24 Questions
What is an allele that has more than one effect on phenotype called?
Pleiotropic
What is a characteristic of many inherited disorders in humans, including cystic fibrosis and sickle cell anemia?
Pleiotropic effects
What happened when Lucien Cuenot tried to obtain a pure-breeding yellow strain in mice?
The yellow mice had a lethal developmental defect
What is a result of a lethal allele in the homozygous state?
Death
What is an example of a pleiotropic allele?
Yellow fur in mice
What happens to the proportions of traits in the F2 generation when a lethal allele is present?
The proportions are 2/3 dominant heterozygous and 1/3 homozygous for the normal allele
What can be traced back to a single gene defect in cystic fibrosis?
Multiple symptoms
What is a pleiotropic effect?
An allele that affects more than one trait
What is the main limitation of Mendel's model of inheritance?
It assumes a simple dominant/recessive mode of inheritance
What type of inheritance is characterized by multiple additive contributions by many genes, resulting in continuous variation?
Polygenic inheritance
What is an example of a trait that exhibits polygenic inheritance?
Human height
What is the term for a single gene affecting multiple traits?
Pleiotropy
Why did the F2 ratios deviate from the classic 3:1 and 9:3:3:1 ratios in some cases?
Because of gene expression not adhering to a simple dominant/recessive mode
What is the relationship between genotype and phenotype in polygenic inheritance?
Multiple genotypes correspond to multiple phenotypes
Why is human height often used as an example of polygenic inheritance?
Because it exhibits a range of continuous values
What is the term for the study of the interaction between genes and their environment to produce a phenotype?
Epigenetics
What is the term for when a single gene defect leads to multiple symptoms?
Pleiotropy
What is an example of pleiotropy in humans?
Cystic fibrosis
What is the term for when the heterozygote shows an intermediate phenotype between the two homozygotes?
Incomplete dominance
What is the consequence of incomplete dominance in the F2 generation of a dihybrid cross?
3 genotypes and 3 phenotypes
What is the term for when both alleles of a gene are equally expressed in the heterozygote?
Co-dominance
What is the example of co-dominance in humans?
ABO blood group system
What determines the ABO blood type in humans?
An enzyme that adds sugar molecules to proteins on the surface of red blood cells
What is the consequence of dihybridism with incomplete dominance in the F2 generation?
9 genotypes and 9 phenotypes
Study Notes
Eye Color and Pleiotropy
- Human eye colors are not simply determined by two genes, but rather by at least four genes.
- Pleiotropy refers to the phenomenon where a single gene affects more than one trait.
- Examples of pleiotropy include a single allele that is dominant for yellow fur in mice, but recessive for a lethal developmental defect.
Pleiotropy and Lethal Alleles
- A pleiotropic allele can have multiple effects on the phenotype.
- A lethal allele is an allele that leads to death in the homozygous state.
- Lethal alleles can change the Mendelian proportions, resulting in 2/3 dominant heterozygous and 1/3 homozygous for the normal allele at F2.
Extensions to Mendel
- Mendel's results were initially overlooked, but later rediscovered in 1900.
- Extensions to Mendel's model include the realization that gene expression does not always adhere to a simple dominant/recessive mode.
- Many phenotypes are influenced by more than one pair of genes, modifying the classic 3:1 and 9:3:3:1 F2 ratios.
Polygenic Inheritance
- Polygenic inheritance refers to the phenomenon where multiple genes affect a single trait.
- Examples of polygenic inheritance include human height, which is influenced by multiple genes.
- Polygenic inheritance results in continuous variation, rather than discrete values.
Incomplete Dominance and Codominance
- Incomplete dominance occurs when the heterozygote is intermediate in appearance between the two homozygotes.
- Codominance occurs when no single allele is dominant, and the heterozygote shows aspects of both homozygotes.
- Examples of incomplete dominance include the crossing between red and white Four O-Clock flowers, resulting in pink flowers.
- Examples of codominance include the crossing between two parents of blood group A and B, resulting in IA IB blood group.
Dihybridism with Incomplete Dominance
- Dihybridism with incomplete dominance results in complex F2 phenotypes and genotypes.
- There are two cases of dihybridism with incomplete dominance, involving single or multiple couples of alleles.
The Human ABO Blood Group System
- The gene that determines ABO blood types encodes an enzyme that adds sugar molecules to proteins on the surface of red blood cells.
- These sugars act as recognition markers for the immune system.
Learn about the complex genetics of human eye color and how pleiotropy affects multiple traits. Discover how multiple genes contribute to eye color and the challenges of predicting pleiotropic effects.
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