Download TECHNICAL NOTE 4.1

TECHNICAL NOTE 4.1
Genetics and Heredity
When viewed under the microscope, each human cell has the same general
structure, a round ball that is filled with various particles (called organelles), and
a smaller ball, somewhere in the middle, called the nucleus. The nucleus houses
all of the “programming code” for the organism. The code for our observable
characteristics (phenotype) such as hair and eye color, foot size, etc., is crammed
into the nucleus.This code is called DNA (deoxyribonucleic acid).
An organism’s basic complement of DNA is called its genome. DNA is essentially a long chain of molecules (nucleotide base pairs, the so-called building
blocks) that is wound into a double helix. Clusters of base pairs are known as
genes, and genes code for a specific function (e.g., the protein that regulates
hair color). There exist thousands of discrete genes within the millions of base
pairs of DNA. And how does all of this DNA fit into the nucleus? It does so by
dividing itself from one long strand of DNA into 23 pieces of DNA that are coiled
up into chromosomes. Thus, genes are simply the functional regions of chromosomal DNA.
Identifying and locating each of these genes is the grand goal of the Human
Genome Project.While researchers have discovered the sequence of base pairs
that make up human DNA, they are nowhere near finished identifying each
discrete gene, and they are further still from understanding the role of each
individual gene.
As mentioned, all of our programming code (DNA) exists as 23 chromosomes.
However, within the nucleus we find 46 chromosomes. These 46 chromosomes
exist as pairs, with each pair containing the same sequence of DNA. Each pair of
chromosomes contains one derived from the mother and the other from the
father. Thus, for example, the gene coding for skin color (melanin content) will
code for darker skin from a mother of African descent and lighter skin from a
father of European descent, with the result being a child whose skin color lies in
between.
Of course, this genetic material must be transmitted to the offspring in order
for inheritance to work. This pathway is through the combination of the sex or
reproductive cells (i.e., the ovum or unfertilized egg and the sperm cell). The
sex cells are different from the rest of the human cells in that each has only 23
chromosomes. It is the contribution of each parent to the genetic make-up of
the offspring that brings our total to 46 chromosomes. If a parent were able to
pass on all of its genes, then the offspring would be a clone of the parent.
Because some of a parent’s genetic make-up may be detrimental (e.g., one
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parent may lack the gene to enable color vision), it is more advantageous to have
a mix of both parents’ genes. In sexual reproduction, when a sperm fertilizes an
egg, a cellular process termed meiosis takes place. It is during this event that
each parent’s contribution of 23 chromosomes undergoes a complex process of
division and replication, with the end result being a single cell with 46 chromosomes that contains genetic material (DNA) from each parent. This one cell is
the offspring of the two parents and rapidly undergoes division to become the
embryo, fetus, and child of the two parents. This represents the chemical basis
for heredity.
Of the 46 chromosomes (23 pairs) typically found in the human cell, one is
referred to as the sex chromosome (the rest are referred to as autosomes and are
numbered 1 through 22). As previously noted, the sex cell has only a single set
of the 46 chromosomes that are found paired in other cells: 22 autosomes, and
one sex chromosome, yielding a total of 23. The pairing of this lone sex chromosome determines sex, with males having an X chromosome paired with a
Y chromosome (XY) and females having two X chromosomes (XX). It is the
Y chromosome that carries the genes associated with male features (e.g., height,
male genitalia development, hair distribution mediated through the production
of testosterone; Chan & Rennert, 2002). The fact that males are overrepresented
in crime statistics and crimes of a sexual nature makes the Y chromosome of
particular interest in some theories of criminal behavior.
Reference
Chan, W-Y. & O.M. Rennert (2002). “Molecular Aspects of Sex Differentiation.” Current Molecular
Medicine, 2, 25–37.