Select incorrect statement:

Question

Accepted Answer

The correct option is 2: XO-chromosomal abnormalities in human being caused Down's syndrome.

This statement is factually incorrect, making it the right answer for a question asking to select the *incorrect* statement. Down's syndrome is a well-known chromosomal disorder, but it is caused by the presence of an extra full or partial copy of chromosome 21, a condition known as **Trisomy 21** (represented as 47,XX,+21 or 47,XY,+21). The XO-chromosomal abnormality (where an individual has only one X chromosome and no second sex chromosome, represented as 45,XO) is the specific genetic basis for a completely different condition known as **Turner syndrome**, which affects females. Turner syndrome is characterized by a set of distinct features, including short stature and the absence of ovarian development, and is unrelated to the developmental and physical characteristics associated with Down's syndrome (Trisomy 21). Therefore, the pairing of the XO-chromosomal abnormality with Down's syndrome is a biological error.

The incorrect option is 1: Heterozygous represents a pair of contrasting traits of a character.

This statement is biologically accurate, describing the essence of a heterozygous genotype, and is therefore not the correct answer to the 'select incorrect statement' question. In genetics, a 'character' refers to a heritable feature (e.g., plant height), and 'traits' are the variant forms of that character (e.g., tall or dwarf). These contrasting traits are controlled by different versions of the gene, called alleles (e.g., T and t). A heterozygous individual (genotype Tt) possesses both the dominant and the recessive allele, meaning their genetic makeup literally represents the pair of contrasting traits for that character.

The incorrect option is 3: Law of segregation is based on the fact that alleles do not show any blending.

This statement accurately reflects one of the fundamental pillars of Mendelian genetics and the Law of Segregation (Mendel's First Law), making it a correct statement. Mendel observed that even in a hybrid (heterozygous) organism, the two alleles for a trait remain distinct and do not contaminate or blend with each other. This purity is what allows the recessive trait to reappear unchanged in the F2 generation after being masked in the F1 generation. The law dictates that these pure alleles will segregate, or separate, during gamete formation so that each gamete receives only one allele.

The incorrect option is 4: Hemophilia a is sex linked recessive disease.

This is a scientifically accurate statement regarding the inheritance pattern of Hemophilia A, classifying it as a correct statement and thus not the answer to the question. Hemophilia A is a classical example of an X-linked recessive disorder. The gene responsible for producing clotting Factor VIII is located on the X chromosome. Because males have only one X chromosome, they are much more frequently affected if they inherit the defective allele. Females, having two X chromosomes, must inherit two copies of the defective allele to express the disease, but they can be asymptomatic carriers.