For most diploid species, sex is determined by one of the homologous pair of sex chromosomes. In humans and fruit flies, the sex chromosomes are X and Y (XX=female ; XY=male). In birds, female is WZ and male is ZZ. There are many ways of determining sex:
Since in humans, Y chromosome carries very little genetic information, sex-linked genes usually refer to X-linked genes (genes on X chromosome). To determine if a gene is sex-linked, you perform pedigree analysis, use Drosophila, or perform a reciprocal cross.
Probability can be sued to predict the types fo progeny that will result from a monohybrid or dihybrid cross. The Punnett square is a graphical representation of these possible outcome. Phenotypes are the result of the genotype of an organism, more than one genotype may result in the same phenotype. Distinct segregation patterns result from monohybrid, dihybrid, and test-crosses.
Probability (expected frequency): probability of an outcome…# of times event is expected to happen/# of opportunities (trials)
The sum of all the probabilties of all possible events = 1
General rules:
Step 1: For outcome “A,†what events must happen? Multiply the probability of each event.
Step 2: Are there different ways to get outcome “A� Calculate the probabilities of each different way and add them up.
Multiplication Rule (Product Rule). The probability of two independent events, A and B, being realized simutaneously is given by the product of their separate probability
Prob{A & B} = Prob{A}â‹…Prob{B}
The probability of indepdent events occuring together is the product of the possibilties of the individual events: p(A and B) = p(A)p(B)
If you roll two dice, what is the chance of getting two 5’s? A 5 on the 1st die and a 5 on the 2nd die? 1/6*1/6
Addition Rule (Sum Rule). The probability of the realization of one or the other of two mutually exclusive events, A and B, is the sum of their separate probability. The probability of either of two mutually exclusive events occurring is the sum of their individual probabilites.
Prob{A or B} = Prob{A}+Prob{B}
If you roll two dice, what is the probabiltiy of two 5’s or two 6’s? Probability of a 5 on 1st die and a 5 on 2nd die or a 6 on 1st die and a 6 on 2nd die.
1/36+1/36=1/18
The Punnett Square is a way of depicting the product rule. Using Mendel’s Law of segregation, we know that both alleles are equaly likely to occur. So for a cross:
1/2R 1/2 r
1/2R 1/4RR 1/4 Rr
1/2r 1/4 rr 1/4 Rr
1/4 RR 1/4 Rr 1/2 Rr
t T
t tt tT
T Tt TT
| 60 red gum bals nd 40 green gum balls. If you buy one gum ball, what is the probability of getting a red one? |
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| # of red gum balls/total # of gum balls = 60/100 = .6 (.06 x 100% = 60%) |
| How many (ratio) progeny will have the same phenotypes of one of their parents from such a cross: AaBBCcDdee x aaBbccDdEE? |
| They must be either A_B_C_D_ee, or aaB_ccD_E_ … (½)(1)(½)(¾)(0) + (½)(1) (½)(¾)(1) = 3/16 |
| What is the probability of one green and one red gum ball if we have two quarters? Green first then red or red first then green. When not specifying order, we must figure out each way of getting the outcome. |
| p(green, then red) = .4*.6 and p(red, then green) = .6*.4 .24+.24=.48 |
| What fraction of the progeny will have large, smooth, purple fruit? SsPpLl x ssPPLl Texture: S-smooth and s-rough; P-purple and p-pink; L-large and l-small. |
| large: p(LL or Ll) = 1/4 + 2/4 = 3/4 smooth: p(Ss) = 1/2 purple: p(Pp or PP) = all = 1 large, smooth, purple p(L_S_P_) = 1/2 * 1 * 3/4 = 3/8 |
| What fraction of the progeny will not have large, smooth, purple fruit? SsPpLlxssPPLl |
| large, smooth, purple = p(L_P_S_) = 3/8. Not large, smooth, purple = 1-3/8 = 5/8 |
III. Biosynthetic Pathways
In the 40’s Beadle and Tatum suggested the “one-gene-one-enzyme†hypothesis which explained how genes can code for a biosynthetic pathway ultimately producing a useful compound.
Rule of Thumb
• The compound that rescues most number of mutants is the last one in the pathway.
• The mutant that is rescued by most supplements is the first one in the pathway.
A B C D
Mut 1 - + + +
Mut 2 - - + +
Mut 3 - - - +
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