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Questions and Answers
What is an allele that has more than one effect on phenotype called?
What is an allele that has more than one effect on phenotype called?
- Recessive
- Lethal
- Pleiotropic (correct)
- Dominant
What is a characteristic of many inherited disorders in humans, including cystic fibrosis and sickle cell anemia?
What is a characteristic of many inherited disorders in humans, including cystic fibrosis and sickle cell anemia?
- Pleiotropic effects (correct)
- Single gene defects
- Dominant alleles
- Recessive alleles
What happened when Lucien Cuenot tried to obtain a pure-breeding yellow strain in mice?
What happened when Lucien Cuenot tried to obtain a pure-breeding yellow strain in mice?
- He was able to obtain a pure-breeding yellow strain
- The yellow mice had a lethal developmental defect (correct)
- The mice became extinct
- The yellow allele was recessive
What is a result of a lethal allele in the homozygous state?
What is a result of a lethal allele in the homozygous state?
What is an example of a pleiotropic allele?
What is an example of a pleiotropic allele?
What happens to the proportions of traits in the F2 generation when a lethal allele is present?
What happens to the proportions of traits in the F2 generation when a lethal allele is present?
What can be traced back to a single gene defect in cystic fibrosis?
What can be traced back to a single gene defect in cystic fibrosis?
What is a pleiotropic effect?
What is a pleiotropic effect?
What is the main limitation of Mendel's model of inheritance?
What is the main limitation of Mendel's model of inheritance?
What type of inheritance is characterized by multiple additive contributions by many genes, resulting in continuous variation?
What type of inheritance is characterized by multiple additive contributions by many genes, resulting in continuous variation?
What is an example of a trait that exhibits polygenic inheritance?
What is an example of a trait that exhibits polygenic inheritance?
What is the term for a single gene affecting multiple traits?
What is the term for a single gene affecting multiple traits?
Why did the F2 ratios deviate from the classic 3:1 and 9:3:3:1 ratios in some cases?
Why did the F2 ratios deviate from the classic 3:1 and 9:3:3:1 ratios in some cases?
What is the relationship between genotype and phenotype in polygenic inheritance?
What is the relationship between genotype and phenotype in polygenic inheritance?
Why is human height often used as an example of polygenic inheritance?
Why is human height often used as an example of polygenic inheritance?
What is the term for the study of the interaction between genes and their environment to produce a phenotype?
What is the term for the study of the interaction between genes and their environment to produce a phenotype?
What is the term for when a single gene defect leads to multiple symptoms?
What is the term for when a single gene defect leads to multiple symptoms?
What is an example of pleiotropy in humans?
What is an example of pleiotropy in humans?
What is the term for when the heterozygote shows an intermediate phenotype between the two homozygotes?
What is the term for when the heterozygote shows an intermediate phenotype between the two homozygotes?
What is the consequence of incomplete dominance in the F2 generation of a dihybrid cross?
What is the consequence of incomplete dominance in the F2 generation of a dihybrid cross?
What is the term for when both alleles of a gene are equally expressed in the heterozygote?
What is the term for when both alleles of a gene are equally expressed in the heterozygote?
What is the example of co-dominance in humans?
What is the example of co-dominance in humans?
What determines the ABO blood type in humans?
What determines the ABO blood type in humans?
What is the consequence of dihybridism with incomplete dominance in the F2 generation?
What is the consequence of dihybridism with incomplete dominance in the F2 generation?
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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.
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