Chapter 6

• Incomplete dominance: is a condition in which neither allele for a gene completely conceals the presence of the other; it results in intermediate expression of a trait.
• Homozygous dominant will produce the dominant characteristic.
• Homozygous recessive will produce the recessive characteristic.
• Heterozygous combination will produce a new phenotype, a combination of the dominant and recessive characteristics.
• When representing incomplete dominance, upper case and lower case letters are not used.
• The trait letter is used as a capital and a superscript letter is used to represent the characteristic of that trait.
• Co-dominance is the situation when both alleles are equally expressed in the phenotype, both alleles are dominant.
• Multiple alleles - some situations allow for a gene to be controlled by more than two alleles
• Sickle cell anemia is a human example of co dominance occurring in the blood specifically the hemoglobin.
• Polygenic inheritance is defined as quantitative inheritance, where multiple independent genes have an additive or similar effect on a single quantitative trait.
• Polygenic inheritance is also known as multiple gene inheritance or multiple factor inheritance.
• Polygenic Inheritance: when a trait is controlled by two or more genes, for example, skin colour in humans is controlled by more than four genes.
• Sex Determination: the sex chromosomes are the 23rd pair, they are NOT homologous and are indicated by X and Y.
• Sex-linked Inheritance: when traits are controlled by genes located on sex chromosomes, most are recessive and X-linked, and they are always expressed in males even if they are recessive because they have just one X chromosome, for example, colour blindness, hemophilia.