A person’s unique genetic blue print is contained in the 23 pairs of chromosomes found in every body cell. Conventionally, chromosomes are numbered from 1 to 22 according to their size in descending (large to small). The first pair is the largest. The 23rd pair determines a person’s sex. An XX combination indicates a female and an XY combination a male.
A chromosome is a long thread like structure composed of twisted parallel strands of DNA (deoxyribonucleic acid which carries the genetic instructions in the cell, i.e. the chemical basis of all heredity-directs the growth and development of the many unique characteristics that an individual is endowed with).
The DNA code is carried on each chromosome and is arranged in thousands of segments called genes (the basic unit of heredity that directs the development of particular characteristic). Each gene is a unit of DNA instructions pertaining to some characteristic e.g. eye, hair color, height, right handedness.
At conception, the genes carried n the 23rd chromosome contributed by one’s mother’s ovum are paired with the genes carried on the 23rd chromosome by one’s father’s sperm. Multiple gene pairs are involved in directing many complex features of development, such as brain organization, body build, and even some personality characteristics. Height, for example, is determined by FOUR different pairs of genes. Other simple traits, e.g. eye color, dimples, freckles are determined by the interaction of a single gene pair.
Prenatal development stages
At conception, chromosomes from the biological mother and father combine to form a single cell-the fertilized egg or zygote. Over the relatively brief span of nine months, that single cell develops into the estimated trillion cells that make up a new born baby. The pre natal stage has THREE distinct phases:
- The germinal period-(Zygotic period) represents the 1st two weeks of prenatal development. Zygote undergoes rapid cell division before being implanted on the mother’s uterine wall. Some of the Zygote’s cells eventually form the structures that house and protect the developing fetus and provide nourishment to the mother.
- Embryonic period– Begins with week 3 and extends through week 8. During this time of rapid growth and intensive cell differentiation, the organs and systems of the body form. Genes on the sex chromosomes and hormonal influences also trigger the initial development of the sex organs. Protectively housed in the fluid filled amniotic sac, the embryo’s lifeline is the umbilical cord through which it gets nutrients, oxygen and water and gets rid of carbon monoxide and other wastes. The umbilical cord attaches the embryo to the placenta, a disk shaped tissue on the mother’s uterine wall. The placenta prevents the mother’s blood from mingling with that of the developing embryo, acting as a filter to prevent harmful substances from reaching the embryo.
However, the placenta may not prevent all harmful substances. Harmful agents/substances that cause abnormal development or birth defects are called TERATOGENS. Generally, the greatest vulnerability to teratogens occurs during the embryonic stage, when major body systems are forming. Known teratogens include: exposure to radiation, toxic industrial chemical like mercury, diseases like rubella, syphilis, genital herpes, AIDS, drugs taken by mother like alcohol, cocaine, and heroin.
By end of embryonic stage, the embryo has grown from a cluster of a few hundred cells no bigger than the head of a pin to over an inch in length. It looks human, even though the head accounts for about half its size.
- The fetal period– Final and longest stage of development-. Body systems grow and reach maturity. By end of 3rd month, fetus can move arms, legs mouth and head. Is capable of reflexive actions e..g fanning toes if the sole of feet is stroked and squinting eyelids are touched.
4th month- mother experiences quickening- fetus moves.
5th month- fetus has distinct sleep-wake cycles and periods of activity.
6th month-brain activity is similar to new born
Final two months- weight doubles, fat increases and then growth slows and fetus body systems become more active.