Genetics Overview

Human development: how does it all begin?

The beginning of human development occurs when an egg cell from the mother and a sperm cell from the father combine into one cell in a process known as fertilization. The egg cell provides genetic information from the mother, and the sperm cell provides genetic information from the father. When the genetic information from the parents combines together during fertilization, a genetic blueprint is created in the nucleus of the fertilized egg that is the “DNA blueprint”. The fertilized egg duplicates and divides into two identical cells and a copy of the “DNA blueprint” is reproduced in each cell.

Embryo growth

Once the fertilized egg duplicates and divides into two identical cells, it continues the process of cell duplication and division (mitosis) that will occur repeatedly over millions of times as the embryo grows and develops. The “DNA blueprint” is reproduced with each cell’s duplication and division and is contained within each new cell’s nucleus. The “DNA blueprint” in the nucleus is the source of all genetic commands necessary for the fertilized egg’s cell division and the development of the fertilized egg into an early embryo.

The genetic commands from the “DNA blueprint” in the nucleus of each cell instruct the cells how to grow and what to do. As the cells continuously duplicate and divide by mitosis they also undergo a transformation or developmental (differentiation) into specialized cells that have different functions. Some cells differentiate or turn into nerve cells that become our early brain and nervous system; other cells differentiate into cardiac cells that become the early heart, and still others differentiate into the early skin and tissues that make up the rest of our body. The “DNA blueprint” in the nucleus is the conductor of an orchestra of life that continuously directs embryonic growth, through cell mitosis, and embryonic development, through cellular differentiation, into a fetus, and ultimately a baby.

Chromosomes: the DNA blueprint

The “DNA blueprint” in the nucleus of each cell is organized into 46 threadlike strands of DNA called chromosomes. The 46 chromosomes are arranged as 23 pairs of nearly identical chromosomes. The chromosome pairs are numbered 1 to 22, from the longest chromosomes labeled as #1 to the smallest chromosomes labeled as #22. The two unnumbered chromosomes are the sex chromosomes, the X and the Y, that determine the sex of the individual. A female has 22 pairs of chromosomes and two X chromosomes, and a male has 22 pairs of chromosomes and a single X and a single Y chromosome to unite as the 23 pairs of chromosomes in the nucleus of each cell.

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Genetics and your baby

During the process of fertilization, one set of 22 chromosomes with an X chromosome is provided as the maternal genetic information derived from the egg, and the other 22 pairs of chromosomes with the additional X or Y chromosome are the paternal genetic information derived from the sperm. As the cells continuously duplicate and divide by mitosis, the same 23 pairs of chromosomes that are originally united in the nucleus of the fertilized egg are reproduced in the nucleus of every new cell. Therefore, a baby inherits half of its genetic information in the 23 chromosomes from the mother, and half of its genetic information in the 23 chromosomes from the father.

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