Q. Which of the following genotypes is homozygous recessive?
A. ss, Bb, TT, XY
B. SS, BB, TT, XX
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).
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