Duchenne muscular dystrophy is an X-linked recessive disorder. The pedigree pattern described is characteristic: the disorder affects males almost exclusively and is transmitted through unaffected female carriers (the mother). The presence of an affected maternal uncle is a strong clue. There is no male-to-male transmission, as fathers pass their Y chromosome to their sons.

Cystic fibrosis is a classic example of an autosomal recessive (AR) disorder. The key features in the vignette are that the parents are unaffected but have an affected child, indicating that the condition skips a generation. This pattern occurs when both parents are heterozygous carriers. AR disorders affect males and females with equal frequency. The parents being from the same village increases the likelihood they share a common ancestor and thus carry the same recessive allele (consanguinity).

When two heterozygous carriers (e.g., Aa) for an autosomal recessive condition have a child, the possible genotypes for the offspring are AA, Aa, aA, and aa, each with a 25% probability. The child will be affected only with the genotype 'aa' (25% chance). The child will be phenotypically normal with genotypes 'AA' (homozygous normal) or 'Aa'/'aA' (heterozygous carrier). The total probability of being phenotypically normal is the sum of these possibilities: 25% (AA) + 50% (Aa/aA) = 75%.

This problem requires two steps. First, determine the probability that the child inherits the disease-causing allele. Since the mother is affected by an autosomal dominant disorder, there is a 50% (0.5) chance she will pass the allele to her child. Second, account for the incomplete penetrance. If the child inherits the allele, there is a 70% (0.7) chance they will express the phenotype. The overall probability is the product of these two probabilities: 0.5 * 0.7 = 0.35, or 35%.

Variable expressivity describes the phenomenon where individuals with the same genotype exhibit different phenotypes in terms of severity or combination of symptoms. In this family, all three affected members have the genotype for neurofibromatosis type 1, but the type and severity of their symptoms vary widely. Incomplete penetrance would mean an individual with the genotype shows no signs of the disease at all. Locus heterogeneity means mutations in different genes cause the same disease. Germline mosaicism explains how unaffected parents can have an affected child with a dominant disorder.

Pleiotropy is the genetic principle where a single gene influences multiple, often seemingly unrelated, phenotypic traits. In Marfan syndrome, the defective fibrillin-1 protein affects connective tissue throughout the body, leading to the diverse skeletal, ocular, and cardiovascular manifestations described. Variable expressivity refers to differences in the severity of a phenotype, whereas pleiotropy refers to the variety of different phenotypes.

Germline mosaicism occurs when a mutation arises in a precursor germ cell of one parent. This parent is phenotypically normal because the mutation is not in their somatic cells, but a proportion of their gametes (sperm or eggs) carries the mutation. This explains how unaffected parents can have multiple children with a new autosomal dominant disorder. A single de novo mutation is less likely to explain two affected children. Incomplete penetrance would imply the parents have the gene but are asymptomatic, which is contradicted by the negative genetic test on their somatic cells.

The question specifically asks for the probability that a male child will be affected. A male child inherits his X chromosome from his mother and his Y chromosome from his father. Since the mother is a carrier (X'X), she has one normal X and one affected X' chromosome. There is a 50% chance she will pass on the affected X' chromosome to her son. Therefore, the probability that any given son will have hemophilia A is 50%. The 25% probability would apply to the risk for any child (male or female) to be an affected male.

The woman is affected with an autosomal recessive disorder, so her genotype is homozygous recessive (aa). Her partner is not a carrier, so his genotype is homozygous dominant (AA). All of their offspring will have the genotype Aa, making them obligate heterozygous carriers. Since PKU is a recessive disorder, heterozygous individuals are phenotypically normal. Therefore, there is a 0% chance their child will have the PKU phenotype. Note: The child could be affected by maternal PKU syndrome if the mother's diet is not controlled during pregnancy, but the child's genotype will not cause the disease.

The man's parents are both heterozygous carriers (Ww). The possible genotypes of their offspring are WW, Ww, wW, and ww. Since the man is phenotypically normal, he cannot have the 'ww' genotype (which corresponds to the disease). Therefore, we must consider only the three possible non-affected genotypes: WW (homozygous normal), Ww (carrier), and wW (carrier). Among these three possibilities, two result in him being a carrier. Thus, his probability of being a carrier is 2/3.