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Question
According to Mendelian studies, a recessive allele of a particular gene requires 2 identical copies for its expression. In which of the following cases can a single recessive allele express itself?
According to Mendelian studies, a recessive allele of a particular gene requires 2 identical copies for its expression. In which of the following cases can a single recessive allele express itself?
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Solution
The correct option is D When it is present in hemizygous condition
From Mendelian experiments, it was established that a particular character usually had 2 different variants or traits. One of the traits which was able to express itself in both F1 and F2 generations, was named as the dominant trait and the other which disappeared in the F1 generation but reappeared in the F2 generation, was termed as the recessive trait. It was also established that alleles were responsible for the expression of the variants. The dominant trait could express itself in both homozygous (in the presence of both identical alleles) and heterozygous condition (in the presence of non identical alleles). The recessive trait however, required 2 copies of the recessive alleles to be expressed. But these heterozygous or homozygous conditions hold true when homologous chromosomes are taken into account. Homologous chromosomes are the ones which are morphologically similar and contain alleles for the same genes at the same loci.
In case of pairing of autosomes and 2 X chromosomes (in case of most females), the concept of homologous pairing can be applied. In these cases, 2 copies of the recessive alleles must be present on both the homologous chromosomes for their expression.
But in the pairing of X and Y chromosomes, this concept cannot be applied. This is because X and Y chromosomes are morphologically different. They also don’t carry the same genes at the same loci. In such cases, alleles present on the X and Y chromosomes express themselves independently, regardless of whether a recessive allele is present or a dominant allele is present. If an X-linked recessive allele is present, it will express itself even if a single copy of it is present. It is because the Y chromosome won’t contain any counterpart (neither identical nor opposite allele). This condition is known as hemizygous condition. It can be seen in the case of male Drosophila flies which have white eyes. The recessive allele for eye colour is present on the X chromosome and the corresponding Y chromosome has no allele on it. We see the recessive trait to be expressed even in the presence of a single copy of the allele.
From Mendelian experiments, it was established that a particular character usually had 2 different variants or traits. One of the traits which was able to express itself in both F1 and F2 generations, was named as the dominant trait and the other which disappeared in the F1 generation but reappeared in the F2 generation, was termed as the recessive trait. It was also established that alleles were responsible for the expression of the variants. The dominant trait could express itself in both homozygous (in the presence of both identical alleles) and heterozygous condition (in the presence of non identical alleles). The recessive trait however, required 2 copies of the recessive alleles to be expressed. But these heterozygous or homozygous conditions hold true when homologous chromosomes are taken into account. Homologous chromosomes are the ones which are morphologically similar and contain alleles for the same genes at the same loci.
In case of pairing of autosomes and 2 X chromosomes (in case of most females), the concept of homologous pairing can be applied. In these cases, 2 copies of the recessive alleles must be present on both the homologous chromosomes for their expression.
But in the pairing of X and Y chromosomes, this concept cannot be applied. This is because X and Y chromosomes are morphologically different. They also don’t carry the same genes at the same loci. In such cases, alleles present on the X and Y chromosomes express themselves independently, regardless of whether a recessive allele is present or a dominant allele is present. If an X-linked recessive allele is present, it will express itself even if a single copy of it is present. It is because the Y chromosome won’t contain any counterpart (neither identical nor opposite allele). This condition is known as hemizygous condition. It can be seen in the case of male Drosophila flies which have white eyes. The recessive allele for eye colour is present on the X chromosome and the corresponding Y chromosome has no allele on it. We see the recessive trait to be expressed even in the presence of a single copy of the allele.
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