Crossing and Probabilities

Transcription

1 Let us review Crossing and Probabilities RR= homozygous dominant for Rr = homozygous recessive for white. P gen: RR x rr F1 gen: Rr F2 gen: Purple: 1RR, 2 Rr = total 3 Purple White : 1 rr = 1 total white Ratio Mendel s Segregation Model Mendel s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses. The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of An uppercase letter represents a, and a lowercase letter represents a recessive allele. Heterozygous Phenotype The has its full effect in the heterozygote, while the has no effect on flower color in the heterozygote. Therefore, a Ww flower has a purple phenotype. phenotype genotype WW or Ww

2 Mendel's hypothesis also explain the 3:1 ratio of purple to white flowers he observed in the F2 generation. Since the F1 hybrids are Ww - 1/2 of their gametes will have the W allele - 1/2 will have the w allele. - This gives us a 3:1 ratio in the F2 generation Looking at Genotype We can see that if a carrying a W allele fertilizes an egg cell carrying a W allele, the offspring will be WW and therefore have purple flowers. This particular genotype will occur in 1/4 of the offspring. 1/2 of the offspring will inherit one W allele and one w allele (Ww). They will also have purple flowers since purple is to white. (Typically, any color is dominant to white) The remaining of the F 2 plants will inherit a, w allele from each parent, giving it a genotype of ww and a phenotype of white flowers. Genotype vs. Phenotype Because an organism's appearance does not always reveal its genetic composition, we need to distinguish between an organism s expressed, or physical traits ( ), and its genetic makeup ( ). In Mendel's pea plants, flower color phenotype was purple or white but there are different genotypes WW, Ww, ww

3 Allele pairs Remember from our study of mitosis and meiosis, that each diploid cell has two sets of. It is on these homologous chromosomes that the allele pairs for each are found. Determining Genotype from Phenotype How can we tell the of an individual with the dominant phenotype? In Mendel's pea plants those with WW or Ww both appeared purple. So if we have a pea plant with purple flowers, how do we determine which genotype the plant has? Testcross A testcross involves breeding the individual whose genotype we are trying to determine with a recessive individual. If any offspring display the recessive phenotype, the parent must be. Law of Segregation... and Beyond Remember Mendel's 4th Hypothesis? Law of : A sperm or egg carries only one allele for each trait because allele pairs separate from each other during gamete formation. Mendel did not stop there... He devised a second law of by observing two traits at a time (e.g. seed color and seed shape.) Mendel wanted to see if these different traits affected each other or not, in other words, did they behave independently of each other or not?

4 Dihybrid Cross Mendel crossed 2, true-breeding parents differing in two characteristics. Such a cross produces dihybrids in the F1 generation, which are heterozygous for both characteristics. A dihybrid cross, a cross between two F1 dihybrids, can determine whether two characteristics are transmitted to offspring together or. The Law of Independent Assortment Using a dihybrid cross, Mendel developed his second law of inheritance. The law of independent assortment states that each pair of alleles segregates of other pairs of alleles during gamete formation. This law of independent assortment applies only to genes on, non-homologous chromosomes. Genes located near each other on the same chromosome tend to be inherited together. Watch this video: Notes:

5 Dihybrid cross two characteristics: Pod color G: green g: yellow Pea color Y: yellow y: green Turn and talk are these characteristics independent of one another? How can you tell? The Law of Independent Assortment The alleles of one gene segregate into gametes independently of another gene s alleles. Think of this like a coin toss: If you toss a coin, what is the probability of it coming up heads? Now, if you toss a second coin, what is the probability of it coming up heads? When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss!

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9 Do Now: Grab your ipad and open up your notes from yesterday!! Try to answer the questions on the google slides *Remember you will receive a copy of all work done on peardeck!* Rules of Probability Mendel s laws of segregation/independent assortment reflect the rules of probability. Remember from math... The probability scale ranges from 0 to 1. An event that is certain to occur has a probability of 1. An event that is certain not to occur has a probability of 0. The probabilities of all possible outcomes for an event must add up to 1.

10 Using Probability Rules When Solving Genetics Problems There are two rules of probability which are helpful when solving problems in genetics: 1) The Rule 2) The Rule The Multiplication Rule The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities...what??? Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele This concept is best grasped through practice! For Help watch this video!!! Take Notes

11 Now let us consider the probability that an F2 pea plant will be heterozygous (Gg) for flower color. There are two ways in which F1 gametes can combine to produce heterozygous offspring. The allele can come from the egg and the recessive allele from the sperm, or... The dominant allele can come from the sperm and the allele can come from the egg. The Rule of Addition The rule of addition states the probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways. KEY WORD IS OR... Let s watch the rest of the video Kickin it up a notch. In a cross between AaBbCc x AaBBCC what is the probability that the offspring will be AABbCc or AABBCC? Trihybrid Cross Practice! We can use the rules of probability to solve complex genetics problems. If we crossed two organisms both having the genotype AaBbCc, what is the probability that an offspring of this cross will have the genotype aabbcc? Since each allele pair assorts independently, we can treat this trihybrid cross as three separate monohybrid crosses:

12 Aa X Aa: Probability of aa offspring = Bb X Bb: Probability of bb offspring = Cc X Cc: Probability of cc offspring = aabbcc Because the segregation of each allele pair is an independent event, we can use the multiplication rule to calculate the probability of an offspring being aabbcc. What is the probability? Answer is: We can combine the rules of multiplication and addition to solve complex problems in Mendelian genetics. Let s determine the probability of finding two recessive phenotypes for at least two of three traits resulting from a trihybrid cross between pea plants that are PpYyRr and Ppyyrr. There are five possible genotypes that fulfill this condition: ppyyrr, ppyyrr, Ppyyrr, PPyyrr, and ppyyrr. We would use the rule of multiplication to calculate the probability for each of these genotypes and then use the rule of addition to pool the probabilities for fulfilling the condition of at least two recessive trait.