Test: Inheritance of One Gene (NCERT) - NEET MCQ

The inheritance of flower colour in the Antirrhinum majus (snapdragon or dog flower) is an example of incomplete or partial dominance. Incomplete dominance is the phenomenon in which neither of the two alleles of a gene is completely dominant over the other. In a cross between true-breeding redflowered (RR) and true-breeding white-flowered plants (rr), the F1 plants obtained were pink (Rr) coloured. When the F1 plants were self-pollinated, the F2 generation resulted in the ratio, 1(RR) Red : 2(Rr) Pink : 1(rr) White. The phenotypic ratios had changed from the normal 3:1 dominant : recessive ratio to 1:2:1. R was not completely dominant over r and this made it possible to distinguish Rr (pink) from RR (red) and rr (white).

To determine the genotype of a tall plant of F₂ generation, Mendel crossed the tall plant from F₂ gneration with a dwarf plant. He called this a test cross. In a typical test cross an organism (pea plants) showing a dominant phenotype whose genotype is to be determined is crossed with the recessive parent instead of self-crossing. The progenies of such a cross can easily be analysed to predict the genotype of the test organism. Normal test cross ratio for a monohybrid cross is 1:1 and for a dihybrid cross is 1:1:1:1.

Genes are the units of inheritance and contain the information that is required to express a particular trait in an organism. Alternating form if a single gene which code for pair of contrasting traits are known as alleles, i.e, tall and drawf are alleles determining the height of pea plant.

Incomplete dominance is reported in flowers of Merabilis jalapa (four o' clock)When red flower are crossed with pink flowers 50 % red flowers and 50 % pink flowers are obtained

F₁ or first filial or progeny is the generation of hybrids produced from a cross between the genetically different individuals called parents. A cross between homozygous and heterozygous parents for a single locus will produce 1:1 ratio of phenotypic features in F₁ generation.

If father has blood group 'A', mother has blood group 'B', and the child with blood group 'O' appears in progeny, this means that the parents are heterozygous.

• This case where three alleles are present in one gene is called co-dominance, The gene I has three forms and when either A or B is expressed in the gene then either A or B blood group is expressed respectively.
• When both A and B are present then it results in the AB blood group. The i allele represents the recessive form and codes for O blood group and will be expressed only in the dominant recessive genotype expression.

Cross between two heterozygous tall pea plants can be shown as follows:

Coat colour in short-horned cattle is an exampl of co-dominance. If a cattle with black coat is crossed with a cattle with white coat, the F₁ hybrids possess neither black nor white coat colour, but have roan coat colour, where black and white patches appear separately. The effect is produced due to juxtaposition of small patches of red and white colour. Hence the alleles which are able to express themselves independently when present together are called codominant alleles and the inheritance pattern is called co-dominance. On inbreeding, the roan hybrids produce three types of cattle — red, roan and white in the ratio of 1:2:1.

Thalassemia-autosomal-linked recessive
AA-Normal
Aa-Carrier
aa-Disease
Affected = 1/4 ​= 25%
An autosomal recessive disorder like thalassemia is caused by abnormalities in both members of a pair of genes. Both parents may carry a single dose of the gene on one chromosome but have a normal gene on the other chromosome, which prevents the appearance of the disease in the carrier parent. A child must get one abnormal gene from each parent in order to be affected by the disease. Parents who are both carriers of an autosomal recessive disorder have a 25% risk of producing an offspring affected by the disease and a 50% risk of producing normal-appearing carriers.

The factor of an allelic or allelomorphic pair which is unable to express its effect in the presence of its contrasting factor in a heterozygote is called recessive factor or allele, e.g., the allele ′t′ in hybrid tall pea plant Tt. The effect of recessive factor becomes known only when it is present in the pure or homozygous state, e.g., tt in dwarf pea plant.

In heterozygous individuals or hybrids, out of the two factors or alleles representing the alternate traits of a character, one is dominant and expresses itself in the hybrid or F₁ generation. The other factor or allele is recessive and does not show its effect in the heterozygous individual (Principle of dominance).

When heterozygous plants are self-pollinated, 50% of progeny would be parental type. Hence, 600 seeds would have parental genotype.

This is an example of a test cross in which a cross is made between heterozygous tall and homozygous dwarf individuals, tall and dwarf plants are obtained in the same proportions.

Given, that Y is dominant for yellow fur and y is recessive for grey fur, the cross between Yy and Yy will produce ratio of 1YY:2Yy:1yy. But as Y is lethal in homozygous condition, hence phenotypic ratio would be 2 yellow (Yy):1 grey (yy).

The cross between blue (BW) and black (BB) forms can be shown as :

Thus, black and blue individuals are produced in the ratio of 1:1

The cross between black mouse (BB) and a brown mouse (bb) and the inbreeding of their hybrid progeny is shown below

Hence, phenotypic raio is 3 black : 1 brown i.e, 75 % of the progeny would have black coat.

The principlem of segregation is the most fundamental principle of heredity that has universal application with no exception. Some workers like Bateson call the principal of segregation as the principle of purity of gametes because segregation of the two Mendelian factors of a trait result result in ametes receiving only one factor out of a pair. As a result gametes are always pure for a character. It is also known as law of non-mixing of alleles and can be demostrated through a monohybrid cross.

A cross between heterozygous long-winged flies and (homozygous) vestigial winged flies represents an example of test cross, in which the exact Mendelian ratio of 1:1 is obtained, i.e. 96 long-winged flies and 96 vestigial winged flies.

When both parents have blood group AB(I^AI^B) then offsprings can have blood groups A(I^AI^A),B(I^BI^B) or AB(I^AI^B) but not O as allele i is not present in any of the parents.