Genetics Regulates Fetal Development

Genes contain the information as to how the cells should perform. They are the units of inheritance. All living organisms have two copies of each gene, but they transmit only one to the offspring- Mendel’s Law. of inheritance.

The genes are organized into the long segments of DNA. During the cell division they get compacted along with some proteins to form an intricate structure called chromosome.

The chromosome is a single piece of coiled DNA that contains an individual’s specific genetic representation.

Every cell in our body has 46 chromosomes arranged in 23 pairs, of which there is only one pair of sex chromosomes (XX/XY). The remaining 22 pairs are called autosomal chromosomes.

Each member of autosomal pair corresponds to the other in size, shape and the heredity functions. Both, ovum and sperm cells contain only one set of chromosomes each, Meiosis division.

After conception, the fertilized egg, zygote, again has 46 chromosomes arranged as 23 pairs, and a child inherits one copy of each chromosome pair from each parent. Thus the hereditary blueprints for development are passed on unchanged from generation to generation.

Zygote is a single cell. It splits by series of mitosis and forms a few identical cells. These cells are then transformed into various body tissues and organs. This intricate progression and vast transformation towards fetal development is monitored by the inherited genes.

So, it is not at all surprising that the incidence of inherent disorders is as high as 2.5-6 percent. They are the most important cause of fetal loss, and a major cause of death and disabilities in infants and children.

Development of Twin Fetuses

During the first 3 days of conception, all the cells formed as a result of serial mitotic splits of zygote are capable of initiating development of any organ system.

Therefore, during this period if any one of these cells separate out from the main group of cells, it independently gives rise to one more baby. This is how monozygotic twins are formed.

Monozygotic twins are the two babies that arise from a single zygote. They both have exactly the same genotype and are therefore commonly known as identical twins. The worldwide incidence of monozygotic twins is about 3 in every 1000 deliveries.

Modes of gene expression

In dominant-recessive pattern of inheritance one allele (one member of gene pair) dominates another so that only its phenotype (physical characteristics or traits resulting) is expressed.

Dominant gene is so called because it has more power to express phenotypically, and masks the effect of its allele, a less powerful gene.

Whereas, a recessive gene is a less powerful gene. It is not expressed phenotypically when paired with a dominant allele.

Co-dominance inheritance is also known as incomplete dominance. Here both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other- for example, AB blood group phenotype.

Another type of co-dominance occurs when one of the two heterozygous alleles is stronger than the other but fails to mask all of its effects, as seen in sickle-cell anemia.

Genetic Imprinting is a process in which particular gene pairs are biochemically marked so that only one parent’s allele is expressed, regardless of its composition.

Polygenic Inheritance can be described as development of human characteristics that are influenced by many pairs of alleles, for example, height, weight, intelligence, skin color, temperamental attributes, susceptibility to cancer and so on.

Sex linked Inheritance is determined by genes located on the sex chromosomes.

With this advanced knowledge of genome and mode of inheritance, there have been advancements in prenatal evaluation of the
genetics by screening of a zygote and gene therapy if required. This forms a major step towards prevention of genetically related chronic disabilities in children.

Also read Genetic Disorders, a child health challenge

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