Incomplete Dominance this occurs when the expression of one allele does not completely mask the expression of another. the result is that a heterozygous organism has a phenotype that is a blend of the two parental phenotypes Example: Colour in snapdragons Red is dominant = R White is recessive = r Rr = pink coloured flowers
Co - Dominance this occurs when there are 2 alleles equally dominant and they are both expressed equally and independently the result is that an organism with the heterozygous genotype exhibits both versions of the trait in a mix both alleles are represented with a capital letter for the trait, with different superscripts to designate the version of the allele Example: Coat color in horses. Black is dominant = C B White = dominant = C W C B C W = both black and white hairs
Multiple Alleles some genes have more than 2 possible alleles in this case there are complex inheritance patterns such as an order of dominance (hierarchy of alleles) Example: Coat colour in mice Yellow A y Agouti light belly A i Agouti A + Black and Tan a l Black a Order of dominance: A y > A l > A + > a l > a
Example: Blood Typing Blood Typing is an example of inheritance based on co dominant multiple alleles In humans ABO Blood Typing, there are three alleles that account for creating a person s blood type. Two of these alleles are dominant and one allele is recessive. I A Blood Type A allele (Dominant) I B Blood Type B allele (Dominant) i Blood Type O allele (Recessive) Blood Type (Phenotype) A B AB O Genotype I A I A or I A i O I B I B or I B i O I A I B i i
Lethal Alleles the presence of some alleles can cause death to an individual during embryonic development this happens due to a mutation in that allele that results in the failure to produce a protein vital to life dominant recessive alleles are generally quickly removed from the population, however recessive lethal alleles only cause death in the homozygous recessive genotype Example: Brachydactyl a genetic disorder where a recessive allele causes shorter than normal fingers in the heterozygous state the allele is lethal when homozygous recessive BB = normal fingers Bb = short fingers (bracydactyl) bb - lethal in a monohybrid cross ¼ of the offspring will die before development is complete
Polygenic Inheritance this occurs when there is more than one gene responsible for a trait (and therefore more than one protein) this results in continuous variation within the population Example: Skin pigmentation several genes produce pigment proteins that together result in the color of someone s skin as there are several genes and varying levels of pigmentation produced the result is large, continuous variation within the population
Gene Linkage Linkage occurs when genes are on the same chromosome. Since it is chromosomes that separate into the gametes during meiosis (Mendel s Law of segregation) and not the individual genes, any alleles for genes that exist on the same chromosome must travel, and therefore be inherited together Remember that sex-linked genes are on the X chromosome (one of the sex chromosomes). linked genes do not show the same phenotypic ratio as non-linked genes The number of linkage groups is the same number as the pairs of homologous chromosomes an organism possesses Recombination occurs when crossing-over has broken linkage groups This was discovered by Thomas Hunt Morgan who studied the genetic inheritance pattern in the genes for wing size and body color of fruit flies (Drosophila melanogaster). Chromosome mapping was originally based on the frequencies of recombination between alleles.