A patient with Down’s syndrome is likely to be subfertile rather than infertile.Down’s syndrome:The clinical features of Down’s syndrome include:- Face: upslanting palpebral fissures, epicanthic folds, Brushfield spots in iris, protruding tongue, small ears, and round/flat face- Flat occiput- Single palmar crease, pronounced ‘sandal gap’ in the first interdigital space of the feet.- Hypotonia- Congenital heart defects (40-50%)- Duodenal atresia- Hirschsprung’s diseaseThe cardiac complications in these patients include:- Endocardial cushion defect (40%)- Ventricular septal defect (30%)- Secundum atrial septal defect (10%)- Tetralogy of Fallot (5%)- Isolated patent ductus arteriosus (5%)The complications that occur later in the life of the patient include:- Subfertility: Males are almost always infertile due to impaired spermatogenesis. Females, however, are usually subfertile and have an increased incidence of problems with pregnancy and labour.- Learning difficulties- Short stature- Repeated respiratory infections (+hearing impairment from glue ear)- Acute lymphoblastic leukaemia- Hypothyroidism- Alzheimer’s disease- Atlantoaxial instability

Penetrance is defined as the percentage of individuals having a particular mutation or genotype who exhibit clinical signs or phenotype of the associated disorder or genotype.Gaucher disease is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Hereditary diseases follow specific inheritance patterns according to the type of gene involved. Mutations in the genes which are on the sex chromosome, chromosome X are called X-linked diseases. X-linked recessive conditions result when both the copies of X-chromosomes are defective, which happens in the case of females since males have only one X- chromosome. Thus, these conditions are more common among males, and examples include colour blindness, haemophilia, Lesch-Nyhan syndrome, and hunter’s syndrome, etc. Hunter’s syndrome is a type of lysosomal storage disease which is also known as mucopolysaccharidosis II. Vitamin D resistant rickets is a X-linked dominant disease. Neurofibromatosis type 1 and 2 are autosomal dominant conditions while Wilson’s disease is autosomal recessive.

Based on the clinical scenario provided, the most probable diagnosis for this patient would be tuberous sclerosis which is an autosomal dominant condition.Among the options provided, only Neurofibromatosis Type 1 is inherited by an autosomal dominant pattern. Other options:- Beckwith-Wiedemann syndrome is an example of an imprinting disorder and results from abnormalities of inheritance or methylation of chromosome 11p15.- Cystic fibrosis is an autosomal recessive condition.- Duchenne Muscular Dystrophy is an example of an X-linked recessive condition.- Hypophosphataemic (Vitamin D resistant) rickets is an X-linked dominant condition.

Cystic fibrosis is a progressive, genetic disease. A defective gene causes a thick, sticky build-up of mucus in the lungs, pancreas, and other organs. In the lungs, the mucus clogs the airways and traps bacteria leading to infections, extensive lung damage, and eventually, respiratory failure. In the pancreas, the mucus prevents the release of digestive enzymes that allow the body to break down food and absorb vital nutrients. Since the digestive enzymes are not being made, the food is not completely digested making them bulky, smelly, and hard to flush away. Finger clubbing is a prominent feature of cystic fibrosis on most patients.

The property described in the question is heterogeneity.Heterogeneity is when a single phenotype or genetic disorder can be caused by multiple numbers of genetic mutations.Other options:- Pleiotropy occurs when a single gene influences multiple phenotypic traits. The underlying mechanism is that the gene codes for a product that is used by various cells. A classic example of pleiotropy is the human disease PKU (phenylketonuria).- The penetrance of a disease-causing mutation is the proportion of individuals with the mutation who exhibit clinical symptoms. For example, the gene responsible for a particular autosomal dominant disorder has 70% penetrance, then 70% of those with the mutation will develop the disease, while 30% will not.

Haemophilia is a X-linked recessive disorder. In X-linked recessive inheritance only males are affected. An exception to this seen in examinations are patients with Turner’s syndrome, who are affected due to only having one X chromosome. X-linked recessive disorders are transmitted by heterozygote females (carriers) and male-to-male transmission is not seen. Affected males can only have unaffected sons and carrier daughters.

Cystic fibrosis has an autosomal recessive pattern of inheritance, meaning that a person might be a carrier of the disease without developing it. If the unaffected partner is a carrier, then there is a 50% chance of inheritance and another 50% chance of having a child who is a carrier. However, if the partner is not a carrier, the offspring will not develop the disease but the possibility of being a carrier raises up to 100%.

Point mutations are the type of mutations in which only a single nucleotide of the DNA is either deleted, substituted or a new single nucleotide is inserted into the DNA, causing alterations in the original normal DNA sequencing. The examples of point mutations include hemochromatosis, sickle cell disease, and Tay-Sach’s disease. Huntington’s disease is a trinucleotide repeat disorder. Down’s syndrome is characterized by an extra copy of chromosome 21, while Klinefelter syndrome is marked by an extra X chromosome. Fragile X syndrome is also a trinucleotide repeat disorder.

Types of DNA mutations and their impactpoint mutation:Substitution: One base is incorrectly added during replication and replaces the pair in the corresponding position on the complementary strand as in Sickle cell anaemiaInsertion: One or more extra nucleotides are inserted into replicating DNA, often resulting in a frameshift as in one form of beta-thalassemiaDeletion: One or more nucleotides is skipped during replication or otherwise excised, often resulting in a frameshift as in Cystic fibrosis.Chromosomal mutation:Inversion: One region of a chromosome is flipped and reinserted as in Opitz-Kaveggia syndromeDeletion: A region of a chromosome is lost, resulting in the absence of all the genes in that area as in Cri du chat syndromeDuplication: A region of a chromosome is repeated, resulting in an increase in dosage from the genes in that region as in some cancers and Charcot-Marie toothTranslocation: A region from one chromosome is aberrantly attached to another chromosome as in One form of leukaemiaCopy number variation:Gene amplification: The number of tandem copies of a locus is increased as in Some breast cancersExpanding trinucleotide repeat: The normal number of repeated trinucleotide sequences is expanded as in Fragile X syndrome, Huntington’s disease