Affected individuals lack the tyrosinase enzyme which converts DOPA to melanin pigment responsible for skin colour. It is caused due to autosomal recessive genes

(i) Alteration or mutation in a single gene causes Mendelian disorders. These disorders are transmitted to the offsprings on the same line as the Mendelian pattern of inheritance.
(ii) Some examples for Mendelian disorders are Thalassemia, albinism, phenylketonuria, etc. These disorders may be dominant or recessive and autosomal or sex linked.

(i) Pedigree is a "family tree", drawn with standard genetic symbols, showing the inheritance pathway for specific phenotypic characters.
(ii) Pedigree analysis is the study of traits as they have appeared in a given family line for several past generations.

Karyotyping is a technique through which a complete set of chromosomes is separated from a cell and the chromosomes are arranged in pairs. An idiogram refers to a diagrammatic representation of chromosomes.

i) The genes present in the non-homologous region of the Y chromosome are called Y - linked genes or holandric genes.
ii) The Y-linked genes have no corresponding allele in X - chromosome.
iii) These genes are transmitted directly from father to son, because males inherit the Y chromosome from the father.
Iv) Eg. Gene for Hypertrichosis (excessive development of hairs on pinna of the ear).

(i) A dominant X - linked gene is necessary for the formation of colour sensitive cells, the cones. which helps to distinguish colours.
(ii) A recessive form of the gene is incapable of producing the cones. Hence the affected person is unable to distinguish red and green colour.

(i) It is a disease caused by a recessive X -linked gene.
(ii) If a normal female marries a colour blind male or vice versa, the gene is carried in the X - chromosome of the female (female has 2 X Chromosomes).
(iii) If both the chromosomes carry the receive gene she becomes colour blind.
(iv) If one X chromosome caries the recessive gene she may not suffer from the disease but will be a carrier carrying the gene to the next generation.
(v) Haemophilia follows the characteristics criss - cross pattern of inheritance.
(vi) The females are carriers of the disease and world transmit the disease to 50% of their son's even if a male parent is normal.

(i) In Drosophila the sex is determined by the balance between the genes'for femaleness located on the 'X' chromosome and those for maleness located on the 'auto somes' (A).
(ii) Hence the sex of an individual is determined by the ratio of its X chromosome to that of the autosome A sets. This ratio is termed as sex index and is expressed as
Sex Index \(=\frac{Number\ of\ X\ chromosomes(X)}{Number\ of\ X\ automosome}\)
change in this ratio leads to a changed sex phenotype Eg: Drosophila

(i) SRY stands for sex determining region Y.
(ii) It is a gene found in the euchromatin regions of the Non-combining region of Y chromosome.
(iii) It codes for testes determining factor (TDF) present in testes of Males. This gene does not occur in X chromosome.

(i) If the mother is Rh negative and foetus is Rh positive, anti D antibodies should be administered to the mother at 28th and 34th week of gestation as a prophylactic measure.
(ii) If the Rh negative mother delivers Rh positive child then anti D antibodies should be administered to the mother soon after delivery.
(iii) This develops passive immunity and prevents the formation of anti D antibodies in the mothers blood by destroying the Rh foetal RBC before the mother's immune system is sensitized.
(iv) This has to be done whenever the woman attains pregnancy.

In ABO blood group, each allele (I^A and I^B) produces a transferase enzyme. I^A allele produces N-acetyl galactose transferase and can add N-acetyl galactosamine (NAG) and Ballele encodes for the enzyme galactose transferase that adds galactose to the precursor (i.e. H substances) In the case of I^o/I^o allele no terminal transferase enzyme is produced and therefore called "null" allele arid hence cannot add NAG or galactose to the precursor.

People with AB blood group have both 'A' antigen and B antigen in their RBCs. Therefore they can receive blood from 'A' group, 'B' group or 'O' group individuals their blood will not produce antibodies against any of them. Therefore people with 'AB' blood group are called as universal recipients.

People with 'O' blood group lack both 'A' and 'B' antigens. Therefore when 'O' .blood is donated to another individual, his blood will not produce any antibodies against it. Therefore people with 'O' blood group ate called as universal donors.