Genes are chunks of DNA that contain heritable factors. "Heritable" means that these factors have been passed on to you by your parents. Factors is a synonym for genetic traits. Examples of traits include hair color, eye color, lip shape, earlobe structure, etc. It is estimated that humans have 21,000 genes that are organized into chromosomes.
The image to the left (nih.gov) shows how DNA is packaged into chromosomes. This process is called supercoiling in which the chromatin ("loose DNA") winds around proteins called histones to form nucleosomes. Nucleosomes coil again to form solenoids, and solenoids coil once more to form looped domains. Looped domains coil to form chromosomes. You can find a picture of it at the bottom on this page.
Genes are found at particular places on chromosomes. This place is called a locus (plural-loci). Geneticists have mapped out the loci of each sequence of DNA in order to identify what the gene controls. The example in the text talks about the gene that controls the protein transducin which enables color vision is found on chromosome 1. You can read about it on p. 121, and it explains the significance of the base change and its effects. We possess two copies of each gene in our bodies; one from Moms and the other from Pop. Theoretically, you should find the same gene at the same locus on each chromosome. |
Alleles are versions of genes. An allele is a specific form of a gene that differs from other alleles by one or more nitrogen bases (C G A T). The example of transducin given in the text (p. 121) shows that the "correct" sequence includes a C at position 235, but the mutation has a T in place of the C. This makes an enormous difference, because we know T pairs with A and C pairs with G. This change of base results in the formation of an incorrect protein and thus, a person is literally colorblind. Alleles allow for single traits to have variations.
Read about cystic fibrosis on page 122-123 and ear wax on page 123.
Read about cystic fibrosis on page 122-123 and ear wax on page 123.
If you'll recall the lessons on transcription and translation, you will remember how important it is for each letter in the genetic code to be in the correct place. If the bases are misplaced or substituted with a different base, the results can be dramatic, as with the color vision example, or with the example in the text regarding cystic fibrosis.
Examine the worked example on page 123-124. |
Mutations are random, rare changes in genetic material. One type involves a change in the base sequence in DNA. If replication works correctly, this doesn't happen. But sometimes it does. If a T is put in place of an A, the corresponding bases on mRNA are altered during transcription. This means that a different amino acid is placed in the growing polypeptide chain. Mutations can have both positive and negative effects on a person's health.
Read about sickle cell disease on page 127-128.
Read about sickle cell disease on page 127-128.