Chromosomes and Heredity
Why do children so often resemble their parents? Why do some brothers and
sisters share similar traits, while others are very different? To a large degree,
it's a function of the genes — which are the basic units of heredity — they
have in common. How does this happen? Let's find out a little bit about what
genes are and how we inherit them.
Your body is made up of trillions of cells. In some
ways, your cells can be very different from each
other. For example, they can specialize in a
particular function, such as carrying oxygen (red
blood cells), absorbing food (intestinal cells),or
sensing light (cells in your eyes).
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In other ways, your cells have a lot in common.
For instance, at the center of almost all of your
cells is a ball-shaped structure called the
nucleus, inside of which are 46 thread-like
structures called chromosomes. These
chromosomes contain the estimated 35,000
genes that, in many ways, make us who we are.
Chromosomes
A chromosome is a long strand of DNA, packaged
together with proteins and other kinds of
molecules. Each chromosome has a centromere,
which plays an important role during cell division
and also divides each chromosome into a short
arm and a long arm. Scientists can tell different
chromosomes apart based on their size, the
relative lengths of their arms, distinctive staining
patterns, and other characteristics.
Humans have two types of chromosomes: sex
chromosomes and autosomes. Two sex
chromosomes determine the sex of an individual,
and they are called the X chromosome and the Y
chromosome.
If you are female, you have two Xs, and if you are male, you have one X and
one Y (although there are genetic conditions in which this varies). The
autosomes comprise the other 22 chromosomes. The longest of the
autosomes is referred to as chromosome 1, the next largest as chromosome
2, and so on, down to the smallest autosomes, chromosomes 21 and 22.
Each cell nucleus contains two copies of each autosome (44 chromosomes),
plus two sex chromosomes (either two Xs or an X and a Y) for a total of 46.
With few exceptions, the chromosomes and genes found within any two cells
of your body will be identical.
The mystery as to why you resemble your family members is solved by
discovering how you inherited your chromosomes from your parents.
Cell Division
Each chromosome within our body (except the
chromosomes within cells that develop into sperm
or eggs) is created by making a copy of a
previously existing chromosome. This occurs
during the process called mitosis, during which
cells divide for growth or repair. Before each
division, the cell makes an identical copy of each
chromosome and during mitosis, each of the two
new cells receives a complete set of 46
chromosomes.
Each new cell has the same set of chromosomes
and the same genetic information as the "parent"
cell. This explains why almost every cell in your
body has the same genetic information.
A slightly different process takes place during the
production of egg and sperm cells. When an egg
and a sperm unite at fertilization, their nuclei
unite to form the nucleus of a human zygote. If
the sperm and egg carried 46 chromosomes, like
the rest of the body's cells, then the zygote would
have 92, which would be incompatible with life.
To prevent this, a special type of cell division,
called meiosis, takes place.
The process of meiosis begins with a single cell
containing 46 chromosomes and results in four
reproductive cells (sperm or eggs), each of which
carries 23 chromosomes. An important feature of
these four cells is that the combination of genes
they carry on their 23 chromosomes is a unique mix of the genes present in
the original single cell.
You resemble your parents because half of the instructions — genes — for
making you came from your father and half from your mother. Similarly,
your brother or sister also received half of their genetic instructions from
each parent, but the set they received is somewhat different from the set
you received. That's why they may resemble you, but they are not identical
to you. Identical twins receive exactly the same combination of genes and
chromosomes.
DNA and Proteins
As discussed in the Basic Genetics section, DNA is a large molecule packaged
in chromosomes in the nucleus of cells. The DNA molecule contains genes
that direct the production of proteins.
Proteins are molecules that play a critical role in
the structure, function, and regulation of your
body's cells, tissues, and organs. Every protein is
made up of a chain of building blocks called
amino acids.
The code that is carried by DNA determines which
amino acids will come together in what order to
form a given protein. Genes act, or "express,"
themselves by dictating the order of amino acids
used to make proteins.
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The proteins made by some genes are needed by
all cells, but different sets of genes may be
switched on or off in different cells. This leads to
different collections of proteins being made and
results in different structures, appearances and
functions. In addition to determining what
proteins are made, the DNA in a cell also controls
how much of a protein will be made and under
what circumstances.
Heredity
If you studied several family trees and traced the inheritance of a given trait
in the families, you would find that unique patterns exist. Several factors are
involved in determining patterns of inheritance, including where the trait-
causing gene is located (on the autosomes or sex chromosomes) and
whether one or two copies are necessary for a given trait to be expressed.
Genes may exist in more than one form, each of
which is called an allele; the most common form
of a gene is called its "wild type." No matter how
many forms (or alleles) a gene has, each person
inherits only two of them — one from the mother
and one from the father.
Genotype (the pair of alleles a person has at a
specific location in the genome) affects
phenotype (the observable effect of the allele,
such as eye color; in the case of medication, how
the person reacts to a drug).
Gene variants (alleles) may change the gene so
that it codes for a protein that works just as well
or better than the protein coded for by the wild
type. However, variant alleles can also change a
protein so that it no longer works as well or does
not work at all.
To illustrate, let's consider genes that code for the
proteins (called enzymes) that break down nonnaturally occurring chemicals in our body (for
A Pair of Genes
example, air pollutants, poisons, or medicines) so
they can be safely excreted.
Suppose that in a certain population, such as everyone living in the United
States, such a gene is present in two forms, called "A" and "a." We would say
this gene has two alleles. Allele "A" codes for an enzyme that breaks down
Drug X quite efficiently, and it is most common in the population (wild type).
Allele "a" resulted from a mutation (change in the order of DNA bases) in
allele "A" at some point in evolution, and the enzyme it produces doesn't
break down Drug X at all.
While people who carry two copies of the "A" allele can take Drug X safely,
people who carry two copies of the "a" allele will not be able to break down
Drug X effectively, so large amounts of the drug will stay in their bodies for a
long time, which might lead to serious side effects. People who carry one
copy of "A" and one of "a" may have an intermediate response (depending
on how the enzyme is affected). This example illustrates how important a
person's genes can be when it comes to being treated safely and successfully
with a medicine.
Inheritance Patterns
Each gene is present on every chromosome
(generally), so a pair of chromosomes contains
two copies of the same gene. The two copies may
be identical or different. In cases where there are
variations of a gene, one of the alleles can take
precedence over (or override) the other. In the
classic case observed by Mendel, the result of
crossing a white-flowered pea with a purpleflowered pea was a purple offspring — not a pale
purple, which would suggest mixing or blending of
the two genes. The gene for purple color was
dominant over the gene for white color, which
was called recessive.
A person who has two identical alleles for a gene
is said to be homozygous for that gene. A
person with two different alleles is said to be
heterozygous. The dominant allele is usually
represented by a capital letter and the recessive
allele by a lower-case letter (ie, A, a).
A person (or pea plant!) with two dominant alleles
is called homozygous dominant for that gene —
and that pea plant would have purple flowers. With one dominant and one
recessive allele, the person would be heterozygous — and the pea would
have purple flowers. If two recessive alleles are present, the person is
homozygous recessive — and only in this case, with both alleles coding for
white flowers, would the pea plant have white flowers.
These are the most common patterns of inheritance:

Autosomal dominant inheritance is caused by a mutation in a gene
located on an autosomal chromosome and occurs when one autosomal
allele masks the expression of another allele. A dominant gene from just
one parent will result in the phenotype, which may be eye or hair color
or may be a serious condition such as Huntington Disease. See an
example here.

Autosomal recessive inheritance also is caused by a mutation in a gene
located on an autosome, but in this case, two copies of the recessive
gene are needed for the trait to be expressed. Cystic fibrosis and
diseases affecting metabolism (such as phenylketonuria/PKU) are
autosomal recessive conditions. See an example here.

X-linked recessive inheritance is caused by a gene on the X
chromosome rather than on an autosome. Females have two X
chromosomes, so they need to inherit two copies of the allele to express
the trait; if they have just one copy, they are a carrier of the trait, but
do not usually exhibit it (although they may have mild symptoms in
some cases). Males are affected by X-linked recessive traits because
they have only X chromosome, so they need only one variant gene to
express the trait. Hemophilia is an example of an X-linked recessive
disease — it affects men most severely. See an example here.

Complex inheritance involves the additive effect of many genes
interacting with each other and with the environment. Common diseases
such as heart disease, obesity, osteoarthritis and asthma are not
inherited according to Mendel's patterns, but result from an interplay of
environmental factors (such as diet, exercise, smoking, and exposure to
pollutants) with susceptibility genes