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DIHYBRID CROSS - GUINEA PIG
Crossing of pure black, rough haired with double recessive white, smooth haired Guinea pigs.
Character - I – Colour Black ---White
Character - II - Nature of the hair Rough ---Smooth
A. Crossing of ‘p’generation:
When pure double dominant black,rough haired Guinea pig is crossed with double recessive white, smooth haired pig, all F1 individuals are dihybrid black and rough haired. It indicates
that black and rough are dominant over white and smooth hair respectively.
B. Crossing of F1 dihybrid individuals:
When dihybrid black and rough haired Guinea pigs are crossed, in F2 , 4 types of phenotypes are produced. They are
i. Double dominant - Black rough -9/16
ii. Recombinant - I Black smooth - 3/16
iii. Recombinant -Il White rough -3/16
iv. Double recessive - white smooth -1/16
In F2, the formation of recombinants indicates that every contrasting form behaves independently. Hence mE2, in addition to parent combinants (Black rough, white smooth), two new combinants (Black smooth, white rough) are also produced.
MONO HYBRID CROSS-ANIMAL EXAMPLE
Scientists conducted hybridisation experiments mainly on Drosophila (Fruit fly) and Guinea pig. Morgan and Castle mainly did genetic experiments on Drosophila. Because a pair of fruit flies will produce many generations with in small time.
The principles of dominance and segregation are explained by monohybrid cross.
1. Monohybrid cross - definition
A cross made to study the inheritance of one character or two . contrasting forms or two different alleles is known as monohybrid cross.
Animal example:
Crossing of black and white Guinea pigs is the monohybrid cross. In this cross, the inheritance of colour character or black and white contrasting forms or ‘B’ and ‘b’ alleles inheritance is observed.
2. Phenotypic explanation
When pure black and white Guinea pigs were crossed, all ‘F1’ individuals are black in colour. These are monohybrids. By observing ‘F1’ individuals, e can explain the principle of dominance. 
A. Principle of dominance - Definition:
The principle of dominance states that when two contrasting forms (Black, White) were crossed,’the contrasting from that appeared in ‘F1’ generation is called dominant form(Black). The contrasting form that did not appear in ‘F1’ generation is called recessive contrasting form (white). The above experiment indicates that the black contrasting form is dominant over white form in Guinea pigs
B. Crossing of ‘F1’ individuals:
When ‘F1’ Black Guinea pigs were crossed, in ‘F2’ both black and white are produced. They (Black and White) are in the ratio of 3/4 or 75%: 1/4 or 25%.
GENE HYPOTHESIS OF MONOHYBRID CROSS IN PISIUM
According to Mendel, the individual contains two alleles for each character. Every organism receives on allele from male parent and another allele from female parent. The alleles are represented by english letters.
A. Genes for tall and Dwarf:
The gene that controls tallness is ‘T. The gene that controls dwarfness is ‘t’.
B. Genotypes of parents:
The genotype of homozygous tall is ‘TT. The genotype of pure dwarf plant is ‘tt’.
C. Gametes of ‘P’ generation:
Pure tall (TT) produces 1 00% one type of gametes. Every gamete carries ‘T’ to the progeny.
Pure dwarf pisum plant tt also produces 1 00% one type of gametes. Each gamete carries ‘t’ to the progeny.
D. Formation of monohybrids:
‘T’ type of gametes fuse with ‘t’ type of gametes. Hence all ‘F1’ individuals are ‘Tt’. As ‘T’ allele is dominant over’t’ allele, all ‘F1’ individuals are phenotypically tall. These are monohybrids. Hybrids are always heterozygous. 
E. Formation of gametes from ‘F1’ or hybrid tall (Tt): The two alleles ‘T’ and ‘t’ of the hybrid tall seperate during gamete formation. If ‘T’ enters into one gamete, automatically t’
enters into another gamete. This tells us that the monohybrid produces two types of gametes. They are 50% ‘T’ type and 50% ‘t’ type.
F. Formation of F individuals:
Male parent produces two types of Pollengrains, ‘T’ and ‘t’. Like wise female parent also produces two types of ovules ‘T’ and ‘t’. When ‘T’ and ‘t’ types of ovules are fertilized by ‘T’ and ‘t’ types of pollen grains, the ‘F2’ generation produced is of 1/4 n 2/4 Tt and 1/4 if (1/4 pure tall, 2/4 heterozygous tall, 1/4 homozygous dwarf).
So, monohybrid ‘F2’ genohypic ratio is I : 2: 1. As the ‘t’ ovule is fertilized by ‘t’ pollen grain, dwarf ‘if’ individual is produced. This indicates that ‘t’ is seperated from ‘T’ in ‘F1’during
gametogenesis. This seperation is called law of segregation. So, the formation of 1/4 recessive progeny in ‘F2’ is explained by law of segregation.
Mendel did 7 monohybrid crosses and observed the inheritance of ‘7’ characters. Each character appears in two morphological forms. He got the same ratios 3 : 1 phenotypic and 1 : 2 : 1 genotypic ratios for all the ‘7’ characters.
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PRAWN CULTURE
PRAWN-CULTURE
Prawns occur in marine, fresh and brackish water habitats. They are extremely delicious, hence find a good export market. Maharashtra occupies the first place with 50% of total prawn production. Kerala occupies second position with 32% of production, Andhra Pradesh occupies the third place with 6% production in prawn culture. Of late, prawns are cultured extensively in the brackish water ponds in coastal districts of Andhra Pradesh. Prawns are exported in deep- frozen, canned, dried and even in powdered condition. Prawns constitute 90% of exported fish of high commercial value.
Prawns can be grouped into two categories.
1. Penacid prawns and 2. Non-penacid prawns.
1. Penacid prawns
The species of prawns which belong to penaeus and its associated species are called penacid prawns. These are generally marine prawns. The important penicid prawns are:
i. Penaeus mondon (tiger prawn), 
ii. Penaeus indicus (white prawn),
iii. Metapenaeus monoceros,
iv. Metapenaeus dobsoni,
vi. Metapenaeus brevicornis, (yellow prawn), (jinga prawn) and
vi Metapenaeus affinis
vii. Parapenaeus spa
2. Non-penacid prawns
The species of prawns which do not belong to penaeus species are called Non-penacid prawns. These prawns are either fresh water or brackish water forms. The non-penacid prawns are
I. Palaemon tenuipes, 
ii. Palaemon styliferus,
iii. Hippolysmata ensisotris
iv. Acetus indicus (paste shrimp).
About 45% of Penaeus sps. are used as food. For every 100 gms. of penaeus flesh 19gm. of proteins, 1 gm. of fat, 90 K.cal of energy, 325 mg. of calcium, 280 mg. of phosphorus, 5.5 mg. of Iron and some vitamins are present. In palaemon sps. 100 gms. of flesh consists of 20gm. of proteins, 1 gm. of fat, 90 K.cal. of energy, 280 mg. of phosphorus and 16 mg. of calcium.
Macrobrachium rosenbergi, the largest fresh water prawn grows very fast in brackish water. Hence, this prawn has a good commercial value. Certain other cultivable prawns are also used extensively in brackish water culture. The good financial returns in this line are attracting many farmers. As a result, prawns are grown extensively in the paddy fields and brackish water ponds in our state.
Macrobrachium rosenbergi occurs in brackish waters in
monsoon and post monsoon seasons in Kerala. It occurs from December to July in the brackish waters located along the east coast.
Stake nets, cast nets, trawl nets, boat seines and dip nets are the gear used to capture prawns.
The fishing seasons vary for marine prawns in west coast and those of east coast. In west coast, November to May is considered as the season for fishing prawns. In east coast, December to August forms the season. The most important prawns in the reccent days are penaeus monodon and penaeus indicus. They are having highest capacity of growth. These two species of prawns are also having high commercial value. Hence they constitute most important cultivable species of India ‘today. Besides there are number of species in penaeus, Metapenaeus and Parapenaeus genera which are cultivable.
Extensive research work on prawn culture has revealed that the annual income from one hectare is about Rs. 7000/- excluding the expenditure. In Kerala state number of agriculturallsts are being attracted towards it and started prawn culture in their fields. The prawn culture is also being taken up in brackish waters in that state. Recently, the prawn culture is also started extensively in fields and brackish waters in Andhra Pradesh.
FISH CRAFT-FISH GEAR
FISH CRAFT
The vessels used for transport and collecting fish is termed as fish craft. Different varieties of crafts are used both in fresh water and marine waters to capture fish. Dhoni, Sagadam, Coracle, Catamarans and Dugout canoes are used in fresh water reservoirs and riverine systems.
The catamarans are also used in marine waters. They are different types. 1. Orissa and Ganjam types. 2. Andhra type, 3. Coramandal type and 4. Boat catamarans.
Mechanisation of fishing craft brought many advantages in capture fisheries. Installation of engines takes the craft faster and farther into the marine waters, It saves labor and helps in handling the gear.
FISH GEAR
The various devices used for capturing fish are termed as fish gear. Spears, harpons, lines, traps, bag nets and drag nets are the common gear used for capturing fish. 
The traps, cast nets, stake nets, dip nets are commonly used for capturing fresh water fish.
Boat seins and shore seins are used for catching marine fish. The deep sea fish are captured by using trawl nets with the help of rechanised boats. The gill nets are used to capture sardines while the line nets are used to capture tunas
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