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 on a Texas ranch, 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?
Practice★☆☆Not started
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.
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.
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 (a classic school-garden experiment): why does neither color dominate the other, as happened with Mendel's pea plants?
Practice★☆☆Not started
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.
In Andalusian hens, plumage color shows incomplete dominance: BB is black, Bb is blue and bb is white. Cross two blue hens and state the expected genotypic and…
In Andalusian hens, plumage color shows incomplete dominance: BB is black, Bb is blue and bb is white. Cross two blue hens and state the expected genotypic and phenotypic ratios in the chicks.
At a school 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 type in the human population?
Practice★☆☆Not started
The alleles $I^A$, $I^B$ and $i$ are involved in blood types. State which genotypes correspond to blood types A, B, AB and O.
The alleles $I^A$, $I^B$ and $i$ are involved in blood types. State which genotypes correspond to blood types A, B, AB and O.
A heterozygous blood-type-A woman ($I^A i$) and a heterozygous blood-type-B man ($I^B i$) have a child. State all the blood types the child can have and the probability…
A heterozygous blood-type-A woman ($I^A i$) and a heterozygous blood-type-B man ($I^B i$) have a child. State all the blood types the child can have and the probability of each one.
Color blindness affects roughly 1 in 12 males but far fewer females in the US: why would one trait show up so differently in boys and girls?
Practice★☆☆Not started
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: X-linked recessive inheritance
Practice★★☆Not started
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.
A couple attends a genetic counseling consultation: the man has normal vision and so does the woman, but she is concerned because her maternal grandfather was…
A couple attends a genetic counseling consultation: the man has normal vision and so does the woman, but she is concerned because her maternal grandfather was color-blind (her maternal grandmother and her father had normal vision and had no family history). The man claims there is nothing to worry about, stating: "Since I am not color-blind, it is impossible for us to have a color-blind daughter or son."
a) Reasonably deduce the probability that the woman is a carrier based on the data from her maternal family.
b) Refute or validate the man's claim using genetic arguments for daughters and sons separately, showing the possible crosses.
c) Calculate the final probability that the first child born to this couple is a color-blind male.
In humans, color blindness is determined by an X-linked recessive allele ($X^d$), while the Rh factor is inherited by an autosomal gene where the positive allele…
In humans, color blindness is determined by an X-linked recessive allele ($X^d$), while the Rh factor is inherited by an autosomal gene where the positive allele ($Rh^+$) is dominant over the negative allele ($Rh^-$). A woman with normal vision and Rh-positive blood type, whose father was color-blind and Rh-negative, has children with a man with normal vision who is heterozygous for the Rh factor.
a) Deduce the parents' genotypes, justifying the woman's based on information about her father.
b) Carry out the necessary genetic analysis to calculate the probability that they have a son who is both color-blind and Rh-negative. Express the result as a fraction and as a percentage (with a decimal point).
c) Explain, without needing to develop the entire Punnett square for daughters, why the probability of having a color-blind daughter in this couple is exactly 0%.