4 Flashcards
define genotype
the genetic constitution of an organism
- defined in relation to a particular gene or gene combinations
- eg Aa, Bb
define phenotype
feature of the organism as observed
- used when discussing a trait of an organism that varies
- eg size, fur colour, mRNA expression level
define genome
the entirety of an organism’s DNA
- includes genes and non-coding regions
state 3 sources of genetic variation
- mutation
- independent assortment
- recombination
define a mutation
a stable change in the DNA sequence
how often do mutations occur?
at a low rate
- mutation rate varies in ways that are partially predictable
what are the different possible effects of mutations on fitness?
- neutral (won’t matter for the fitness of the organism)
- deleterious (weakly detrimental up to lethal)
- beneficial
state the 4 main 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
- it is random with respect to effects on fitness
- it is not ‘summoned’ to make things better - rate depends on the type of mutation, and can also vary among genes
- the environment can affect the mutation rate (eg mutagens, high temp)
what are the 4 types of mutation?
- point mutations (a simple substitution of a nucleotide)
- insertions/deletions (‘indels’) - adding/removing a nucleotide
- changes in repeat number
- chromosomal rearrangements (eg inversions)
describe a mutation involving a change in repeat number
- when there is a repeated motif (eg ATG) within the gene, the motif may accidentally be added/removed an extra time
- the repeated motif makes it harder for the replicative machinery to replicate
describe an example of a chromosomal rearrangement
inversions:
- flipping of the nucleotide sequence
- occurs when there is a double break in DNA strand; when enzymes try to put the two strands back together, it is hard to know which one was forward and which one was backwards
give an example of a method that can be used to identify a new mutation?
a trio study:
- two parents (reproducing organisms) are selected
- their genome and the genome of their offspring is sequenced
what is the rate of new mutation in humans?
per base pair of DNA: 16 in every billion nucleotides each generation
per individual genome (two copies of our 3 billion base pair genome): approx. 96 new mutations per zygote
for the entire human population (8 billion): every base pair in the genome mutated about 126 times over per generation
why do some mutations not lead to amino acid changes?
we have more codons than amino acids - so sometimes mutations don’t make a difference to the amino acid sequence (= silent substitution)
give an animal example of how single mutations can cause profound effects on traits
- Antp Hox gene mutations in Drosophila
- results in an extra pair of wings, for example, or a set of legs, instead of antennae, growing from the fly’s head
describe G6DP deficiency in humans?
Glucose-6-phosphate dehydrogenase
most common enzyme deficiency in humans
- causes severe anemia
- also protects against malaria
- 2 amino acid replacement mutations in the coding sequence of G6PD are associated with disease allele (A-)
what are polymorphisms?
- A gene is said to be polymorphic if more than one allele occupies that gene’s locus within a population.
- each allele must also occur in the population at a rate of at least 1% to generally be considered polymorphic.
what are allozymes?
Alloenzymes are variant forms of an enzyme which differ structurally but not functionally from other allozymes coded for by different alleles at the same locus.
give a flow chart for how mutations in G6PD lead to genetic variation
DNA (polymorphisms) -> proteins (allozymes) -> phenotype (polyphenisms)
define independent assortment
the alleles of two (or more) different genes get sorted into gametes independently of one another.
how does independent assortment in segregation during meiosis generate diversity?
- allows different combinations of parental chromosomes
- 2^n, where n=sets of chromosomes
how can we tell if a mutation has increased or decreased fitness from a fitness distribution bar chart?
if the mutation leads to a fitness less than 1 (which is the fitness of the ancestor) then they are detrimental
describe how recombination during meiosis (metaphase) further contributes to variation
there is synapsis of bivalents, leading to crossing over at chiasmata and recombinant chromosomes.
how did scientists believe that heredity worked before Mendel?
performatifs (1700s):
- spermists/ovists believed only one parent contributed to inheritance
theory of blending inheritance (1800s)
- postulated that factors from both parents mix together irreversibly
what is the problem with blending inheritance for evolution by natural selection?
there would be no way for a beneficial mutation to increase in frequency across generations
describe Mendel’s experiment with diagrams
- hybrid cross of pure-breeding lines (yellow and green)
- self fertilised (all yellow)
- offspring were 3/4 yellow and 1/4 green
state the key conclusions from Mendel’s pea experiments
- inheritance is determined by discrete particles, genes
- each diploid organism carries two copies (alleles) of each gene.
- alleles can exhibit dominance/recessivity
- gametes contain only one allele per gene - gametes fuse to make offspring
- sperm/pollen fuses with egg/ovule - offspring inherit one gamete from each parent at random
- one allele per gene at random from each parent
describe phenotypic polymorphisms with simple mendelian genetic causes
- common in nature
- direct correspondence between trait and its genetic basis
- easy to track selection and evolution
what are the two types of genetic variation?
discrete variation:
- mendelian genetics
- genes of major effect, dominance, and recessiveness
- involves a spread of alleles and a change in allele frequency
continuous variation
- quantitative genetics
- many genes each with alleles of small effect, important environmental effects
- selection response as change in average trait value
define discrete variation
Discrete variation refers to traits with a finite number of phenotypes, often controlled by single genes
define continuous variation
Continuous variation, on the other hand, displays an infinite number of phenotypes across a continuum and is usually polygenic.
how is the phenotype of a quantitative trait established?
- often affected by many factors
- complex polygenic inheritance
AND - environmental factors
