A. Development of the zygote
The man or woman we see around us was in the beginning mere cell in the father and mother. Through sexual intercourse between the father and the mother, sperms are ejaculated into the mother’s vagina by the father and if it is the ripe time for release of ovum by the mother, fertilization can occur and the beginning of the boy or girl that will grow to a man or woman starts.
At the time of ejaculation, if a mature sperm meet with a mature ovum or ova released by the woman fertilization can occur. The woman drops one mature egg (ovum) from of her ovaries into the fallopian tube once a month, Nnodim, (2001). It is usually done during her ovulation period. Ovulation period is that time in which a woman releases a mature egg (ovum) from her ovaries into the fallopian tube each month. Fertilization is the fusion of a male sperm with the woman’s ovum. As fertilization occurs at the ovum and the product is a Zygote that journeys through the fallopian tube to the womb for implantation – is attachment of the Zygote to the uterus.
The zygote travels through the fallopian tube to the woman’s womb to be attached. As the zygote travels to the uterus cells division occurs and after about seven days it gets to the uterus.
At this stage, the zygote or fertilized egg is called a blastocyst Dacey & Travers, (1996).
As the woman receive and house the zygote, the uterus becomes spongy or porous. The fertilized egg as it travels to the womb undergoes cell division.
The first cell division is called or known as mitosis. The cell division makes the zygote to get billions of cells that soon become specialized into muscle, bone, skin, blood and nerve cells. Through the cell divison, the number of chromosomes remains the same Dacey & Traver (1996). This is done until blastocyst is formed and it is the blastocyst that is implanted into the uterine walls. While the second cell division is called melosis.
Through this cell division the number of chromosomes turns halves. Instead of each sex cell to receive 46 chromosomes; it gets 23 chromosomes. That is, in this division, 23 chromosomes go to one cell and the rest 23 chromosomes go to another cell. This is the sex cell. According to Dacey & Travers, (1996), “the male reduction division begins to occur just before puberty”. But for the female since she produces only one mature egg each month; provision for indefinitely large number of eggs is not normally, a woman releases only “300 to 400 mature ova in her life time” * Dacey and Traver, (1996).
HUMANS at Glance
In human fertilization, a released ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete)—combine to form a single 2n diploid cell called the zygote. Once the single sperm enters the oocyte, it completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the sperm in its own pronucleus. The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygote's chromosome number temporarily 4n diploid. After approximately 30 hours from the time of fertilization, fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called blastomeres.
Between the stages of fertilization and implantation, the developing human is a preimplantation conceptus. There is some dispute about whether this conceptus should no longer be referred to as an embryo, but should now be referred to as an embryo, which is terminology that traditionally has been used to refer to plant life. Some ethicist and legal scholars make the argument that it is incorrect to call the conceptus an embryo, because it will later differentiate into both intraembryonic and extraembryonic tissues, and can even split to produce multiple embryos (identical twins). Others have pointed out that so-called extraembryonic tissues are really part of the embryo's body that is no longer used after birth (much as milk teeth fall out after childhood). Further, as the process of the embryo splits to form identical twins – leaving the original tissues intact – a new embryo is generated, rendering it no different from the process of cloning an adult human. However, the National Institutes of Health has made the determination that the traditional classification of pre-implantation embryo is still correct.
After fertilization, the conceptus travels down the oviduct towards the uterus while continuing to divide mitotically without actually increasing in size, in a process called cleavage. After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the embryonic stage of development.
B. GENE
A gene is a sequence of DNA or RNA which codes for a molecule that has a function. During gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the intermediate template for a protein that performs a function. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits. These genes make up different DNA sequences called genotypes. Genotypes along with environmental and developmental factors determine what the phenotypes will be. Most biological traits are under the influence of polygenes (many different genes) as well as gene–environment interactions. Some genetic traits are instantly visible, such as eye color or number of limbs, and some are not, such as blood type, risk for specific diseases, or the thousands of basic biochemical processes that constitute life.
Genes can acquire mutations in their sequence, leading to different variants, known as alleles, in the population. These alleles encode slightly different versions of a protein, which cause different phenotypical traits. Usage of the term "having a gene" (e.g., "good genes," "hair colour gene") typically refers to containing a different allele of the same, shared gene. Genes evolve due to natural selection or survival of the fittest of the alleles.
The concept of a gene continues to be refined as new phenomena are discovered. For example, regulatory regions of a gene can be far removed from its coding regions, and coding regions can be split into several exons. Some viruses store their genome in RNA instead of DNA and some gene products are functional non-coding RNAs. Therefore, a broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affects an organism's traits by being expressed as a functional product or by regulation of gene expression.
The term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1905. It is inspired by the ancient Greek: γόνος, gonos, that means offspring and procreation.