31 Years NEET Previous Year Questions: Principles of Inheritance & Variation - 1 - NEET MCQ

The production of gametes by the parents, the formation of the zygotes, the F₁ and F₂ plants can be understood from a diagram called Punnett Square. It helps in identifying the possible allelic combinations.

Fig: Punnett Square

Line in NCERT: "the production of gametes by the parents, the formation of the zygotes, the F and F₂ plants can be understood from a diagram called Punnett Square as shown in Figure 4.4."

Thomas Hunt Morgan, who studied fruit flies, provided the first strong confirmation of the chromosome theory. “Morgan discovered a mutation that affected fly eye color. The chromosome theory of inheritance states that genes are found at specific locations on chromosomes, and that the behavior of chromosomes during meiosis can explain Mendel's laws of inheritance.

Line in NCERT: "Following this synthesis of ideas, experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for the variation that sexual reproduction produced."

Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes.

Line in NCERT: "His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome."

This phenomenon is known as Polyploidy, wherein the cells contain more than two paired (homologous) sets of chromosomes, which is often seen in the case of plants.

The major cause of polyploidy is the non - disjunction of sister chromatids during meiotic recombination.

Fig: Polyploidy can make fruits bigger in size

Line in NCERT: "Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and, this phenomenon is known as polyploidy."

When two genes in a dihybrid cross are situated on the same chromosome, the proportion of parental gene combinations are much higher than the non-parental or recombinant type. This is also called incomplete linkage.

Line in NCERT: "the proportion of parental gene combinations were much higher than the non-parental type."

Sickle cell anemia is not an X-linked recessive gene disorder. It is a point mutation caused by a point change in the sixth nitrogenous base pair.

Statements (a), (b) and (c)  are correct.

Hence, option D is correct.

Line in NCERT: "Haemophilia: This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied." "Sickle-cell anaemia: This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous)." "Phenylketonuria, Thalassemia, etc. It is important to mention here that such Mendelian disorders may be dominant or recessive."

• Dominance means in a heterozygous pair of alleles, one allele suppresses the influence of another and dominates the trait.
  Example: Tt is tall because of the dominance of the T allele.
• Codominance means when in a heterozygous pair both the alleles are equally dominant.
  Example: Blood group AB.
• Pleiotropy means when a gene influences more than one character.
  Example: The appearance of a vestigial wing and eye colour in a fruit fly.
• Polygenic inheritance is defined as quantitative inheritance, where multiple independent genes have an additive or similar effect on a single quantitative trait.

Hence, Option B is correct.

Phenotypic ratio : 3: 1 (Tall :Dwarf) Genotypic Ratio: 1:2:1 (Homozygous Tall : Heterozygous Tall : Dwarf) 

Line in NCERT: "From the Punnett square it is easily seen that 1/4th of the random fertilisations lead to TT, 1/2 lead to Tt and 1/4th to tt. This leads to a phenotypic ratio of 3/4th tall: (1/4 TT + 1/2 Tt) and 1/4th tt, i.e., a 3:1 ratio, but a genotypic ratio of 1:2:1."

Hemophilia A and hemophilia B are inherited in an X-linked recessive pattern. The genes associated with these conditions are located on the X chromosome, which is one of the two sex chromosomes.

• In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
• In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, it is very rare for females to have hemophilia.

Line in NCERT: "Haemophilia: This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied."

The given pedigree shows the autosomal recessive disorder. In this disorder, the individual inherits two mutated genes, one from each parent. This disorder is usually passed on by two earners.

The carriers have a 25% chance of having an unaffected child with normal genes, 50% chance of having an unaffected child who also is a carrier and a 25% chance of having an affected child with recessive genes.

Line in NCERT: "By pedigree analysis one can easily understand whether the trait in question is dominant or recessive."

Mendel used seven pea plant traits in his hybridization experiments:
(i) Flower colour (purple or white)
(ii) Flower position (axil or terminal)
(iii) Stem length (long or short)
(iv) Seed shape (round or wrinkled)
(v) Seed colour (yellow or green)
(vi) Pod shape (inflated or constricted)
(vii) Pod colour (yellow or green). 

Hence, Pod length is not included.

Alleles are defined as alternative form of same gene. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent.

Fig: Contrasting alleles for eye color on the same gene

Line in NCERT: "Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene."

• The most common cause of variations is recombination in the organism which are reproduced sexually. 
• During meiosis in sexual organisms, two homologous chromosomes cross over and exchange genetic material. 
• It is a process by which pieces of DNA are broken and recombined to produce new combinations of alleles.
• This recombination process creates genetic diversity at the level of genes that reflects differences in the DNA sequences of different organisms.

Fig: Recombination in Homologous chromosomes

Seven pairs of contrasting characters were selected in pea plant and studied by Mendel in his experiment. 

Line in NCERT: "Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits."

• Epistasis is the phenomenon of suppression of phenotypic expression of a gene by a non-allelic gene which shows its own effect.
• A dominant epistatic allele suppresses the expression of a non-allelic gene whether the latter is dominant or recessive.

Example: fruit colour of Summer Squash (Cucurbita pepo) is governed by a gene that produces yellow colour in the dominant state (Y-) and green colour in the recessive state (yy). 

Fig: Epistasis in Squash Fruit

According to Hardy-Weinberg equilibrium,

p² + 2pq + q²  = 1

where p² is the frequency of homozygous genotype AA
q² is the frequency of genotype aa
pq is the frequency of genotype Aa

There are 1000 individuals, out of which 360 belong to genotype AA.

p²  = 360/1000 = 0.36

Then p = 0.6.

So, the correct answer is '0.6'.​

As color blindness is an autosomal recessive genetic disorder, for it is present at X -chromosome. Thus, according to the situation given in the question, a man whose father was color blind (will be, i.e, XY normal) marries a woman whose mother was color blind and father was normal (i.e, this woman will be a carrier) according to the cross given in the first figure.
Thus, when marriage will happen between a normal man and a carrier woman, in that case, the percentage of a male child to be colorblind is 25% (this can be easily observed from the second figure).So, the correct answer is '50%'.

Line in NCERT: "Colour blindness is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X chromosome. Males have only one X chromosome and females have two."

A cross is made between two pea plants in which one shows Red dominant flowers and the other genotype is not known. In the next generation, 75% of plants show red flowers and 25% of progeny are white flowers, i.e. the phenotype of parents is Rr and Rr.
Thus, both are heterozygous.

Fig: Cross between two heterozygous red dominant flowers

Line in NCERT: "the F2 resulted in the following ratio 1 (RR) Red: 2 (Rr) Pink: 1 (rr) White."

Line in NCERT: "Colour blindness is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour."

Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. Aneuploidy arises due to non-disjunction of homologous chromosome.
Aneuploidy is of four types:

• Monosomy = 2n – 1
• Nullisomy = 2n – 2
• Trisomy = 2n + 1
• Tetrasomy = 2n + 2

Line in NCERT: "Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy."