Selective Breeding

SELECTIVE BREEDING

·                Selective Breeding and natural selection are similar because both involve selection of which animals or plants survive to breed.

·                Selective breeding and natural selection are different because In selective breeding the farmer / breeder does the selection. In natural selection it is the survival of the fittest in a habitat that leads to selection.

Method of Selective Breeding in Plants :

·                Hybrid G was produced by selective breeding. Individual plants from pure lines of A and B were selected (for size of cobs) and crossed to produce hybrid E.

·                Similarly, individual plants from pure lines of C and D were selected and crossed to produce hybrid F.

·                Plants from hybrids E and F were then selected for their cob size, and crossed to produce hybrid G.

·                Crossing would be done by transfer of pollen from anthers to stigmas of plants.

·                Cob G is larger, it has more seeds and the seeds are more even in size.

Note : To show genetic differences between hybrid C and hybrid G, the sequence of genes are different.

Aims of selectively breeding plants :

           

·                Plants have resistance to disease, so they are not killed by fungi, bacteria, etc.

·                Plants are better suited to climate, so can grow well in a particular location.

·                Plants have a better balance of nutrients (sugar, starch, protein, oil) to produce more nutritious food, or have a high vitamin content etc.

·                Plants that give high yield and have crops with bigger grains

·                Plants that grow rapidly giving an early crop.

·                Drought resistance

·                Plants with strong stems to help the plant survive the wind or heavy rain

Method of Selective Breeding in Animals :

·                Choose a cow with the best characteristics and give hormone / FSH injections to cause multiple ovulations.

·                Collect ova and use IVF to fertilise with sperm collected from a bull with the best characteristics.

·                Separate cells of embryos that develop and produce large numbers of embryos.

·                Screen for sex (males) and implant into surrogate mother cows.

Note : To choose cows for selective breeding in a dairy farm, milk yield and milk to feed conversion rate are important.

MICROPROPAGATION :

·                Very small pieces of the plant called explants are obtained.

·                Explants are trimmed to about 0.5 – 1 mm.

·                Explants are grown in vitro in sterile agar medium, containing nutrients and hormones.

·                Auxins cause growth of callus and roots.

·                When the explants have grown, they are transferred to compost in a greenhouse.

Advantages :

·                Plants can be grown rapidly in large numbers as only a few cells are needed.

·                Plants can be produced at any time of year since grown inside.

·                Large numbers of plants can be stored easily until needed.

·                All are genetically identical and have desired characteristics. For example,

Disadvantages :

·                All plants have same genes, so all are susceptible to same diseases / could all be affected at same time.

Note : All plants produced by micropropagation are genetically identical as all have same genes since they are produced by mitosis from cells of the same parent plant

CLONING :

Clone : An organism arising from asexual reproduction from a single parent cell, with identical genetic makeup (genotype) to parent.

·                Diploid nucleus from the udder cell of an adult sheep is obtained.

·                An ovum is enucleated.

·                The diploid nucleus is inserted into the ovum by a shock / spark of electricity, thus, in Vitro fertilisation occurs.

·                Embryo is implanted in the uterus of a surrogate mother.

·                For example, Dolly

Disadvantages :

·                Cloned animals, like ‘Dolly’, may have genetic defects