Measuring Genetic Variability Flashcards
What are the two types of characters
Discrete characters → Mutually exclusive categories
e.g flower colour, fur colour
Quantitative → individuals vary continuously
e.g height, length, skin tone (melanism)
What do genotypes affect
Most phenotypes are partly determined by genotype, which accounts for resemblance between parents and offspring
What are genes, loci, and alleles
Gene = a discrete unit of inheritance
Locus (plural = loci) = a hypiothetical position / location of a gene
Allele = a variant in a gene
Example: the colour (phenotype) of a pea flower (which is dilpoid) is a discrete character, determined at a single locus, by genes of 2 possible alleles P or p
What are dominant vs recessive alleles
Refers to the attributes of an Allele
A dominant Allele (uppercase letter) is one that only requires one copy to be present and expressed in the phenotype
A recessive Allele (lowercase letter) requires two copies to be expressed in the phenotype
What is segregation
‘segregation’, During meiosis only one copy of each loci is contributed to the gamete (with equal probability)
Then the two gametes fuse into the diploid zygote with 2 alleles for each loci (one from each parent)
What is homozygous vs Heterozygous
If they are both the same allele, that loci is Homozygous (PP)or(pp)
If they are different alleles, that loci is heterozygous (Pp)
What is Mendelian inheritance
Dominance of some Alleles explains why some characteristics in a daughter population aren’t present in the parent generation
This pattern of inheritance discovered by Gregor Mendel
This simple pattern of inheritance of discrete phenotypes as “Mendelian”
What is Independent assortment
Independent assortment is when during gametogenesis (meiosis) in a diploid germ cell, each copy/allele at a locus is equally likely to be passed to a given gamete.
Punnett Squares can be used to explicitly list all the genotype possibilities under independent assortment
What happens when there are two loci in a gene
With something like a two loci genotype with (YyRr)
9 possible genotypes are possible
YYRR (1), YYRr (2), YYrr (2), YyRR (4), YyRr (1), Yyrr (2), yyRR (1), yyRr (2), yyrr (1)
with four possible phenotypes
YR, Yr, yR, yr
what are the two important stats
- mean - the average or ‘middle’ value
- variance - the dispersion or ‘spread’
What is the mean of a discrete variable of character equivalent to
Mathematically, the mean of a discrete variable of character is equivalent to the proportion (a number that ranges from 0 - 1)
Applied to any given genotype of allele, the population mean is simply the proportion, referred to as genotype frequencies and allele frequencies
How do you calculate allele frequency
Allele frequencies are denoted p and q
The allele frequency is usually calculated by
P = (2PP + Pp) / (PP+Pp+pp)
p = (2pp + Pp) / (PP+Pp+pp)
What happens in quantitative characters
For many continuously varying (or quantitative or polygenic) characters many loci contribute to phenotype with each locus possibly having many possible alleles.
What are Phenotypic vs Genetic variances
Phenotypic variance: a measure of the total observable variation among individuals
Genetic variance: a measure of the genetic contribution to the difference among individuals
What is Heritability
Heritability h^2 is the ration of
Genetic variance / phenotypic variance
measure of average parent-offspring resemblance
What happened in 1858
In 1858 Darwin and Wallace both announced their co-discovery of descent with modification
How do genes tie into evolution
Evolution is the natural outcome of unequal reproduction among individuals that bear heritable characters
The Inheritance of characters is based in genetics
How was blending inaccurate
But this model was not always accurate on its predictions as offspring often had the characteristic of one of their parents and characters were not all converging on the average since variation persists
What was the blending genetic model
Up to the 1800s inheritance was not understood. Though cells had been discovered the function of the nucleus and its contents was not known.
A prevailing model at the time was based on “blending” where offspring would be an intermediate of parental characters.
What was Gregor Mendel theory
The Particulate hypothesis
is the idea that parents pass on discrete heritable units called genes.
Gregor Mendel correctly inferred the mechanism of inheritance, despite not being able to “see” genetic material and DNA was not discovered
Direct a genome
In a diploid genome the genome is divided into loci (a specific part of the genome)
Each loci will have two alleles, one from each parent.
More than two Alleles are possible for each Loci, each Allele is a different option, example eye colour
What are homozygous vs heterozygous
If they are both the same Alleles the Loci is Homozygous
If they are two different Alleles the Loci is Heterozygous
What does lowercase vs uppercase mean with alleles
Dominant = uppercase
Recessive = lowercase
What is the principle of Independent assortment
The allele is passed at one loci has no bearing on which allele is passed on at another loci
Imagine that in pea plants the colour of a flower is based on a single locus, where P = Purple, and p = white
What are the possible genotypes and phenotypes
PP = purple, Pp = purple, and pp = white
In this situation, two white flowered plant will always lead to another white flower.
- genotypes Pp is heterozygous
- and genotypes PP and pp are homozygous
If two Pp individuals fertilized, the ration of possible genotypes in the offspring would be _____
and the ration of phenotypes would be _____
PP:Pp:pp = 1:2:1
Purple:white = 3:1
What statistical attributes are Evolutionary geneticist concerned with
Evolutionary geneticists are primarily concerned with 2 statistical attributes of a population
- mean - the average or “middle” value
- Variance - the dispersion or “spread”
With Alleles (a discrete variable), the mean frequency of an allele is actually simply its proportion
What is Co-dominance
Co-dominance is when the resulting characteristic in the offspring is a blend of the two parents
Ex: red+white flower → pink flower
This can happen when there are two dominant alleles both being expressed at the same time
R+W → 1/2R + 1/2W
What can happen when there are more than 2 alleles for a loci
For some loci there are more than 2 alleles which influence the phenotype
for example in human blood, there is an A, B, and o allele. (A+B dominant, o recessive)
resulting in blood types A, B, o, and AB
What is linkage
In reality independent assortment does not always hold
The closer loci and genes are to one another (physically on a chromosome) the more likely it is that these will be inherited together during meiosis.
What is Epistasis
When a phenotypic effect of a gene/allele at one locus depends on the gene/allele at another locus
Ex
The effect of the B/b alleles on coat colour in laborador retrievers depends on the E/e locus. ee makes the coat yellow, if its Ee or EE than the B/b alleles influences the coat colour to be various browns
What is Pleiotrophy
when phenotypic effects of a gene/allele at one locus affects more than one character
What are quantitative characters
For some characters like human height, many loci contribute to the phenotype, with each locus having many possible alleles. They act roughly additively. This is why human height exists on a range.
These characters are called quantitative, and vary continuously
What is variance
The variance is a value defined as the sum of each individuals squared deviation from the mean divided by the ideal number of individuals
(∑(n-n)^2 )/N
standard deviation
what is the mean
the mean is the average or middle value
(∑n)/N
What is h^2
Heritability, h^2, is the ration (or proportion) of
genetic variance / Phenotypic variance
roughly a measure of average parent offspring resemble
Genetic variance is a measure of the genetic contribution to the differences among individuals
Phenotypic variance is a measure of the total observable variability among individuals
Besides genotypes what can effect Phenotypes
Other things that affect phenotypic variance include:
- environmental background
- indirect genetic effects
- Gene interactions (Epistasis)
- Epigenetic effects (Gene expression controlled outside of the nucleus)
