6.1.2 Patterns of Inheritance COMPLETE Flashcards
DEFINITION- Gene
A length of DNA that codes for a polypeptide or protein
DEFINITION- Allele
Form of a gene
DEFINITION- Locus
The position of a gene on a chromosome
DEFINITION- Dominant
An allele that when present will always express itself in the phenotype
DEFINITION- Recessive
An allele that will only express itself hen no dominant allele is present and there are two recessive
DEFINITION- Codominant
Where two alleles of the same gene are both expressed in the phenotype
DEFINITION- Homozygous
The alleles of a gene in a cell or organism are both the same
DEFINITION- Heterozygous
The two alleles of a gene are different
DEFINITION- Genotype
The alleles present for a particular trait
DEFINITION- Phenotype
The way the genotype is expressed, the characteristics of an organism
DEFINITION- Homologous
Chromosomes that have the same genes at the same loci
DEFINITION- Sex linkage
Males have XY females have XX chromosomes. So females can be carriers of recessive diseases but males will suffer from it if its on the X.
DEFINITION- Autosomal Linkage
The two genes are both on the same chromosome so they will not show independent assortment in meiosis, therefore allele combinations tend to remain the same.
Two types of Variation
Interspecific- Between species
Intraspecific- Within species
Causes of Genetic Variation
Random mutations- Chromosomes - DNA Recombination alleles- Crossing over P1 -Independent assortment chromosomes M1 -Independent assortment chromatids M2 -Random Fertilisation
Environmental Variation
Phenotypic variation caused by the environment, i.e. language spoken.
Etiolation- Plants grow tall and spindly when grown in the dark
Chlorosis- Plants develop yellow leaves due to lack of chlorophyll form lack of minerals (mg, Fe)
Diet- Body mass, height ect.
Discontinuous Variation
Phenotypes fall into distinct categories with few intermediates, often monogenic and displayed on bar charts.
Little effect from the environment
i.e. blood groups, pea colour
Continuous Variation
A continuous range of values between to extremes, usually produce a normal distribution curve. Controlled by many genes (polygenic) and they tend to have an additive effect on the phenotype. Influenced by the environment more.
i.e. Height, leaf size, heart rate.
Monogenic inheritance
Produces 1 homozygous tall, 2 heterozygous tall and 1 homozygous short.
Mendels Laws
1st- Organisms characteristics are determined by pairs of factors (genes) only one of each pair can be represented in a single gamete
2nd- Each pair of contrasted characters may be combined with another pair, independent assortment
Test cross
Used to test the genotype of an organism showing the dominant characteristic, if any offspring show the recessive characteristic the parent must be heterozygous
Codominance
Homozygous red= CrCr
Homozygous white= CwCw
Codominant= CwCr
Ratio is 1:2:1
Multiple Alleles
Characteristics that are coded by more than 2 alleles of a gene e.g. blood groups.
Sex linked inheritance
X chromosome is much bigger than Y so contains many more genes, diseases found on the X will more likely lead to males suffering with the disease and females carrying. e.g. Haemophilia
Haemophilia
A recessive sex linked condition which prevents normal blood clotting, a carrier women genotype would be XHXh whereas a males is XhY so they’ll have the condition. He cannot pass it on to his son as his son will receive his Y chromosome.
DEFINITION- Population
Group of individuals of the same species in a given area that can interbreed
DEFINITION- Gene Pool
All the alleles for the characteristic of the whole population
DEFINITION- Gene Frequency
The number of specific alleles
DEFINITION- Evolution
The change in characteristics of organisms and therefore gene frequencies over time.
Conditions that prevent gene frequencies changing
- Large population to stop genetic drift
- No migration (isolated)
- No mutation
- Random Mating
- No natural selection
Hardy Weinberg letter meanings
p= frequency of dominant allele in population q= frequency of recessive allele in population p2= frequency of homozygous dominant individuals q2= frequency of homozygous recessive individuals 2pq= Frequency of heterozygous individuals
Hardy Weinberg order
- Find q2
- Then square root to find q
- p+q=1 so find p
- 2pq to find frequency of heterozygous individuals
Dihybrid Crosses
The inheritance of two genes simultaneously, assuming they’re on different chromosomes and that independent assortment takes place. i.e. round and yellow= RY, wrinkled and green= ry.
Ratio is 9:3:3:1
Autosomal linkage
Most of the offspring remain the same as parents due to the genes being on the same chromosome. some variance when chiasmata forms between them leading to crossing over
Chi squared
Find O (observed) then E (expected) then minus them, square that answer and divide by E
Critical Value and significance
If critical value is bigger than x2 you accept the null hypothesis as any differences are due to chance alone.
Find the critical value
p= 0.05 DF= Number of columns - 1
DEFINITION- Epistasis
The interaction with genes at different loci, more than one gene controls the same characteristic. So often one gene blocks the expression of the alleles of the other gene.
DEFINITION- Epistatic Gene
A gene that prevents the expression of alleles of a genes at a second locus
DEFINITION- Hypostatic Gene
Genes phenotype is masked by an epistatic gene
Recessive epistasis
Ratio is 4:3:9 (W:Y:O)
Colourless ——> Yellow——> Orange
To reach yellow pigment Aa/AA gene is required for the enzyme, for orange Bb/BB gene is needed.
aaBB will show epistatis as will be colourless as can’t reach the yellow pigment.
Dominant epistasis
Where the dominant allele of the epistatic gene masks the expression of the hypostatic one. May code for an inhibitor which prevents the transcription of the hypostatic gene or code a protein that inhibits enzyme reactions or alter the precursor molecule so that second enzyme has no substrate.
Ratio is 12:3:1 (W:Y:O)
DEFINITION- Selection pressure
An environmental factor that confers greater chance of survival and hence reproduction for some members of a population and lower chances for others
DEFINITION-Carrying capacity
The maximum number of organisms in a population that the environment can sustain
Environmental Resistance
Factors that limit the growth of a population such as food, light, water.
Stabilising Selection
If the environment is stable then stabilisation selection occurs, the same alleles are selected and the gene pool remains the same, extremes are selected against in favour of intermediates
Directional Selection
If the environment changes there will be a change in selection pressure, directional selection is an evolutionary force where one extreme will be selected against and the other will have an advantage, overtime allele frequency shifts towards the extreme.
Genetic Drift
Random fluctuations can occur in allele frequency within a population, by chance. Most likely in small populations where theres geographical separation. It occurs because f the randomness of reproduction, in extreme cases an allele may be fully eradicated.
Genetic Bottleneck
A natural disaster or disease may kill a large percentage o the population, by chance this may lead to particular allies being lost from the population. Genetic diversity is reduced
Speciation
Large populations can be split by an isolating mechanism, geographical or reproductive. The selection pressures may be different or genetic drift might occur, changing the gene pool. Eventually the subgroups may be unable to reproduce.
Geographical Isolation
- Physical barrier e.g. mountain separates them
- Different selection pressures
- Genetic drift, selection and mutation lead to changes in allele frequency
- ALLOPATRIC SPECIATION
Reproductive Isolation
- 2 groups may be isolated from breeding even if living together, e.g. mechanical problems, different courtship
- SYMPATRIC SPECIATION
Ecological Barriers
-Insects living in different trees in a wood
Temporal/ Seasonal Barriers
-Live in the same habitat but active during different seasons, e.g. two species of flowering plants.
Artificial Selection
- Alleles selected by humans
- Fast
- Benefits Humans
- May be detrimental to the organism
- Few traits selected
- Changes gene frequency
Example of Artificial selection
Dairy farmers breed select cows who produce high milk yields
Problems with artificial selection
- Reduces genetic diversity
- Inbreeding often occurs more likely to have recessive disease
- Selected characteristic may cause health problems
- If environment changes well have a reduced gene pool.
Artificial selection solutions
- Breeders may decide to cross two varieties to increase hybrid vigour
- Gene banks created, e.g. rare breeds and seed banks
