9.2.1 - Resource Page 5T Y P E S O F I N H E R I T A N C E

COMPLETE DOMINANCEIn 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

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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:

`From Wikipedia, the free 

encyclopedia 

Complete dominance[edit] In complete dominance, the 

effect of one allele in a 

heterozygous genotype 

completely masks the effect of 

the other. The allele that masks 

the other is said to 

be dominant to the latter, and the allele that is masked is said 

to berecessive to the former.[5] Complete dominance therefore means that the 

phenotype of the heterozygote 

is indistinguishable from that of 

the dominant homozygote. 

A classic example of 

dominance is the inheritance of 

seed shape (pea shape) in 

peas. Peas may be round 

(associated with allele R) or wrinkled (associated with allele r). In this case, three combinations of alleles (genotypes) are possible: RR and rr are homozygous and Rr is heterozygous. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant to allele r, and allele r is recessive to allele R. 

Incomplete dominance[edit] 

 

This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the 

R allele blends with the white petal trait of the r allele so that plants with the Rr genotype have pink flowers. 

Incomplete dominance (also called partial dominance) occurs when the phenotype of the heterozygous 

genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. For 

example, the snapdragon flower color is either homozygous for red or white. When the red homozygous 

flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink 

snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in 

the four o'clock plant wherein pink color is produced when true­bred parents of white and red flowers are crossed. Inquantitative genetics, where phenotypes are measured and treated numerically, if a 

`From Wikipedia, the free 

encyclopedia 

Complete dominance[edit] In complete dominance, the 

effect of one allele in a 

heterozygous genotype 

completely masks the effect of 

the other. The allele that masks 

the other is said to 

be dominant to the latter, and the allele that is masked is said 

to berecessive to the former.[5] Complete dominance therefore means that the 

phenotype of the heterozygote 

is indistinguishable from that of 

the dominant homozygote. 

A classic example of 

dominance is the inheritance of 

seed shape (pea shape) in 

peas. Peas may be round 

(associated with allele R) or wrinkled (associated with allele r). In this case, three combinations of alleles (genotypes) are possible: RR and rr are homozygous and Rr is heterozygous. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant to allele r, and allele r is recessive to allele R. 

Incomplete dominance[edit] 

 

This Punnett square illustrates incomplete dominance. In this example, the red petal trait associated with the 

R allele blends with the white petal trait of the r allele so that plants with the Rr genotype have pink flowers. 

Incomplete dominance (also called partial dominance) occurs when the phenotype of the heterozygous 

genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. For 

example, the snapdragon flower color is either homozygous for red or white. When the red homozygous 

flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink 

snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in 

the four o'clock plant wherein pink color is produced when true­bred parents of white and red flowers are crossed. Inquantitative genetics, where phenotypes are measured and treated numerically, if a 

Genotype Phenotype

RR - homozygous dominant round peas

rr - homozygous recessive wrinkled peas

Rr - heterozygous round peas

9.2.1 - Resource Page 5T Y P E S O F I N H E R I T A N C E

INCOMPLETE DOMINANCEIncomplete 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.

Genotype Phenotype

RR - homozygous dominant red petals

rr - homozygous recessive white petals

Rr - heterozygous pink petals

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MULTIPLE ALLELISM or POLYGENICWhile 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.

9.2.1 - Resource Page 5T Y P E S O F I N H E R I T A N C E

CO-DOMINANCECo-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.

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.

Genotype Phenotype

RR - homozygous dominant red petals

rr - homozygous recessive white petals

Rr - heterozygous red and white petals

heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is 

said to exhibit no dominanceat all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other. 

When plants of the F1 generation are self­pollinated, the phenotypic and genotypic ratio of the F2 generation 

will be 1:2:1 (Red:Pink:White) for both generations.[6] 

Co­dominance[edit] 

 

Co­dominance in a Camellia cultivar 

 

A and B blood types in humans show co­dominance, but the O type is recessive to A and B. 

Co­dominance occurs when the contributions of both alleles are visible in the phenotype. 

Co­dominance, where allelic products co­exist in the phenotype, is different from incomplete dominance, 

where the quantitative interaction of allele products produces an intermediate phenotype. For example in 

Co­dominance, a red homozygous flower and a white homozygous flower will produce offspring that have 

red and white spots. When plants of the F1 generation are self­pollinated, the phenotypic and genotypic ratio 

of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete 

dominance. Again, note that this classical terminology is inappropriate – in reality such cases should not be 

said to exhibit dominance at all. 

Multiple alleles[edit] Although any individual of a diploid organism has at most two different alleles at any one locus (barring 

aneuploidies), most genes exist in a large number of allelic versions in the population as a whole. If the 

alleles have different effects on the phenotype, sometimes their dominance interactions with each other can 

be described as a series. 

For example, coat color in domestic cats is affected by a series of alleles of the TYR gene (which encodes the enzyme tyrosinase). The alleles C, cb, cs, and ca (full colour, Burmese, Siamese, and albino, respectively) produce different levels of pigment and hence different levels of colour dilution. The C allele (full colour) is completely dominant over the last three and the ca allele (albino) is completely recessive to the first three. [9] [10] [11] 

    

heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is 

said to exhibit no dominanceat all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other. 

When plants of the F1 generation are self­pollinated, the phenotypic and genotypic ratio of the F2 generation 

will be 1:2:1 (Red:Pink:White) for both generations.[6] 

Co­dominance[edit] 

 

Co­dominance in a Camellia cultivar 

 

A and B blood types in humans show co­dominance, but the O type is recessive to A and B. 

Co­dominance occurs when the contributions of both alleles are visible in the phenotype. 

Co­dominance, where allelic products co­exist in the phenotype, is different from incomplete dominance, 

where the quantitative interaction of allele products produces an intermediate phenotype. For example in 

Co­dominance, a red homozygous flower and a white homozygous flower will produce offspring that have 

red and white spots. When plants of the F1 generation are self­pollinated, the phenotypic and genotypic ratio 

of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete 

dominance. Again, note that this classical terminology is inappropriate – in reality such cases should not be 

said to exhibit dominance at all. 

Multiple alleles[edit] Although any individual of a diploid organism has at most two different alleles at any one locus (barring 

aneuploidies), most genes exist in a large number of allelic versions in the population as a whole. If the 

alleles have different effects on the phenotype, sometimes their dominance interactions with each other can 

be described as a series. 

For example, coat color in domestic cats is affected by a series of alleles of the TYR gene (which encodes the enzyme tyrosinase). The alleles C, cb, cs, and ca (full colour, Burmese, Siamese, and albino, respectively) produce different levels of pigment and hence different levels of colour dilution. The C allele (full colour) is completely dominant over the last three and the ca allele (albino) is completely recessive to the first three. [9] [10] [11] 

    

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SEX-LINKEDIn 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.__________________________________________________________________________