Topic 7
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What is a genotype
Genetic constitution of an organism. All the alleles it has for a gene
Phenotype
The expression of genes and its interaction with the environment
Homozygous
A pair of homologous chromosomes carrying the same alleles for a single gene
Heterozygous
A pair of chromosomes carrying two different alleles for the same gene
Recessive allele
An allele only exoressed if no dominant allele is present
Codominant
Both alleles are eqully dominant and expressed in the phenotype
Multiple alleles
More than two alleles for a single gene
Sex linkage
A gene whose locus is on the x chromosome
Autosomal linkage
Genes that are located on the same chromosome. Not sex chromosome
Epistatis
When one gene modifies or masks the expression of a different gene at a different locus.
Monohybrid
Genetic inheritence cross of characteristics determined by one gene
Dyhybrid
Genetic inheritance cross for a chsaracteristic determined by two genes
What is chi squared
Chi squared is a statistic to investigate differences between frequencies It can be used to determine whether there is a significant difference between the frequency you expect and the frequency you observe
How to work out chi squared
(Observed frquency - part expected ratio)^2 / expected frequency Then add both values together to get a chi squared value
What is a dihybrid crossing
A genetic cross where the inheritance of two genes is considered at the same time
Ratio of each phenotype
9-3-3-1 In a dihybrid cross, if ttwo heterozygous parents for both genes are crossed you will always get this ratio
Crossing over in meiosis
Crossing over results in new combinations of alleles in the gamtes This means predicted gametes in the punnet may differ
What is autosomal linkage
When two genes are located on the same chromosome. This means that the alleles for ech gene linked on the same chromosome will be inherited together. This impacts the predicted gametes
What is epistasis
When one gene influences the expression of another gene
What is the diybrid ratio
9:3:3:1 ratio
Autosomal linkage ratio
3:1
How is autosomal linkage possible
This is the only possible because crossing over must hace occured during meiosis to make new combinations of gamtes. If ratio isnt 9:3:3:1 then crossing over has occured
Pedigree diagrams key points
• family tree of the inheritance of one or more genes. • the individual at the top are the oldest • circles represent females, squares males
What is the evidence tat the condition is caused by a recessive allele
Two parents do not have the condtion but the child does. Therefore both parents must be carriers of the recessive allele but due to the presence of the dominant allele they do not have the condition themselves