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Question

A pedigree of an inheritable disease is shown below. 

The correct answer is
X-linked recessive

The pedigree described in the question is designed to determine the pattern of inheritance for a specific disease. Let's analyze the pedigree step-by-step:

  1. In the pedigree chart, males are represented by squares, and females by circles. Affected individuals are filled, and unaffected are open. Carrier states, if represented, are often shown with half-filled shapes.
  2. In X-linked recessive inheritance:
    • Males (with only one X chromosome) will express the disease if they inherit the affected X chromosome.
    • Females (with two X chromosomes) must inherit two affected X chromosomes to express the disease; if they inherit only one, they become carriers.
    • Affected males pass the affected X chromosome to all their daughters (making them carriers if the mother is unaffected) and none to their sons.
  3. Check the generations in the pedigree chart:
    • First Generation: One male is affected which suggests that the disease might be X-linked since he could have inherited the X chromosome from a carrier mother.
    • Second Generation: An affected male from an unaffected father points to an X-linked pattern, as he must have received the affected X from his carrier mother.
    • The pattern where no affected sons come from an affected father supports the X-linked recessive inheritance mode.
  4. Verify against the options:
    • Autosomal dominant: Not possible, as not all generations show affected offspring from affected parents.
    • X-linked dominant: Unlikely, as affected fathers would pass the disease to all daughters.
    • X-linked recessive: Fits the observed pattern where mainly males are affected, and females are carriers.
    • Autosomal recessive: Unlikely given the skewed gender distribution of affected individuals.

Thus, the correct answer is X-linked recessive.

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Important Questions from Mendelian Inheritance

  1. Mendel's 'law of segregation' applies to the segregation of __________ during gamete formation.
  2. Fabry disease in humans is a X-linked disease. The probability (in percentage) for a phenotypically normal father and a carrier mother to have a son with Fabry disease is ________.
  3. The allele associated with albinism in humans is recessive ($c$). The probability that an albino male ($cc$) and a carrier female ($Cc$) will have an offspring with normal skin pigmentation is _________. 

    (Round off to one decimal place)

  4. The blood group of the mother is A⁺ and that of the father is AB⁺. Which of the following statements is/are correct?
  5. Assuming independent assortment and no recombination, the number of different combinations of maternal and paternal chromosomes in gametes of an organism with a diploid number of 12 is ________.
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