Lecture 16 Flashcards
Where Does Heritable Variation
Come From?
1) Mutation
2) Segregation and independent
assortment (Mendel’s laws)
3) Recombination
Mutation
stable change in the DNA sequence
* Occurs at a low rate
– Varies in ways that are partially predictable
* Different possible effects*:
– Neutral
– Deleterious (weakly detrimental up to lethal)
– Beneficial
*on fitness vs. traits unrelated to fitness
Characteristics of mutation
Mutation is an inevitable phenomenon
– Despite cellular mechanisms to correct errors
during DNA replication
* Mutation is not directed toward an outcome
by the organism or by the environment
– Random with respect to effects on fitness
– Not “summoned” to make things better
* Rate depends on the type of mutation
– Can also vary among genes
* Environment can affect mutation rate
– E.g. mutagens, high temperature
Mutation and the Structure of DNA
1) Point mutations
2) Insertions/deletions (“indels”)
3) Changes in repeat number
4) Chromosomal rearrangements (e.g. inversions)
ATGCAGT ATCCAGT
ATGCAGT ATGGCAGT
ATGATGATGATG ATGATGATGATGATG
ATGCAGT TGACGTA
What is the Rate of New Mutation
in Humans
Per base pair of DNA:
* 0.0000000123 = 12.3 in every billion nucleotides
each generation
* Per individual genome (two copies of our 3
billion base pair genome):
* Approx. 74 new mutations per zygote
* For the entire human population (7.9 billion):
* Every base pair in the genome mutated about 97
times over on average
G6PD Deficiency in Humans
Most common enzyme deficiency in humans
* Can cause severe anemia due to red blood cell
breakdown (with certain foods)
* Also protects against malaria
G6PD Deficiency in Humans
2 amino acid replacement mutations in the coding
sequence of G6PD associated with disease allele
– Analysis of human DNA sequence diversity
Heredity before Mendel:
Organisms Resemble their parents,
BUT HOW?
Preformationism (1700s)
– Spermists/ovists believed
only one parent
contributed to inheritance
* Theory of blending
inheritance (1800s)
– Postulated that factors
from both parents mix
together irreversibly
Key Conclusions from Mendel’s
Pea Experiments
1) Inheritance is determined by discrete particles
* Genes
2) Each diploid organism carries two copies of each gene
* Alleles can exhibit dominance / recessivity
3) Gametes fuse to make offspring
* Sperm / pollen with egg / ovule
* Gametes contain only one allele per gene
4) Offspring inherit one gamete from each parent at random
* One allele per gene at random from each parent
Independent Assortment in Segregation
During Meiosis Generates Diversity
- Allows different combinations of parental
chromosomes
– e.g. 2 sets of chromosomes: 22 = 4 possible combinations
of gametes, 22 * 22 = 16 possible zygote combinations - Humans: 23 sets of chromosomes
– 223 = 8.4 million different combinations - Fern Ophioglossum petiolatum: 510 sets of
chromosomes
– 2510 = 3.4 x 10153 different combinations
Genotype
Genetic constitution of an organism
– Defined in relation to a particular gene or gene combinations
– e.g. Aa, AaBB
Phenotype
Feature of the organism as observed
– Used when discussing a trait of an organism that varies
– E.g. size, fur color, enzyme activity, mRNA expression level
Genome
The entire organism’s DNA
– Includes genes and non-coding regions
Simple Mendelian Genetic
Polymorphism
Common in nature
* Direct correspondence between
trait and its genetic basis
* Easy to track selection & evolution
Phenotypic Variation* in Human Height
Most traits vary continuously, not with discrete categories
-approx. 80% of height variation is due to genetic differences
in populations with healthy nutrition and sanitation
