CONDITIONS - CHROMOSOMAL ABNORMALITIES
KLEINFELTER SYNDROME
Male with an extra X chromosome so: 47,XXY
Sex chromosome aneuploidy.
Do not produce enough testosterone before birth and during puberty: small testicles, tall body with long limbs, failure to develop secondary sexual characteristics. May have breast tissue development, may have delay in reaching motor milestones, speech delay, learning difficulties.
As an adult have a low sex drive, less muscular, osteoporotic tendencies and infertility.
They may also have mosaic Kleinfelter syndrome 1/10, so may be mildly affected depending on the extent.
DI GEORGE SYNDROME
Chromosome 22q11.2 deletion.
1/4,000 to 1/6,000 live births, but it may be higher.
Small head circumference, epicanthic folds, mildly droopy eyes, broad nasal bridge, cleft lip, open mouth appearance, prominent cup shaped ears, learning difficulties.
DOWN SYNDROME
Trisomy 21
The most common aneuploidy (in addition to Trisomy 13, Trisomy 18 and aneuploidies involving sex chromosomes).
1/700 foetuses are affected - but early miscarriage and termination of pregnancy TOP reduce the prevalence of this condition among neonates.
about 50% born with congenital heart disease, the most common being atrio-ventricular septal defect AVSD, risk of hypothyroidism, coeliac disease and deafness increased. More likely to develop Alzheimers disease as adults.
95% of cases are due to non-disjunction in meiosis, but 4% due to unbalanced translocation involving chromosome 21, 1% are mosaic cases.
Risk of non-disjunction increases with age, by forty it is approximately 1%.
TURNER SYNDROME Short stature, Coarctation of aorta, delayed puberty.
Usually do not have learning difficulties.
Abnormally developed ovaries, they fail to enter puberty and require hormone treatment.
COPY NUMBER VARIANTS
Are not always associated with a phenotype, it depends on the area of the genome involved. IT may be associated with learning difficulties, and autism spectrum disorder in some, and no effects in others. This is incomplete penetrance. The severity may also be different, this is variable expressions.
GERMLINE MOSAICISM
May be the explanation for cases where there is a higher proportion of abnormalities in the offspring of individuals who are not themselves affected by the same abnormalities.