Extensions of Mendelism Questions and Answers

Q: Human ABO blood group system is the example of

The ABO system is an example of multiple alleles.A single gene, I, controls the ABO blood groups, and it has three alleles in the population: \(I^A\), \(I^B\) and i. Any person carries only two of them, but the existence of more than two alternative forms of one gene is what multiple allelism means. \(I^A\) and \(I^B\) are also codominant.Epistasis and complementation involve interactions between two different genes, and polygenic inheritance involves many genes acting together on a continuous trait such as height.

Q: Difference in chromosomal determination of sex between Drosophila and human is because :

The difference is that in Drosophila the ratio of X chromosomes to autosomes decides sex, whereas in humans the Y chromosome determines maleness.In Drosophila an X:A ratio of 1 (XX with two sets of autosomes) gives a female and a ratio of 0.5 gives a male, so an XO fly is a (sterile) male and an XXY fly is a female; the Y is needed only for male fertility. In humans the SRY gene on the Y chromosome switches on testis development, so XXY (Klinefelter) individuals are male and XO (Turner) individuals are female.The other options reverse the two systems: a mere pair of X chromosomes does not decide sex in the fly, the Y does not determine maleness in Drosophila, and a single X without a Y gives a female, not a male, in humans.

Q: The phenotypic ratio obtained in F2 is 2 : 1 in the progeny of cross due to presence of :

A 2 : 1 ratio is due to a lethal allele.Crossing two heterozygotes normally gives 1 AA : 2 Aa : 1 aa. If the allele is lethal when homozygous, one class dies before it can be counted, and the survivors appear in a 2 : 1 ratio. The classic case is the yellow coat allele AY in mice (Cuenot): yellow x yellow gives 2 yellow : 1 non-yellow, because AYAY embryos die.A plain dominant allele gives 3 : 1, a recessive one also 3 : 1, and a co-dominant (or incompletely dominant) one 1 : 2 : 1. Only lethality removes a class and leaves 2 : 1.

Q: In a hypothetical diploid animal species, there is a locus with two alleles A and B. If A and B are co-dominant, then how many different phenotypes are possible in this species ?

3 phenotypes are possible.With two alleles at one locus there are three genotypes: AA, AB and BB. When the alleles are co-dominant, both are fully expressed in the heterozygote, so AB shows the products of both A and B and looks different from either homozygote. Each genotype therefore has its own phenotype:\[ \text{AA},\ \text{AB},\ \text{BB}: 3\ \text{phenotypes} \]Two phenotypes would be the result with complete dominance, where AB looks like one homozygote. Four would need a third allele or a second locus.

Q: A single gene can often have multiple effects on the phenotype of an organism, affecting a number of apparently unrelated traits. This phenomenon is known as :

The phenomenon is pleiotropy.Pleiotropy is when one gene affects several, apparently unrelated traits. Phenylketonuria is a classic example: one defective enzyme causes intellectual disability, light pigmentation and other effects. Sickle-cell anaemia also shows it.Epistasis is the masking of one gene's effect by another gene. Polygenic inheritance is the reverse of pleiotropy: many genes together control a single trait, such as height. An epigenetic effect is a heritable change in gene expression without a change in DNA sequence.

Human ABO blood group system is the example of

A.epistatic gene
B.multiple alleles
C.complementation
D.polygenic inheritance

Exam Year(s)

MPPSC Forest Service - 2023,

Answer: B

The ABO system is an example of multiple alleles.

A single gene, I, controls the ABO blood groups, and it has three alleles in the population: \(I^A\), \(I^B\) and i. Any person carries only two of them, but the existence of more than two alternative forms of one gene is what multiple allelism means. \(I^A\) and \(I^B\) are also codominant.

Epistasis and complementation involve interactions between two different genes, and polygenic inheritance involves many genes acting together on a continuous trait such as height.

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Difference in chromosomal determination of sex between Drosophila and human is because :

A.In Drosophila, the ratio of X chromosome to autosome determines maleness or femaleness whereas in human, the Y chromosome determines maleness.
B.As seen in human, mere presence of two X chromosomes triggers female developmental pathway in Drosophila.
C.In both human and Drosophila the Y chromosome determines maleness
D.As seen in Drosophila, presence of single X chromosome in the absence of Y chromosome can tigger maleness in human.

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The difference is that in Drosophila the ratio of X chromosomes to autosomes decides sex, whereas in humans the Y chromosome determines maleness.

In Drosophila an X:A ratio of 1 (XX with two sets of autosomes) gives a female and a ratio of 0.5 gives a male, so an XO fly is a (sterile) male and an XXY fly is a female; the Y is needed only for male fertility. In humans the SRY gene on the Y chromosome switches on testis development, so XXY (Klinefelter) individuals are male and XO (Turner) individuals are female.

The other options reverse the two systems: a mere pair of X chromosomes does not decide sex in the fly, the Y does not determine maleness in Drosophila, and a single X without a Y gives a female, not a male, in humans.

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The phenotypic ratio obtained in F2 is 2 : 1 in the progeny of cross due to presence of :

A.Dominant allele
B.Recessive allele
C.Lethal allele
D.Co-dominant allele

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: C

A 2 : 1 ratio is due to a lethal allele.

Crossing two heterozygotes normally gives 1 AA : 2 Aa : 1 aa. If the allele is lethal when homozygous, one class dies before it can be counted, and the survivors appear in a 2 : 1 ratio. The classic case is the yellow coat allele A^Y in mice (Cuenot): yellow x yellow gives 2 yellow : 1 non-yellow, because A^YA^Y embryos die.

A plain dominant allele gives 3 : 1, a recessive one also 3 : 1, and a co-dominant (or incompletely dominant) one 1 : 2 : 1. Only lethality removes a class and leaves 2 : 1.

Discuss in Forum

In a hypothetical diploid animal species, there is a locus with two alleles A and B. If A and B are co-dominant, then how many different phenotypes are possible in this species ?

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: C

3 phenotypes are possible.

With two alleles at one locus there are three genotypes: AA, AB and BB. When the alleles are co-dominant, both are fully expressed in the heterozygote, so AB shows the products of both A and B and looks different from either homozygote. Each genotype therefore has its own phenotype:

\[ \text{AA},\ \text{AB},\ \text{BB}: 3\ \text{phenotypes} \]

Two phenotypes would be the result with complete dominance, where AB looks like one homozygote. Four would need a third allele or a second locus.

Discuss in Forum

A single gene can often have multiple effects on the phenotype of an organism, affecting a number of apparently unrelated traits. This phenomenon is known as :

A.Epistasis
B.Polygenic inheritance
C.Pleiotropy
D.Epigenetic effect

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

The phenomenon is pleiotropy.

Pleiotropy is when one gene affects several, apparently unrelated traits. Phenylketonuria is a classic example: one defective enzyme causes intellectual disability, light pigmentation and other effects. Sickle-cell anaemia also shows it.

Epistasis is the masking of one gene's effect by another gene. Polygenic inheritance is the reverse of pleiotropy: many genes together control a single trait, such as height. An epigenetic effect is a heritable change in gene expression without a change in DNA sequence.

Discuss in Forum

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