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Chi-square test

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 A chi-square test is a valuable tool that aids the investigator in determining the goodness of fit. The test takes into account the size of the sample and the deviations from the expected ratio. It not only can be used for samples of different sizes but can be adapted to ratios with a different number of classes, like two classes for monohybrid crosses and four classes for dihybrid crosses. Essentially, the chi-square test is a mechanism by which deviations from a hypothetical ratio are reduced to a single value based on the size of the sample. This allows the investigator to determine the probability that a given sum of deviations will occur by chance. Expected values are obtained from the total size of the sample. If the hypothesis is that a 1:1 ratio results from a cross, the total is divided into two equal parts. For any other expected ratio, the total is divided into appropriate proportions. Chi-Square Formula Degrees of freedom (df) = n-1 where n is the number of classes Ste...
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Probability in Genetics

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 Probabilities are mathematical measures of likelihood. Probability refers to the chance or likelihood that a specific outcome will occur in an event in a particular situation. In other words, the probability is a way of quantifying (giving a specific, numerical value to) how likely something is to happen. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. The values of probability range from 0 to 1. A probability of 1 for an event means that it is guaranteed to happen, while a probability of 0 for an event means that it is guaranteed not to happen. For example, a coin is tossed four times and it appears to head all the times (4/4), the probability of getting head is 1. If it appears tail all the times (0/4), the probability of a head is 0. If it appears twice head and twice tail (2/4), then the probability is 0.5. Probabilities can be either empirical, meaning that they a...
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Pleiotropy

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 The term pleiotropy is derived from the Greek words pleio, which means "many," and tropic, which means "affecting." Genes that affect multiple, apparently unrelated, phenotypes are thus called pleiotropic genes. Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is genes that code for a product that is either used by various cells or has a cascade-like signaling function that affects various targets. During his study of inheritance in pea plants, Gregor Mendel made several interesting observations regarding the color of various plant components. Specifically, Mendel noticed that plants with colored seed coats always had colored flowers and colored leaf axils. Mendel also observed that pea plants with colorless seed coats always had white flowers and no pigmentation on their axils. In other words, in Mendel's pea plants, seed coat color was always associated with specific flower and axil colors. Today...
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Pseudoalleles

 If we assume that the mutant genes like vestigial and antlered are not allelic but located at different loci instead of locating at the same locus in different chromosomes, so closely linked that there is no crossing over between them, the mutant gene will suppress the expression of the adjacent normal allele to a certain extent. These closely linked genes are called pseudo alleles and this suppression is the result of the position effect. Thus, visible or apparent cases of allelism may be explained on the assumption of close linkage. Pseudoalleles refer to closely linked and functionally related genes. A cluster of pseudoalleles is known as pseudoallele series or a complex locus or a complex region. Characteristics of pseudoalleles 1. Pseudoalleles govern different expressions of the same character. In other words, they are functionally related. 2. Pseudoalleles are considered to occupy a complex locus which is divided into sub loci. Thus, they occupy different positions, but on ...
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The ‘Rhesus’ (Rh) blood group in man

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 A very interesting series of alleles affecting the antigens of human blood has been discovered through the work of Landsteiner, Wiener, Race, Levine, Sanger, Mourant, and several others. The original discovery was that the red blood cells are agglutinated by a serum prepared by immunizing rabbits against the blood of Rhesus monkey (Macaca rhesus). The antigen responsible for this reaction was consequently called as Rhesus factor and the gene that causes this property was denoted as R-r or Rh-rh. The Rh factor is simply a protein that is found on the covering of the red blood cells. If the red blood cells have this protein, then it is called Rh-positive. If the blood cells don't have this protein, then it is called Rh-negative. Just as everyone inherits ABO genes, every person inherits one Rh factor gene from each parent. The Rh-positive gene is the dominant gene when paired with an Rh-negative gene. Interest in this factor was stimulated by Levine’s study of a characteristic form ...
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Blood group genetics and Bombay phenotype

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 The gene I encode a glycosyltransferase, an enzyme that modifies the carbohydrate content of the red blood cell antigens. Allele IA codes for N-acetylgalactosamine transferase, allele IB codes for galactose transferase. These alleles vary at four nucleotides leading to two different proteins (enzymes) differing in four amino acids. Allele i has single nucleotide deletion in the initial coding region, leading to a frameshift and no formation of glycosyltransferase enzyme. Allele IA IB codes for both the enzymes and produce both N-acetylgalactosamine transferase and galactose transferase. These sugars (N-acetylgalactosamine and galactose) are added to a basic sugar molecule on RBC called mucopolysaccharide or H antigen which is made of galactose, N-acetyl glucosamine, and fucose. The production of mucopolysaccharide is controlled by another gene called H. The individual with genotype HH and Hh will produce functional mucopolysaccharide on the surface of RBC, whereas an individual wi...
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Examples of Multiple Alleles

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 Another example of multiple alleles is the eye color in Drosophila. The wild eye color in Drosophila is red. There is a wide variation in eye color in Drosophila. The first white-eyed mutant was discovered and later on, several other colors of the eye were reported. The main eye colors include wild, white, cherry, blood, eosin, apricot, ivory, and cream. The wild color is dominant over all other eye colors and exhibits a 3:1 ratio in the F2 generation. The cross between individuals of other eye colors exhibits intermediate expression in F1 and true expression only in homozygous conditions. This variation in eye color was initially considered to be due to multiple alleles of the same gene. However, later on, it was found to be due to pseudo-alleles. 4. Self-Incompatibility alleles in plants The most common example of multiple alleles in plants is the series of self-incompatibility alleles. Such alleles were reported in Nicotiana and later on, they were found in several other plant ...
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