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. A gene is a portion of DNA that determines characteristics. Through meiosis and reproduction these genes can be transmitted from one generation to another. Genes are the basic units of heredity — they have in common. How does this happen?
Our body is made up of trillions of cells. In some ways, these 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, these 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.
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.
Heredity
Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism. Through heredity, variations exhibited by individuals can accumulate and cause a species to evolve. The study of heredity in biology is called genetics, which includes the field of epigenetics.
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.
0 comments:
Post a Comment