This is the 2007 version. Click here for the 2017 chapter 10 table of contents.

Genetic Influences

One of the most amazing findings from modern science is that your entire body, including your complicated brain, is based on patterns built into DNA molecules. Development really begins, for the individual organism, when DNA from two parents is combined.

To what does a gene correspond, physically?

A gene corresponds, physically, to a sequence of units called nucleotides on the DNA molecule. Each nucleotide contains a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: The four bases (adenine, thymine, guanine, and cytosine) form into pairs..."A" with "T" and "G" with "C." Each base pair (AT, TA, GC, CG) forms a "rung" on the ladder. A single gene controls the construction of a single molecule such as a protein, which may have many diverse effects in the organism.


An artist's conception of the DNA molecule. Under a microscope DNA looks more like a clump of spaghetti.

What are chromosomes?

Chromosomes are structures in the nucleus of a cell that contain the DNA for genetic transmission. Each chromosome contains about 20,000 active genes.

Chromosomes are often displayed in a karyotype, which is a photomicrograph (photograph taken through a light microscope) showing the chromosomes. An example of a karyotype is shown below.


A karyotype

What is a karyotype?

Humans have 46 chromosomes grouped in 23 pairs. The so-called sex chromosomes are shown as the 23rd pair in the karyotype. The XX pattern is typical of females. The XY pattern shown here is typical of males. If this karyotype were from a female, the last pair would consist of identical-looking X-shaped chromosomes.

Why does a child often bear strong resemblance to a grandparent?

Except for the XY chromosomes of the male, chromosomes come in matched pairs. In each pair, one chromosome comes from the father, one from the mother, so each person receives 50% of his or her genes from the mother, 50% from the father.

There is no rule saying 25% must come from each grandparent. In the 50% from each parent, genes from one or another grandparent may be more numerous or more dominant. Therefore a child can receive a disproportionate share of genes from one grandparent or another, by the "luck of the draw." If the genes are those that affect appearance, personality, and/or distinctive skills, a child may resemble one grandparent more than others.


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