genotype while creating psychological and family consequences when prevention is limited.

Close relatives are more likely to inherit the same rare allele from a shared ancestor. If two related carriers reproduce, the offspring probabilities for each pregnancy are still generated by the same segregation rules, but the prior probability that both parents carry the allele is elevated. Genetic counseling should communicate this increased risk without implying that related parents will necessarily have an affected child or that unrelated parents have zero risk.

Screening and diagnosis are also distinct. Screening is applied broadly to identify people with elevated probability or a biochemical signal and may produce false positives or false negatives. Diagnostic testing asks a more specific question, often using sequence or functional evidence. Newborn PKU screening is valuable because the harmful phenotype can be prevented most effectively before neurological damage becomes evident.