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.

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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).

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.

To understand how we end up with a given set of genes, we need to learn more about chromosomes.

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

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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.

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.

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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.

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.

A Pair of Genes

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 non-naturally occurring chemicals in our body (for 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 purple-flowered 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.
  See a comparison here.