D) In Mendelian patterns

Explanation: Single gene disorders typically follow Mendelian patterns of inheritance and can be either dominant or recessive, autosomal or sex-linked.

C) 15%

Explanation: Around 15% of Turner syndrome cases are associated with isochromosome X, which is composed of two copies of the long arm (i(Xq)).

B) Maternal age

Explanation: The risk of having a child with Down syndrome increases with maternal age, with a live-birth risk of under 1 in 1000 in a 25-year-old woman, rising to 1 in 100 at a maternal age of 40.

C) Translocation

Explanation: The 't' in the notation 't(2;3)(p21;q29)' represents a translocation, indicating an exchange of genetic material between chromosome 2 at region p21 and chromosome 3 at region q29.

B) A condition where an individual has genetically distinct cell lines originating from different zygotes

Explanation: Chimerism is defined as a condition in which an individual has two or more genetically distinct cell lines originating from different zygotes.

B) Normal development unless a gene is disrupted

Explanation: Balanced translocations usually result in normal development unless the break occurs within a gene or separates a gene from its controlling elements, which can lead to an observable phenotype.

B) Dominant

Explanation: Gain-of-function variants usually lead to a new or enhanced function of the protein and are almost always dominant.

B) 50%

Explanation: In autosomal dominant disorders, an affected individual has a 50% chance of passing the mutant allele to their offspring.

D) Aneuploidy

Explanation: Aneuploidy is a numerical chromosomal abnormality, not a structural one. Structural abnormalities include translocations, deletions, duplications, and inversions.

C) Nondisjunction

Explanation: Nondisjunction is the process where homologous chromosomes fail to separate during meiosis, resulting in a germ cell containing 24 chromosomes rather than the normal 23.

B) A translocation involving acrocentric chromosomes

Explanation: Robertsonian translocations are a specific type of translocation involving only the acrocentric chromosomes (13, 14, 15, 21, and 22).

B) Introduction of a premature stop codon

Explanation: Frameshift mutations alter the reading frame of the gene, often leading to the introduction of a premature stop codon and resulting in a truncated, non-functional protein.

B) 46

Explanation: A normal diploid human cell contains 46 chromosomes, which includes 22 pairs of autosomes and 1 pair of sex chromosomes.

B) Trisomy 21

Explanation: Trisomy 21, also known as Down syndrome, is the most common viable trisomy and affects around 1 in every 650 live births in the absence of prenatal screening.

C) X chromosome inactivation

Explanation: In normal female cells (46,XX), one randomly selected complete copy of the X chromosome is switched off in a process known as X-inactivation, ensuring that both males and females express the same number of X-encoded genes.

B) 22q11.2 deletion

Explanation: DiGeorge syndrome is associated with a 22q11.2 deletion, which results in heart defects, palatal abnormalities, characteristic facial appearance, learning difficulties, and immune problems.

B) Deletion or inactivation of one gene copy

Explanation: Haploinsufficiency occurs when one copy of a gene is deleted or inactivated, and the remaining copy cannot maintain normal protein function.

B) Variable expressivity

Explanation: Variable expressivity refers to the phenomenon where individuals with the same genetic condition exhibit a range of symptoms and severity.

A) Triploidy

Explanation: The most common form of polyploidy in humans is triploidy, where there are 69 chromosomes present.

B) Klinefelter syndrome

Explanation: Klinefelter syndrome is associated with a 47:XXY karyotype and is characterized by tall stature, small testes, gynecomastia, sparse facial hair, infertility, and mild reduction in IQ.

C) The presence of an abnormal number of chromosomes

Explanation: Aneuploidy is defined as an abnormal number of chromosomes and includes conditions such as trisomy and the presence of additional structurally abnormal chromosomes.

C) 1 in 2500

Explanation: Turner syndrome, resulting from monosomy X (45,X or 45,X0), has an incidence of approximately 1 in 2500 in newborn females.

A) Unequal sister chromatid exchange

Explanation: The most common cause of chromosomal duplications is unequal sister chromatid exchange due to chromosomal misalignment during meiosis.

D) Disease skips generations

Explanation: Autosomal recessive inheritance often results in the disease skipping generations, as two copies of the mutant allele are required for the phenotype to be expressed.

B) Gain of function

Explanation: Gain of function variants result in a protein acquiring a new or enhanced function, often leading to dominant inheritance patterns.

C) Trisomy 18

Explanation: Trisomy 18, also known as Edwards syndrome, is associated with severe congenital malformations and affected babies usually die within the first few months of life.

B) Total number of chromosomes and sex chromosomes

Explanation: The notation '46,XX' in cytogenetic reports indicates the total number of chromosomes (46) and the sex chromosomes (XX), which is typical for a female.

D) Substitution

Explanation: Substitution mutations involve the replacement of one nucleotide by another, which can lead to changes in the DNA sequence and potentially affect protein function.

B) 1 in 500

Explanation: It is estimated that around 1 in 500 individuals carry a reciprocal translocation.

B) They are often lethal

Explanation: Large chromosomal deletions (≥4 Mb) can be identified by conventional karyotyping and are often lethal due to the significant loss of genetic material.

A) Affected individuals in every generation

Explanation: Autosomal dominant inheritance is characterized by the presence of affected individuals in every generation, as only one copy of the mutant allele is sufficient to cause the disease.

B) Characteristic facial appearance

Explanation: Pallister-Killian mosaic syndrome is characterized by hypotonia, a characteristic facial appearance, sparse hair, and intellectual impairment due to the presence of an extra, abnormal isochromosome 12p (i(12p)).

B) Trisomy

Explanation: Trisomy is the clinical term for the presence of a single extra chromosome, which can occur in any chromosome but is most commonly seen in chromosomes 13, 18, 21, X, and Y.

C) They occur when breaks in chromosomes are misrepaired

Explanation: Structural chromosomal abnormalities are very variable and occur when breaks in chromosomes are misrepaired, leading to either balanced or unbalanced rearrangements.

B) Disruption of RNA splicing

Explanation: Variants at the intron-exon boundary can disrupt RNA splicing, leading to the exclusion of exons or inclusion of intronic sequences, which can result in an altered protein product.

B) Polyploidy

Explanation: Polyploidy refers to cells possessing an extra copy of the haploid genome, such as triploidy (69 chromosomes) or tetraploidy (92 chromosomes).

C) Patau syndrome

Explanation: Patau syndrome, or trisomy 13, is characterized by the presence of an extra chromosome 13 and is associated with severe congenital malformations.

C) 50% to 70%

Explanation: Chromosomal abnormalities are detected in an estimated 50% to 70% of miscarriages, indicating a significant role of these abnormalities in early pregnancy loss.

D) Single base pair substitutions

Explanation: Single base pair substitutions, also known as point mutations, are the most common type of variant observed in single gene disorders.

C) An exchange of material between non-homologous chromosomes

Explanation: Reciprocal translocations are the most common type of translocation and usually involve the exchange of material between non-homologous chromosomes.

B) Increased risk of recurrent miscarriages or having a child with congenital abnormalities

Explanation: Carriers of balanced translocations have an increased risk of recurrent miscarriages or having a child with congenital abnormalities due to the possibility of the fetus inheriting the unbalanced form of the translocation.

C) Chromosomes break and the resulting fragment attaches to another chromosome

Explanation: Translocations occur when chromosomes break during meiosis and the resulting fragment becomes attached to another chromosome.

B) A chromosome that has broken and fused into a circular structure

Explanation: A ring chromosome is a circular structure formed when a chromosome breaks in two places and the broken ends fuse together.

C) Mosaicism

Explanation: Mosaicism occurs when a percentage of cells will be trisomy 21, and the baby is said to be a mosaic. This condition involves the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg.

A) Turner syndrome

Explanation: Turner syndrome is associated with a 45:X karyotype and is characterized by short stature, webbing of the neck, congenital heart defects, and delayed or absent puberty.

C) Early abortion

Explanation: The absence of the X chromosome, leaving only the Y chromosome, is incompatible with embryonic development and will always result in early abortion.

C) A chromosome with either duplicated short or long arms

Explanation: Isochromosomes are unbalanced structural anomalies consisting of either two long arms or two short arms, arising when the centromere divides transversely rather than longitudinally during meiosis.

B) Loss of function

Explanation: Autosomal recessive disorders usually involve loss of function mutations, where both copies of a gene carry a variant that leads to a non-functional protein.

C) Spontaneous abortion

Explanation: Triploidy usually results in spontaneous abortion, although there have been occasional reports of live births of affected infants who present with growth restriction and congenital malformations and die shortly after birth.

C) Mosaicism

Explanation: Mosaicism occurs when an individual has two cell populations derived from a single zygote, each with a different genotype.

C) Triploidy

Explanation: Triploidy is characterized by the presence of an extra set of chromosomes, which can come from either the father (diandry) or the mother (digyny).