DNA and Traits
Source: Pearson Interactive Science 7.2
DNA DNA is a chemical that stores the instructions for makingan organism. The molecule forms long strands and is shapedlike a twisted ladder. Because it is so long, DNA wraps up into tightstructures called chromosomes. Each species has a characteristicnumber of chromosomes.
Chromosomes have many segments called genes that encodethe information needed to make a protein. Proteins are chemicalsthat 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. Whetherit is a muscle cell in your little finger or a sensor in your nose thatsmells garlic, the DNA is the same. What changes is the combinationof genes that are expressed. Just as you can use the same cookbookto make a variety of meals, your cells can express differentgenes to carry out different jobs. Your sex cells are the exception.They have only half the usual number of chromosomes, but thesecells are not active unless fertilization occurs.
Traits A trait is the way an organism looks or acts. Traits areuseful in classifying organisms, and they enable species to surviveover over generations. Heredity is the process of passing traits fromone generation to the next. All living things inherit traits. Bacteriainherit genes that make them round or helical or rod-shaped. Dogsinherit genes help them herd sheep or swim well. Chickens inheritgenes that make them lay green or brown eggs.
Not all of an organism’s characteristics are inherited from itsparents. Environmental factors have a strong influence as well. Forexample, you may inherit a tendency to be tall. But a childhooddisease may stunt your growth. Other traits are learned. You canlearn how to walk a tightrope or play the banjo by practicing. Adog can learn how to sit up, beg, and roll over. A rat can lean topress a button to get a treat. These skills are learned traits. They arenot passed on through heredity.
Sexual Reproduction In sexual reproduction, sex cellsfrom two parents combine to form a zygote during fertilization.An egg cell from the female parent combines with a sperm cellfrom the male parent. Offspring produced by sexual reproductionhave a unique combination of DNA because they have a mixtureof the mother’s and father’s DNA. This is an important advantageof sexual reproduction. Shuffling the gene combinations increasesspecies’ chance for survival, even under changing or uncertainconditions.
The process that determines which parts of the DNA are putinto the sperm or egg cell is random. On top of that, it is randomwhich egg and sperm come together to form the zygote. When youlook at it this way, it’s not at all surprising that some people lookdifferent from their family members. This is true for all sexuallyreproducing forms of life. For example, purebred dogs may lookmuch like their parents, but there are always some differences.
Sexual reproduction takes more energy and time than asexualreproduction. It also requires two parents of opposite sexes, whichcan be a problem if there aren’t many members of the species in anarea. However, these disadvantages appear to be offset by advantagesof producing genetically unique offspring.