Back Cross

BACK CROSS

DEFINITION :-  It is a genetic cross between a hybrid organism and one of the original parental types is called back cross.

EXAMPLE OF BACK CROSS:-

      A monohybrid cross between a homozygous black guinea pig and homozygous white guinea pig gives F1 generation of heterozygous black offspring. The back cross between the  F1 heterozygous black individual and the homozygous black parent will produce black guinea pig only.

RESULT OF A BACK CROSS 

The result of the back cross depends on the parental type. A back cross between a heterozygous F1 individual and a homozygous dominant parent gives the progeny having the dominant trait only  as in the above  figure shows. 

                   On the other hand,, a back cross between F1 heterozygous and a homozygous recessive parent produces offspring with both the genotype as described in test cross.

USE OF BACK CROSS

The back cross is used by the plant and animal breeders as a rapid method of  purifying the desired stock.

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TEST CROSS

TEST CROSS 

It is a cross between an organism of an unknown genotype and a homozygous recessive organism.

Purpose of test cross

An organism showing the recessive phenotype must have a homozygous genotype for the recessive allele. For example, a white Guinea has bb genotype and a dwarf pea 🫛 plant has tt genotype. However, an organism displaying a dominant phenotype may have a homozygous genotype or a heterozygous. For instance, a black guinea pig may have BB genotype or Bb genotype and a tall pea plant may have TT or Tt genotype. It may interested to a breeder to know the genotype of such an organism.

Result of a Test Cross

1) If the test cross yield offspring of which 50%show the dominant character and 50% show the recessive character,i.e , F1 ratio is 1:1, the individual test is heterozygous. This is so because the individual showing the recessive trait must have received one recessive allele from each parent.

Example1

    

2)  If all the offspring of the test cross show the dominant trait, the individual being tested in homozygous dominant with genotype BB for guinea pig and TT for pea plant.

Example 2

Use of Test Cross

It reveals the genotype of an organism displaying the dominant phenotype. It was devised by Mendel and continues to be an important tool of geneticists.

Types of Test Cross

It is of two types:-

a) Monohybrid test cross - It deals with a single character and gives F1 ratio of 1:1.

b) Dihybrid test cross - It deals with the two traits at a time and gives F1 ratio of 1:1:1:1 for a cross between double heterozygote and double recessive homozygote, BbSs* bbss guinea pig, RrYy*rryy pea plants . 

Mendel's Principle (Law) of Dominance

Mendel's Principle (Law) of Dominance 

Law of Dominance states that one allele or pair of alleles, which control a character, usually expresses itself and prevents the expression of the other . The allele which expressed in a hybrid is called dominant allele and the other unexpressed one is recessive allele.

Explanation:- 

Cross a homozygous tall pea plant having the genotype TT with a dwarf pea plant having the genotype tt . F1 generation  consists only of tall plants, having genotype Tt. The expressed allele T for tallness is dominating over the unexpressed allele t for dwarfness. The fact that the allele for dwarfness is present in F1 plant can be verified by inbreeding them when F2 progeny will consist of both tall and dwarf plants in the ratio of 3:1.

Degree of Dominance:-

In pea 🫛 plant, Dominance is complete as the allele for tallness fully suppresses the expression of dwarfness whereas in the4 o'cloc
k plant, the flower colour shows incomplete dominance. Thus, the degree of Dominance in Law of Dominance is 1:2:1 i.e

3:1. 

The monohybrid cross which showing Law of Dominance in pea plant and in Guinea pig as expressed as follows:- 

 

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Sexual Reproduction and Events/ Process in sexual reproduction

 Sexual Reproduction:-

     It involves the production of male gametes and female gametes either by same parent or different parent.

Bisexual:- when the two types of different gametes are produced by the same individual , it is regarded as bisexual.

Unisexual:- When the male gametes are produced  from male parent and female gamete produced from female parents the parents are regarded as unisexual or heterosexual .

                          During the process of sexual reproduction , the male and female gametes fuse to form the zygote, which develop into a new organism.All the organisms shows the diversity or some phases while grow to a certain maturity in their life before  they start reproducing sexually. The phases are:-

juvenile  phase:-   The period of growth between their birth up to their  reproductive maturity,

Vegetative Phase:- In plants, the period of growth between seed germination upto initiation of flowering.

Reproductive Phase:- The later part, 

S

when organism start reproducing sexually, is called reproductive  phase.

Events in sexual reproduction

In Spite of their morphological, anatomical and physiological differences, the fundamental events in sexual reproduction are same. These events can be categorized as follows:-

i) Pre-fertilization events  ii)Fertilization  and  (iii) Post fertilization

i) Pre-fertilization events

The formation of two types of gametes - male and female gametes , inside the gametangia, is called gametogenesis. The reproductive units in sexual reproduction is gametes.The gametes are generally of two types- male gamete and female gamete. The gametes of all the organisms are usually haploid cells,i.e possess single cell or n number of chromosomes .Different types of gametes are as follow :-

a) Isogametes or Homogametes:-the two gametes are morphologically similar.i.e, they appear similar in size, shape, and activity. example - in some algae. It is not possible to distinguish them into male and female gametes.

b) Heterogametes :- In these type of organisms, the male and female gametes are morphologically distinct.      

c) Microgametes or Spermatozoa:- The male gametes are called microgametes or spermatozoa. These are minute and motile so that they swim to the female for fertilization

d)  Macrogametes or Ova :- The female gametes are termed as macrogametes or ova.These are usually large, non-motile and often have a store of food to nourish the developing embryo. 

Sexuality in organisms

    The male and female gametes are in most of the animals produced by male and female gametes respectively. Such animals are termed as unisexual or dioecious.

In some animals such as liver fluke, earthworms, leech, both kind of gametes are produced by single individual. Such animals are called bisexual, monoecious or hermaphrodite.

Cell Division during gamete formation

          

The gametes are generally formed by meiotic divisions. Therefore they are haploid or n number of chromosomes. In sexual reproduction,  the male and female gametes fuse to form zygote  by the process of fertilization. the two haploid gametes form the diploid or 2n number of chromosomes. The zygote give rise to offspring by the mitotic divisions . Thus, offspring is also a diploid like its parents which formed haploid gametes by meiosis for its creation.Meiosis and fertilization are the two important events in sexual reproduction that keep the number of chromosomes constant from generation to generation.

Gamete Transfer:-

    Once the male and female gametes are formed inside the gametangia, they are allowed to come in contact with each other to facilitate their fusion (syngamy/fertilization).

    In plants transfer of pollen grains from another to the stigma , is called pollination. It is of two types -self and cross pollination.

In self pollination transfer of pollen grains to stigma occurs within the same flower e.g peas whereas in the cross pollination transfer of pollen grains are transferred from the anthers of male flower to the stigma of female flower through the air, water,insects or animals .

 

In case of animals, the male gametes are transferred to the female gametes by three distinct pattern depending on whether fertilization and embryonic development occur within or outside the maternal  body:-

External fertilization and external development:-

This type of pattern is found in many aquatic animals, such as Obelia,Nereis, Labeo and Frog.Parents release sperms and eggs into the surrounding water, where fertilization occurs and zygote develop into offspring.For this pattern to succeed, male and female shed their gametes at the same time and place and in large numbers.The parents may or may not make a physical contact for releasing the gametes.

Internal Fertilization and external development:-

Sperms are passed from the male to the female with an intromittent organ.The female reproductive tract provides a confined, protected place where sperms and eggs can easily meet without any danger.The zygote passing down to the female reproductive tract to the exterior can acquire secretion, membrane or shell for protection of the developing embryo.

Internal Fertilization and internal development:-

Internal development provides additional advantage to the embryo.The mother's body provides exactly the right chemical conditions, warmth and nourishment also.

As the mother carries the embryo wherever she goes,it is not vulnerable to predators who attack externally developing eggs.

ii) Syngamy and fertilization

It is the fusion of the two entire gametes to form a zygote. The whole description is given in my earlier post.

iii) Post fertilization events

All the events that occur during sexual reproduction after the fusion of gametes are called post -fertilization events.

a) The zygote:-

Fusion of two haploid (n)gametes results in the formation of single diploid (2n) zygote. The zygote is the first cell of diploid organism including human beings. However there are certain organism where zygote undergoes meiosis. Whether the zygote divides by meiosis to form haploid spore or divides by meiosis to develop diploid by depends upon the type of life cycle occurs in the organism.

b) Embryogenesis:-

The process of development of embryo from the zygote is the known as embryogenesis. Development of offspring from reproductive units , such as buds or fragments or in asexual reproduction is called blastogenesis.Both embryogenesis and blastogenesis have the same target to develop a new individual by the process office and cell division and differentiation.

In animals, the development of zygote takes place either inside the body of female parent or outside. On this basis the animals are grouped into two categories:--  oviparous and viviparous.

Oviparous:-

These animals lay fertilized or unfertilized eggs.e.g , reptiles,and birds lay fertilized eggs in a safe place in the environment.The eggs are covered by hard calcareous shell . After passing a variable incubation period, the young ones hatch out from the eggs.

Viviparous 

 These animals give birth to young ones. The zygote develops into young ones inside the body of the female.

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Characteristics and significance of Sexual Reproduction

Sexual reproduction

            Sexual reproduction is the type of reproduction which involves the fusion of haploid female and male gametes which results the formation of diploid zygote.

What are the characteristics of Sexual reproduction?

Sexual reproduction has the following basic features:

1. It is generally biparental.
2.  It involves the fusion of  formation of male and female gamete.
3. Mostly there is fusion of male and female gamete (fertilization).
4. Cell division are meiotic during gamete formation and mitotic during the development of  zygote into the offspring.
5. The offspring are not generally identical/same to the parent. They show variation as they as they receive the character of their parent. So, Sexual reproduction is the source of variety of population.
6. Multiplication is lower than the asexual reproduction.
7. Sexual Reproduction may not result in an increase in number of individuals.
8. It involves the meiosis and fusion of gametes.
9. There are three events in the Sexual reproduction-  i) Pre-fertilization ( Includes gametogenesis and gamete transfer) (ii) Syngamy  or Fertilization (iii) Post- fertilization ( includes formation of zygote and embryogenesis)

What is the significance or importance of sexual reproduction?

Sexual reproduction has the dual significance for the species.

• It results in the multiplication and perpetuation(continuation) of the species.
• It contributes to the evolution of the species by introducing variation in the population much more rapidly rather than the asexual reproduction.

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Types of Asexual reproduction- Multiple Fission, Budding, Spore formation

Asexual Reproduction & its types

Definition  of asexual reproduction and its Ist types i.e Binary fission has been discussed in earlier post so, now we are discussing about the other three types of asexual reproduction which are- Multiple fission, Budding and Spore formation.

MULTIPLE FISSION

Multiple fission is the division of the parent into many small daughter individuals simultaneously.

                                      

Example 1- Multiple fission in plasmodium

              

   In Plasmodium ( a malarial parasite), multiple fission takes place in the active adult(schizont) as well as in the encysted zygote(sporont). The process is known as the schizogony in the former case and is termed as sporogony.The daughter individuals are called merozoites in the former case and sporozoites in the latter case. 

Example 2- Multiple fission in Amoeba

              The products of multiple fission become individually surrounded by resistant coats, the cyst walls, before their release from the parent, the multiple fission is here called sporulation, and the encysted products are termed as spores.The spore remain inactive during unfavourable conditions such as desiccation and extremes of temperature. On the return of favourable conditions, each cyst hatches into a small Amoeba, which gradually grows into an adult. Sporulation not only brings about the  multiplication, but also enables the organisms to (i) survive  during unfavourable condition, and (ii) disperse to new localities with air.

Budding 

         Formation of a daughter individual from a small projection , the bud, arising on the body is called budding.

Example 1 - Budding in YEAST

         Yeast is the unicellular fungi. It produces asexuallyby the formation of spores. During the process of budding in yeast, the parent cell develops  a small protuberance which grows in size. The parent nucleus divides and one of the two daughter nuclei goes into bud. Finally the bud get detached and grows into new individual.

Example 2 - Budding in HYDRA

          In Hydra, the buds arise from the surface of the  parents body. Each bud grows and gradually assumes the form ans size of the parent. When full- grown , the new individual may separate from parent and become an independent animal.

SPORE FORMATION

Spores are the propagules which  germinate to produce new individuals. There are several kinds of spores Some of them are given below:

Zoospores:- The zoospores are special kind of motile and flagellated spores produced inside the sporangia .They are usually naked(without cell wall). The flagella help them to swim in aquatic habitat for proper dispersal. The Reproduction by zoospores occurs in some lower fungi- phycomycetes(e.g Achlya, Albugo, Phytophthora,etc.)

Sporangiospores:- These are non- motile spores produced inside the sporangia, Sometimes these spores are also known as endospores.They are usually dispersed by wind and germinate to produce new mycelium (e.g Rhizopus, Mucor, etc.)

Chlamydospores:- These are thick walled resting  spores produced directly from hyphal cells. They store reserve food material and  are capable of withstanding long unfavourable conditions.

Oidia:- The breaks up into its component cells or small pieces which behave like spores.They are thin walled and do not store food material. The oidia are usually formed under the conditions of excess water,sugar and certain salts. They germinate immediately after liberation and cannot survive under unfavourable conditions.

Conidia:- These are non- motile spores produced singly or in chains by constriction at the tip or lateral side o special hyphal branches called conidiophores.They are produced exogenously, dispersed by wind and germinate directly by giving out germ tubes.

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Asexual reproduction - definition, types, Binary fission

 Asexual Reproduction

Definition:-Production of offspring by a single parent without the formation and fusion of gametes is called Asexual reproduction. The youngones received all the genes from its single parent.Asexual reproduction is also known as agamogenesis or agamogeny, due to mitotic cell division it is also termed as somatogenic reproduction.

Clone ;- Asexual Reproduction produces identical offspring .so, they are commonly referred as clones.

Occurrence:- Asexual Reproduction occurs most commonly in unicellular organisms such as, Monerans and Protista, and in plants and some animals also.It doesn't occur in highest non vertebrates and all vertebrates.

Types of Asexual Reproduction

Asexual Reproduction occurs in different ways in different organisms. Some common types are discussed below:- Binary Fission. Multiple Fission, Budding, Spore formation

1.BINARY FISSION:-

                     It is the division of parent into two small , nearly equal sized daughter individuals.

(i)  EXAMPLE  :- Binary fission in bacteria-

It  is the most common method of asexual reproduction in actively growing bacteria. The cytoplasm and nucleoid divide into two without mitosis. During the division of nucleoid, the DNA replicates while attached to mesosomes. A new mesosome develops which get attached to the daughter chromosome. At the same time the cell wall and plasma membrane starts growing transversely inwards from the middle of the dividing cell between the two mesosomes The dividing septum originates centripetally and separates the parent cell into two nearly equal daughter cells, each having a nuclear body. These cells may separate from each other or may remain attached for some time to form chain.

                 Under favourable conditions, the process of binary fission completes in about 20-30 minutes. Thus, a very large number of bacterial cells are produced in just 24 hours. 

EXAMPLE 2   Binary fission in Protozoan Protists:

In protozoans, The replicated chromosomes are separated by intranuclear(closed) mitosis and the nucleus divides by furrowing. Then the cytoplasm gradually constricts between the two separating nuclei, ultimately forming two equal sized daughter individuals, each having nucleus. the offspring grow to the size of the parent before dividing. e.g it occurs in  Amoeba, Paramecium, Euglena and Trypanosoma.

           It is explained as the three types of binary fission.

Binary fission is of 3 types with regards ti the plane of division:

• Irregular
• Longitudinal
• Transverse

i) Irregular Binary Fission:- 

                                 It occurs in amoeba . Plane of division of cytoplasm is variable , but always in right angle to the elongated dividing nucleus.

ii) Longitudinal Binary fission:-

                           It occurs in flagellates such as Euglena. The cytoplasm splits lengthwise from before backward , forming two similar daughter individuals.

iii) Transverse Binary Fission:-

                           It occurs in ciliates such as paramecium. The cytoplasm divides transversely between two sets of nuclei, forming two dissimilar daughter individuals.\

Example  3     Binary Fission in Planarians

Planarians divide by transverse binary fission. The worms fixes the posterior part of the body firmly to the substratum and moves the anterior part forward. Thus, the anterior part exerts a pull over the posterior part, which holds on . Due to the pull, middle part is stretched, constricts and finally breaks. The anterior part regenerates the posterior part , and the posterior part regrows the anterior part. Thus, two complete worms are formed.

Importance or significance of vegetative propagation

 vegetative propagation        

Vegetative propagation is the process by which new plants grow from various sources, including seeds, cuttings, and other plant parts. Plant propagation can refer to both man-made or natural dispersal of seeds.

What is the importance of vegetative propagation

       OR

  What is the significance and uses of vegetative propagation? 

 Importance/significance and uses of vegetative propagation:- 

1.   Vegetative propagation is the only method of reproduction in thse plants which have lost their capacity to produce seed, e.g Banana, Seedless grapes, Rose, Oranges, Figs,Pineapple and many more.
2.  Plants which produce small quantities of viable seeds (e.g Cynodon dactylon - doob grass)are mostly propagated vegetatively.
3.  This method is more rapid, easier, and less expensive so it is useful for those oplants which have poor seed viability or prolonged seed dormancy to reproduce mostly by this method. 
4.  The greatest advantage of vegetative propagation is is that a plant biotype(original plant)can be retained and multiplied indefinitely without can change or variation. 
5.  Effects of physico-chemical treatments (such as application of some chemical or x-rays)and environmental variations(i.e, high tempetature or low temperature)can be studied more conveniently in those plants which propagate vegetatively. 
6.  vegetative propagation by plant tissue culture method has been applied for the production od disease free plants. 
7.  This is a technique which is also applied for the production of an unlimited number of plants within a relatively short time. 
8.  It helps to improve the quality of seeds or plants. 
9. The desired characteristics such as color, size, and aroma of flowers and fruits can be produced by vegetative propagation.
10.  The fruit-bearing plants produce fruit faster than plants grown from seeds.

Artificial Method of Vegetative Propagation

Artificial Method of Vegetative Propagation This method of vegetative propagation are made by man - made or by horticulturists and developed by plant growers for the production of commercial production of crops. They are called artificial method. Some of the artficial method of vegetative propagation are as below:-

i) Cutting:- THe small piece of any plant organ (stem, leaves, roots)used for propagationis called cutting. a) leaf cutting :-It is used to propagateBegonia, Bryophyllum, Kalanchoe etc. b) Root cutting:- I is mostly used to Propagate Citron and Tamarind . c) Stem cutting :- This method is most commonly used in artificial propagation.About 20-30 cm long pieces of stem are planted in natural position in the soil for the proper sprouting. Some factors suc as age of parent plant, length and diameter of the cutting, season and type of plants are also taken to the consideration for the propagation of the plantor the species.It is used in some plants such as: grapes, sugarcane, Rose, Bougainvillea etc.

ii) Layering In this method, roots are artificially induced on the stem branches before they detached from the parent plant. It is of two types:a)Mound Layering b) Air Layering a) Mound Layering:- In this method, the lower branch of stem is bentdown and partially defoliate. An injury is made in the defoliate portion. now the injured and defoliate portion is covered with a light layer of moist soil. After a few days the pegged portion develop adventitious plants. Now the rooted branch is cut and separate from the plant and grown into a new plant. It is used in Jasmine, Grape vine, Strawberry and cherries etc.

ii) Air Layering or (Gootee) :- In this method, a ring of bark is removed(girdled) or a slit at an upward angle is made at the base of an aerial branch. The girdled portion is covered with moist moss or grafting clay( 2 parts clay, 1 part cow dung, some pieces of hay, cotton and water) and wrapped with the moist polythene sheet . the wrapped portion is called gootee. The girdled portion of the branch inside the gootee developsroots within a period of month or two. Now the branch is cut and planted in the soil after removing polythene. This method is used in Litchi, Pomegranate,Orange, Lemon etc.

iii) Grafting:- In this method,two plants are joined in such a way that they as one plant. Grafting is done between the two closely related dicotyledonous plants.The rooted supporting portion of one plant , called stock is joined with a twig of another plant called scion. Generally, the root stock belongs to the wild variety which resistance to the disease and pests and possess an efficient root system for absorption of water and minerals. The scion is ddrived from the plant possessing better characters. Example: the scion of Dussehri mango is grafted on the stock of desi mango . Types of Grafting : Grafting is of four types. a)Tongue or Whip b)Wedge c)Crown d)side grafting

a)Tongue or Whip Grafting :- In this type, both stock and scion are cut obliquely at about the same angle.

b)Wedge Grafting:- A V-shaped notch is made on stock and a wedge shaped cut is made on scion.

c) Crown Grafting:- Several scions having wedge shaped cut are grafted on the slits at the top of stock. d) Side Grafting:- Single scion having wedge shaped cut is inserted in a lateral slit of the stock. e.g Mango,Apple, Roses, Peach, Plum,Rubber, etc. iv) Bud Grafting:- Tis method is similar to grafting except the scion , in this case, it consist of a bud along with a smal portion of bark having intact cambium. The living tissue of scion bud is inserted into an incision made in the stock so that it reaches uptte cambium of the stock.the bud portion should remain exposed outside in the air. The joint is thoroughly sealed with bandage.The complete operation should be performed during rainy season to ensure survival of the bud . After 3-5 weeks, the bud begins to grow. As soon as the bud sprouts, the stock is cut above the level of graft, e.g Rose. Apple etc.

v)Propagation by Plant tissue culture or Micropopagation:- This method includes propagation of plants by culturing the cells,tissues and organs, called tissue culture. Initially, the culturing of cells or tissues results in the formation of an undifferentiated mass of cells, called callus, which later differentiates to produce a large number of plantlets, These plantletsare then transfered to separate pots or nursery to obtain a large number of plants. Tissue culture technique is useful to obtaining the virus free plants,disease free plants, homozygous diploids and in commercial micropropagation of orchids, chrysanthemum, Gladiolus and other ornamental plants. This method isemployed for quic multiplication of plants.

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