This simply states that during the formation of gametes that pairs of alleles will segregate independantly of one another. This means that when considering two genes (with 2 alleles each, therefore 4 alleles all together), all combinations of gametes genotypes will occur.
The Product Rule
This permits one to calculate the probability of two independent events both occuring. Simply multiply the probability of one event by the other. The probability of an individual having a genotype of yy is therefore
1/2 X 1/2 = 1/4
However, the probability of an individual having a genotype of Yy is not as simple, as these are two mutually exclusive events; because one parent must give Y and another must give y and not anything else.
The Sum Rule
To calculate the probability of two mutually exclusive events both occuring, you add the individual probabilities.. So the Probability of having Yy is
1/2 + 1/2 = 1/4
Multiple traits can be analyzed via these methods. Consider two individuals heterozygous for 3 traits being crossed
AaBbCc X AaBbCc
The Probability that the offspring will have a genotype of aaBBcc is
1/4 x 1/4 x 1/4 = 1/64
Because the events are independent and thus only the Product Rule applies.
The probability of having the phentoype ABc can also be calculated
3/4 X 3/4 X 1/4 = 9/64
Chi-square Analysis
This type of analysis allows one to determine how close to the theoretical results one's experimental results are, and if something other than statistical chance is creating unexpected results. The formulae is
Where E is the expected results and O is what is actually observed. The answer to the equation is then compared to a chi-squared table, using the degrees of freedom and the above value to determine the 'p' or proability for the experiment. For genetic analyses, if there are two phenotypic classes then there is one degree of freedom. The p values inform you as to theprobability that the variations were due to chance, a
nd usually p values lower than 0.05 are considered a cut off. That p value would mean that there is a less than 0.05 percent chance that the variations were statistically insignificant.Patterns of Inheritance and Pedigrees
A pedigree is a diagram showing the inheritance patterns of traits. A horizontal pattern of inheritance indicates that a trait is rae and recessive. It occurs in several members of any particular generation, but doesn't tend to occur generation to generation. A vertical pattern of inheritance is where the trait is in every generation. Often these are dominant traits, however a very common recessive allel can give the same pattern. Approximately half of the family will get the trait when its within the family, and everyone that is affected will have an affected parent.
Incomplete Dominance
The appearance of the intermedaite pink trait is a result of gene dosage, gene A produces a red pgiment, gene 'a' produces a white pigment. Homozygotes are either red or white, but heterozygotes are pink, because they have both a red and a white pigment. This is called Incomplete Dominance and is sometimes called 'blending inheritance'
Co-Dominance
Human blood types illustrate the concept of co-dominance. There are three alleles, Ib, Ia, and i. Regardless of what other alleles are part of teh genotype, so long as allele Ib is present, a particular sugar is added to the coats of red blood cells. If Ia is present, then a different sugar is added. So when both are present, both are added, and when neither is present, and the genotype is ii, no sugars are added.
A child with type A blood born to a mother with type B blood could not have come from a father with types O or B blood. This is why blood typing is sometimes used in paternity testing.
Some genes are not simply dominant to each other, but infact occur ina dominance series, such that A >ax>af>a
Other alleles can infact be leathal when present in the homozygous condition.
In general, by examining the ratios of crosses, one can determin ehow the genes are acting. A 12:3:1 ration indicates that Dominant Epistasis is occuring. The 9:7 ration indicates that complementation between multiple genes is occuring.
Chromosomes
In the somatic cells of an individual, there is a fixed number of chromosomes, humans have 46 and flies have 8 for example. The chromosomes are present in pairs, this condition is called the diploid condition or the 2N condition. Gametes, however, are haploid, they posses just the 1N number of chromosomes. In humans the gametes possess 23 chromosomes.
Mitosis is the process by which new cells are produced, the end result is two daughter cells with identical sets of chromosomes.
- Chromosomes duplicate, with copies remaining attached to each other, these attached copies are called 'sister chromatids'
- The Sister chromatids line up in the center of the cell
- They seperate to opposite poles of the cell
- The cellular cytoplasm divides, resulting in two new cells
In meiosis, there are two rounds of division and a duplication. Meiosis is often called 'reductive division'. A diploid cell that completes meiosis will yeild haploid cells.
- The chromosomes duplicate into Sister Chromatids
- Homologous duplicated chromostomes pair (aka synapsis).
- Homologous chromosomes(but not sister chromatids) will exhange material (aka crossing over)
- Paired homologous duplicated chromosomes (aka tetrads) line up independently
- Homologous duplicated chromosomes seperate from their homolog, forming daughter cells. These cells are not identical. This is what makes up the basis of mendellian Independent Assortment)
- Duplicated chromosomes divide (iow, sister chromatids seperate), forming 4 daughter cells overall, each with half the original chromosomal content (iow, they are now haploid).
The Chromosomal Theory of Inheritance
When chromosomes were first observed in teh stains of cells, their function was not known and their behaviour wasn't understood. Over time, their complicated and bizzare movements noted above were observed and in 1902, Sutton hypothesized that these chromosomal bodies infact carry genes. He based this hypothesis largely the fact that Mendels theoretical Laws could be explained by the movements of the chromosomal bodies.
Chromosomes:
- Occur in pairs
- Seperate in gametogenesis
- But reconcile in fertilization
- And they assort independantly
The linkage between sex determination and the chromosomes strengthened this hypothesis.
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