COMPLETE DOMINANCE In complete dominance, the effect of one allele completely masks the effect of the other. The allele that masks the other is called dominant, and the allele that is masked is called recessive. Complete dominance means that the phenotype of a heterozygous individual (Aa) is indistinguishable from a homozygous dominant (AA) individual. Autosomal Dominant Inheritance Autosomal Recessive Inheritance Example: Inheritance of seed shape in pea plants. Peas may be round, associated with the dominant allele R, or wrinkled, associated with the recessive allele r. In this case, there are 3 possible genotypes or combinations of alleles and 2 possible phenotypes: round peas wrinkled peas round peas
INCOMPLETE DOMINANCE Incomplete dominance occurs when the phenotype of the heterozygous genotype (Aa) is distinct and often intermediate or blend of the phenotypes of the homozygous genotypes (AA, aa). Example: Inheritance of petal color in snapdragon flowers. Petals may be red, white, or pink based on the genotype or combination of alleles. red petals white petals pink petals
MULTIPLE ALLELISM or POLYGENIC While some genes have only 2 allele versions, most genes have a large number of allele versions. IF the alleles have different effects on the phenotype, the expression of the trait may come in a range or spectrum. Also, many traits are controlled by more than 1 gene, and therefore more than 2 alleles. Example: Inheritance of fur color in cats. Fur color is affected by several alleles of the TYR gene. The alleles C (full color), cb (burmese), cs (siamese), and ca (albino) produce different levels of pigment and hence different color combinations and variation. Example: Inheritance of blood type in humans. The human blood types are A, B, O, and AB all determined by different alleles.
CO-DOMINANCE Co-dominance occurs when the contributions of both alleles are visible in the phenotype. Example: Inheritance of petal color in the Camellia cultivar flower. Petals may be red, white, or red and white spotted based on the genotype or combination of alleles. When an individual inherits a dominant allele that codes for red petals and recessive allele that codes for white petals, both phenotypes show up and the flower has red and white petals. red petals white petals red and white petals Example: Inheritance of feather color in chickens. Chickens may be white, black, or speckled based on the genotype or combination of alleles. When an individual inherits one of each type of allele, both white and black phenotypes show up and the chicken is speckled. Example: Inheritance of blood type in humans. The different human blood types are A, B, O, and AB. A and B are co-dominant. A person with the phenotype blood type A could have genotypes AA or AO. A person with the phenotype blood type B could have genotypes BB or BO. A person with the phenotype blood type O must have genotypes OO. A person with the phenotype blood type AB has genotypes AB which is co-dominant, both blood types are expressed.
SEX-LINKED In humans and other mammal species chromosomes come in pairs. Humans have 23 pairs of chromosomes - 22 pairs are called autosomes and the last pair is the sex chromosomes. Sex is determined by 2 sex chromosomes called the X chromosome and Y chromosome. Human females are typically XX; males are typically XY. Genetic traits on the X and Y chromosomes are called sex-linked, because the genes are found on the sex chromosomes, not because they are characteristics of one sex or the other. The term sex-linked almost always refers to X-linked traits. Females (XX) have 2 copies of every gene found on the X chromosome because they have 2 X chromosomes, while males (XY) have only one copy of each gene on the X chromosome because they have only 1 X chromosome.