Test: Inheritance of Two Genes (NCERT) - NEET MCQ

Cross involving two contrasting character is called dihybrid cross or a two factor cross. The two factors of each trait assort at random and independent of the factors of other traits at the time of meiosis (gametogenesis) and get randomly as well as independently rearranged in the offspring producing both parental and new combinations of traits. This forms the basis of the law of independent assortment given by Mendel.

NCERT Topic: Law of independent assortment

NCERT Line: "When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters."

A. Dihybrid test cross - The typical phenotypic ratio resulting from a dihybrid test cross, where two traits are considered, is 9:3:3:1. Therefore, A matches with (i).

B. Law of segregation - This law states that allele pairs segregate during gamete formation, and randomly unite at fertilization. It doesn't directly relate to the phenotypic ratios or the concepts of dihybrid cross, purity of gametes, or multiple alleles. However, it can be associated with the purity of gametes because segregation ensures that each gamete carries only one allele for each gene. So, B matches with (iv).

C. Law of independent assortment - This law states that genes for different traits can segregate independently during the formation of gametes. This is usually illustrated with a 1:1:1:1 phenotypic ratio in the case of a test cross involving two different traits. So, C matches with (iii).

D. ABO blood group in man - The ABO blood group system in humans is an example of multiple alleles because there are three alleles involved: So, D matches with (v).

Therefore, the correct matches would be:

• A - (i)
• B - (iv)
• C - (iii)
• D - (v)

A typical Mendelian dihybrid cross between yellow round (dominant) and green wrinkled (recessive plants) can be represented as follows:

Thus, round green (green round) offsprings are represented by genotypes yyRR and yyRr (or RRyy and Rryy).

Types of gametes = 2ⁿ, where n is number of heterozygous loci. Thus, man will produe 2² = 4 types of gametes and woman will produce 2³ = 8 types of gametes.

A cross between a tall plant with yellow seeds (TtYy) and a tall plant with green seeds (Ttyy) will be:

Pnenotypic ratio is
Tall and yellow : Tall and green : Dwarf and yellow : Dwarf and green :: 3 : 3: 1: 1 or 3/8, 3/8,1/8,1/8.

The phenotypic ratio of monohybrid F₂ generation in case of Mirabilis jalapa is 1 Red : 2 Pink : 1 White, due to incomplete dominance.

The law of independent assortment applies only to factor or genes present on different pairs or distantly on the same chromosome or pairs of homologous chromosomes. The principle or law of independent assortment can be studied by means of dihybrid cross, e.g., between pure breeding pea plants having yellow round seeds (YYRR) and pure breeding pea plants having green wrinkled seeds (yyrr).

NCERT Topic: Law of independent assortment

NCERT Line: "In the dihybrid cross (Figure 4.7), the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1."

Gametes produced by AaBb parent would be 25 % AB, 25 % aB, 25 % Ab and 25 % ab.

Plant H is formed by fusion of gametes yR and YR and hence has the genotype YyRR. Plant N is formed by fusion of gametes YR and yR and hence will have the same genotype as plant N i.e., YyRR.

Given that recombination percentage is 7% and 5%, therefore, total recombinants would be 7 + 5 = 12%. It is known that one map unit is the distance that yields 1% recombinant chromosomes. Hence the distance between two non-allelic genes is 12 map units. 

Since one parent is homozygous AABBCC  the genotype formed will be only 1 type i.e. ABC. The other parent is heterozygous for all genes. About 1/8th or 12.5% of total offspring will be heterozygous at all three-locus.

Recombinants obtained from the cross are 16 red eye striped thorax flies and 12 pink eye plain thorax flies.

Total number of flies =200

Total number of recombinants =28

Therefore, percentage of recombinants 

NCERT Topic: Linkage and recombination

NCERT Line: Not Found

As per linkage experiments carried out by Morgan, the two linked genes do not always segregate independently of each other and F₂ ratio deviated very significantly from 9:3:3:1 ratio (expected when two genes are independent). Hence, if linkage was known at the time of Mendel, he would riot have been able to explain law of independent assortment.

NCERT Topic: Mendel's law of independent assortment

NCERT Line: "Later, it was found that Mendel's law of independent assortment does not hold true for the genes that were located on the same chromosomes."

When plant YyRr is self pollinated, 9:3:3:1 ratio of phenotypes will be observed.

NCERT Topic: Inheritance of two genes

NCERT Line: "The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round: 1 wrinkled."

When YyRr is crossed with double recessive parent then genotypic and phenotypic ratio will be 1:1:1:1

Phenotypic ratio = 1 yellow round : 1 yellow oval : 1 green round : 1 green and oval. This represent a test cross.

NCERT Topic: Inheritance of two genes

NCERT Line: "Based upon such observations on dihybrid crosses (crosses between plants differing in two traits) Mendel proposed a second set of generalisations that we call Mendel's Law of Independent Assortment."