Crossing over and independent assortment

Adenine and guanine

Why was complementary pairing of DNA such an important discovery?

It provides a mechanism for copying genetic information.

Describe the “redundancy” of the genetic code, and who many codons and amino acids are used?

The same amino acid can be specified by more than 1 codon (usually 4 codons). 64 codons are used, and there are 20 amino acids.

What’s the difference between a transition and transversion mutation? 

A transition is a purine/purine or pyrimidine/pyrimidine point mutation. A transversion is a purine/pyrimidine or pyrimidine/purine point mutation.

Transitions are more common because they cause little disruption in the double helix and often don’t get noticed as an error.

None, they are the same thing: point mutations that result in amino acid change

9.  How were mutation rates traditionally estimated and did this method over or underestimate the rate of mutation, and why?

Traditionally, mutation rates were estimated by the number of offspring per generation that had observable mutant phenotypes. These were underestimations of mutation.

When scaled on a per genome per cell basis how greatly do mutation rates vary between bacteria and vertebrates?

Not very much. Equal mutation rates. Due to generation time and population size.

Studied hermaphroditic roundworm Caenorhabditis elegans and its descendents. Looked at individuals from generation 280, 353, and 396. Found a mutation rate of 2.1 mutations/genome/generation.

Small numbers of bases that had been inserted or deleted from the genome.

Was the mutation rate higher or lower for the mitochondrial genome and why?

Higher. Mitochondrial DNA lack some DNA repair enzymes found in nuclear DNA.

Both species and populations have different mutation rates.

Is there natural selection on mutation rates? Give an example of a study that addressed this hypothesis 

Yes; E. coli populations with high mutation rates do well in a new or fast changing environment, but populations with low mutation rates do well over time and in stable environments.

30%?

Alleles that have no affect on fitness

Gene duplications

Retrotransposition and Unequal Cross Over

How could you detect a gene duplication even that took place by retrotransposition?

Retrotransposed genes lack introns and code for a poly A tail

.01 duplications/gene/million years; similar to the rate of point mutations

When did humans and chimps diverge, and what fraction of the genomes of humans and chimps has been duplicated in one or the other species?

There may be no change; both are used. The duplicated gene might accumulate mutations without consequences to the phenotype. It might change function over time and become an entirely new gene.            

Similar protein function and structure suggest that the globin gene family might be a product of gene duplication.

Paralogus: genes that are duplicated and then diverge in sequence (within the same species)

Orthologous: genes that are homologous but have diverged after a speciation event (among different species)

What is the largest gene family recorded to date and what function are these genes involved in? 

1,296 paralogous genes involved in mouse olfactory reception

Linkage is the tendency for alleles of different genes to assort together at meiosis. Heterozygous inversions do not allow the sequences to align properly during meiosis I for crossing over events.

Flies have different frequencies of inversion in varying climates. One reason might be that alleles inside inversions affect body size (NS selects big body size in cold and small body size in hot areas)

Genome duplication results in polyploidy (2 or more chromosome sets)

Polyploidy often results in reproductive isolation from the parent population.

As frequent as point mutations (2 out of every 100,000 offspring are tetraploid)     

+/+  (susceptible), +/Δ32 (susceptible but slower progression to AIDS), Δ32/ Δ32 (mostly resistant).      

DNA extraction, PCR and Gel Electrophoresis, Δ32 alleles showed a shorter band of 371 bp than the + allele (403 bp)

20.9%, (10/241 = 4.1%)

What are two methods of estimating heterozygosity? 

PCR and Gel Electrophoresis. Direct Sequencing. The average frequency of heterozygotes across loci, or the fraction of genes that are heterozygous in the genotype of the average individual.

What aspect of the Cystic Fibrosis disease makes it difficult to treat clinically?

Genetic variation is high, hundreds of alleles are present in most populations.

Why is there so much genetic variation in populations?  Give two reasons.

Selectionist theory: Genetic diversity is maintained by natural selection – in favor of rare individuals, heterozygotes, or different alleles in different times and places.

Neutral Theory: most of the alleles at most polymorphic loci are functionally and selectively equivalent and are maintained by genetic drift (it is not eliminated by selection)