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Questions and Answers
What is the expected phenotype ratio when yellow mice are crossed with each other?
What is the expected phenotype ratio when yellow mice are crossed with each other?
- 2:1 (correct)
- 4:1
- 3:1
- 1:1
What characteristic defines the lethal gene observed by E. Baur in Snapdragon?
What characteristic defines the lethal gene observed by E. Baur in Snapdragon?
- Variegated leaves (correct)
- Thorny stems
- Large seeds
- Red flowers
What type of offspring results from selfing the 'golden' variety of Snapdragon?
What type of offspring results from selfing the 'golden' variety of Snapdragon?
- Two golden for every green (correct)
- No offspring produced
- Equal numbers of golden and green
- All golden
Why can yellow mice never be obtained in a homozygous condition?
Why can yellow mice never be obtained in a homozygous condition?
What is the characteristic breeding condition of green mice from the golden variety?
What is the characteristic breeding condition of green mice from the golden variety?
What type of inheritance pattern is demonstrated by the yellow mice?
What type of inheritance pattern is demonstrated by the yellow mice?
What do the findings by W.E. Castle and C.C. Little confirm about Cuénot's work?
What do the findings by W.E. Castle and C.C. Little confirm about Cuénot's work?
In a testcross involving yellow mice, what is revealed about the yellow-colored offspring?
In a testcross involving yellow mice, what is revealed about the yellow-colored offspring?
What defines a lethal allele?
What defines a lethal allele?
Which statement is true regarding dominant lethal genes?
Which statement is true regarding dominant lethal genes?
What type of lethal gene results in a delayed death after the organism has lived for some time?
What type of lethal gene results in a delayed death after the organism has lived for some time?
How do synthetic lethal genes operate?
How do synthetic lethal genes operate?
Which type of lethal allele is influenced by environmental conditions?
Which type of lethal allele is influenced by environmental conditions?
What is a characteristic of semi-lethal alleles?
What is a characteristic of semi-lethal alleles?
Which of the following is an example of a fully dominant lethal allele?
Which of the following is an example of a fully dominant lethal allele?
Which of the following best describes essential genes?
Which of the following best describes essential genes?
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Study Notes
Lethal Alleles
- Lethal alleles are mutations causing non-functional essential proteins, leading to death.
- They can be dominant, recessive, or synthetic (lethal only in combination).
- Essential genes are crucial for survival.
Dominant Lethal Genes
- Expressed in both homozygous and heterozygous conditions.
- Rarely detected due to rapid elimination from populations.
- Examples: Coat color inheritance in mice (L. Cuenot, 1905) and Huntington's disease in humans.
Synthetic Lethal Genes
- Combinations of two mutant alleles causing death when present together, but not individually.
- Help scientists identify gene function by studying the phenotypes of synthetic lethal mutants.
Types of Lethal Alleles
- Early Onset: Lethal during embryogenesis.
- Late Onset: Delayed effects, causing death later in life.
- Conditional: Lethal only under specific environmental conditions.
- Semi Lethal: Kills some individuals but not all.
History
- L. Cuenot (1905) documented a lethal gene in mouse coat color inheritance.
- W.E. Castle and C.C. Little supported Cuenot's findings, marking the first documented recessive lethal allele.
- E. Baur (1907) observed a lethal gene in Snapdragon (Antirrhinum), linked to variegated leaves.
Lethal Gene in Snapdragon (Antirrhinum)
- E. Baur (1907) discovered a lethal gene causing variegated leaves.
- Self-pollination of "golden" snapdragons resulted in a 2:1 ratio of golden and green offspring instead of the expected 3:1 ratio.
- Green snapdragons are homozygous recessive and breed true, while golden ones are heterozygous.
Lethal Gene in Mouse Coat Color
- L. Cuenot (1905) discovered a lethal gene affecting mouse coat color, where "yellow" was dominant to "brown".
- Yellow mice were never found in homozygous condition.
- Crossing yellow mice resulted in a 2:1 ratio of yellow to brown offspring, as the yellow allele was lethal in homozygous condition.
- Breeding experiments revealed that yellow mice were heterozygotes.
Examples of Lethal Alleles
- Lethal genes hindering embryo development lead to a 2:1 offspring ratio in yellow mice inheritance.
- The yellow coat color in mice is linked to a lethal allele, resulting in a departure from the expected Mendelian 3:1 ratio.
- A testcross reveals yellow mice to be heterozygotes.
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