This is an aspect of psychology which deals with the development, growth and behavior in human beings right from the time of conception to the period of adolescence when most of the functions of the body become matured. It also deals with the factors which determine what a child will become in future.
Importance of Chromosome
Chromosomes are important because they contain the entire (or at least the vast majority of) genetic information for an organism. The organisms DNA is contained within the chromosome as a long series of nucleotides that are organized into genes. Each chromosome has thousands of gene DNA sequences that are organized into chromosomal structures through the help of different proteins. Chromosomes are organized by these proteins into very compact structures called chromatin that can fit into the nucleus of a cell. These chromosomal protein also help the DNA of the cells undergo different functions such as replication for cell division and unwinding different genes for use in protein synthesis.          
The collections of chromosomes in plants, animals and humans carry two complete sets of identification information. The duplicate set allows for cell division and reproduction. Genes on chromosomes can be defective in a number of ways and are closely related to number of genetically induced disorders, including down syndrome, Tay-sachs disease, Huntington’s disease, Kslinefelter syndrome and sickle cell anemia
Ovum: This is the female reproductive cell or gamete of animals, which is capable of developing, usually only after fertilization, into a new individual. The female reproductive cell or gamete of plant
Embryo: An embryo is an organism in the early stages of development which cannot survive on its own. A fertilized egg may be considered an embryo until around the eighth week of pregnancy. Embryo in animals typically indicate any pre-natal stage of development, including those in wombs or eggs.
Foetus: This is a prenatal human between its embryonic state and its birth. The fetal stage of development tends to be taken as beginning at the gestational age of eleven weeks.    

The role of both heredity and environment in shaping the cognitive facuity (Intelligence) of a child.

Researcher finding confirming that intelligence is a by product of our heredity revolves around finding specific genetic markers and associating in levels of parent and siblings.
Genetic markers on intelligence are believed to be found in chromosomes 4,6 and 22. The genetic marker in chromosome 6 appears in1/3 of children with high 10, and only 1/6 of children with average 1Qs.
Identical twins appear to posses more similar 1Q level than fraternal twins. Arthur Jensen in 1969 reviewed research studies about intelligence and found that the 1Q levels of identical twins are 32% more similar than the 1Q levels of fraternal twins. He also found that the 1Q levels of identical twins reared together are just 11% more similar than the 1Q levels of identical twins reared apart. Thus, Jensen concludes that the environment plays very minimal role in developing intelligence.  

Environmental influences on intelligence  

Environmental influences come in different packages-family, ethnicity socio-economics status, gender and education.
The family has a critical role in elevating 1Q scores. Communication received during the first 3 years of life is the primary predicator of Stanford Binet 1Q score at age 3. More over, simply moving or exposing children to families with better environment increase their 1Qs by 12 point.  

Stages of Human Development

i. Conception
ii. Embryo
iii. Foetus
These are known as the prenatal stages
iv. Stages of childhood development can be divide into:
Neonatal:     From both to four weeks
Infancy:        From four weeks to two years
Pre-school period:   From two to six years Middle
Childhood Period:    From six to nine years
Pre –Adolescence Period:  From nine years to puberty  
v. Sense Organ Development of the Infant
1. Smell
2. Taste
3. Organic sensitivity
4. Skin Sensitivity
5. Vision
6. Hearing
Vi. child hood Development   – physical development area of physical development are:
1. The nervous system
2. The growth of muscles
3. The growth of endocrine gland
4. Body homeostasis
5. Development of prehension
6. Development of writing and drawing
7. Development of laterality  

Principle of human development

Every species follows a pattern of development peculiar to that species.
Genetic study of children over a period of time has shown that development follows a fixed pattern and the pattern is influenced by experience. Every child has a uniave pattern of growth such patterns are however, a part of an established order of nature. The identified principle that are true to human development are referd to as the general principle of development”
1. Cephalocondal growth patterns According to this principle, development spread over the body according to this principle, development spread over the body from head to foot.this means that improvement in structures and functions come first in the head area, than in the think and leg region. The organs in the area of the head development and mature first before the organs in other areas.
The child is first able to see, hear sound and jingle before using his hands and legs in a meaningful ways.
2. Proxismodistal growth pattern this principle holds that development proceeds from the central axis of the body towards the extremites.
In the foetus, the head and trunk are well developed before the rudimentary limb buds appear. The am buds  gradually appear and develop into the had and lastly fingers before  his had and finger respectively and can use the latter as a unit before he/she can control the movements of his/her finger structures therefore proceeds function.
3. The principle of differentiation: development proceeds from simple to complex, from homogenous to heterogenous and from general to specific.
At conception, the mother egg, cell and the sperm from the father fuse to from the zygote. The zygote itself contains 23 pair of chromosomes. It starts to divide itself into 2 then 4 to 8 up to billions of cells that from a body be iit circulatory, muscular or skeletal. In both mental and motor responses general activities always process specific activities in any postnatal life, the infant can more its whole body but incapable of specific responses. The body can wave his arms in general movement before he/she is capable of any specific reaching.
4. The principle of asynchronous growth or split growth the principle holds that the changes that occur in the body proportion are due to asyrichronous or split growth. This means that the different growth; and that each reaches its own mature size at its own time. Growth in all parts of the body is however continous and concurrent e.g a child’s brain does not stop growing while his/her muscles are growing.
Asynchronous growth. Is particularly obvious when different parts of the body are compared e.g the muscles, bones lungs and genetials increase approximately 20 timees during the growth years, while the eyes and the brain which are relatively more developed at birth increase much less. The eye balls complete their growth during the first 5 years and the brain also completes its growth during the first 10 years but the heat and some other internal organs require more than 20 years completing their growth.
5. Principles of complexity of growth. Growth is an external complex process. It has different collective aspects other areas. The effect of this is that it is not easy to specify causal relationships since there can be other causes other areas i.e causes  for growth retardation  in a child may be traced to reasons other than malnutrition.        
REFRENCES:
Durojaiye, M.O.A (1976) a new introduction to Education psychology, London, evans. Eke, e & Esuman, J.K, child development in a changing cultural context, Ibadan: Heineman (in press) Stones, e, (1966) an introduction to education psychology New York spectrum books Bsewer, r.f (1976) psychology applied to teaching, Houghton Mifflin  company U.S.A Lovell k (1973) education psychology and children hodder and strongten U.K