Symptoms:
1. Severe mental retardation
2. Defective development of the CNS.
3. Increased separation between the eyes.
4. Flattened nose, ears are malformed
5. Mouth is constantly open and the tongue protrudes.

Intersexes: 
1. Intersexes refers to the individuals having the characteristics of both female and male sexes and their sexual anatomy does not seem to fit the typical definition of male or female.
Supersexes: 
1. Supersexes ar formed as a result of abnormal combination of sex chromosomes. 
2. Example : Super males in humans human beings have 44+XYY

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.

1. The sex determining mechanism of humans is XX-XY type.
2. In humans, out of 23 pairs of chromosomes present, 22 pairs are exactly same in both males and females known as autosomes (body).
3. The 23rd chromosome is the sex chromosome. (i.e) Pair of X-chromosome (XX) is present in females and males having one X and one Y chromosome (XY).
4. During, spermatogenesis, in case of males two types of gametes are produced (x and y).
5. The 50% total sperms produced carry X-chromosomes and the rest 50% carry Y - chromosomes besides autosomes. This condition is called male diagamety.
6. The Y containing sperms and X containing sperms are called androsperms and gynosperms respectively.
7. Female produces only one type of ovum with an X-chromosome. If an ovum fertilizes with a sperm carrying X chromosome the zygote develops into a female (XX) and if an ovum is fertilized with sperm carrying Y-chromosome, zygote thus, formed will be male (YY).

8. Hence, the genetic makeup of sperm, which fertilizes the ovum determines the sex of a child there are 50% chances of having either a male or female in each progeny.
9. Thus, it is by chance phenomenon, so women should not be blamed for giving birth to a girl child

Genetic basis of blood grouping in human population
1. ABO blood grouping in human shows the phenomenon of codominance.
2. Alleles are able to express themselves independently when present together.
3. More than two alleles govern the same character.
4. Karl Landsteiner discovered two kinds of antigens called antigen 'A' and antigen 'B' on the surface of RBC's of human blood.
5. In 1902 two of Landsteiner's students Von De Castelle and Sturli discovered fourth rarest blood group 'AB' (universal recipient).
6. Bernstein in 1925 discovered that the inheritance of different blood groups in human beings is determined by a number of multiple allelle series.
7. The three autosomal alleles located on chromosome are concerned with the determination of blood group in any Person.
8. The gene controlling blood types has been labeled as 'L' (after the name of the discoverer, Landsteiner) or I (from iso -agglutination).
9. The I gene exists in three allelic forms, I^A, I^B and I^o specifies A antigen.
10. I^B allele determines B antigen and I^o allele specifies no antigen.
11. Individuals who possess these antigens in their fluids such as the saliva are called secretors
Genetic basis of the human ABO blood groups
Genotype ABO blood group phenotype Antigens present on red blood cell Antibodies present in blood plasma I^AI^A Type A A Anti -B I^AI^O Type A A Anti -B I^BI^B Type B B  Anti -A I^BI^O Type B B Anti -B I^AI^B Type AB A and B Neither Anti - A nor Anti -B I^OI^O Type O Neither A nor B Anti - A and anti -B