Lethal Alleles and Their Types

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Questions and Answers

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?

Variegated leaves (correct)

Thorny stems

Large seeds

Red flowers

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?

The yellow allele is lethal in homozygous state. Signup and view all the answers

What is the characteristic breeding condition of green mice from the golden variety?

They are homozygous for a recessive trait. Signup and view all the answers

What type of inheritance pattern is demonstrated by the yellow mice?

Autosomal dominant Signup and view all the answers

What do the findings by W.E. Castle and C.C. Little confirm about Cuénot's work?

The lethality of the yellow allele. Signup and view all the answers

In a testcross involving yellow mice, what is revealed about the yellow-colored offspring?

They are heterozygous. Signup and view all the answers

What defines a lethal allele?

An allele that produces a non-functional protein essential for survival Signup and view all the answers

Which statement is true regarding dominant lethal genes?

They kill both homozygous and heterozygous individuals. Signup and view all the answers

What type of lethal gene results in a delayed death after the organism has lived for some time?

Late Onset Signup and view all the answers

How do synthetic lethal genes operate?

They only cause death when paired with another specific mutation. Signup and view all the answers

Which type of lethal allele is influenced by environmental conditions?

Conditional Signup and view all the answers

What is a characteristic of semi-lethal alleles?

They cause death in only a fraction of the population. Signup and view all the answers

Which of the following is an example of a fully dominant lethal allele?

Huntington's disease Signup and view all the answers

Which of the following best describes essential genes?

They are necessary for the survival of an organism. Signup and view all the answers

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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Description

Explore the fascinating world of lethal alleles, including dominant, recessive, and synthetic lethal genes. This quiz covers essential concepts about gene function and the types of lethal alleles that can lead to early or late onset conditions, as well as their historical context. Test your knowledge on how these mutations can impact survival and inheritance!