Lecture 4: Evolutionary Significance of Genetic Variation Flashcards
Where does heritable variation come from?
Mutation, independent assortment, recombination.
What is mutation
- a stable change in the DNA sequence
- occurs at a low but variable rate
- not directed by organism or environment
Mutations can be…
neutral, deleterious, lethal, or beneficial
What can affect rate of mutation?
Environmental factors like high temperature, mutagens.
Point mutation
Mutation affecting only one nucleotide in gene sequence. Involved substitution of one nucleotide.
ATGCAGT –> ATCCAGT
Frameshift mutation
Insertion/deletion. Extra nucleotide inserted in sequence affects all codons after insertion or deletion
ATGCAGT –> ATGGCAGT
Silent mutation
Type of point mutation. Because of the redundancy of genetic code, there is no change in the amino acid.
Missense mutation
Type of point mutation. The substitution changes the amino acid.
Nonsense mutation
Type of point mutation. Occurs when nucleotide substitution results in an amino acid codon changing to a stop codon (makes polypeptide too short and thus non functional)
Duplication Mutation (changes in repeat number)
codon is abnormally copied
Translocation mutation
Chromosomal level mutation where there is a translocation of groups of base pairs from one part of chromosome to the other
Inversion mutation
Chromosomal level mutation which occurs when a segment on the chromosome is cut out, turned around and reinserted backwards.
What is the rate of new mutation in humans?
- per base pair per generation -> 1.2 x 10^-9
- 72 new mutations per individual
- in entire human pop. EACH base pair is being mutated on average 92 times every generation
At which letter in a codon does a mutation have the greatest effect?
First letter. Mutation of third letter is most likely to end up as a silent mutation.
Number of fitness-affecting mutations per diploid genome each generation of humans?
1.6 - 3 (greater than fruit fly, worm, plant, and rat)
What is G6PD?
Glucose-6-Phosphate Dehydrogenase is an enzyme involved in the function of red-blood cells. Deficiency of this enzyme is the most common deficiency in humans and causes severe anemia but protects against malaria.
What is G6PD deficiency caused by?
The deficiency allele is associated with 2 amino acid replacement mutations in coding sequence of G6PD.
A-allele that has reduced enzyme activity has the sequence Met Asp Leu while the wild type has Val Asn Leu .
How does Independent Assortment affect diversity?
Independent assortment of parent chromosomes during meiosis allows different combos. Shuffling of chromosomes makes millions of different combos.
What other than independent assortment generates diversity?
recombination.
What did spermists/ovists believe in the 1700s? What is blending theory?
They believed only one parent contributed to inheritance. Blending theory in 1800s suggested parents genes mixed together irreversibly.
What is Gregor Mendel known for?
Experiments with pea plants. He discovered fundamental laws of inheritance—that genes come in pairs and we inherit two distinct units, one from each parent.
What were Mendel’s conclusions from pea experiments (4)?
- Inheritance determined by discrete particles (genes)
- Each organism carries two copies (alleles) of each gene
- Organisms form gametes (pollen/sperm, eggs/ovules) containing only one allele
- Offspring inherit one allele from each parent at random.
Genotype
Genetic constitution of an organism—used in relation to a particular gene or gene combo ex. Aa, AaBB
Phenotype
The organism as observed—used when discussing a trait or feature of an organism that varies.
Genome
The organism’s DNA including both genes and non-coding regions.
Discrete trait
Simple, mendelian genetics, One phenotype or the other is apparent. You have it or you don’t (tongue rolling).
Continuous trait
complex inheritence, multiple genes factor into the corresponding phenotype (height)
R. A. Fisher, J. B. S Haldane, and S. Wright
Initiated theoretical population genetics in the 30s. Showed that continuous variation and darwinian natural selection were consistent with mendel’s laws. Also demonstrated evolutionary significance of genetic variation.
What is the relationship between gene number and phenotypic variability?
The more genes involved in creating the phenotype the more variation we see. If aa codes for blue and AA/Aa codes for yellow (two phenotypes), in another case Aa (heterozygote) might code for green. Two genes involved would give 5 different phenotypes.
What are quantitative traits affected by?
Genes and environment. It is very difficult to make simple connections between genotype and phenotype as it’s hard to control for environmental differences in humans.
Discontinuous variation
genes of major effect, dominance and recessiveness, spread of alleles. You can deduce phenotype from genotype.
Continuous variation
many genes of minor effect, important environmental effects, selection response.
