Genetic Disorder

Given: A set of fundamental genetic terms to be defined.

Definitions:

(i) Dominant:
A dominant allele is one that expresses its effect (trait) in the phenotype even when only one copy is present (i.e., in heterozygous condition). It masks the effect of the recessive allele. It is conventionally represented by a capital letter (e.g., $A$).

(ii) Recessive:
A recessive allele is one whose effect is masked by the dominant allele when both are present together. It expresses itself only when present in homozygous condition (i.e., two copies of the recessive allele are present). It is represented by a lowercase letter (e.g., $a$).

(iii) Homozygous:
An organism is said to be homozygous for a particular gene locus when it carries two identical alleles at that locus on homologous chromosomes. It can be homozygous dominant ($AA$) or homozygous recessive ($aa$). Homozygous organisms breed true for that trait.

(iv) Heterozygous:
An organism is said to be heterozygous for a particular gene locus when it carries two different alleles at that locus on homologous chromosomes (e.g., $Aa$). The dominant allele is expressed in the phenotype, while the recessive allele remains hidden. Such organisms are also called carriers.

(v) Phenotype:
Phenotype refers to the observable physical, biochemical, or physiological characteristics of an organism that result from the interaction of its genotype with the environment. Examples include height, eye colour, blood group, and skin colour. Two organisms with different genotypes may have the same phenotype (e.g., $AA$ and $Aa$ both show the dominant phenotype).

(vi) Genotype:
Genotype refers to the complete genetic constitution (allelic composition) of an organism for a particular trait or for all traits. It represents the actual set of alleles present in the organism's genome. For example, for a gene with alleles $A$ and $a$, possible genotypes are $AA$, $Aa$, or $aa$. The genotype determines the potential phenotype of the organism.

Given: Down syndrome — a chromosomal disorder.

Origin (Cause):
- Down syndrome is caused by trisomy of chromosome 21, meaning the affected individual has three copies of chromosome 21 instead of the normal two, giving a total chromosome number of $2n = 47$.
- It arises due to non-disjunction during meiosis (usually meiosis I or II) in one of the parents, most commonly in the mother during oogenesis.
- Non-disjunction means the failure of homologous chromosomes (or sister chromatids) to separate properly during cell division.
- The risk of Down syndrome increases significantly with advanced maternal age (above 35 years).
- In rare cases (~4%), it can result from Robertsonian translocation, where chromosome 21 attaches to another chromosome (usually chromosome 14).
- Karyotype notation: $47, +21$ (or $47, XX, +21$ in females; $47, XY, +21$ in males).

Symptoms:
1. Physical features: Short stature, round and flat face, small nose, upward-slanting eyes with epicanthal folds (Mongoloid appearance).
2. Hands: Short, broad hands with a single palmar crease (simian crease), short fingers.
3. Intellectual disability: Mild to moderate mental retardation; IQ typically ranges from 25–75.
4. Delayed development: Delayed motor milestones (sitting, walking) and speech development.
5. Congenital heart defects: Present in approximately 40–50% of cases.
6. Hypotonia: Poor muscle tone (floppiness) at birth.
7. Increased susceptibility to respiratory infections, leukaemia, and Alzheimer's disease in later life.
8. Infertility: Males are usually infertile; females may occasionally be fertile.
9. Protruding tongue and open mouth appearance.

Treatment:
- There is no cure for Down syndrome as it is a chromosomal condition.
- Management is symptomatic and supportive:
1. Early intervention programmes: Speech therapy, physiotherapy, and occupational therapy to improve motor and communication skills.
2. Special education: Tailored educational programmes to maximise intellectual potential.
3. Surgical correction of congenital heart defects if present.
4. Regular medical monitoring for associated conditions (thyroid disorders, hearing/vision problems, infections).
5. Psychological and social support for the individual and family.
- Prenatal diagnosis (amniocentesis, chorionic villus sampling, or triple/quadruple marker test) can detect the condition before birth.

Conclusion: Down syndrome is the most common autosomal chromosomal disorder, caused by trisomy 21, and is managed through multidisciplinary supportive care.

Given: Klinefelter syndrome — a sex chromosomal disorder.

Origin (Cause):
- Klinefelter syndrome occurs in males and is caused by the presence of one or more extra X chromosomes.
- The most common karyotype is $47, XXY$ (total 47 chromosomes), though variants like $48, XXXY$ or $48, XXYY$ also exist.
- It arises due to non-disjunction of sex chromosomes during meiosis in either parent:
- If non-disjunction occurs in the mother during meiosis I or II, an egg with two X chromosomes ($XX$) is formed; when fertilised by a normal $Y$-bearing sperm, the result is $XXY$.
- If non-disjunction occurs in the father during meiosis I, a sperm with both $X$ and $Y$ ($XY$) is formed; when fertilised with a normal $X$-bearing egg, the result is $XXY$.
- Frequency: Approximately 1 in 500–1000 live male births.

Symptoms:
1. Hypogonadism: Small, firm testes (testicular atrophy) and reduced testosterone production.
2. Infertility: Azoospermia (absence of sperm) — the most consistent feature; affected males are almost always infertile.
3. Gynaecomastia: Abnormal development of breast tissue in males.
4. Tall stature with long legs and a relatively short trunk.
5. Sparse body and facial hair due to androgen deficiency.
6. Mild intellectual disability or learning difficulties (especially in language and reading) in some individuals.
7. Feminine body contour with broad hips.
8. Delayed or incomplete puberty.
9. Increased risk of osteoporosis, type 2 diabetes, and autoimmune disorders.

Treatment:
- There is no cure as it is a chromosomal condition.
- Management is hormonal and supportive:
1. Testosterone replacement therapy (TRT): Started at puberty; helps develop secondary sexual characteristics (facial hair, muscle mass, deepening of voice), improves mood, energy, and bone density.
2. Treatment of gynaecomastia: Surgical removal (mastectomy) if required.
3. Fertility treatment: Assisted reproductive techniques such as Testicular Sperm Extraction (TESE) combined with Intracytoplasmic Sperm Injection (ICSI) may help some men father children.
4. Speech therapy and educational support for learning difficulties.
5. Psychological counselling for the patient and family.
6. Regular monitoring for associated conditions (metabolic syndrome, osteoporosis).

Conclusion: Klinefelter syndrome ($47, XXY$) is the most common sex chromosomal disorder in males, caused by non-disjunction, and is primarily managed with testosterone replacement therapy.

Given: Turner syndrome — a sex chromosomal disorder.

Origin (Cause):
- Turner syndrome occurs in females and is caused by the complete or partial absence of one X chromosome.
- The most common karyotype is $45, X$ (or $45, XO$) — only one X chromosome is present, giving a total of 45 chromosomes.
- Other variants include mosaicism ($45, X / 46, XX$), isochromosome of the long arm of X ($46, X, i(Xq)$), or deletion of part of the X chromosome.
- It arises due to non-disjunction of sex chromosomes during meiosis in either parent:
- An egg or sperm lacking a sex chromosome is produced; when such a gamete fuses with a normal $X$-bearing gamete, the result is a $45, X$ individual.
- Non-disjunction is more commonly of paternal origin (loss of the paternal sex chromosome).
- Frequency: Approximately 1 in 2500 live female births.
- It is the only viable monosomy in humans.

Symptoms:
1. Short stature: The most consistent feature; affected females are significantly shorter than average.
2. Gonadal dysgenesis: Ovaries are replaced by streak gonads (fibrous tissue), leading to primary amenorrhoea (absence of menstruation) and infertility.
3. Webbed neck (pterygium colli): Excess skin on the sides of the neck.
4. Lymphoedema: Swelling of hands and feet, especially at birth.
5. Broad chest (shield chest) with widely spaced nipples.
6. Low posterior hairline and low-set ears.
7. Cubitus valgus: Increased carrying angle of the arms.
8. Congenital heart defects: Coarctation of the aorta and bicuspid aortic valve are common.
9. Renal anomalies: Horseshoe kidney.
10. Normal intelligence in most cases, but may have difficulties with spatial perception and mathematics.
11. Failure of secondary sexual development (no breast development, no pubic hair) due to oestrogen deficiency.

Treatment:
- There is no cure as it is a chromosomal condition.
- Management is hormonal and supportive:
1. Growth hormone (GH) therapy: Administered during childhood to increase final adult height.
2. Oestrogen replacement therapy (ERT): Started at the age of puberty (around 12–13 years) to induce secondary sexual characteristics (breast development, uterine growth, menstruation) and prevent osteoporosis.
3. Cyclic oestrogen-progesterone therapy: To maintain regular menstrual cycles.
4. Fertility treatment: Most women with Turner syndrome are infertile; however, egg donation with in vitro fertilisation (IVF) can allow pregnancy in some cases.
5. Surgical correction of cardiac defects if present.
6. Regular monitoring for associated conditions (hypothyroidism, hypertension, hearing loss, diabetes).
7. Psychological counselling and educational support.

Conclusion: Turner syndrome ($45, X$) is a sex chromosomal monosomy in females caused by non-disjunction, characterised by short stature, gonadal dysgenesis, and infertility, and is managed primarily with growth hormone and oestrogen replacement therapies.

Given: Structural chromosomal abnormalities — changes in the structure of chromosomes.

Introduction:
Structural chromosomal abnormalities arise due to chromosome breakage followed by incorrect rejoining of the broken segments. They can be induced by mutagens (radiation, chemicals) or occur spontaneously. The major types are described below:

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(i) Deletion:
- Definition: Loss of a segment from a chromosome.
- Types:
- *Terminal deletion:* Loss of a segment from one end of the chromosome.
- *Interstitial deletion:* Loss of an internal segment of the chromosome.
- Effect: Loss of genes present in the deleted segment; usually lethal in homozygous condition.
- Example: Cri-du-chat syndrome — deletion of the short arm of chromosome 5 ($5p^-$); affected children have a cat-like cry, intellectual disability, and microcephaly.
- Notation: Represented as $del$ followed by the chromosome number and region, e.g., $del(5)(p15)$.

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(ii) Duplication:
- Definition: A segment of a chromosome is present more than once (repeated) in the same chromosome.
- Types:
- *Tandem duplication:* The duplicated segment is adjacent to the original segment in the same orientation.
- *Reverse tandem duplication:* The duplicated segment is adjacent but in reverse orientation.
- *Displaced duplication:* The duplicated segment is located at a different position on the same or different chromosome.
- Effect: Gene dosage imbalance; generally less harmful than deletion.
- Example: Charcot-Marie-Tooth disease type 1A — duplication of a region on chromosome 17.

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(iii) Inversion:
- Definition: A segment of a chromosome is detached, rotated by 180°, and reinserted at the same position.
- Types:
- *Paracentric inversion:* The inverted segment does not include the centromere (both breaks are on the same arm).
- *Pericentric inversion:* The inverted segment includes the centromere (breaks are on both arms).
- Effect: The gene sequence is reversed; may disrupt gene function if the break occurs within a gene. Carriers may produce unbalanced gametes during meiosis.
- Example: Pericentric inversion of chromosome 9 is a common polymorphism in humans.

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(iv) Translocation:
- Definition: Transfer of a chromosomal segment from one chromosome to a non-homologous chromosome.
- Types:
- *Simple (non-reciprocal) translocation:* A segment from one chromosome is transferred to another non-homologous chromosome.
- *Reciprocal translocation:* Segments are exchanged between two non-homologous chromosomes.
- *Robertsonian translocation:* Fusion of the long arms of two acrocentric chromosomes (13, 14, 15, 21, 22) at their centromeres, with loss of the short arms. Results in 45 chromosomes in the carrier.
- Effect: May cause gene disruption or position effects; carriers of balanced translocations are usually phenotypically normal but have a high risk of producing unbalanced offspring.
- Example: Chronic Myelogenous Leukaemia (CML) — reciprocal translocation between chromosomes 9 and 22, producing the Philadelphia chromosome $t(9;22)$. Also, Robertsonian translocation involving chromosome 21 can cause familial Down syndrome.

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(v) Isochromosome:
- Definition: A chromosome in which both arms are identical (either both are long arms or both are short arms), formed by transverse division of the centromere instead of longitudinal division.
- Effect: Results in duplication of one arm and deletion of the other.
- Example: Isochromosome of the long arm of X, $i(Xq)$, seen in some cases of Turner syndrome.

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(vi) Ring Chromosome:
- Definition: Formed when both ends of a chromosome break and the broken ends join together to form a ring structure, with loss of the terminal segments.
- Effect: Loss of genes from both ends; ring chromosomes are unstable during cell division.
- Example: Ring chromosome 13 or ring chromosome X.

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Summary Table:

| Abnormality | Description | Example |
|---|---|---|
| Deletion | Loss of chromosomal segment | Cri-du-chat syndrome |
| Duplication | Repetition of a segment | Charcot-Marie-Tooth disease |
| Inversion | Segment reversed 180° | Inv(9) polymorphism |
| Translocation | Segment moved to non-homologous chromosome | Philadelphia chromosome (CML) |
| Isochromosome | Both arms identical | Turner syndrome variant |
| Ring chromosome | Ends join to form a ring | Ring chromosome X |

Conclusion: Structural chromosomal abnormalities result from chromosome breakage and abnormal rejoining. They alter the gene content or arrangement and can lead to various genetic disorders depending on the genes involved.