Genes & Alleles
- The genotype is an organism’s genetic composition
- The phenotype is an organism’s characteristics, often visible, which occur as a result of both its genotype and the impact of its environment
- Genes are a sequence of DNA that code for a polypeptide
- Genes can exist in 2 or more different forms called alleles.
- In diploid cells, chromosomes occur in pairs called homologous chromosomes. This means the alleles at a specific locus can be homozygous if they are both the same type of allele or heterozygous, if both the alleles are different.
- An allele is dominant if it is expressed in the phenotype of an heterozygous individual.
- An allele is recessive if it is not expressed in the phenotype of an heterozygous individual.
- An allele is codominant if it is expressed, along with the other allele, in the phenotype of a heterozygous individual.
Monohybrid Inheritance
- Monohybrid inheritance is the inheritance of a single gene
- A test cross be used to work out the unknown genotypes of individual organisms
- In the test cross the unknown genotype is crossed with a homozygous recessive individual. If all the offspring have the dominant phenotype, the unknown genotype was homozygous dominant for the trait. If half the offspring have the recessive phenotype, the unknown genotype was heterozygous.
Dihybrid Inheritance
- Dihybrid inheritance involves the inheritance of two different characteristics simultaneously
- During a dihybrid cross, alleles are independently assorted during gamete formation. A punnet square can show all possible genotype and phenotypes of offspring:
- In a dihybrid F1 generation cross, the phenotypic ratio for the F2 generation is always 9:3:3:1
Linkage
- Autosomal linkage occurs if two or more genes are located on the same autosome (non-sex chromosome). The two genes are less likely to be separated during crossing over, resulting in the alleles of the linked genes being inherited together.
- For example, if GN & gn are linked in heterozygous grey bodies and normal winged individuals (GgNn), you get a 3:1 phenotypic ratio
- Sex linkage occurs when there is a gene on the X chromosome, not present on the Y chromosome.
- This means that males are more likely to exhibit recessive disorders like haemophilia
Epistasis
- Epistasis is the interaction between two non-linked genes which causes one gene to mask the expression of the other in the phenotype
- Epistatic genes can work antagonistically (against each other) or in a complementary fashion
- When a gene suppresses another gene, the gene doing the suppressing is called the epistatic gene. The gene which is being suppressed is called the hypostatic gene.
- Antagonistic epistasis can be either recessive or dominant.
- In dominant antagonistic epistasis, the expression of the dominant allele of the epistatic gene prevents the expression of the hypostatic gene. This means that any genotypic combination with either one or two of the dominant alleles for the epistatic gene will suppress the expression of the hypostatic gene
- Recessive epistasis occurs when the presence of two copies of the recessive allele at the first locus prevents the expression of another allele at a second locus.
- In complementary epistasis, the two genes work together, for example, they may encode two enzymes that work in succession.
Complementary Epistasis Example
- An example of complementary epistasis is in the inheritance of coat colour in mice.
A/a is the epistatic gene
AA & Aa produces coloured fur
aa produces no pigment- white fur
B/b is the hypostatic gene
BB & Bb encodes for black coloured fur
bb produces encodes for agouti coloured fur
AA & Aa produces coloured fur
aa produces no pigment- white fur
B/b is the hypostatic gene
BB & Bb encodes for black coloured fur
bb produces encodes for agouti coloured fur
This produces a 9:4:3 phenotypic ratio
Chi-squared Test
- If during an experiment, an unexpected result is obtained, we need to determine whether this unexpected result is due to chance or attributable to a specific cause (significant or not).
- The chi-squared test is a type of statistical test that allows us to calculate whether the difference between the results we observe and the results we expected is significant
- The null hypothesis assumes that any difference that occurs between the expected and observed results is due to chance.
O is the observed numbers (no units)
E is the expected numbers (no units)
E is the expected numbers (no units)
- The Χ^2 value is then compared to a critical value, found from a chi-squared table by looking at the p-value and degrees of freedom
- The degrees of freedom is the number of categories (or classes) minus one
- The p-value is normally taken as 0.05, meaning that there is a 5% probability that the result is due to chance only
- If Χ^2 < critical value, then the results are not significant (are due to chance). The null hypothesis is accepted
- If Χ^2 > critical value, then the results are significant (are attributable to a specific cause). The null hypothesis is rejected