The specific position of a gene on a chromosome, the two alleles of a gene are found at the same loci on the chromosome pairs
Phenotype
Observable characteristics of an organism which are as a result of genotype and environment
Genotype
The alleles present within cells of an organism, for a particular trait or characteristic
Dominant
Only a single allele is required for the characteristic to be expressed, that is the allele is always expressed in the phenotype
Recessive
The characteristic is only expressed if there is no dominant allele present
Homozygous
Two identical alleles
Heterozygous
Two different alleles
Codominance
Both alleles contribute to the phenotype
Linkage
Genes for different characteristics, located at different loci on the same chromosome are linked
Monogenic inheritance
A phenotype or trait is controlled by a single gene
Dihybrid cross
Inheritance of two genes
Sex linkage
Expression of an allele dependent on the gender of the individual as the gene is located on a sex chromosome
Sex-linked conditions
Haemophilia
Duchenne muscular dystrophy
Autosomal linkage
Genes which are located on the same chromosome and tend to be expressed together in the offspring
Codominance
When both alleles are expressed in a heterozygote, that is, both alleles contribute towards the phenotype
Codominance example
Blood type
Epistasis
The interaction of different loci on the gene, one gene locus affects the other gene locus. One gene loci can either mask or suppress the expression of another gene locus.
Recessive epistasis
The presence of a recessive allele prevents the expression of another allele at a second locus. Recessive epistasis gives the ratio of 9:3:4.
Dominant epistasis
A dominant allele at one locus completely masks the alleles at a second locus. Dominant epistasis gives a ratio of 12:3:1.
Chi-squared test
A statistical test which can be used to establish whether the difference between observed and expected results is small enough to occur purely due to chance.
Chi-squared test
Can be used if the sample size is sufficiently large, over 20
Can only be used for discontinuous variation data in the form of raw counts
Can be used to determine whether the null hypothesis is correct or not
Null hypothesis
The assumption that there is no difference between observed and expected results
Chi-squared value is less than critical value
Null hypothesis is accepted as the difference due to chance is not significant
Chi-squared value is greater than critical value
Null hypothesis is rejected as the difference between observed and expected results is not due to chance, and is significant
Types of mutations
Insertion/deletion mutations
Point mutation/substitution
Nonsense mutation
Missense mutation
Silent mutation
Insertion/deletion mutations
One or more nucleotide pairs are inserted or deleted from the sequence, altering the sequence of nucleotides after the insertion/deletion point (frameshift)
Point mutation/substitution
One base pair is replaced by another
Nonsense mutation
A translation is stopped early, giving rise to a truncated polypeptide due to premature introduction of a stop codon
Missense mutation
A codon change which results in the production of a different amino acid, thus resulting in altered tertiary structure of the protein
Silent mutation
A codon change which does not affect the amino acid sequence produced
Neutral effects of mutations
The mutation causes no change to the organism, either because it occurs in a non-coding region of DNA, is a silent mutation, or the change in tertiary structure of the protein has no effect on the organism
Beneficial mutations
Humans developed trichromatic vision
Harmful mutations
Mutation in the CFTR protein causes cystic fibrosis
Chromosomal mutation
Down's syndrome - a third copy of chromosome 21 is present
Transcriptional control
Gene expression can be controlled at the transcriptional level
Lac operon
A length of DNA composed of structural genes and control sites which controls the expression of beta-galactosidase responsible for hydrolysis of lactose in E.coli
Lac operon
Consists of a promoter region, operator region, and structural genes
The inhibitor is coded for by a regulator gene located outside the operon which binds to the operator region
Concentration of glucose is high and concentration of lactose is low
Transcription of the structural genes is inhibited due to binding of the repressor to the operator region
Concentration of glucose is low and concentration of lactose is high
Lactose binds the repressor, causing the shape of its active site to change and making it ineffective, so it can no longer bind to the operator region and transcription of the structural genes takes place