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