3Multiple Alleles A gene may have many alleles not just 2 But, a single diploid individual can only carry 2 alleles
4DominanceWhere there is complete dominance the phenotype of the heterozygote is the same as that of the homozygote dominant.Incomplete or partial dominance can lead to the expression of intermediate phenotypes.Andalusian blue, colour produced by CB/CW heterozygote from Black x White chickens.
6CodominanceIn codominance both phenotypes are expressed in the heterozygote, not an intermediate type as in incomplete dominanceIn ABO blood group system heterozygous individuals produce both A & B antigens.
14Epistasis Genes interact to mask another genes phenotype. No new phenotypes are produced.Can be dominant or recessive.
15Agouti 9:3:4 ratio Rodent coat colour and pattern. Agouti, albino or black.Albino lacks any coat colour.Black lacks yellow colour in agouti pattern.But, you can’t have a coat pattern without any colour! so even A/- c/c will be albino, and the recessive c/c masks the A allele
17Flower colour, 9:7 ratioWhite variety crosses sometimes produce purple flowered F1Purple F1 x F1 gives 9:7 purple:whiteDue to interaction between dominant allelesPurple C/- P/-Duplicate recessive white
19Lethal Genes Yellow coat colour in mice is an essential gene. Yellow heterozygotes bred to non-yellow mice give 2:1 yellow non-yellow ratio.Yellow AY/AY homozygotes are aborted.Yellow dominant for coat colour.Recessive with respect to lethality.
21Environmental Effects Fur colour in Himalayan rabbits;above 30°C all whiteat 25°C normal pattern with dark extremeties.Cooled below 25°C, more dark patches.
22NeurofibromatosisPenetrance, the phenotype does not always reflect the genotype.Expressivity, the degree to which the phenotype is expressed for a given genotype.
23Continuous VariationMany phenotype characters are the product of the action of a large number of genes.So the phenotype is the product of independent assortment in the individual genes.In a population we will see gradual differences, or continuous variation.
24Linkage Non-independent assortment of genes Linked genes, located on same chromosomeGenetic recombination, where progeny show new combinations of alleles not present in the parents
26Total flies = 2,441Total parental phenotypes = 1,541Total recombinant phenotypes = 900% recombinants = 36.9If there was independent assortment we would predict 50% recombinantsConclusion, the genes are linked
28Linkage MapsUse linked genes to determine physical location on chromosome.The greater the separation between 2 genes the higher the probability that crossover will occur.1% recombination = 1 map unit (mu or cM)Multiple cross-overs can also occur
30Avoiding multiple cross-overs Use closely-linked genes (within 10 mu)Use a three-point test cross including a third allele.The products of a double crossover will be the least frequent.Can help establish the order of the genes on the chromosome.Interference can happen between cross-overs.