Inheritance And Evolution Flashcards
Gene
A base sequence of DNA that codes for the amino acid sequence of a polypeptide which results inn a characteristic
Allele
One or more alternative versions of the same gene
Genotype
The genetic constitutions of an organism - the different alleles it has
Phenotype
The expression of the genetic constitution and its interaction with the environment - what characteristics an organism has as a result of its genes and the effects of the environment on them
Gene locus
The position on a chromosome where a particular gene is found
Dominant alleles of a gene
An allele whose characteristic appears in the phenotype even when there is only one copy
Recessive alleles of a gene
An allele whose characteristics only appears in the phenotype if two copies are present
Co dominant alleles of a gene
An allele whose characteristics appear together with another allele in the phenotype because neither allele is recessive
Multiple alleles
More than two alleles of a gene
Diploid
A cell with two of each type of chromosome
Homologous pairs of chromosomes
Haploid
A cell with only one of each type of chromosome
Heterozygous genotype
Two different alleles at the same gene locus on each of a pair of chromosome
Homozygous genotype
Two of the same alleles at the same gene locusts each of a pair of chromosomes
Monohybrid inheritance
The inheritance of a single characteristic controlled b one gene with two or more alleles
Dihybrid inheritance
The inheritance of two characteristics which are controlled by different genes
Sex linked characteristic
Alleles that code for the characteristics are located on the sex chromosome (X OR Y)
Autosomal chromosomes
A chromosome that is not a sex chromosome
Autosomal linkage
When two genes coding for different characteristics are located on the same chromosome and consequently are inherited together.
Epistasis
When the alleles of one gene mask the expression of alleles on another gene.
Dominant epistasis
Just one copy of the epistasis allele will block expression of the second gene
Recessive epistasis
Two copies of the epistasis allele are required to block expression of the second gene.
Dihybrid crosses with no linkage
Inheritance of two different characteristics controlled by two different genes found on two different chromosomes
In meiosis 1 homologous pairs segregate independently this mean that
Genes on different chromosomes are inherited independently
Epistasis
3
An allele of one epistasis gene masks (blocks)
The expression of a second gene
One characteristic controlled by two genes
Epistasis can be either recessive epistasis or dominant epistasis
Recessive epistasis
Only a homozygous recessive genotype on the epistatic gene masks the expression of the second gene.
Dominant epistasis
Occurs when the epistatic allele is dominant, the organism can be heteros or homozygous to show the effect
Mutation
Random changes in base sequences of genes to give new allele
Meiosis
Crossing over and independent segregation of homologous pairs
Homologous pairs form
Bivalents at the start of meiosis 1
Crossing over occurs
Sections of non sister chromatids are exchanged to give new combinations of alleles in the gametes. Chiasma are the points a which the crossing over occurs
During meiosis 1
Homologous pairs are separated
The homologous pairs segregate independently giving many combinations of maternal and paternal alleles in the gametes.
Random fusion of gametes
Any gamete from one parent is equally likely to fuse with any gamete from the other parent regardless of the allele combinations they contain.
Autosomal genes
Yess carries on chromosome 1-22
How many genes on X chromosome in humans
Roughly 2000
Y chromosome
Much smaller
Carries only 78 genes
In males the X chromosome
I inherited from ty mother therefore all males with a recessive sex linked condition must have a mother who is a carrier of the condition (or has the condition)
All females receive an X chromosome from
Their father- if he has a recessive sex linked condition then the daughter will be a carrier of the condition (or have the condition)
Males have only one
X chromosome and will have a recessive sex linked condition with just one recessive allele
Females have two
X chromosomes and will only have a sex linked recessive condition if homozygous for the recessive allele
Dihybrid crosses with no linkage
Inheritance of two different characteristics controlled by two different genes found on two different chromosomes
In meiosis 1 homologous pairs segregate independently
This means that genes on different chromosomes are inherited independently
Epistasis
3
An allele of one epistatic green masks the expression of a second gene
One characteristic controlled by two genes
Can either be recessive epistasis or dominant epistasis
Linkage
Inheritance of genes located on the same autosomal chromosomes
Chi squared test
Statistical test used to compare actual and expected results to determine whether the differences are due to chance or are significant
Degrees of freedom
Number of categories - 1
Population
A group of organisms of the same species living in a particular place at a particular time, the organisms are free to interbreed with one another
Gene pool
The complete range of alleles present in a population
Allele frequency
How often an allele occurs in a population expressed as a percentage
Genotype frequency
How often a genotype appears in a population expressed as a percentage
Hardy-Weinberg model can only be used if allele frequency doesn’t change over time, following conditions must apply
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Large population- less likely for genetic drift
Mating is random
One allele does not confer an advantage so that natural selection will not occur
There are no emigrants or immigrants
No mutations occur
Equation for allele frequency
P + q = 1
Equation for genotype frequency
P^2 + 2pq + q^2 = 1
P
Dominant allele
Q
Recessive allele
P^2
Homozygous dominant genotype
2pq
Heterozygous genotype
Q^2
Homozygous recessive genotype
Calculating actual allele frequency
When alleles are co dominant and the heterozygous phenotype is distinguishable from the homozygous phenotype
Intraspecific variation
Between members of a population and between membres of a species
Interspecific variation
Between members of different species
Causes of phenotypic variation
2
Genetic variation between individuals due to different individuals possessing different allele combinations
Environmental differences between individuals
Causes of genetic variation
2
Gene mutations to create new alleles
Crossing over of non sister chromatids
of homologous pairs in meiosis 1
Independent segregation of homologous chromosomes in meiosis 1
Random fusion of gametes
Evolution is
The change in allele frequency in a population over he course of many generations
Two causes of evolution
Natural selection
Genetic drift- in small populations
Natural selection
Changes allele frequency in populations over the course of many generations
Natural selection
7
- individuals subject to selection pressure such as competition, predation, disease
- phenotypic variation due to genetic variation, different combinations of alleles
- there is differential reproductive success
- individuals with advantageous alleles have reproductive success
- process repeated over many generations
- frequency of advantageous alleles in gene pool increases
- population becomes more adapted to its environment
Stabilising selection
4
Occurs in a stable environment
Individuals with extremes of phenotypes have lower reproductive success
Mean phenotype remains unchanged but more of the population becomes closer to the mean
Standard deviation and range is reduced
Directional selection
3
Occurs when the environment changes
Individuals with phenotype of one extreme have higher reproductive success
Mean phenotype in the population shifts towards the favoured extreme over many generations
Disruptive selection
3
Occurs when the environment favours individual of both extreme
Individuals with phenotype of both extreme have increased reproductive success
After many generations it may result in two different breeding populations- speciation
Genetic drift
The random changes in frequency due to chance rather than natural section
Genetic drift process
4
Individuals in a population show variation in their phenotype
By chance those with one allele happen to have more offspring so more of the population carry the allele
By chance the process is repeated, those with the allele produce more offspring and the frequency of the allele increases in the population
If enough differences inn allele frequency build up over many generations genetic drift can lead to speciation
Genetic drift is. More evident in small populations because
Chance differences are less likely to cancel each other out. In a small population,small populations have a bigger effect
Genetic drift increases the genetic diversity of a population
Speciation
The development of a new species from an existing species
A population becomes a new species when
It’s members can no longer interbreed with and produce fertile offspring with the original population
Two forms of speciation
Allopatric and sympatric
Allopatric speciation
Occurs when two populations become physically separated and there is no interbreeding / gene flow between the populations
Allopatric speciation process
6
A sub group becomes geographically isolated by physical barrier
Isolated population experiences different environmental conditions an directional selection will occur changing allele frequency
Different random mutations will occur in isolated and original population
Genetic drift will occur changing allele frequency in isolated population
After many generations, individuals from the isolated populations allele frequency wil be so different that they willl no longer be able to interbreed to produce fertile offspring with the original population
So many differences in gene pool that the two populations are reproductively isolated
Sympatric speciation
When two populations are still occupying the same habitat but a change has occurred to stop them interbreeding with one another
Two populations become reproductively isolated and stop interbreeding
Mechanisms of reproductive isolation
Sympatric speciation
3
Seasonal changes= due to a mutation some plants may flower at a different time of the year , they can therefore only interbreed with plants flowering at the same time
Mechanical changes= gene mutations which lead to changes in size and functioning of the genitalia making mating between two sub groups physically impossible
Behavioural changes= a sub group may develop. A different courtship ritual that only attracts mates within the sub group
