6.2 - Genetic Inheritance Flashcards
How is variation caused?
Genetic variation:
Sexual reproduction
Mutations
Environmental variation:
Any variation caused by anything other than genetic differences
E.g. scars, direction roots grown in response to sunlight, water etc.
What is most variation caused by a combination of?
A combination of genetic and environmental factors
Phenotype def
Appearance of a characteristic
Phenotype def
Appearance of a characteristic
Genotype def
The genetic makeup of an organism
- influences phenotype
Monogenic def
Determined by a single gene
Gene locus def
Determined by a single gene
Heterozygous def
Having different alleles at the same gene locus on a pair of homologous chromosomes
E.g. Bb
Homozygous def
Having identical alleles at the same gene locus on a pair of homologous chromosomes
E.g. BB, bb
True-bred def
Homozygous (dominant or recessive)
Recessive def
Masked by dominant alleles (little letter)
F1 def
First generation of offspring (to original parents)
F2 def
Second generation (offspring of F2)
Two forms of variation
Continuous
Discontinuous
Continuous variation info
- no defined categories/distinct groups
- there is a range - any value possible
- caused by more than one gene (polygenic) and, often, the environment
- the greater the no. of gene loci contributing to characteristic, greater range in variation
- quantitative
Discontinuous variation info
- discrete categories - no intermediates, e.gl eye colour
- usually caused by one gene (monogenic)
- genes at diff loci interwct to influence one charcteristic and cause discontinuois variation (epistasis)
- no environmental effects cause it
- qualitative
How to test the genotype of a plant
Test cross with a homozygous recessive, e.g. tt for short plants
If ant offspring are small then the plant is heterozygous (dominant), Tt
T is tall - dominant
What does dihybrid inheritance involve?
Looks at the simultaneous inheritance of two characteristics (controlled by different gene loci)
E.g. seed colour and seed shape
The inheritance of one does not affect the other
What are the three blood groups?
A
B
O
What are the 4 blood group phenotypes and genotypes
AB
Bo
Ao
oo
O is recessive to A and B
B is recessive to A, but dominant to O
What letter is usually used for blood groups in punnet squares
I
Stands for immunoglobulin
Chromosomes for different genders
Male: XY
Female: XX
Male chromosome info
Chromosomes are not fully homologous
Y chromosome lacks many of the genes that code for characteristics on the X chromosome
Males can be neither homozygous or heterzygous for these genes
This makes males more susceptible to genetic diseases (In top portions of X and Y chromsomes)
Two diseases caused by sex-linked genes
Haemophilia
Colour blindness
Haemophilia info
Gene causing haemophilia found in X chromosome but not the Y
Functional allele: Xh
Faulty allele:
Female carriers (heterozygous) do not have the disease
Colour blindness info
Gene causing red-green colour blindness is found on tye X chromosomes but not the Y
Functional allele: XB
Faulty allele: Xb
Female carriers (heterzygous) do not have the disease
Tortoiseshell cats info
A gene for cat coat is sex-linked
The two alleles are:
Orange - C^O
Black - C^B
The two alleles are both dominant - they are co-dominant
When both black and orange alleles are present, coat colour is called tortoishell.
What is autosomal linkage?
When 2 or more genes on the same chromosome are inherited together
- they do not undergo independent assortment
- autosomes are the non-sex chromsomes
What are autsosomes?
The non-sex chromosomes
Ratio of dominant and recessive, etc. (not sure about rest) in dihybrid crosses
9:3:3:1
Epistasis def
An interaction of non-linked genes (on different chromosomes) where one masks the expression of the other.
Epistasis reduces the number of phenotypes in the F2 generation and so reduces variation.
Key terms and info for Epistasis
Epistasis reduces the number of phenotypes in the F2 generation and so reduces variation.
The alleles that are masking the effect of the alleles of the other gene are called epistatic alleles
The alleles whose effect is being masked are called the hypostatic alleles
What are epistatic alleles
The alleles that are masking the effect of the alleles of the other genes
What is recessive Epistasis?
Where the homozygous recessive alleles of a gene locus 1 are epistatic (prevent the expression of) to both alleles on gene locus 2 (hypostatic)
What is a “true bred” gene?
A gene that contains homozygous alleles
Dominant epistatis def
Where the dominant alleles of gene locus 1 are epistatic (prevent the expression of) to both alleles on gene locus 2 (hypostatic)
Epistasis by complementary gene action
The genes can work together
Ratios to look out for in genetic inheritance
• 3:1 phenotypic ratio for 2 genes/traits ‐ autosomal linked and no crossing over will have occurred
• A NON 9:3:3:1 phenotypic ratio showing 4 phenotypes for 2 genes/traits ‐ autosomal linked and crossing over will have occured
> The further apart the gene loci for the linked genes, the more likely crossing over is and the higher the number of recombinant phenotypes
• Any phenotypic ratio where there is a distinct male/female difference ‐ sex linkage
• Any modification of a 9:3:3:1 phenotypic ratio will show epistasis
> 9:3:4 recessive epistasis
> 12:3:1, 13:1 dominant epistasis
> 9:7 or 9:3:4 epistasis by complementary gene action
• 9:3:3:1 ratio ‐ generally dihybrid inheritance of non linked genes
Go to textbook to revise topic from start
What does a ratio of 3:1 show in inheritance
Shows ratio for 2 genes/traits
- autosomal linked and no crossing over occurs
What 9:3:3:1 ratio shows
Shows 4 phenotypes for 2 traits/genes
- autosomal linked and crossing over will have occurred
- further apart the gene loci for linked genes, more likely crossing over is and higher number of recombinant phenotypes
What does any phenotypic ratio where there is a distinct male/female difference show
Sex linkage inheritance, e.g. genetic diseases like haemophilia more common in men
What a modification of a 9:3:3:1 phenotypic ratio shows
Shows Epistasis
9: 3:4 - recessive Epistasis
12: 3:1, or 13:3 - dominant Epistasis
9: 7 or 9:3:4 - Epistasis by complementary gene action
What a 9:3:3:1 ratio also shows
Generally dihybrid inheritance of non-linked genes
What is Hardy-Weinberg pricniple
A principle that is used to predict allele frequencies within a population
When can Hardy-Weinberg principle be applied?
- large population
- random mating
- no selective advantage for genotype
- no gene mutation, migration or genetic drift
Hardy-Weinberg equations
p + q = 1
p^2 + 2pq + q^2 = 1
Two types of selection/natural selection
Stabilising selection
Directional selection
Stabilising selection info
Occurs when organisms’ environment doesn’t change
Favours intermediate phenotypes (Goldilocks, over extremes)
Reduces variation in a population
E.g. animals with very short/long fur selected against in constant temps, compared to animals with medium length fur
- higher frequency of alleles for mid length fur
Directional stabilisation info
Occurs when environment changes
Favours a new (extreme) phenotype
Causes a change in population mean phenotype
- e.g. climate change - temp decreases.
- animals with longer fur more likely to survive, breed and pass on allele for long fur
- over time allele for long fur becomes more frequent in population
Genetic drift def
Random changes in allele frequency in small populations
Genetic drift info
Happens in small populations (with a gene pool)
Chance mutations that are neither beneficial/harmful cause change in frequency of alleles
The population alleles can ‘drift’ from original
- only happens in small populations because each individual forms a larger proportion of gene pool and therefore has a greater effect on the gene pool.
It is also easier to ‘lose’ a gene from a small gene pool
2 types of genetic drift
Genetic bottleneck
Founder effect
Genetic bottleneck info
An event, e.g. flood, rapidly reduces the number of a population
Some alleles lost from population at random
Genetic variation reduced - genetic drift
Founder effect
A small number of individuals from an original larger population establish a new population
Some alleles lost from population at random (these could be advantageous)
Genetic variation reduced - genetic drift
Could lead to inbreeding - due to reduced alleles in gene pool
Also could lead to more homozygous recessive reproduction - more genetic diseases etc.
Speciation def
The splitting of a population of a species into 2 isolated populations that over time undergo genetic changes which result in reproductive isolation and therefore the formation of 2 different species.
How does speciation occur?
Via isolation
Two isolating mechanisms that cause speciation
Geographical isolation
Reproductive isolation
Two types of speciation
Allopathic - geographical isolation
Sympatric- reproductive isolation
Geographical isolation info (allopatric)
Populations are physically separated, e.g. by water, mountains, fences, etc.
Barrier prevents gene flow between populations
Genetic changes occur is species
- caused by genetic drift, mutations or natural selection
The populations become genetically so different
- they can no longer interbreed to produce fertile offspring (reproductively isolated)
- new species have been formed
Reproductive isolation info (sympatric)
Several things can lead to individuals in a population becoming reproductively isolated:
- behavioural changes e.g. changes to sleep patterns, courtship behaviours, etc.
- Biological changes e.g. size differences, genitalia differences
- genetic changes e.g. change in chromosome number prevents zygote viability
Once populations can no longer interbreed to produce fertile offspring (reproductive isolation)
- new species has been formed
Explain why a collection of small islands remote from a mainland provides optimal conditions for speciation.
(2 Marks)
- geographical isolation (don’t accept allopatric)
- different species on different islands have different selection pressures
- species on islands can only reproduce with organisms/species on the same island
- small populations/gene pools
Suggest how different isolating mechanisms allowed tame dogs to evolve separately from wolves.
(3 Marks)
Geographic:
- wolves avoid human settlements
Behavioural:
- differences in courtship/pheromones
Mechanical:
- idea of different size of wolves and some small dogs
Gamete incompatibility:
- possibility of different chromosome number
Seasonal/temporal:
- Different breeding seasons/times
Artificial selection def
Selective breeding of organisms in order to produce desired phenotypes in an organism
- often to benefit humans
Humans chose parents with desired phenotypes and therefore desired alleles
- interbreed them to produce offspring with higher frequency of these phenotypes
Repeated over many generations
Stages of artificial selection - simple
Male and female with desired characteristic chosen
Male and female interbred
Best offspring selected and interbred
This is repeated over many generations
Examples of selective breeding/artificial selection in animals and plants
Animals:
- highest yield of milk, meat
Plants:
- highest yield of crops,
Problems with natural selection/selective breeding
- Inbreeding
- Ethical considerations of selective breeding
Artificial selection problems - inbreeding info
Artificial selection problems - ethical considerations info
