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.

1. In this method of sex determination, the homogametic male possesses two 'X' chromosomes as in certain insects and certain vertebrates like fishes, reptiles and birds producing a single type of gamete.
2. The female sex consists of a single 'Y' chromosome. Thus the females are heterogametic and produce two types of eggs.
3. To avoid confusion with the XX-XO and XX-XY trues of sex determination, the alphabets 'Z' and 'W 'are used here instead of X and Y respectively.
4. Heterogametic females are of two types, ZO-ZZ type and ZW-ZZ type.

1. The X chromosome which is common to male and female carries a number of genes. Such genes are called sex linked genes and the characters controlled by them are termed as sex-linked characters. The inheritance of sex linked genes or traits is known as sex linked inheritance.
2. Sex linked inherited traits are more common in males than females, because males are hemizygous (XY) and therefore express that traits when they inherit one mutant allele.
3. Any male receiving X linked recessive allele from his mother will express the trait as his y  chromosome has no corresponding allele.
4. If a recessive X linked gene causes disorder then more males than females have the disorder because the disorder is caused by a single allele in males. Their y chromosome does not carry any corresponding allele. So that sex linked recessive characters are mole in moles.

It is characterized by multiple and severe body malformations as well as profound mental deficiency. Small head with small eyes, cleft palate, malformation of the brain and internal organs are some of the symptoms of this syndrome.

(i) Sexual separation of the defectives.
(ii) Sterilization of the defectives
(iii) Control of immigration and
(iv) Regulation of marriages

Symptoms:
1. Severe mental retardation (Phenylpyruvic acid which damages the brain).
2. Light pigmentation (reduction of color) of skin and hair.

The genes present in the differential region of Y chromosome are called Y- linked or holandric genes. The Y linked genes have no corresponding allele on X chromosome. The Y linked genes inherit along with Y chromosome and they are phenotypically express only in the male sex.

In human beings a dominant X - linked gene is necessary for the formation of colour sensitive cells, the cones. The recessive form of this gene is incapable of producing colour sensitive cone cells. Homozygous recessive females (X^cX^c) and hemizygous recessive males (X^cY) are unable to distinguish red and green colour. The inheritance of colour blindness can be studied in the following two types of marriages.
(i) Marriage between colour blind man and normal visioned woman:
A marriage between a colour blind man and a normal visioned woman will produce normal visioned male and female individuals in Fl generation but the females are carriers. The marriage between a Fl normal visioned carrier woman and a normal visioned male will produce one normal visioned female, one carrier female, one normal visioned male and one colour blind male. Colour blind trait is inherited from the male parent to his grandson through carrier daughter, which is an example of criss-cross pattern of inheritance.

(ii) Marriage between normal visioned man and colour blind woman:
a) If a colour blind woman (X^CX^C) marries a normal visioned male (X^Cy), all F₁ sons will be colourblind and daughters will be normal visioned but are carriers.
b) Marriage between F₁ carrier female with a colour blind male will produce normal visioned carrier daughter, colourblind daughter, normal visioned son and a colourblind son in the F₂ generation.

Application of the laws of genetics for the improvement of human race is called eugenics.
Two methods of Eugenics are:
(i) Constructive method or Positive eugenics
(ii) Restrictive method or Negative eugenics
(i) Positive eugenics: Positive eugenics attempts to increase consistently better or desirable germplasm and to preserve the best germplasm of the society. The desirable traits can be increased by adopting the following measures:
(a) Early marriage of those having desirable traits
(b) Subsiding the fit and establishing sperm and egg banks of precious germ plasm.
(c) Educating the basic principles of genetics and eugenics.
(d) Improvement of environmental conditions.
(e) Promotion of genetic research.
(ii) Negative eugenics: Negative eugenics attempts to eliminate the defective germplasm of the society by adopting the following measures:
(a) Sexual separation of the defectives
(b) Sterilization of the defectives
(c Control of immigration and
(d) Regulation of marriages

(i) Certain characters are controlled by nonnuclear genomes found in chloroplast, mitochondria, infective agents and plasmids. These characters do not reveal Mendelian pattern of inheritance.
(ii) The inheritance of the extra chromosomal genes are found to exhibit maternal influence.
(iii) Although both male and female parents contribute equally to the zygote in terms of chromosomal genes, the female parent usually contributes the zygote's initial cytoplasm and organelles, since the sperms contain very little cytoplasm. If there are hereditary units in the cytoplasm, these will be transmitted to the offsprings through the egg, so the offsprings exhibit maternal effect.
(iv) The cytoplasmic extranuclear genes have a characteristic pattern of inheritance which do not resemble the genes of nuclear chromosomes and is known as extra chromosomal or extra nuclear or cytoplasmic inheritance and exhibit maternal influence.
(v) In extra nuclear inheritance, male and female parents contribute equally their nuclear genes to the progeny but do not make equal contribution of' extra chromosomal genes hence, the crosses can yield different (or) non Mendelian results. Extra chromosomal inheritance in Limnaea peregra.
(vi) Limnaea peregra is a freshwater snail. The shell of these animals are spirally coiled. The coiling of the shell is clockwise (dextral) or anticlockwise (sinistral).
(vii) Both type of coilings are produced by two. different types of genetically controlled cleavages namely, dextral cleavage and sinistral cleavage.
(viii) In Limnaea, dextral coiling is normal and Sinistral coiling is a mutant character.
(ix) Direction of coiling is determined by a pair of nuclear genes, D( dextral) and d (sinistral). The gene for dextral (D) being dominant over sinistral coiling (d). In Fig. a dextral snail provides the eggs and a sinistral snail provides the sperm. The offsprings are all dextral (Dd), in the F1 generation.
(x) When the F1 heterozygous dextral individual (Dd) were self crossed the F2 generation showed dextral coiling with genotype of IDD, 2Dd and l dd.
(xi) When a reciprocal cross is made the F1 individuals have Dd genotype but are coiled sinistrally, as in the female parent. In both the crosses the Flare phenotypically similar to the female parent, though the offsprings in both crosses have the same genotype Dd. This is because the genotype of the maternal parent determines the phenotype of the offspring.
(xii) When the F1 sinistral individuals were self crossed, the shell coiling in the F2 generation, were all dextral. This is because the genes do not segregate in the F2 generation. Only in the F3 generation segregation occurs in the ratio of 3 dextral: 1 sinistral.
(xiii) Reasons: The type of cleavage depends on the organization of the egg which is established before the maturation division of the oocyte nucleus and by the influence of the maternal genotype.
(xiv) The direction of coiling of the shell depends upon the orientation of the mitotic spindle during the first cleavage.
(xv) Obviously, maternal control affects only one generation. In each generation the coiling is dependent on the maternal genotype.

(i) Fisher and Race hypothesis: 
Rh factor involves three different pairs of alleles located on three different closely linked loci on the chromosome pair. This system is more commonly in use today, and uses the 'Cde' nomenclature.

2. In the above Figure, three pairs of Rh alleles (Cc, Dd and Ee) occur at 3 different loci on homologous chromosome pair- 1.
3. The possible genotypes will be one C or c, one D or d, one E or e from each chromosome
For e.g. CDE/cde; CdE/cDe; cde/cde; CDe/CdE etc.
4. All genotypes carrying a dominant 'D' allele will produce Rh positive phenotype and double recessive genotype 'dd' will give rise to Rh negative phenotype.
(ii) Wiener Hypothesis 
1. Wiener proposed the existence of eight alleles (R¹, R², R^O, R^Z, r, r¹, r¹¹, r^y) at a single Rh locus.
2. All genotypes carrying a dominant 'R allele' (R¹, R^2, R^O, R^Z) will produce 'Rh- positive' phenotype and double recessive genotypes (rr, rr¹, rr¹¹, rr^y) will give rise to Rh^- negative phenotype.

Heterogametic Males:
In this method of sex determination the males are heterogametic producing dissimilar gametes while females are homogametic producing similar gametes.
It is of kinds XX-XO type and XX-XY type.
(i) XX-XO Type
(a) This method of sex determination is seen in bugs and some insects such as cockroaches and grasshoppers.
(b) The female with two X chromosomes are homogametic (XX) while the males with only one X chromosome are heterogametic (XO).
(c) The presence of an unpaired X chromosomes determines the male sex. The males with unpaired 'X' chromosome produce two types of sperms, one half with X chromosome and other half without X chromosome.
(d) The sex of the offspring depends upon the sperm that fertilizes the egg.
 
(ii) XX-XV type (Lygaeu Type):
This method of sex determination is seen in human beings and in Drosophila.
(a) The females are homogametic with XX chromosome, while the males are heterogametic with X and Y chromosome.
(b) Homogametic females produce only one kind of egg, each with one X chromosome, while the heterogametic males produce two kinds of sperms some with X chromosome and some with Y chromosome.
(c) The sex of the embryo depends on the fertilizing sperm. An egg fertilized by an 'X' bearing sperm produces a female, if fertilized by a 'Y' bearing sperm, a male is produced.