Deviation from mendelism - class-XII

Description: deviation from mendelism
Number of Questions: 15
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Tags: biology classical genetics botany genetics
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Incomplete dominance was discovered by

  1. Correns

  2. Mendel

  3. Johannsen

  4. Bateson

Correct Option: A

Karl Correns discovered incomplete dominance in which none of the factors of a gene is dominant, the phenotype of the heterozygous dominant individual is the blend of dominant and recessive traits. He studied flower colour in Mirabilis jalapa and showed that a monohybrid cross between two pure varieties one with red flower petals another with white flower petals gives pink-flowered F$ _1$ plants. If these pink-flowered F$ _1$ plants are crossed, the F$ _2$ plants appear in a ratio of 1 : 2 : 1, having red, pink, or white flower petals, respectively. Pink-flowered plants are heterozygotes with a petal colour intermediate between the red and white colours of the homozygotes. Here, dominant allele (R) specifies red pigment colour, and recessive allele specifies no colour (r) and the flower petals have a white background colour. The pink heterozygotes (Rr) have about half the red pigment of the flowers in red homozygotes (RR) because the heterozygotes carry only one copy of “R” allele that produces colour, while the homozygotes have two copies.

When both the alleles are equally expressed in hybrid, the phenomenon is known as

  1. Dominance

  2. Bivalence

  3. Codominance

  4. All of the above

Correct Option: C

Genes are units of hereditary information that are located on segments of chromosomes. The genes for a specific trait may exist in different forms known as alleles. An example of an allele or a gene is flower color. A flower may have alleles that make it red or pink and so on. Alleles that always show up when they are present are known as dominant alleles. Alleles that are masked or hidden by dominant alleles are known as recessive alleles. In some situations both alleles are expressed equally. A genetic scenario where neither allele is dominant or recessive and both get expressed is known as 'codominance'.
Blood types are excellent examples of codominance. 

Inheritance of blood group is a condition of
1. Co-dominance
2. Incomplete dominance
3. Multiple allelism
4. Multiple gene

  1. 1, 2

  2. 2, 4

  3. 2, 3

  4. 1, 4

  5. 1, 3

Correct Option: E

Incomplete dominance is the condition when the dominant allele is not completely dominant over the recessive resulting in a mixture of dominant-recessive phenotype in heterozygous condition. When a single trait is governed by more than one gene, it is said to be under control of multiple genes. Human blood group inheritance is the example of codominance and multiple alleles. It is governed by three alleles of gene "I", namely I$^{A}$, I$^{B}$ and I$^{O}$. I$^{A}$ and I$^{B}$ show codominance while allele "I$^{O}$" is recessive to both "I$^{A}$" and "I$^{B}$". This gives total six genotypes and four phenotypes : A (I$^{A}$ I$^{A}$ and I$^{A}$ I$^{O}$), B (I$^{B}$ I$^{B}$ and I$^{B}$ I$^{O}$) , AB (I$^{A}$ I$^{B}$) and O (I$^{O}$ I$^{O}$). Correct option is "E".

In a cross between a red flowered plant and a white flowered plant, all offsprings produced bear pink flowers. This is because

  1. Red is dominant

  2. White is dominant

  3. Pink is dominant

  4. Red and white exhibit incomplete dominance

  5. Red and white exhibit codominance

Correct Option: D

When a red flowered plant is crossed with a white flowered plant, ideally the offsprings should bear either red and white flower. However, in the case where the trait  flower color exhibits incomplete dominance, all the offsprings will give forth pink colored flowers due to the blending of traits.

Phenotypic and genotypic ratio in F$ _2$ generation in incomplete dominance is

  1. 1 : 2 : 1 and 1 : 2 : 1

  2. 9 : 6 and 1 : 2 : 1

  3. 9 : 6 and 3 : 1

  4. 9 : 3 : 3 : 1 and 1 : 2 : 1 : 4 : 1 :1 : 2 : 1 : 2 : 1

Correct Option: A

Incomplete dominance is the condition when none of factors of a gene is dominant, the phenotype of heterozygous dominant individual is blend of dominant and recessive traits. For example flower colour in Mirabilis jalapa shows incomplete dominance and the monohybrid cross between two pure varieties gives 1 : 2 : 1 phenotypic ratio in F$ _2$ generation which is 3 : 1 in otherwise dominant traits. This is because the heterozygous genotypes do not show the dominant trait but they are blend of dominant and recessive phenotype. A cross between two pure breeding genotypes that differ in two pairs of trait (dihybrid cross) followed by selfing of F$ _1$ hybrid gives 9 : 3 : 3 : 1 phenotypic ratio and 1 : 2 : 1 : 4 : 1 : 1 : 2 : 1 : 2 : 1 genotypic ratio. Option B and C show both dihybrid (9 : 6), a dihybrid cross gives total 16 types which make these twoptions wrong.

What would be the proportion of dwarf plant with pink flowers. If heterozygous tall plant with pink flower is test crossed ? A plant taken for test cross character of length showing complete dominance and character of flower colour showing incompete dominance

  1. $4/16$

  2. $6/16$

  3. $3/16$

  4. None of these

Correct Option: A

Heterozygous tall plant with pink flower is test crossed with homozygous recessive partner, as a result of which different phenotypes are produced in the F1 offspring. The cross is between TtPp and ttpp which is shown in the Punnett Square.

 Male/Female  TP Tp  tP  tp 
 tp  TtPp  Ttpp  ttPp ttpp 

The percentage of dwarf plant with pink flowers (ttPp) is thus 4/16 or 1/4.

So the correct option is '4/16'.

Dominant and recessive alleles express themselves together in

  1. Dominance

  2. Co-dominance

  3. Pseudodominance

  4. None of the above

Correct Option: B

Codominance occurs when two alleles are equally dominant, and both appear in a phenotype. The classic example is human blood type, in which the A and B alleles are codominant, so anyone with both will have the phenotype AB.

Choose the correct answer from the alternatives given.
The allele for pea comb (P) in chickens is completely dominant to the allele for single comb (p). The alleles for black feather colour (B), and white feather colour (B) show codominance, so that BB individuals possess blue feathers. If chickens heterozygous for both pairs of genes are mated, what proportion of offspring are expected to be pea combed and white feathered? 

  1. 9 / 16

  2. 3 / 16

  3. 1 / 16

  4. 2 / 16

Correct Option: B

In a cross of PpBb and PpBb,

The individuals with Pea-comb and white feathers include PPbb, Ppbb and Ppbb.
So, the correct option is '3/16'.

Which of the following is not the Mendel's principle of inheritance?

  1. Law of incomplete dominance

  2. Law of dominance

  3. Law of segregation

  4. Law of independent assortment

Correct Option: A

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. In the early 1900s, German botanist Carl Correns (1864–1933) would conduct similar research on four o'clock plants. While Mendel's work laid a foundation, it isCorrens who is credited with the actual discovery of incomplete dominance. In his work, Correns observed a blend of colors in flower petals. So, the correct option is 'Law of incomplete dominance'.

$F _2$ generation in a Mendelian cross showed that both genotypic and phenotypic ratio are same as 1 : 2 : 1. It represents a case of  

  1. Monohybrid cross with complete dominance

  2. Monohybrid cross with incomplete dominance

  3. Co-dominance

  4. Dihybrid cross

Correct Option: B

Incomplete dominance is a type of intermediate inheritance in which one allele is not completely dominant over its paired allele. In this, the third phenotype is observed in heterozygous condition. In monohybrid cross with incomplete dominance, the phenotypic and genotypic ratio is 1:2:1, where one part is with dominant phenotype, 2 parts are with intermediate phenotype and 1 part is with recessive phenotype. For example, a snapdragon flower shows red (dominant), white (recessive) and pink (intermediate) flowers.

Thus, the correct answer is 'Monohybrid cross with incomplete dominance.'

In Mirabilis jalapa , the percentage of plants having pink and red flowers would be ________ when pink flower plant is test crossed.

  1. 25% pink and 25% red

  2. 50% pink and 50% red

  3. 50% pink and 0% red

  4. 0% pink and 50% red

Correct Option: B
 Gamete  R
 r Rr- Pink flower  rr- White flower 

Mirabilis jalapa is a plant showing incomplete dominance which means that when in F$ _1$ generation intermediate phenotype appears instead of the two parent phenotypes. 

While, the test cross is defined as a cross between an F$ _1$ plant and the homozygous recessive parent.
So, for plants to be pink in Mirabilis jalapa the plant has to be Rr in genotype. (where R is dominant on r while RR gives Red color and rr gives white color flowers in a plant).
Desired cross: Rr x rr 
So, the phenotype ratio is: 1:1::Pink flower:White flower
So, the correct answer is '50% pink and 50% red'.

Different forms of a gene are called

  1. Heterozygotes

  2. Alleles

  3. Complementary genes

  4. Supplementary genes

Correct Option: B

A.Heterozygotes – Organisms with a pairs of contrasting characters are called Heterozygotes.

B.Alleles - An alternative form of a character in a pair, controlled by a single gene is called Allele. It is dominant either in homozygous or heterozygous state.

C.Complementary gene – Two genes present on separate loci that interact together to produce a dominant phenotype is called Complementary gene. In the F2 generation 9:7 ratio.

D.Supplementary gene – genes which include two pairs of non-allelic genes. Both of them are involved in affecting the same character. Out of the non-allelic genes, one gene is dominant and can express by itself. Second gene is also dominant but expresses only when it is supported by the presence of the first gene. In the F2 geneartion 9:3:4 ratio.

So, the correct option is ‘Alleles’. 

A cross between yellow flowered and round fruit (both dominant) bearing plant and white flowered elongated fruit (both recessive) bearing plant yielded 20 plants in $F _1$ progeny which formed 960 plants in $F _2$ generation. The number of plants with yellow flowers and round fruits in $F _1$ and $F _2$ generations would be

  1. 20, 960

  2. 20, 540

  3. 10, 180

  4. 10, 60.

Correct Option: B

class=A cross is made between pure yellow flowered  and round fruit(YYRR) with white flowered elongated fruit(yyrr). In the F1 generation 20 progeny with yellow flowered and round fruits(YyRr) are formed. When they are subjected to self pollination followed by self fertilization, in the F2 generation, 960 progeny are formed. In these 540 plants are yellow flowered round fruits, plants are yellow flowered elongated fruits, 180 fruits are white flowered round fruits and 60 plants are white flowered elongated fruits the correct option is 20, 540

Two plants of Summer Squash both having circular fruits are crossed. $F _1$ plants had discoid fruits. $F _2$ generation has 3 types of fruits, discoid, circular and long in the ratio of

  1. 9 : 3 : 4

  2. 9 : 6 : 1

  3. 12 : 3 : 1

  4. 7 : 6 : 3

Correct Option: B

A modified dihybrid mendelian ratio of 9 : 3 : 4 indicates

  1. Supplementary genes

  2. Complementary genes

  3. Lethal genes

  4. Epistalic genes

Correct Option: A

A.Supplementary genes – A pair of non allelic genes, one of which produces its effect independently in the dominant state while the dominant allele of the second gene is without any independent effect but is able to modify the effect of the former to produce to a new trait are called Supplemetary genes. Due to these genes in the F2 generation, 9:3:4 ratio of progeny are obtained. It is a modified Mendel dihybrid ratio.

B.Complementary genes – Two genes present on separate loci that interact

together to produce dominant phenotypic character; neither of them if present alone can express itself are called complementary genes. Due to these genes in the F2 generation, 9:7 ratio of progeny are obtained. It is a modified Mendel dihybrid ratio.

C.Lethal genes – A pair of genes present in the homozygous conditions, causes the death of the individual are called Lethal genes. Due to the genes, in F2 generation 2:1 ratio of progeny are obtained.

D.Epistatic genes – A gene which masks the action of another gene is termed as epistatic gene and process is called epistasis. Due to these genes in the F2 generation, either 9:3:4  or 12.3.1 ratio of progeny are obtained. It is a modified Mendel dihybrid ratio.

So, the correct option is ‘supplementary genes’.     

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