5 - Complementation in diploids

Allelic forms of a gene 

Pattern of inheritance involving two genes controlling two different phenotypes 

• Obeys Mendel's second law (Principle of Independent Assortment)

Basic terminology 

• Homozygous - when both alleles at a given diploid locus are the same - eg. AA or aa

• Heterozygous - when there is one dominant and one recessive allele present at a diploid locus eg. Aa 

• Homozygote - an individual who is homozygous at the locus in question 

• Heterozygote - an individual who is heterozygous at the locus in question 

• Hybrid - derived from two genetically different parents 

• Monohybrid - a hybrid or heterozygous for one gene 

• Dihybrid - a hybrid or heterozygous for two different genes  

• True breeding - homozygous at the loci/locus in question 

Principle of segregation 

Rules from monohybrid crosses - results in a 1:2:1 ratio of genotypes  in the F2 

Result in a 3:1 (dominant:recessive) ratio of phenotypes in the F2 

Dihybrid cross

• Cross involving 2 genes controlling 2 different traits

• Some offspring having recombinant phenotypes not seen in the parental generation 

• This is because of ROB - each bivalent can orientate independently in the metaphase 1 plate 

• Mendel worked on genes on separate chromosomes 

• If they were linked on the chromosomes they could only be segregated by crossing over 

• During gamete formation different genes segregate independently from each other when on different chromosomes 

• Can use punnett squares to determine possible genotypes and phenotypes and their ratios 

• Crosses that obey the 9:3:3:1 ratio are obeying Mendel's Principle of Independent Assortment 

Branched line diagrams 

• More practical than punnett squares ‘

• Good if you want all the genotypes or phenotypes and their probabilities 

Example: phenotypes in the F2 resulting from a dihybrid cross between yellow round and green wrinkled peas 

Dihybrid genotypic ratios

Number of genotype classes = 3ⁿ

Where n = number of genes each with 2 alleles 

• For dihybrid cross - the 16 genotypes can be ordered into 9 classes based on no. of dom homozygous; recessive homozygotes and heterozygotes 

Polygenic inheritance 

1. Complementation and the identification of genes controlling the same phenotype in diploids 

2. Interactions between genes controlling the same phenotype - epistasis 

Identification of more than one gene involved in controlling the same phenotypic trait: 

• Use mutational analysis to identify genes involved in controlling the same trait 

• Generate and identify sets of mutants 

• Complementation analysis is a key approach for deciding this 

Diploid complementation 

• Two genes may have different functions in the generation of the same phenotype 

• Eg. a molecular product, made by 2 enzymes in a biochemical pathway 

• Presence of one wild type allele for both genes complements each other to give a wild type phenotype ie: 

Eg. complementation between two genes (e and b) in Drosophila body colour 

Eg. complementation between two genes involved in eye colour in Drosophila 

• Large number of mutations alter the normal red eye colour

How would you determine whether the mutation in each strain is dominant or recessive?

• Cross each mutation to the wild type, red strain and observe the phenotypes of the progeny

• A dominant mutation is one that appears in a heterozygote

• If the progeny shows red eyes - mutation is recessive 

• The the progeny shows brown eyes - the mutation is dominant 

How would you determine how many different genes are affected in the six mutant strains?

Set up pairs of crosses between the mutants to perform complementation tests:

• Mutations in the same gene will produce brown-eyed (mutant) progeny and belong to the same complementation group 

• Mutations in different genes will produce red eyed progeny and will belong to different complementation groups 

• Counting the number of different complementation groups will give the number of genes affected 

How would you determine which mutants, if any, are allelic?

• Allelic mutations are those that are members if the same complementation group

How would you determine whether any of these mutants are alleles of genes already known to affect eye colour?

• Cross each mutant to known mutants of eye colour genes and test for complementation