Chromosomal disorders

Reference: NCERT Class XII, page 76.

Down's syndrome is caused by non-disjunction of the $21^{st}$ chromosome (an autosome).

An abnormal ovum carrying an extra X chromosome (22 + XX), fertilised by a normal X-bearing sperm, gives a 44 + XXX baby.
So the cause is formation of abnormal ova in the mother.

Turner's syndrome is caused by the absence of one X chromosome: 45 chromosomes with XO. Such females are sterile, with rudimentary ovaries.

Klinefelter's syndrome is caused by the presence of an additional X chromosome in a male, giving the karyotype 47, XXY, i.e. 44 autosomes + XXY.
Such a person has overall masculine development, but feminine features such as gynaecomastia (development of breasts) are also expressed, and the individual is sterile.
So Klinefelter's syndrome - 44 autosomes + XXY is the correct match.
Erythroblastosis foetalis is due to Rh incompatibility between an Rh negative mother and an Rh positive foetus; the Rh gene is autosomal, not X-linked.
Down syndrome is trisomy of chromosome 21 (45 autosomes + XX or XY). The condition 44 autosomes + XO is Turner's syndrome.
Colour-blindness is a recessive X-linked trait, not Y-linked.

Cri-du-chat (cat-cry) syndrome is a chromosomal disorder caused by a structural change, a deletion, and not by a change in chromosome number.
It is caused by the loss (deletion) of a part of the short arm (p arm) of chromosome 5; it is written as 5p-.
The affected baby has a characteristic high-pitched cry like the mewing of a cat, a small head (microcephaly), widely spaced eyes and severe mental retardation.
Fertilisation of an XX egg by a Y-bearing sperm gives XXY, which is Klinefelter's syndrome.
Trisomy of the 21st chromosome is Down's syndrome.
The deletion is in the short arm, not the long arm, of chromosome 5.
So cri-du-chat syndrome is caused by the loss of half of the short arm of chromosome 5.

Down's syndrome is caused by the presence of an additional copy of chromosome 21 (trisomy 21), so the total number of chromosomes becomes 47.
It results from non-disjunction of chromosome 21 during gamete formation, and its frequency increases with the age of the mother.
The affected individual is short with a small round head, a furrowed tongue and a partially open mouth, a characteristic palm crease, and retarded physical, psychomotor and mental development.
Turner's syndrome is 45, XO; superfemaleness is XXX (an extra X chromosome, not chromosome 21); triploidy is the presence of three complete sets of chromosomes (69).
So the woman is characterised by Down syndrome.

Trisomy of chromosome 21 (47 chromosomes) causes Down's syndrome, earlier called mongolism.
Features include short stature, a small round head, a furrowed tongue, a partially open mouth, a broad palm with a single crease, and mental retardation.
Turner's syndrome is 45, XO and Klinefelter's syndrome is 47, XXY.
So it is called Down's syndrome.