Genetics in Agricultural Breeding Programs

Natural Selection

• Mechanism for evolution in natural populations

• Organisms with best traits suited to the environmental factors affecting a population are most likely to survive and reproduce.– Results in the inheritance of the same well-suited

traits

• Important traits in natural selection-disease resistance, size, color pattern/camouflage, etc.

Natural Selection

• Types of Natural Selection– Stabilizing selection– Directional selection– Disruptive selection

Stabilizing Selection

• Individuals with the average or norm for a trait have an advantage over other forms of the trait– Example: gray moths (norm) are favored over

black and white moths

Directional Selection

• Individuals with one extreme or less common version of a trait are favored over other forms of the trait.– Example: Black moths are favored over gray or

white moths

Disruptive Selection

• Multiple extremes or alternative forms of a trait are favored over the norm– Example: Black moths and white moths are

favored over gray moths

Selective Breeding

• Method of breeding plants and animals utilized in agriscience to produce offspring that possess certain characteristics desirable to agriculturists– Utilized for generations-produced the first domestic

animals in early civilizations

Selective Breeding

• Used to select for a variety of traits including:– Muscling/Size– Fat content– Breeding Capability– Color– Speed/Agility– Temperament– Milk Production

Selective Breeding

• Methods for selective breeding:– Artificial insemination– Pen/field breeding– Isolation Breeding-inbreeding– Mechanical pollination of plants– Hybridization of plants and animals

Selective Breeding

• Selective breeding is accomplished much quicker in plants than animals due to growth rates and ease of propagation/production

Selective Breeding Techniques

Objective: Summarize the process and purpose of selective breeding techniques

Techniques in Animals

• Progeny testing is used to determine the value of male livestock by analyzing the transmission of traits to offspring and its breeding potential

• Artificial insemination has proven to have the greatest impact on animal breeding since the first domestication of livestock.

Artificial Insemination

• Semen is collected from male individuals – methods vary widely by the type of animal– Artificial vaginas are used for larger mammals.

• Semen is checked for viability and flash frozen in specialized straws.– Straws should be placed in storage tanks filled with liquid

nitrogen, till use.– Semen stored under proper conditions has proven viable

decades later (-320ºF)– Female is treated with prostaglandin (hormone) to induce

estrus or heat– Semen straws are immediately thawed in water (99ºF) and

soon after inserted into the mother using a specialized release gun.

Sperm Sexing

• Utilizes a cytometer cell sorter to separate male sperm from female sperm– Sperm with X chromosomes (female sperm) weigh

more (contain more DNA) than those with a Y and can thus be dyed and separated because they absorb more dye

Invitro-Fertilization

• Method of removing eggs from a mother for fertilization under laboratory conditions.– A large number of eggs are collected from the

ovaries of an outstanding female and fertilized in Petri dishes under laboratory conditions

– Embryos can be inserted into surrogate mothers or frozen for later use

– Offers the most control and requires the least amount of semen.

Birth Through Surrogate Mothers

• Hormones are used to cause the female to superovulate (produce a large number of eggs).

• Original mother undergoes artificial insemination

• Fertilized eggs are removed by a process called FLUSHING to be placed in other female animals for development.

Cross-Pollination in Plants

• Method used to select particular parents for the production of seed in plants.

• Process– Plants possessing desirable characteristics are

selected and carefully monitored– Pollen can be gathered from male plants (or flowers)

months or in some cases, years in advance and refrigerated for storage

– Flowers on the female part must be covered prior to opening, and if capable of self-pollination, must have the stamens removed.

– Once the female flowers open, pollen from the male should be placed on the stigma, and the flower covered again.

• Record keeping is critical in crossing plants.

Selecting Plants and Animals for Breeding

Observe Patterns of Heredity

• The occurrence of genetic disorders in offspring or parents is an indicator that the parent may have a recessive gene for the disorder

• Though genetic recombination is random, some animals are more likely to transmit genes than others

• Keeping careful breeding records improves effectiveness

Select Animals Carefully

• Along with a good genetic background, animals used in selective breeding should be:– Healthy-old injuries or illnesses are not a factor

unless they are a result of genetic propensities or impair breeding capabilities

– Carefully monitored-nutrition levels, pests and stress can all reduce breeding viability. Some very good specimens are completely isolated.

Select Animals Carefully

• Hybrids should be avoided, since traits expressed in the organism are rarely transmitted to offspring– The process of inbreeding isolates genes for only a

single generation, as many are recessive.

Carefully Plan Breeding Crosses

• Plants can be crossed not only within species (interspecific), but also within genus (intergeneric), and even, in rate cases family (interfamilial)

• Animals are usually limited to crosses within the same species

Methods for Producing Selective Breeding

Programs

Inbreeding

• Crossing organisms that are genetically related– Crossing two plants to produce an f1 generation,

then crossing two of the f1 offspring to create an f2 generation

Backcrossing

• Crossing offspring from a cross with one of the previous parents, or a similar organism, to maximize the expression of certain traits.– Often used after intergeneric crosses to produce

offspring that possess more characteristics from one genus.