Via
BSCI 106
Spring 2000
Lecture 5. Patterns of Inheritance II: More Mitosis, Meiosis, Mendel
1. More 1 locus traits with dominance: test cross
a. Test cross: a way to test whether an individual that shows the dominant trait is a homozygote or heterozygote for the dominant allele
b. cross the unknown genotype with at least one dominant allele with a homozygous recessive:
B_ x bb
c. get a different outcome in the progeny if the unknown was BB than if it was Bb.
2. More monohybrid crossing: one locus, incomplete dominance
a. With partial (incomplete) dominance, heterozygote can be distinguished from both homozygotes.
b. Usually, heterozygote is intermediate with partial dominance
c. Sometime, heterozygote may not be a "blend" of the two parents (see book for example)
d. Punnett square with incomplete dominance-- genotype and phenotype ratios (what's the difference between this and complete dominance?)
3. Use of probability theory to determine the expected frequencies of genotypes and phenotypes
a. Probabilities range between 0 (no chance) and 1 (certainty)
b. In a diploid organism, the probability that a gamete will receive either of the two possible alleles at a given locus from a parent = ??
c. Basic true facts about probability:
Probability of X AND Y = (Probability of X) x (probability of. Y)
Probability of X or Y = (Probability of X) + (probability of. Y)
d. forming a zygote-- two gametes meet to make a diploid zygote.
Make this cross:
Parents: BB x bb:
F1: all Bb WHY??
F2 cross F1 Bb x Bb
probability of a BB is 1/2 x 1/2 = 1/4
Probability of a bb is 1/2 x 1/2 = 1/4
Probability of a Bb is (1/2 x 1/2) + (1/2 x 1/2)
e. if parents are Bb x bb, what is the prob. of getting a Bb offspring? BB? Bb? bb?
4. The dihybrid cross: two loci on different chromosomes
a. What if parents differ at two (or more) loci: following two (or more) traits at once
b. Dihybrid cross RRGG (round, yellow) x rrgg (wrinkled, green)
i. make F1-- all RrGg
ii. will alleles of maternal and paternal origin segregate together?
iii. if so, predict only RG or rg gametes
iv. HOWEVER, Mendel saw RG, Rg, rG, rg gametes
c. Seeing all possible combinations of alleles from the different parental origins led Mendel to say that "particles" do NOT segregate in groups
d. Mendel's Law of Independent Assortment (2nd Law): Alleles of different genes assort independently of one another during gamete formation (NOTE: for this to work, genes must be on different chromosomes). Why does this occur?? (Hint: think about how tetrads line up in metaphase I of meiosis!)
5. Genotype ratios in dihybrid cross and the 9:3:3:1 phenotype ratio
a. Punnett square-- it works but it is cumbersome
b. Probabilities: the way to figure these ratios out!
c. RrGg x RrGg-- what is the probability of an RRGg progeny?
Probability of RR progeny = (1/2 x 1/2) = 1/4
Probability of Gg progeny = (1/2 x 1/2) + (1/2 x 1/2) = 1/2
Probability of RR ANDGg?? 1/4 x 1/2 = 1/8
d. what's the probability of RRgg? rrGg?
e. can use probabilities on phenotype ratios too! What is the probability ofthe wrinkled yellow phenotype?
Prob of wrinkled =
Prob. of yellow =
Prob of wrinkled AND yellow =
f. If do this cross-- get closer and closer to predicted genotype and phenotype ratios as sample size gets bigger
6. Law of Independent Assortment
a. applies to genes on different chromosomes
b. get 2n different types of gametes due to independent assortment during metaphase I of meiosis-- 3 loci on different chromosomes, 8 kinds of gametes just from independent assortment
c. Humans have 23 chromosomes. 223 = around 8 million different gametes then different gametes unite-- whew! (this is even without recombination)
7. What if alleles at different loci are on the SAME chromosome (linkage)??
a. No independent assortment
b. act just like cross with only one locus!
c. Ex: F1 is RTrt x RTrt
F2 RT RT
RT rt
rt rt
d. Would Mendel have figured out about independent assortment if some of the traits he studied had been linked and others were on different chromosomes?
8. Beyond Mendel
a. Many traits influenced by >2 genes (e.g. human height, skin color, p 250-251)
i. no discrete categories
ii. particulate inheritance still applies
iii. segregation and independent assortment of several genes leads to a nearly continuous phenotype distribution (Fig. 14.12)
b. Phenotypic expression of an allele can depend on alleles at other loci (epistasis, collaboration, modifiers pp. 250, Fig. 14.11)
c. Recombination = crossing over
i. occurs between homologs during Prophase I of meiosis (Figs. 13.6, 13.9)
ii. greatly increases the number of different kinds of gametes that can be formed, because it unlinks genes on the same chromosome!
iii. One of the major sources of genetic variation in populations
Next time: where does genetic variation come from???