Ans. Mendel selected pea plant because of the following special feature :
- Garden peas were found to differ in certain definite and easily detectable traits.
- The traits of each kind of pea plant were presented in generation after generation because the
plants had bisexual flowers and normally resorted to self pollination. - Artificial cross – pollination could be easily achieved by removing the stamens (emasculation).
- Pea plants had a short life cycle.
- pea plants produced many seeds in one generation.
- Mendel worked on only pure or true breeding traits, which means that a plant on self pollination
produces offspring with the same traits. - pea plants having each of the seven characters he selected were readily available.
Q.2. Differentiate between the following :
(a) Dominance and recessive : (b) Homozygous and heterozygous : (c) monohybrid and dihybrid.
Ans. (a)
| Dominant allele or Factor | Recessive allele or Factor |
| 1. It can express itself in the presence as well as absence of its recessive allele.
2. It codes for a complete, functional polypeptide (enzyme) that expresses it fully. |
1. It can express in the absence but not in the presence of its dominant allele.
2. It codes for incomplete, non- functional poly- heterozygous condition. |
(b)
| Homozygous Individuals | Heterozygous Individuals |
| (i) Have similar alleles for a specific trait (TT or tt).
(ii) Possess dominant or recessive alleles, never both. (iii)Produce only one kind of gametes (T, T or t, t). (iv)Breed true for a specific trait, i.e.. produce offspring with similar genotype and phenotype on inbreeding. (v) Do not have additional vigour. |
(i) Have dissimilar alleles for a specific trait (Tt).
(ii) Possess both dominant and recessive alleles. (iii) Produce two kinds of gametes(T, t). (iv) Do not breed true, i.e.. produce offspring with 3 genotypes and 2 or 3 phenotypes on inbreeding. (v) Have additional Vigour called hybrid vigour or heterosis. |
(c) A breeding experiment dealing with a single character is called a monohybrid cross whereas a breeding experiment dealing with two characters at the same time is called a dihybrid cross.
calculated by using the formula 2’’ where n is the numner of loci, i., 24 means 2 = 16.
usually expresses itself (that is, appears as an observable trait in a hybrid organism) and prevents
the expression of the other. The allele which is expressed in a hybrid is called dominant allele, and
he unexpressed one is termed recessive allele.
recessive characters is known as a test cross.
generation after a cross between a homozygous female and a heterozygous male for a single locus.
what proportions of phenotype in the offspring could be expected to be (a) tall and green (b)
dwarf and green.
In the given cross, G represents grey body (dominant) and g resents black (recessive) L represents
long (dominant) and l represents dwarf (recessive) The F1 generation for the dihybrid cross shows
3 : 1 phenotypic ratio.
(Drosophila melanogaster). He established the principle of linkage., discovered sex chromosomes,
sex -linkage, crossing over, technique of chromosome mapping, etc. He wrote a book, the Theory of
Gene in, 1926, that got genetics recognition as an important branch of biology. He awarded Nobel
Prize for physiology/ medicine in 1933.
of a family to determine how the trait is inherited.
are 3 main mechanisms of determining sex in organisms : genetic or chromosomal, environmental
or non genetic and non chromosomal.
Similarly if mother has blood group B, the genotype may be IB IB (homozygous)or IB Ii
(homozygous).In case a homozygous IA IA father and IB IB mother, then the offspring will have blood
group AB, as follows:
Thus, the genotype of parents of a child having blood group ‘O’ will be IA Ii (male) and IB Ii (female). The possible genotype of the other offspring will be A, B and AB
Q.13. Explain the following terms with example
(a) Co-dominance (b) Incomplete dominance
gene locus on homologous chromosomes ) neither show dominant- recessive relationship nor show
intermediate condition. But both of them express themselves simultaneously, then this condition
is known as co- dominance. Bort alleles which, when present together in heterozygous individual,
express their traits independently instead of showing dominant-recessive relationship are called
codominant alleles.
(b) The expression of the traits of two traits of two pure parents as an intermediate condition or fine
mixture in the F1 hybrids is known as incomplete or partial dominance. It is also called blended or
intermediate or mosaic inheritance.
called point mutation. For example, a change of a single nitrogenous base causes sickle- cell anaemia
that may prove fatal by age 20.
Ans. The two autosomal genetic disorders are as follow –
(1) Down’s syndrome. It is a type of autosomal aneuploidy caused due to trisomy of chromosome
number 21, i.e., presence of an additional copy of chromosome number 21. The main symptoms of
Down’s syndrome are as follows :
(i)sort statured with small round head
(ii) Protruding tongue and ever open mouth
(iii) Physical, psychomotor and mental retardation
(iv) Mongolian eylid fold, short neck and projecting lower lip.
(v)Broad palm with characteristic palm crease
(2)Phenylketonuria. It is a type of autosomal gene mutation caused due to lack of enzyme
phenylalanine hydroxylase. This enzyme converts amino acid phenylalanine into tyrosine. The main
symptoms of this genetic disorder are-
(i) Extreme mental retardation (ii) Hypopigmentation of skin and eczema.