Question

What can you infer about the genotype of parents in crosses I and II?

Answer 1

Step 1: Understanding the Concept:
Mendelian genetics uses phenotypic ratios in offspring to determine the genotypes of the parents.
Step 2: Detailed Explanation:
1. Cross I: The graph shows 500 Tall and 0 Dwarf. This is a \(100\%\) Tall population. This result is obtained when a pure-breeding tall (\(TT\)) is crossed with a pure-breeding dwarf (\(tt\)). All offspring are heterozygous (\(Tt\)) and phenotypically tall.
2. Cross II: The graph shows 375 Tall and 125 Dwarf.
The ratio is \(375 : 125 = 3 : 1\).
A \(3:1\) phenotypic ratio is the characteristic result of a monohybrid cross between two heterozygous individuals (\(Tt \times Tt\)).
Step 3: Final Answer:
Cross I parents: \(TT \times tt\) (or any cross where one is \(TT\)).
Cross II parents: \(Tt \times Tt\).

Answer 2

Step 1: Understanding the Concept:
Genetic crosses are classified by the number of traits being studied simultaneously.
Step 2: Detailed Explanation:
In the provided bar graphs, only one character (height of the plant) with two contrasting traits (Tall and Dwarf) is being monitored.
A cross involving the study of inheritance of one pair of contrasting characters at a time is called a monohybrid cross.
Step 3: Final Answer:
The graphs represent a monohybrid cross.

Answer 3

Step 1: Understanding the Concept:
A test cross is a specific type of backcross where an individual with an unknown genotype is crossed with a recessive parent.
Step 2: Detailed Explanation:
1. Identification: Graph III shows 250 Tall and 250 Dwarf plants. The ratio is \(250 : 250 = 1 : 1\).
2. In Mendelian genetics, a \(1:1\) phenotypic ratio in the offspring of a tall plant and a dwarf plant indicates that the tall parent was heterozygous (\(Tt\)).
\[ Tt (\text{Heterozygous Tall}) \times tt (\text{Homozygous Dwarf}) \rightarrow 50\% \text{ Tall } (Tt), 50\% \text{ Dwarf } (tt) \]
3. Importance: A test cross is used to determine whether a dominant phenotype individual is homozygous dominant (\(TT\)) or heterozygous (\(Tt\)). If the offspring show any recessive traits, the parent must be heterozygous.
Step 3: Final Answer:
Type: Test Cross. Importance: Genotype determination of dominant phenotype individuals.

Answer 4

Step 1: Understanding the Concept:
The first two generations of Mendel's monohybrid cross lead to fundamental laws of inheritance.
Step 2: Detailed Explanation:
1. From Cross I (Tall \(100\%\)): We conclude that in a pair of dissimilar factors, one member of the pair dominates (expressed) and the other is recessive (hidden). This is the Law of Dominance. It explains why only one parental trait is expressed in the \(F_1\) generation.
2. From Cross II (\(3:1\) ratio): We conclude that although the alleles are present together in the \(F_1\) (\(Tt\)), they do not blend. During gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors. This is the Law of Segregation. The reappearance of the dwarf trait in Cross II proves this.
Step 3: Final Answer:
Conclusion: Traits are controlled by discrete units called factors. Principles: Law of Dominance and Law of Segregation.