Dominant, Recessive, and Codominant Traits: Seventh Grade Science
While Gregor Mendel established the baseline rules of simple dominance, many biological traits display more complex inheritance patterns, including incomplete dominance, codominance, multiple alleles, and sex-linked transmission tracked across family pedigree charts.
Beyond Simple Mendelian Dominance
Complete dominance occurs when the dominant allele completely masks the recessive allele in heterozygous individuals. However, as twentieth-century geneticists expanded investigations across diverse organisms, they discovered that alleles can interact in non-Mendelian patterns that generate intermediate or combined phenotypes.
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| MODES OF ALLELIC INHERITANCE |
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| Inheritance Pattern | Phenotypic Expression in Heterozygote |
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| Complete Dominance | Dominant phenotype expressed; recessive masked (Tt) |
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| Incomplete Dominance| Blended, intermediate phenotype between parents (CRCW)|
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| Codominance | Both alleles expressed simultaneously and fully (IAIB)|
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| Multiple Alleles | More than two distinct alleles exist in the population|
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| Polygenic Traits | Multiple independent genes control one trait (Height) |
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Incomplete Dominance: Blended Phenotypes
In incomplete dominance, neither allele is completely dominant over the other. When a heterozygous individual inherits both alleles, they express an intermediate, blended phenotype:
- In snapdragon flowers (Antirrhinum majus), crossing a pure-breeding red flower (CRCR) with a pure-breeding white flower (CWCW) produces 100 percent pink heterozygous flowers (CRCW) in the F1 generation.
- When pink snapdragons self-pollinate (CRCW x CRCW), the parental red and white phenotypes reappear in the F2 generation with a 1:2:1 phenotypic ratio (1 Red : 2 Pink : 1 White), proving that the alleles themselves remain distinct particulate units that do not permanently blend.
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| INCOMPLETE DOMINANCE IN SNAPDRAGONS |
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| Red Parent (CR CR) x White Parent (CW CW) |
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| CR CR |
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| CW | CR CW | CR CW | |
| | (Pink) | (Pink) | |
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| CW | CR CW | CR CW | |
| | (Pink) | (Pink) | |
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| All F1 offspring are Pink (Heterozygous intermediate expression) |
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Codominance: Simultaneous Full Expression
In codominance, both alleles are fully and equally expressed in the heterozygote without blending:
- In roan cattle, an individual inheriting the allele for red hairs (HR) and the allele for white hairs (HW) develops a coat consisting of interspersed red and white hairs growing side-by-side (HRHW).
- Human ABO Blood Groups: The ABO blood group system exemplifies both codominance and multiple alleles. A single gene locus on chromosome 9 has three common alleles: IA, IB, and i. Alleles IA and IB are codominant with each other, and both are completely dominant over the recessive allele i. An individual with the genotype IAIB expresses both A-antigens and B-antigens on their red blood cell membranes, resulting in Type AB blood.
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| THE HUMAN ABO BLOOD GROUP SYSTEM |
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| Blood Group | Possible Genotypes | Antigens on Red Blood Cells |
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| Type A | IA IA or IA i | A-Antigens only |
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| Type B | IB IB or IB i | B-Antigens only |
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| Type AB | IA IB | Both A and B Antigens (Codominant) |
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| Type O | i i | Neither Antigen (Universal Donor) |
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Reading and Constructing Pedigree Charts
A pedigree chart is a standardized biological diagram that tracks the inheritance of a specific trait or disorder across multiple generations of a family. Pedigrees allow clinical geneticists to determine whether a trait is autosomal dominant, autosomal recessive, or sex-linked.
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| STANDARD PEDIGREE CHART SYMBOLS |
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| [ ] Square = Male ( ) Circle = Female |
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| [X] Filled = Affected Male (X) Filled = Affected Female |
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| [ ]---( ) Horizontal line = Mating marriage |
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| | | Vertical descent line connects to children |
| [ ] ( ) Roman numerals indicate generation (I, II, III...) |
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| SAMPLE AUTOSOMAL RECESSIVE PEDIGREE |
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| Generation I: [ ] (Carrier) ====== ( ) (Carrier) |
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| +----------------+----------------+ |
| | | | |
| Generation II: [ ] (X) [ ] |
| (Unaffected) (AFFECTED: aa) (Unaffected) |
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| Clue: Trait skips generations; unaffected parents have affected child |
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Analyzing Modes of Inheritance in Pedigrees
- Autosomal Recessive Traits (e.g., Cystic Fibrosis, Sickle Cell Disease): Frequently skip generations. Affected individuals (aa) are often born to unaffected, heterozygous carrier parents (Aa). Males and females are affected with equal frequency.
- Autosomal Dominant Traits (e.g., Huntington's Disease): Appear in every generation without skipping. Every affected individual has at least one affected parent.
- X-Linked Recessive Traits (e.g., Red-Green Color Blindness, Hemophilia): Affect males far more frequently than females because biological males possess only one X-chromosome (hemizygous, XY). A recessive allele on a male's single X-chromosome is expressed automatically because the Y-chromosome carries no complementary gene. An affected male receives the recessive allele from his carrier mother.
Exercises and Solutions
Exercise 1: Calculating Probabilities in Human ABO Blood Inheritance. A woman with Type A blood whose father had Type O blood marries a man with Type B blood whose mother had Type O blood. (a) Determine the exact genotypes of both the woman and the man, explaining your genetic deduction; (b) Construct a Punnett square for this couple; (c) Calculate the percentage probabilities for every possible blood group phenotype among their future children.
Solution 1: (a) The woman displays Type A blood, meaning her genotype could be either IAIA or IAi. Because her biological father had Type O blood (genotype ii), he could only transmit a recessive i allele to his daughter; therefore, the woman must be heterozygous with the genotype IAi. Similarly, the man displays Type B blood (genotype IBIB or IBi). Because his biological mother had Type O blood (ii), he must have inherited a recessive i allele from her, making his genotype IBi. (b) The cross is IAi x IBi. The woman produces gametes IA and i in equal proportions; the man produces gametes IB and i. The Punnett square yields four distinct genotypic outcomes in equal ratios: IAIB (top left), IAi (bottom left), IBi (top right), and ii (bottom right). (c) The phenotypic probabilities for their future children are: 25 percent Type AB blood (genotype IAIB, demonstrating codominance); 25 percent Type A blood (genotype IAi); 25 percent Type B blood (genotype IBi); and 25 percent Type O blood (genotype ii). This cross demonstrates that a couple with Type A and Type B blood can produce offspring of all four ABO blood types.
Exercise 2: Deducing Patterns of Inheritance from Pedigree Clues. A family pedigree tracking a rare genetic neurological condition reveals the following family data: in Generation I, an unaffected male and an unaffected female produce five children (three girls and two boys). Four of the children are completely healthy, but one boy develops the neurological condition. In Generation II, this affected male marries an unaffected woman with no family history of the disorder, and all four of their children (two boys and two girls) are completely unaffected. However, in Generation III, one of the daughters from Generation II marries a healthy man, and two of their three sons are born with the neurological condition, while their daughter is healthy. Determine whether this disorder is transmitted as autosomal dominant, autosomal recessive, or X-linked recessive, providing three points of pedigree evidence.
Solution 2: The disorder is transmitted as an X-linked recessive trait. The three points of pedigree evidence supporting this conclusion are: First, the disorder skips generations, appearing in Generation I, disappearing entirely in Generation II, and reappearing in Generation III, which rules out an autosomal dominant inheritance pattern; Second, the condition affects males exclusively across all three generations, while females act as unaffected carriers, which is a classic hallmark of sex-linked inheritance; Third, the transmission pattern directly tracks the movement of the X-chromosome: the affected male in Generation II inherited his mutant X-chromosome from his carrier mother in Generation I, passed that mutant X-chromosome to his daughter in Generation II (who became an obligate carrier), who in turn transmitted the mutant allele to two of her sons in Generation III. Because males carry only one X-chromosome and have no counterbalancing allele on the Y-chromosome, they express the recessive disorder.