Sex Determination-In human being,sex is determined by sex chromosome (23^rd pairs). In female,the sex chromosomes are similar in size and shape,,they are referred to  as “X” chromosome hence female have XX in (^23rd) pairs. .Male on the other hand have contrasting (different) size and shape. One is an “X” chromosome while other is a “Y” chromosome. M.ale chromosome is therefore XY.half of the male sperms contain X chromosome and the other half is Y chromosome . Each egg of the female contain an X Chromosome. This implies they if an X- sperm fertilizes an X- egg, the offspring will be a female (XX). Otherwise if Y- sperm fertilizes an X -egg,the offspring is a male,having XY genetic constitution.

SEX-LINKED CHARACTER OR SEX LINKAGE

These are characters that associate themselves with sex chromosome. The gene rresponsible  for such characters are found on the sex chromosome  such characters are called sex-linked characters. Examples are colour blindness, haemophilia and baldness

1. Colour blindness

It is associated with sex. The sufferers are unable to distinguish between colours. All colours unable to distinguish between colour. All colours e.g red and green looks alike to the sufferers of the disease. The gene of the disease is recessive and it is associated with X-chromosome only. The Y- chromosome does not carry the gene that controls colour blindness.If gene C (capital) is present with its allele c

(small letter) the disease effect is not felt.

X^CX^C  –  Normal female

X^CY    _  Normal male
X^c X^c      _  Carrier female

X^c Y     _  Sufferer male(colour blind)

2. Sickle cell Anemia

This is an inherited and genetically linked disorder-The red blood cells of the sufferer of the disease change shape of round look to sickle or cresent form by a lowering in concentration of oxygen due  to insufficient flow of the blood to  the part of the body. This results in sickle crisis with pain in bones and joints. Sickle cell disease /trait is caused by recessive gene connoted by letter ‘S’ and normal situation connected by A. The trait is carried by hemoglobin.

Parent

Both the normal and carrier do not feel the effect of sicked cell.

Some heretic disorder

1. Sickle cell Anaemia

3. Hemophilia

– is a disease in which the correct protein (fibrin) for clothing are not produced. The result is a serious bleeding even after trivial bruises who pass it to possible sufferers (males) don’t always suffer from it in most cases.

iii.        Colour blindness

1. Rhesus factor

This is a type of agglutinogen found in red blood cells. It was first discovered in monkeys when present it is termed rhesus positive (RH⁺)                  .

Lack of it is referred to as rhesus negative (RH^–).

INCOMPLETE DOMINANCE:

Is a condition where neither of the contrasting characters are dominant over each other. This result in a mixture or blend of the two characters. This phenomena is known as incomplete dominance, blending inheritance or co-dominance.

Examples: Andalusian fowl: If a white feather Andalusian fowl (WW) is crossed with a black fowl (BB). The offspring are BW (blue) parents WW  x  BB

CO-DOMINANCE

– ABO blood group in man exhibit co-dominance. The blood group is determined by a single  pair of genes located or a homologous pair of  chromo ones. There are three different alleles., T^A  for antigen  A,   T^B for  antigen B , t^o for  no antigen B, for no  antigen .T^A and T^B  are  dominant while t^o  is recessive.