Genetic Disorder

- Dominant: an allele that expresses itself in the presence of another allele; in a dominant trait, one dominant gene is enough to show the trait.
- Recessive: an allele that expresses itself only when both copies are recessive; in a recessive trait, two copies are needed to show the disorder.
- Homozygous: having two identical alleles for a gene, such as $SS$ or $ss$.
- Heterozygous: having two different alleles for a gene, such as $Ss$.
- Phenotype: the observable characteristics or traits of an organism.
- Genotype: the genetic makeup or allele combination of an organism.

Origin: Down syndrome is a genetic condition caused by the presence of an extra chromosome 21. This is trisomy 21. The karyotype is 47, XX, +21 in females and 47, XY, +21 in males. It usually happens because of non-disjunction during cell division. The chance increases with maternal age.

Symptoms: Common features include flat face, slanting eyes, small mouth, protruding tongue, flattened nose, short neck, short arms and legs, single deep crease across the palm, low IQ, stunted growth, muscular hypotonia, underdeveloped gonads, and sometimes breathing, heart, or hearing problems.

Treatment: There is no single standard cure. Treatment is based on the person’s needs. Early help such as speech therapy, physiotherapy, and nutritional supplements can be useful. With better diagnosis and treatment, life expectancy has increased greatly.

Origin: Klinefelter syndrome affects males and has the genotype 47, XXY. It is caused when the X chromosome fails to separate during meiosis, so an XX ovum fertilized by a Y sperm produces an XXY zygote.

Symptoms: Affected children are usually tall, have reduced facial and body hair, small testes, enlarged breasts, and a coarse voice.

Diagnosis: It is often diagnosed by the Barr body test of a buccal smear. A normal male has no Barr body, but in Klinefelter syndrome one Barr body is seen because of the extra X chromosome.

Treatment: People are often treated with testosterone to develop masculine features. Psychological counselling is also needed to help control depression and aggression.