Genetics

Phenotype/Genotype

• Phenotype is what an animal looks like

• Phenotype = Genetics + Environment (+GxE interactions)

• Genotype = the genetic makeup of the organism

Discovery of Genetics

• Gregor Mendel

• Ahead of that time there was no good concept of transmission of genetic information from one generation to the next.

Gregor Mendel

• Austrian Monk lived 1823-1884

• Presented his observations and experiments on pea plants in 1865

• Discoveries lay unnoticed ~20 years until others independently found the same thing

• He found traits were controlled by discrete “factors” (genes)

Cell Theory of Inheritance

• All plants & animals are made of small building blocks called cells

• Cells composed of:– cell wall– nucleus– cytoplasm

• All cells originate from other cells

Cell Structure

NucleusGolgi Complex

VacuoleLysosome

Endoplasmic Reticulum

Ribosome

Unit of Inheritance

• Gene

• Genes are particular parts of DNA

• Contained in the NUCLEUS

DNA

• Deoxyribonucleic Acid

• Contains the genetic code by the arrangement of 4 base pairs. Up to 600/gene

• Structure of DNA by Watson & Crick won Nobel prize

DNA

• Made up of 4 nucleotides and deoxyribose

• Forms genes• Genes reside on

chromosomes

Chromosomes

• Made up of DNA• Contain many genes

on each chromosome• Not always visibile,

only when they coil up• Occur in pairs in

somatic cells

Sex

• 1 pair of chromosomes determines sex

• Other traits on that chromosome will be “sex linked” traits

• Mammals– Female = XX

– Male = XY

• (different nomenclature for poultry)

Some Terms

• Homozygous– SAME

• Heterozygous– DIFFERENT

More Terms

• Homologous = “member of pair”

• Dipoid = 2n number of chromosomes

• Haploid = 1n number of chromosomes

More more terms

• Dominance = gene always expressed

• Recessive = gene only expressed if not masked

• Codominant or Lack of Dominance = both homologous genes expressed

Angus - Black is dominant, Red is recessive

Shorthorn - Red, White, No Dominance, All patterns, Roan

More more more terms

• Locus = Location on the chromosome of a gene

• Allele = alternate genes that occupy corresponding sites on homologous chromosomes– like black and red for angus cattle

Even MORE Terms

• Kinds of cell division– Mitosis

• The way cells divide in somatic cells

• Results in diploid # of chromosomes

– Meiosis• Cell division in sex cells (ova, sperm)

• Results in haploid # of chromosomes

Mitosis

1- Interphase

2- Prophase

3- Metaphase

4- Anaphase

5- Telophase

Meiosis

• Reduction division

• Occurs only in gametes (sex cells)

• Results in 1/2 the # of chromosomes– (haploid number)– 1 of each pair of homologous chromosomes

No. of Chromosomes by Species

• Horse 64• Donkey 62• Mule 63• Swine38• Sheep 54• Cattle 60• Man 46

• Mink 30• Dog 78• Lion 38• Domestic cat 38• Bengal tiger 38• Chicken 78

Mendellian Genetics

• Explains the segregation and recombination of genes

• Understandable for a small number of traits at a time

• Understandable for traits controlled by 1 or a few genes

• MOST Productivity traits = many genes

Abnormalities

• Mutation– Accidental change in the structure of a gene– Occur with low frequency randomly or from

radiation, chemicals, drugs, etc.

Mutation types- Crossing Over

Mutation types - Deletion

Mutation types - Duplication

Mutation types - Insertion

Are Mutations Good or Bad?

• Usually BAD

• Sometimes NO EFFECT

• Sometimes GOOD– Polled condition in hereford cattle

Prediction

• When traits are controlled by single gene pairs, predicting phenotype from genotype is possible if we know the type of gene action!– Dominance– Recessive– Codominance

• More useful is predicting the GENOTYPE from what we see of the animals (phenotype)– We can make matings and observe the outcome– ONLY finds Statistical Probability in some

cases

• View now the genetic animations for determining the combinations possible!

Livestock Improvement

• Most economically important traits involve several or many genes– Growth

• depends on appetite, gut capacity, metabolism rate– etc etc etc

– Milk production• depends on mammary development, cow size,

appetite, blood supply, – etc etc etc etc

Therefore -- Population Genetics

• Goal is to select animals with many good genes

• Remember P = G + E– So to compare animals, keep the Environment

the same

Rules for Maximum Genetic Improvement

• Have maximum genetic variation

• Spend selection efforts on traits largely influenced by heredity

• Observe (measure) accurately the traits carried by the animal

• Use the selected animal(s) most effectively

1. Have maximum genetic variation

• Uniformity may be good, but limits genetic progress

• Breeding herds exist to provide best genetics for future generations (and improve)

2. Spend selection efforts on traits largely influenced by heredity

• Heritibility h2

• The proportion of variation that can be expected to be transmitted to the next generation

• The relative importance of heredity in influencing certain traits

• Heritability refers to TRAITS not the animal

Heritability estimates

Cattle Swine

• No. of young weaned 10-15 10-15

• % lean cuts 40-50 30-40

• Rate of gain 50-55 25-30

Level of Heritability• Low (5-15%)

– Reproductive traits– Health

• Medium (15-40%)– Conformation score (dairy, beef 25%)– Many production characteristics

• High (40%+)– Carcass characteristics– Growth rate (cattle, sheep)– Mature weight

How much progress can we make?

• Depends on how much better the parents are than the average of the population.

• Two parents, each has ½ the influence

• Depends on the heritability of the trait

• Progress = selection differential * h2

Selection differential

• How much better are the parents than the average of the population they are selected from

Example

Say herd population is 18,000 lbs of milk– Choose a bull with a milking potential of

22,000 lbs of milk– Choose cows with 20,000 lbs milking potential– Bull Cow 22,000 20,000-18,000 18,000---------- ---------- 4,000 2,000½ the genetics comes from bull, ½ from cowSo you can have ½ of 4000 and ½ of 2000

Example

½ the genetics comes from bull, ½ from cow

So you can have ½ of 4000 and ½ of 2000

(4000 + 2000) / 2 = 6000/2 = 3000

Multiply the Selection Differential (3000) by h2

H2 for milk production is 0.25

3000 X 0.25 = 750 lbs of improvement

Add that to the herd avg: 18,000 + 750 = 18,750

Which is the avg of production in the replacements.

Example

• If the replacements = 10% of the herd,

• What is the new herd average?– 90% of herd still averages 18,000– 10% of herd averages 18,750– (18,000)(.90) + (18,750)(.10) = 18,075

• If we replace 20% of the herd– (18,000)(.80) + (18,750)(.20) = 18, 150

• As you can see, progress is slow

• So you must continue to strive to make progress as steadily as you can

If you only selected the bull

• The selection differential on the bulls side is the same (22000 – 18000 = 4000)

• Sel.Diff. On the cow side is 0

• 4000 / 2 = 2000

• (2000 X .25) = 500 which is improvement

• Add 500 to herd average

• (500 + 18000) = 18,500

Let’s do another example

• Suppose a swine herd average is 1.2 inches of backfat– Select a boar with 0.8 inches, gilts with 1.0 in.

(1.2 – 0.8) = 0.4

(1.0 – 0.8) = 0.2

(0.4 + 0.2) / 2 = 0.3

The offspring are expected to be 0.3 better

1.2 inches – 0.3 inches = 0.9 avg of next generation

What influences how much genetic progress you can make?

• Amount of genetic variation

• Heritability

• Accuracy of measurement of information

• Extent of use of selected animal

How extensively you can make use of an animal is influenced by:

• Prolificness– Two years selection in corn could produce

327,680 Billion descendants for planting from 2 kernels selected

– Swine have bigger litters than cattle

How extensively you can make use of an animal is influenced by:

• Generation time– Poultry completes a generation in 7 or 8 months– Sows farrow first at 1 year of age, 2 litters/year– Cow calves first at 2 or 3 years, say avg of 5 yrs

as a realistic practical average

• With long generation interval comes slower rate of improvement

3. Observe/measure accurately the traits carried by the animal

• Desirable traits– Health– Prolificness

• Regular heat periods, enough ova, conceive on 1st service

• Males masculine and progressive, sufficient volume, concentration of sperm

– Long life, longevity (in some animals)• Cows more impt than pigs

Desirable traits, continued

• Efficient growth

• Quality of product

Factors that influence which traits to emphasize

• Choose traits contributing most to long-term profit

• Choose traits for which your herd is lacking

• Choose traits with a degree of heritability

Measure accurately

• Records– Individuals, birth dates, litter size, birth weight– Use a SCALE, not eyeball– Proper ID of animals– Carcass measurements

• Backfat, loin eye area, carcass length, etc.

Animal Id Systems

• Tatoo

• Ear notch

• Ear tag

• Leg band

• Brand

• Freeze brand

• Paint

Age estimation

• Records are best

• Other methods– Teeth– Size of body parts

• Use of Central Testing facilities– Pros– Cons

• Systems for measuring meatiness– Backfat probe (fat depth related to meatiness)

• Metal ruler

• Ultrasonics

– “Fat-o-meter”– TOBEC– Cutout info from relatives– Visual appraisal – OFTEN INACCURATE

Ways of assessing breeding efficiencies of sires beforehand

• Sperm volume, concentration, viability

• Libido

• Scrotal circumference

• In the female, it may be more difficult to assess– Mothering ability– Temperament

Conformation

• Some of the “desired” terms are highly unscientific

• Need adequate leg structure, mammary system, etc.

• Some traits cannot be easily measured

How to consider multiple traits

• Realize selecting for many things means less progress in each

• Selection Thresholds vs Indexes

• Indexes take into account relative economic importance and heritability

Relatives

• Ancestors– Animal gets just ½ its genes from a parent– Is ¼ related to a grandparent– Only 1/8 related to a great grandparent

• Sibs (brothers & sisters)

• Progeny

Progeny Testing

• The BEST info on what genes an animal can pass into its offspring is what is seen in its offspring

When to progeny test

• When you want the best answer badly– Because it takes a long time to develop a parent

(long generation interval)– Expensive procedure– Use for Dairy cattle, Beef cattle– Less used for poultry, swine

• Use of offspring selected on other info can make more rapid progress even if less accurate

Repeatability

• Will sow with large litter 1 have another?

• Does production repeat in next season?

• Depends on the trait

RepeatabilityTrait Beef Sheep

Swine

Prolificacy .10

Birth wt .30 .30

Repro Effic. .10 .20 .10-.16

Weaning wt. .45 .40

Grade at wean .22

Annual wool .50-.60

Use the selected animal most effectively

• AI

• Embryo transfer

Hybrid Vigor = Heterosis

• Increased vigor of crossbreds as compared with the average of the purebred parents– High for “non-additive” traits

• Maternal ability

• Survival

– Low for : carcass, growth rate

Heterosis

• Can NOT be transmitted from one generation to the next

• MUST be recreated in each generation by making the cross

Heterosis

• How important? Everybody? Forever?

Mating Systems

• Random mating– Selected males and females run together

• Example: bulls with cows in range country

• Inbreeding– Mating relatives– Used to concentrate genes (homozygosity)

• Concentrates good genes AND bad genes

• Outbreeding– Similar to crossbreeding, but still within a

breed– Mating animals that are “unrelated”

Crossbreeding

• Mating animals from different breeds

• Need a system to make progress successfully

• Reasons for crossbreeding– Bring in good genes lacking in a breed– Increase vigor, prolificacy

Crossbreeding methods

• Rotational crosses– 2 way– 3 way– How many breeds?– Advantage – can produce your own females

Terminal Cross

• Select a male line and a separate female line

• Sell all resulting offspring

• Purchase males, females

• Advantage:– Male line can be great at carcass & doesn’t

have to be great at maternal traits– Female line can emphasize female traits

The future

• “Breeds” per se became less important in poultry, likely swine will follow

• Identification of specific genes of benefit

• Marker assisted selection

• Transgenics and other biotechnology tools

• “Genomics”

• Maybe “the future is now”