Genetics. Law of Probability. (beginning level)

When calculating the expected result of any genetic cross, we utilize the laws of probability. It is like flipping a coin for heads and tails. Each time you flip a coin, the chance of heads to come up is ½ or 50%, and the chance of tails to come up is also ½ or 50%. In the same manner, if the parent has a genotype Bb, what is the chance that any child will inherit either an allele B or b from the parent? But in the cross Bb x Bb, a child inherits one allele from each parent. What is the chance that a child will inherit a particular allelic combination (genotype)? The Multiplicative Law of Probability states that the probability, or chance, of two or more independent events occurring together is the product of their chance of occurring separately. Thus, the probability of receiving these genotypes is as follows: 1. The chance of BB = (The chance of B) x (The chance of B) ½ x ½ = ¼ 2. The chance of Bb = (The chance of B) x (The chance of b) ½ x ½ = ¼ 3. The chance of bB = (The chance of b) x (The chance of B) ½ x ½ = ¼ 4. The chance of bb = (The chance of b) x (The chance of b) ½ x ½ = ¼ To determine the genotypic ratio (GR) and the phenotypic ratio (PR), we also consider the additive law of probability: the chance of an event that can occur in two or more independent ways is the sum of the individual chances. Therefore, combining the two laws of probability, we can arrive at the genotypic ratio:law of probability in genetics Thus, the genotypic ratio (GR): ¼ BB : 2/4 Bb (add ¼ Bb and ¼ bB) : ¼ bb. By simple inspection of the genotypes, you can now deduce the phenotypic ratio. Genotypes with at least one dominant allele will result in brown eyes while genotype with two recessive alleles will result in blue eyes. Therefore, using the additive law, we conclude that the phenotypic ratio (PR) is ¾ or 75% brown eyes (add ¼ BB and 2/4 Bb) : ¼ or 25% blue eyes (1/4 bb). Test Cross The genotype of an organism will determine its phenotype. So, given the particular gene combination, you can predict the external appearance of an organism. But how would you know, by simple inspection of the phenotype of an organism, its genotype? If the organism exhibits a recessive phenotype (e.g. droopy ears in dogs), then there is no doubt that its genotype is homozygous recessive (ee). But if an organism exhibits a dominant phenotype (e.g. erect ears), you cannot be sure if its genotype is homozygous (EE), or heterozygous (Ee) by merely looking at its appearance. To determine if a dominant phenotype occurs in a homozygous or heterozygous condition, a test cross is used. In a test cross, the individual with the dominant phenotype is crossed with a recessive individual. If the parent with the dominant phenotype is homozygous, all the offspring exhibit the dominant phenotype . But if the parent with the dominant phenotype is heterozygous, the phenotypic ratio is 1:1 . Test cross. Crossing the parent with the dominant trait with a recessive individual will determine its genotype. a) if the parent with the dominant trait is homozygous, the PR is 100%. b) if the parent with the dominant trait is heterozygous, the PR is 1:1. #lawOfProbability #dominant #independentEventsOccurringTogether #dominantPhenotype #genotypicRatio #genotype #Homozygous #recessive #geneticCross #phenotypicRatio #GeneticsLecture #MultiplicativeLawOfProbabilit #Allele #TestCross #Iherb