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Which of the following genotypes is homozygous recessive?

Q. Which of the following genotypes is homozygous recessive?

A. ss, Bb, TT, XY


C. tt, xx, ss, bb

D. tt, xy, ss, bb



Answer: C (tt, xx, ss, bb)


Explanation: Each gene contains two alleles and they occupy same locus on the chromosomes.

From the above question, S, B, T and X are the alleles of four unique genes. In homozygous recessive condition, they will be represented in small letters. Hence, the correct answer is C (tt, xx, ss, bb).

Interestingly, female (XX) contains two copies of each gene on the X chromosomes, so this alleles can be homozygous (XX or xx) or heterozygous (Xx). However, males (XY) will express all the alleles located on the single X chromosome that they inherit from their mother, hence, the ‘recessive’ or ‘dominant‘ concept is an irrelevant in case of male genotype (XY). So, the option D is incorrect even it is represented in small letters (tt, xy, ss, bb).

In general, if two alleles (SS or ss) of the genotype encode a same type of protein and that shows a same type of character, then that genotype is either homozygous dominant (SS) or homozygous recessive (ss). However, recessive allele will only be expressed when it is in a homozygous pair (ss).


Suggestion: Read more about genotype and its types and frequency.





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