In a testcross involving F1 dihybrid flies, more parental-type offspring were produced than the recombinant-typeoffspring. This indicates
- Chromosomes failed to separate during meiosis
- The two genes are linked and present on the same chromosome
- Both of the characters are controlled by more than one gene
- The two genes are located on two different chromosomes
The Correct Option is B
Solution and Explanation
Answer: \(\boxed{\text{The two genes are linked and present on the same chromosome}}\)
Explanation:
- When two genes are located close together on the same chromosome, they tend to be inherited together. This phenomenon is known as linkage.
- In such cases, parental combinations (the same allele combinations as in the parents) are observed more frequently than recombinant combinations.
- Recombinants are produced due to crossing over during meiosis, but if genes are linked, crossing over between them is less frequent.
Incorrect options:
- Option 1: Chromosome separation failure (nondisjunction) would result in abnormal chromosome numbers, not affect parental/recombinant ratios.
- Option 3: Polygenic inheritance affects trait expression, not the parental/recombinant ratio in test crosses.
- Option 4: Genes on different chromosomes assort independently and typically produce a 1:1:1:1 ratio in a testcross — equal parental and recombinant types.
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Concepts Used:
Principles of Inheritance and Variation - Mutation
A Mutation is a change in the sequence of our DNA base pairs caused by numerous environmental stimuli such as UV light or mistakes during DNA replication. Germline mutations take place in the eggs and sperm and can be passed onto offspring, whereas somatic mutations take place in body cells and are not passed on.
Types of Mutations
There are three types of mutations, which are as follows:
Silent mutation
It refers to any change in DNA sequence that has no effect on the amino acid sequence in a protein or the functions that a protein performs. There is no phenotypic indication that a mutation has occurred.
Nonsense mutation
When there is a change in the sequence of base pairs due to a point mutation, that results in a stop codon. This leads to a protein that is either shortened or non-functional.
Missense mutation
A missense mutation occurs when a point mutation causes a change in the codon, which then codes for another amino acid.
The mutation is caused by the following factors:
Internal Causes
When DNA copies incorrectly, the majority of mutations occur. Evolution occurs as a result of all of these mutations. DNA makes a copy of itself during cell division. When a copy of DNA isn't flawless, it's called a mutation since it differs somewhat from the original DNA.
External Causes
When certain chemicals or radiations are used to break down DNA, it causes the DNA to break down. The thymine dimers are broken by UV radiation, resulting in altered DNA.