Alleles are alternative forms of a gene. For example, there is an allele for brown eye color and an allele for blue eye color. Both are alleles for the gene "eye color." Even though a single individual can carry only two alleles for a specific gene, there may be multiple alleles for the trait throughout a population. This example of eye color demonstrates a case in which a gene has multiple alleles. Other examples include fingerprints, skin color and height.

The following are ways in which genes interact with one another to influence the total phenotype.

• epistasis

• polygenic inheritance

• blending
  (quantitative inheritance)
  example: blending of skin color in humans

• X sex chromosome
  1. Barr bodies-inactive X chromosomes in mammalian females
  2. Mary Lyon's hypothesis-the inactivation in the embryo of one of the X chromosomes

• Y sex chromosomes-determine the sex of the individual- distinquished from autosomes that determine other body traits.

• in humans
  1. XX = female
  2. XY = male

• in birds/butterflies/moths
  1. XX or ZZ = male
  2. XY or ZW = female

The presence of certain characteristics and traits are depenendent on the sex of the parent carrying the gene and the sex of the offspring. Color-blindness is a well known example. Color blindness is a recessive trait carried on the X chromosome-fig. 8-7

Have a look at this site for more information on this subject:
Human Genone Project Information

• carrier-an individual that carries the allele for the trait, but does not manifest it phenotypically.

• color blindness

• hemophilia-chapter 34

Since the formulation of Mendel's laws, research has shown that all genes do not segregate independently. Genes on the same chromosome have been shown to be linked.

• linked genes
• linkage groups

Crossing over

During meiosis, chromosomes break apart and link on to other sister chromosomes. This is called crossing over, and results in the separation of alleles on the same chromosomes, breaking apart linkage groups.

• euploidy-changes in chromosome number

• aneuploidy-single chromosome is added or subtracted

• polyploidy-carrying of one or more additional sets of chromosomes

• nondisjunction-the common cause of aneuploidy
  1. trisomy condition
  2. monosomy condition

• gene = nucleic acid sequence that codes for a functional protein or RNA sequence

• introns

• one gene -- one protein hypothesis

• DNA nucleotides (double stranded = double helix)
  1. deoxyribose sugar (fig. 8-11)
  2. phosphate group
  3. one of the following nitrogen bases
     1. adenine*
     2. guanine*
     3. thymine**
     4. cytosine**

        * = purine (fig. 8-12)
        ** = pyrimidine (fig. 8-12)

     Open this site to see the genetic code: Genetic Code Cracker

  4. DNA has complementary base pairing (fig. 8-16)
     1. A binds with T
        (2 hydrogen bonds; fig. 8-16)

     2. G binds with C
        (3 hydrogen bonds; fig. 8-16)

• RNA nucleotides (single stranded)
  1. ribose sugar (fig. 8-11)
  2. phosphate group
  3. one of the following nitrogen bases
     1. adenine*
     2. guanine*
     3. uracil**
     4. cytosine**

        * = purine(fig. 8-12)
        ** = pyrimidine (fig. 8-12)

The parent strands of DNA part. The daughter strands are synthesized using the base sequence of the parent strands as the template.(fig.4-21 & fig. 8-17)

• codons (64)

• amino acids (20)

• DNA nucleotides (4) = A,T,C,G

• triplet code of DNA transcribed into mRNA

• introns vs exons (fig. 8-18)

As ribosomes move along the strand of mRNA, amino acids are added to form the polypeptide chain.

(fig. 8-19)

tRNA (anticodon; fig. 8-20)

• transcriptional control

• translational control

• gene rearrangement

• DNA modification

Scientists are now able to manipulate genetic sequences.

Recombinant DNA refers to the DNA from different species in a single molecule. Open the site below to learn how genetists are using recombinant DNA.

Recombinant DNA

Open this link for more information on PCR: Xeroxing DNA

Open this site to read about the future of genetic research: Genetic Research

Using DNA to solve crime-Open this link to find out more:
The Evaluation of Forensic DNA Evidence

• causes
• frequencies

Neoplastic, or cancer cells, proliferate in an unrestrained manner. Cancer is the result of a genetic changes that take place in cells. Alterations occur in two types of genes:

• tumor suppressor genes
• proto-oncogenes

Once you have completed the lesson, you should go to Assignment 8-1.

E-mail Sylvester Allred at Syl.Allred@nau.edu

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ALL RIGHTS RESERVED