Download DNA And Traits

DNA and Traits
Source: Pearson Interactive Science
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DNA DNA is a chemical that stores the instructions for making an organism. The molecule
forms long strands and is shaped like a twisted ladder. Because it is so long, DNA wraps up into
tight structures called chromosomes. Each species has a characteristic number of chromosomes.
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Chromosomes have many segments called genes that encode the information needed to make a
protein. Proteins are chemicals that do most of the work of a cell, and they contribute to the traits
that you see.
Almost every cell in your body has the same DNA. Whether it is a muscle cell in your little
finger or a sensor in your nose that smells garlic, the DNA is the same. What changes is the
combination of genes that are expressed. Just as you can use the same cookbook to make a
variety of meals, your cells can express different genes to carry out different jobs. Your sex cells
are the exception. They have only half the usual number of chromosomes, but these cells are not
active unless fertilization occurs.
Traits A trait is the way an organism looks or acts. Traits are useful in classifying organisms,
and they enable species to survive over over generations. Heredity is the process of passing traits
from one generation to the next. All living things inherit traits. Bacteria inherit genes that make
them round or helical or rod-shaped. Dogs inherit genes help them herd sheep or swim well.
Chickens inherit genes that make them lay green or brown eggs.
Not all of an organism’s characteristics are inherited from its parents. Environmental factors
have a strong influence as well. For example, you may inherit a tendency to be tall. But a
childhood disease may stunt your growth. Other traits are learned. You can learn how to walk a
tightrope or play the banjo by practicing. A dog can learn how to sit up, beg, and roll over. A rat
can lean to press a button to get a treat. These skills are learned traits. They are not passed on
through heredity.
Sexual Reproduction In sexual reproduction, sex cells from two parents combine to
form a zygote during fertilization. An egg cell from the female parent combines with a sperm
cell from the male parent. Offspring produced by sexual reproduction have a unique combination
of DNA because they have a mixture of the mother’s and father’s DNA. This is an important
advantage of sexual reproduction. Shuffling the gene combinations increases species’ chance for
survival, even under changing or uncertain conditions.
The process that determines which parts of the DNA are put into the sperm or egg cell is random.
On top of that, it is random which egg and sperm come together to form the zygote. When you
look at it this way, it’s not at all surprising that some people look different from their family
members. This is true for all sexually reproducing forms of life. For example, purebred dogs may
look much like their parents, but there are always some differences.
Sexual reproduction takes more energy and time than asexual reproduction. It also requires two
parents of opposite sexes, which can be a problem if there aren’t many members of the species in
an area. However, these disadvantages appear to be offset by advantages of producing
genetically unique offspring.