- Mendelian phenotupic ratio of a monohybrid cross is 3 : 1 and the genotypic ration is 1 : 2 : 1.

- It can be mathematically condensed as (ax + by)².

(i) Terminal flowers    
(ii) Constricted pods    
(iii) Wrinkled seeds    
(iv) Yellow pod.

In the test cross, an individual showing dominant phenotype would be crossed to an individual with recessive phenotype for the same trait.
(i) If the individual is homozygous dominant, all the individuals in the progeny would show the dominant phenotype.

(it) If the individual is heterozygous, the progeny will show dominant phenotype and recessive phenotype in the ratio of 1 : 1.

Thus, the genotype of the individual with dominant phenotype can be ascertained.

In the test cross, an individual showing dominant phenotype would be crossed to an individual with recessive phenotype for the same trait.
(i) If the individual is homozygous dominant, all the individuals in the progeny would show the dominant phenotype.

(it) If the individual is heterozygous, the progeny will show dominant phenotype and recessive phenotype in the ratio of 1 : 1.

Thus, the genotype of the individual with dominant phenotype can be ascertained.

A monohybrid cross is as follows:

\(F_{ 2 }\)

The phenotypic ratio of smooth-seeded to wrinkled-seeded plants, is 3 : 1.
From the Punnett square, it can be seen that one of the smooth-seeded plants is homozygous (55), while the other two are heterozygous (Ss): hence the genotypic ratio is 1 : 2 : 1.

(i) The rest of the progeny will be aaBb and Aabb.
(ii) Yes, the genes are linked.
(iii) Yes, there is crossing over, as 10% of the progeny are recombinants.

(a) 3/4 of the progeny will be black and horned.
(b) 1/4 of the progeny will be red and horned.

Trisomy of 21st chromosome occurs due to non-disjunction, i.e. non-separation of the homologous pair of 21st chromosomes at anaphase I of meiosis during ova formation and ferlilisation of such an ovum with two 21st chromosomes with a normal sperm.
Symptoms of Down's Syndrome:
(i) Partially open mouth with furrowed tongue.
(ii) Broad, flat face with slanting eyes.
(iii) Broad palm with characteristic palm increase.
(iv) Small and arched palate.
(v) Epicanthic eye fold.
(vi) Congenital heart diseases.
(vii) Mental retardation            

(i)No
(ii) Down's syndrome (Mangolism) is ca~ed due to trisomy of chromosome 21. Such children. have 47 chromosomes instead of 46 which is the normal condition.
(iii) Sensitivity towards fellow childem & compassion.

Dominance:
Every gene contains the information to express a particular trait.
In a diploid organism, there are two alternate forms of a gene, or a pair of alleles.
The two alleles may be similar (homo-zygous) or may be different (heterozygous).
One of them may be different due to some changes that it has undergone, which modify the information of that particular allele.
The phenotype will be dependent on the original unmodified allele.

Thalassemia is categorised as a Mendelian disorder, because it. is an autosomal, recessively-inherited disorder, whose transmission follows Mendel's laws/ principles of inheritance.
(i) The disease is caused by mutation or deletion of the alleles and there is reduced synthesis of alpha or beta globin chain of hemoglobin.
(ii) The symptom is anemia, which is due to less synthesis of hemoglobin.