GENETICS The body''s genetic material is contained within the nucleus
of each of its cells. The genetic material consists of coils of DNA (deoxyribonucleic acid) arranged in a complex way to form chromosomes. Human cells contain 46 chromosomes in pairs, including of one pair of sex chromosomes.
Each DNA molecule is a long double helix that resembles a spiral staircase. The steps of the staircase, which determine a person''s genetic code, consist of pairs of four types of molecules called
bases. In the steps, adenine is paired with thymine, and guanine is paired with cytosine. The generic code is written in triplets, so each group of three steps of the staircase codes the prodution of the of the amino acids, which are the building blocks of proteins.
When a part of the DNA molecule is actively controlling some function of the cell, the DNA helix splits ope along its lenght. One strand of the open helix is inactive; the other strand acts as a template against which a complementary strand of RNA (ribonucleic acid) forms. The RNA bases are arranged in the same sequence as bases of the inactive strand of the DNA except hat RNA contains uracil and DNA contains thymine. The RNA copy, called messenger RNA (mRNA), separates from DNA, leaves the necleus, and travels into the cytoplasm of the cell. There, it attaches to ribosomes, the cell''s factories for manufacturing proteins. The messenger RNA instructs the ribosome as to the sequence of amino acids for constructing a specific protein. Amino acids are brought to the ribosome by transfer RNA brings one amino acid to be incorporated into the growing chain of protein.
A gene consists of the code required to construct one protein. Genes vary in size, depending on the size of the protein. Genes are arranged in a precise sequence on the chromosomes; the location of a particular gene is called its locus.
The two sex chromosomes determine whether a fetus becomes male or female. Males have one X and one Y sex chromosome; females have two X chromosomes, only one of which is active.
DNA(deoxyribonucleic acid) is the cell''s genetic material , found in jumbled loosely coiled threads called
chromain in the nucleus of each cell. Just before a cell devides, the chromatin becomes tightly coiled, forming chromosomes.
The DNA molecule is a long, coiled double heliz that resembles a spiral staircase. In it, two strands, composed of sugar (deoxyribose) and phospate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase. in the steps, adenine is paired with thymine, and guanine with cytosine. Each pair of bases is held together by a hydrogen bond. A gene is a segment of DNA that has a particular function and consists of a specific sequence of bases.
X-Chromosome Inactivation
Because a female has two X chromosomes, she has twice as many X-chromosome genes as does a male. This would seem to result in an overdose of some genes. However, one of the two X baby born with an additional autosomal chromosome (a trisomy disorder) usually has many severe physical and mental
abnormalities. Similarly, the absence of one X chromosome usually results in relatively less severe abnormalities(Turner''s syndrome).
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Gene Abnormalities
Abnormalities of one or more genes, particularly recessive genes, are fairly common. Every human being carries six to eight abnormal recessive genes. In the general population, the chance of a person''s having two similar recessive genes is very small, but in children of close relatives, the chace are higher. Chance are slso high among groups that intermarry, such as the Amish or Memonites.
A person''s genetic makeup is called a
genotype. The body''s responsible to having those genes — that is, the expression of the genotype — is called the
phenotype.
All inherited characteristics (traits) are encoded by genes, Some characteristics, such as hair color, simply distinguish people from one another; they are''nt considered as abnormal.
X-linked Genes
Because the Y chromosome in males has very few genes, the genes on the signle X chromosome (X-linked, genes) are virtually all unpaired and therefore expressed, whether they''re dominant or recessive. But because females have two X- linked genes that apply to genes on autosomal chromosomes: Unless both genes in a pair are recessive, only dominant genes are expressed.
If an abnormal X-linked gene is dominant, affected males transmit the abnormality to all of their daughters but none of their sons. The sons of the affeted male receive his Y chromosome, which does''nt carry the abnormal gene. Affected females with only one abnormal gene trasmit the abnormality to half their children, ale or female.
If an abnormal X-linked gene is recessive, nearly everyone with the trait is male. Men transmit the abnormal gene only to their daughters, all of whom become carriers. Carriers mothers do not have the trait but transmit the gene to half their sons, who usually have the trait, but half are carriers.
Red-green color blindness, caused by a common X-linked recessive gene, affects about 10 percent of males its unsual among females. In males, the gene for color blindness comes from a mother who is color-blind or who has normal vision but is a carrier of a color-blind gene. It never comes from the father, who instead supplies the Y chromosome. Daughters of colo-blind father are rarely color-blind but are always carriers of the color-blind gene.