Lethal gene are

Options

A. Causative for appearance of ancestral traits
B. Always recessive
C. Genes present on different chromosomes but influencing single trait
D. Killer in homozygous state.

Answer

**Step-by-Step Text Solution:**

1. **Definition of Lethal Genes**: Lethal genes are specific genes that can cause the death of an organism when present in a homozygous condition. This means that if an individual inherits two copies of a lethal gene (one from each parent), it may lead to the organism's death.

2. **Homozygous Condition**: The homozygous condition refers to having two identical alleles for a particular gene. In the case of lethal genes, both alleles must be the same (either both dominant or both recessive) for the lethal effect to manifest.

3. **Example of Lethal Genes**: A classic example of a lethal gene is found in mice, where a specific gene that determines body color can be lethal when present in a homozygous state. If a mouse inherits two copies of this lethal gene, it may not survive to adulthood.

4. **Options Analysis**:
- The first option states that lethal genes are causative for the appearance of ancestral traits, which is incorrect.
- The second option suggests that they are always recessive genes present on different chromosomes, which is also incorrect.
- The third option states that they are "killer in homozygous state," which accurately describes the nature of lethal genes.

5. **Conclusion**: Based on the definition and examples provided, the correct answer to the question "Lethal genes are" is that they are "killer in homozygous state."