Inherited Change

The X2 (chi-squared) Test If you look at a dihybrid cross between two heterozygous individuals, then we would expect to see a 9:3:3:1 ration of phenotypes in the offspring. However, this is still just a predicted probability!!! To determine whether the differences between the expected numbers of each genotype and the observable numbers are due to chance or something unexpected, we can use the X2 (chi-square) test. This statistical test allows for the comparison of the observed and expected results to see if there is a significant difference between them. To carry out the X2 test, you need the following information: The observed numbers for each genotype [O]. The expected ratio and the expected numbers for each genotype [E]. The difference between the observed and the expected values. [O – E] The square of the difference [(O – E)2] The square of the difference divided by the expected values. [(O – E)2 /E] The value of X2 will be the sum of the previous bullet.

The X2 (chi-squared) Test Once the value of X2 is obtained, we need to figure out what the value means. To do so, we must look at a table that relates X2 values to probabilities. The table below gives the probability that the differences between our expected and observed results are due to chance. In biological experiments, a probability of 0.05 is usually critical. If our X2 value represents a probability of 0.05 or larger, then we can be fairly certain that the differences between our observed and expected results is due to chance (Not significant) If the probability is smaller, then it is likely that the difference is significant and consider what else may have gone wrong with the cross. Degrees of Freedom Probabilty greater than 0.1 0.05 0.01 0.001 1 2.71 3.84 6.64 10.83 2 4.60 5.99 9.21 13.82 3 6.25 7.82 11.34 16.27 4 7.78 9.49 13.28 18.46

The X2 (chi-squared) Test Not only do we have to look at the probabilities, we should also look at the degrees of freedom in the results. This value takes into account the number of comparisons made. To work out the number of degrees of freedom, calculate the number of classes date minus 1. (The classes of data is the number of possible sets of phenotypes. Once the degrees of freedom are calculated, then you can look at the correct portion of the table. EXAMPLE: 144 offspring produced. Purple, cut 86 green, cut 24 Purple, potato 26 green, potato 8

Mutations Most genes have different alleles that are made up of a sequence of nucleotides, each with their own base. The different alleles originally arose due to mutations. Mutations are unpredictable changes in the genetic material of an organism. A gene mutation is a change in the structure of a DNA molecule, which will produce a different allele of a gene. A chromosomal mutation is a change in the structure or number of whole chromosomes in a cell. Mutations can occur with no obvious cause or because of an environmental factor. All types of ionizing radiation (alpha beta, and gamma radiation) can damage DNA molecule, altering the structure of the bases. UV radiation has a similar effect by increasing the chances of mutations (mutagen)

Mutations There are 3 different ways, in gene mutations, in which the sequence of bases in a gene can be altered. Base substitutions occur when one base simply replaces another. Base additions occur when one or more extra bases are added to the sequence. Base deletions occur when one or more bases are lose from the sequence. Base additions or deletions have a very significant effect on the structure and function of the polypeptide that the allele codes for. They are significant because they alter every codon that follows the mutation in the DNA molecule. They are said to be frame shift mutations. Base substitutions usually have no effect at all. They are said to be silent mutations.

Sickle Cell Anemia Sickle cell anemia is caused by a base substitution. Haemoglobin is composed of 2 alpha and 2 beta chains (polypeptides) The gene which codes for the amino acid sequence in the beta chains is not the same for everyone. Most people have the sequence: Val-His-Leu-Thr-Pro-Glu-Glu-Lys For some people the base sequence CTT can be replaced by CAT to give: Val-His-Leu-Thr-Pro-Val-Glu-Lys The change is sequence changes the polypeptide, which does not affect the haemoglobin molecule when it is combined with oxygen. However, the unusual beta chains causes the haemoglobin molecule to be less soluble when it is not combined with oxygen and it therefore becomes sticky.

Phenylketonuria (PKU) Phenylketonuria is a disease that is caused by an abnormal base sequence that codes for an enzyme (phenylalanine hydroxylase). Phenylalanine hydroxylase helps to catalyze the conversion of phenylalanine to tyrosine, which will then be converted to melanin. If phenylalanine hydroxylase cannot be made, then little melanin is formed. Therefore, people with PKU had lighter skin and hair color. A bigger problem that people with PKU have is that too much phenylalanine will accumulate in the blood and tissue fluid. This can cause severe brain damage in children. Brain damage can be prevented by testing before birth and placing the child in a strict diet that does not contain phenylalanine. (NO chocolate!!)

Gene Technology Gene technology (genetic engineering) allows scientists to change the DNA in a cell. An example of how gene technology has been helpful is the use of genetically altered bacteria to mass-produce human insulin. Scientists succeeded in the early 1980s in inserting the gene for human insulin into a bacterium. mRNA carrying the code for insulin production from the β cells in the pancreas was removed. The mRNA is incubated with reverse transcriptase from a retrovirus. However, in order for the insulin genes to stick onto other DNA, they were given sticky ends using restriction enzymes (guanine nucleotides in single stranded DNA at each end). A vector (plasmid in this case) is used to insert the insulin gene in the bacterium. The insulin gene was added to the plasmid by treating the bacteria with enzymes that dissolve their cell walls. They were then centrifuged, and the much smaller plasmids were cut using restriction enzymes and sticky ends with cytosine added. The plasmid and DNA are then mixed, where they pair together and linked with DNA ligase to form recombinant DNA.

Gene Technology 5. The plasmids are now mixed with bacteria (E. coli). The portion of bacteria that took up the plasmid was then separated from the others using antibiotic resistance provided by another gene that was introduced at the same time. 6. The genetically altered bacteria can now be cultured on a large scale. The will secrete insulin, which will then be extracted, purified, and sold to people. Gene technology has also been used to synthesize other human protein hormones or enzymes that can be used in the food industry. In addition, gene technology can introduce genes into any organism. Gene therapy seems promising in replacing defective genes in humans with good genes. This can be very helpful in treating genetic disorders. Human factor VIII is also genetically modified in hamster cells.

Genetic Engineering