Other patterns of inheritance
Mendel's model of complete dominance does not cover the whole of genetics: some flowers produce an intermediate color when crossed, some people show two alleles at once in their blood type, and some diseases affect almost only men because their gene travels on the X chromosome.
Codominance
When a pure white cow is crossed with a pure black bull, the calves are born with clearly defined white and black patches all over their bodies instead of being gray: how can two characteristics be expressed at the same time without one dominating or being diluted by the other?
Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
Explain what codominance is and give a human example in which two alleles are expressed simultaneously.
Explain the difference between codominance and incomplete dominance, and give an example of each one.
Explain the difference between codominance and incomplete dominance, and give an example of each one.
Incomplete dominance
If a pure red-flowered four o'clock plant is crossed with a pure white-flowered one, all the offspring produce flowers of a uniform pink shade: why does neither color dominate the other, as happened with Mendel's pea plants?
Explain what incomplete dominance is and describe which phenotype appears when crossing a red-flowered plant with a white-flowered plant if the trait follows this…
Explain what incomplete dominance is and describe which phenotype appears when crossing a red-flowered plant with a white-flowered plant if the trait follows this pattern.
In the four o'clock plant, flower color shows incomplete dominance: RR is red, Rr is pink and rr is white. Cross a pink-flowered plant with a white-flowered one and…
In the four o'clock plant, flower color shows incomplete dominance: RR is red, Rr is pink and rr is white. Cross a pink-flowered plant with a white-flowered one and state the genotypic and phenotypic ratios of the offspring.
In four o'clock plants, flower color shows incomplete dominance: red (RR), pink (Rr) and white (rr). Complete the Punnett square in the diagram, where two pink-flowered…
In four o'clock plants, flower color shows incomplete dominance: red (RR), pink (Rr) and white (rr). Complete the Punnett square in the diagram, where two pink-flowered plants are crossed, and state the genotypic and phenotypic ratios of the offspring.
Multiple alleles: the ABO blood types
At a Red Cross blood drive, donations are classified into four different blood types (A, B, AB and O), a variety that could not be explained if only two gene variants existed for this characteristic: how many different alleles actually determine blood group in the human population?
A person has blood type O. State their genotype and explain whether they can have any $I^A$ or $I^B$ allele.
A person has blood type O. State their genotype and explain whether they can have any $I^A$ or $I^B$ allele.
For the ABO blood groups there are three different alleles, but each person only has two of them. Explain what multiple allelism is and why this happens.
For the ABO blood groups there are three different alleles, but each person only has two of them. Explain what multiple allelism is and why this happens.
Sex-linked inheritance
Explain what it means for a gene to be linked to the X chromosome and explain why males (XY) have only one allele for those genes, while females (XX) have two.
Explain what it means for a gene to be linked to the X chromosome and explain why males (XY) have only one allele for those genes, while females (XX) have two.
In humans, sex is determined chromosomally: females are XX and males are XY. Explain, using the corresponding cross, why the probability of having a son or a daughter is…
In humans, sex is determined chromosomally: females are XX and males are XY. Explain, using the corresponding cross, why the probability of having a son or a daughter is equal (1:1 ratio) and which parent provides the chromosome that determines the sex of the offspring.
Color blindness and hemophilia: "criss-cross" inheritance
A color-blind man ($X^d Y$) has children with a woman with normal vision who is not a carrier ($X^D X^D$). State the genotypes and phenotypes of the sons and of the…
A color-blind man ($X^d Y$) has children with a woman with normal vision who is not a carrier ($X^D X^D$). State the genotypes and phenotypes of the sons and of the daughters, and explain why this type of inheritance is said to go "criss-cross" from fathers to daughters.