Mendel S Laws Of Inheritance

Step 1: Understanding the Concept:
Human blood groups are determined by three alleles: , , and . and are co-dominant, while is recessive to both. Genotype refers to the genetic makeup, while phenotype refers to the expressed blood group (A, B, AB, or O).
Step 2: Detailed Explanation:
1. Perform a cross between and :
- (Genotype 1, Phenotype: A)
- (Genotype 2, Phenotype: A)
- (Genotype 3, Phenotype: AB)
- (Genotype 4, Phenotype: B)
2. Counting Genotypes: There are 4 distinct combinations ( ).
3. Counting Phenotypes:
- and both result in Blood Group A.
- results in Blood Group AB.
- results in Blood Group B.
- Total phenotypes = 3 (A, AB, and B).
Step 3: Final Answer
There are 4 different genotypes and 3 different phenotypes possible.

Step 1: Understanding the Concept:
This question tests the definitions of Pleiotropy and Co-dominance. Pleiotropy occurs when one gene influences multiple phenotypic traits. Co-dominance occurs when two different alleles are both fully expressed in the phenotype.
Step 2: Detailed Explanation:
1. Statement 1: This is the definition of Pleiotropy. For example, in Phenylketonuria, a single gene mutation leads to mental retardation, reduced hair, and skin pigmentation. This statement is correct.
2. Statement 2: The AB blood group is indeed an example of co-dominance, but the explanation is wrong. It is regulated by two dominant alleles ( and ), not one dominant and one recessive. (or ) is the recessive allele in the ABO system. This statement is incorrect.
Step 3: Final Answer
Only Statement (1) is correct.

Step 1: Understanding the Concept: A test cross is a fundamental genetic cross designed to determine the exact, unknown genotype of an organism exhibiting a dominant phenotype.
Step 2: Key Formula or Approach: Analyze the definition of a test cross and the logical implications of its resulting phenotypic ratios.
Step 3: Detailed Explanation: Statement I correctly defines a standard test cross: it involves crossing an organism with a dominant phenotype (which could be TT or Tt) with a homozygous recessive individual (tt). Thus, Statement I is completely true.
Statement II discusses interpreting the phenotypic results of this test cross.
If the test cross produces a ratio containing BOTH dominant and recessive traits (e.g., a 1:1 ratio), it logically implies that the unknown parent had to pass down a recessive allele, meaning it was heterozygous (Tt), NOT homozygous dominant.
Conversely, if the unknown organism is homozygous dominant (TT), all F offspring will be heterozygous (Tt) and show ONLY the dominant trait (100% dominant, 0% recessive), yielding no mixed ratio.
Therefore, observing a mixed ratio indicates a heterozygous genotype, making Statement II false.
Step 4: Final Answer: Statement I is true, and Statement II is false.