Genetics and Inheritance in Pigs: Key Concepts and Mendelian Laws

How many chromosomes do domestic pigs have?

38 chromosomes, or 19 pairs.

What are chromosomes?

Structures in the cell nucleus that contain DNA and genes.

What is a gene?

A unit of heredity located on DNA that influences a trait.

What are alleles?

Alternative versions of a gene, such as A and a.

What is a genotype?

The genetic makeup or allele combination of an individual.

What is a phenotype?

The observable or measurable expression of a genotype.

What does homozygous mean?

Having two identical alleles for a gene, such as AA or aa.

What does heterozygous mean?

Having two different alleles for a gene, such as Aa.

What is a dominant allele?

An allele that is expressed in the phenotype when at least one copy is present.

What is a recessive allele?

An allele whose phenotype is masked by a dominant allele in a heterozygote.

What are qualitative traits?

Traits whose phenotypes can be classified into groups and are generally controlled by one or two pairs of genes.

What are examples of qualitative traits in pigs?

Hair color, ear shape, and tail type.

What are quantitative traits?

Traits that can be numerically measured and are generally influenced by many pairs of genes.

What are examples of quantitative traits?

Carcass traits, structural traits, growth/performance traits, and reproductive traits.

What type of distribution is associated with polygenic traits?

A normal or bell-shaped distribution.

What is Mendelian inheritance?

The inheritance of traits according to the principles described by Gregor Mendel.

What are the three laws of Mendelian inheritance?

Law of Dominance, Law of Segregation, and Law of Independent Assortment.

What is the Law of Dominance?

A dominant allele can determine the phenotype and mask a recessive allele in a heterozygous individual.

What is the Law of Segregation?

The two alleles for a gene separate during gamete formation so each gamete receives one allele.

What is the Law of Independent Assortment?

Alleles of different genes assort independently during gamete formation.

What offspring result from AA × AA?

100% AA.

What offspring result from AA × aa?

100% Aa.

What offspring result from AA × Aa?

1/2 AA and 1/2 Aa.

What is the genotype ratio of Aa × Aa?

1 AA : 2 Aa : 1 aa, or 1:2:1.

What is the phenotype ratio of Aa × Aa when A is dominant?

3 dominant phenotype : 1 recessive phenotype, or 3:1.

What is the phenotype ratio of Aa × aa when A is dominant?

1 dominant phenotype : 1 recessive phenotype, or 1:1.

What is a monogenic trait?

A trait controlled by one gene pair.

What genotype ratio results from a single heterozygote cross Aa × Aa?

1:2:1.

What phenotype ratio results from a single heterozygote cross Aa × Aa?

3:1 when there is complete dominance.

What is a digenic trait?

A trait involving two gene pairs.

What is the classic double heterozygote cross?

WwGg × WwGg.

What is the phenotype ratio of WwGg × WwGg?

9:3:3:1.

In the 9:3:3:1 dihybrid ratio, what does the 9 represent?

Offspring showing the dominant phenotype for both traits, W- G-.

In the 9:3:3:1 dihybrid ratio, what does the first 3 represent?

Offspring showing the dominant W phenotype and recessive gg phenotype, W- gg.

In the 9:3:3:1 dihybrid ratio, what does the second 3 represent?

Offspring showing the recessive ww phenotype and dominant G phenotype, ww G-.

In the 9:3:3:1 dihybrid ratio, what does the 1 represent?

Offspring recessive for both traits, wwgg.

What is a polygenic trait?

A trait influenced by many pairs of genes.

What happens to trait distributions as more gene pairs influence a trait?

The trait becomes more continuous and tends toward a normal distribution.

What is additive gene action?

Gene effects add together incrementally to influence the phenotype.

What ADG values were used to demonstrate additive gene action?

gg = 1.4 lb/day, Gg = 1.5 lb/day, and GG = 1.6 lb/day.

What is non-additive gene action?

Gene effects do not simply add together; interactions between genes influence the phenotype.

What determines phenotype according to the lecture?

Genotype, gene-gene actions, and environmental factors, especially nutrition.

What does P = G + E mean?

Phenotype = Genetics + Environment.

What is heritability?

The proportion of variation in a trait attributed to genetics relative to total variation from genetics and environment.

What symbol is commonly used for heritability?

h².

What does high heritability generally mean for genetic selection?

Genetic improvement can occur more quickly through selection.

What does low heritability generally mean for genetic selection?

Genetic improvement through selection is slower.

What was the heritability estimate for average daily gain (ADG)?

0.50.

What was the heritability estimate for loin muscle area (LMA)?

0.48.

What was the heritability estimate for muscle color?

0.47.

What was the heritability estimate for backfat (BF)?

0.46.

What was the heritability estimate for ultimate pH?

0.38.

What was the heritability estimate for intramuscular fat (IMF)?

0.25.

What was the heritability estimate for tenderness?

0.20.

What was the heritability estimate for water-holding capacity (WHC)?

0.19.

Which trait in the lecture's table had the highest heritability estimate?

Average daily gain (ADG) at 0.50.

Which trait in the lecture's table had the lowest heritability estimate?

Water-holding capacity (WHC) at 0.19.

How can genetic improvement of lower-heritability traits be hastened?

Identify genetic markers or genes for use in marker-assisted selection (MAS).

What is heterosis?

Hybrid vigor; the average offspring performance is superior, or differs, compared with the average of the two parents for one or more traits.

What is another name for heterosis?

Hybrid vigor.

What type of gene action is associated with heterosis?

Non-additive gene action.

What type of traits particularly benefit from heterosis?

Traits with low heritability.

What are the three types of heterosis discussed in the lecture?

Paternal heterosis, maternal heterosis, and individual heterosis.

What is paternal heterosis?

Heterosis involving boar performance and the performance of their offspring.

What traits are examples of paternal heterosis?

Libido, persistent breeding, and longevity.

What is maternal heterosis?

Heterosis involving sow performance and the performance of their offspring.

What traits are examples of maternal heterosis?

Milk production, litter size, and rebreeding rate.

What is individual heterosis?

Heterosis affecting the performance of the individual crossbred offspring.

What traits are examples of individual heterosis?

Growth rate such as ADG, feed efficiency, and survival rate.

What are the three breeding principles for genetic improvement?

1. Identify economically important traits. 2. Use selection pressure to improve highly heritable traits. 3. Use crossbreeding to improve poorly heritable traits and maximize heterosis.

What are examples of growth/performance traits used in swine breeding?

Growth rate, feed intake, and feed efficiency.

What are examples of reproductive traits used in swine breeding?

Number of pigs born, number of pigs weaned, number of live pigs born, birth weight, weaning weight, and pigs weaned per sow per year.

What are examples of carcass traits used in swine breeding?

Carcass lean percentage, loin eye area, and backfat thickness.

What are examples of structural traits used in swine breeding?

Leg soundness and number of teats.

What is selection pressure?

Using superior animals for breeding while reducing or eliminating reproduction by poorer-performing animals.

What is the goal of a genetic selection program?

Increase the frequency of superior-performing animals while reducing poor-performing animals.

What type of traits respond especially well to selection pressure?

Highly heritable traits.

What is inbreeding?

Mating closely related animals.

What is linebreeding?

Mating relatives within a particular line, such as ancestors, descendants, or cousins.

What can inbreeding do to desirable traits?

It can intensify desirable traits.

What can inbreeding do to undesirable traits?

It can also intensify undesirable traits.

What is inbreeding depression?

A reduction in vigor or performance associated with inbreeding.

Why is inbreeding not normally used extensively in commercial swine production?

It can increase undesirable traits and cause inbreeding depression.

What is outbreeding?

Mating unrelated animals within a breed to avoid inbreeding depression.

What is crossbreeding?

Mating purebred animals from different breeds to produce hybrid offspring.

What are the major purposes of crossbreeding?

Combine desirable strengths from different breeds and take advantage of heterosis.

What is a terminal cross?

A crossbreeding system in which the offspring go to market rather than being retained as replacements.

What is a rotational cross?

A crossbreeding system in which crossbred females are kept as replacement animals and breeds are rotated across generations.

What is a rota-terminal cross?

A system combining rotational and terminal crossbreeding.

What is the major goal of crossbreeding systems?

To take advantage of as much heterosis as possible.

What types of crossbreeding systems were discussed?

Two-breed crosses, three-breed crosses, four-breed crosses, terminal crosses, rotational crosses, and rota-terminal crosses.

What is an example of a two-way terminal cross?

Hampshire boar × Yorkshire sow = HY F1 pigs.

What happens to pigs produced in a two-way terminal cross?

All terminal pigs go to market.

What type of heterosis is maximized in F1 offspring from a two-way terminal cross?

Individual heterosis.

Are replacement gilts produced from the two-way terminal cross in the lecture?

No; Yorkshire replacement gilts must come from elsewhere.

What is an example of a two-way rotational cross?

Hampshire boar × Yorkshire female, then Yorkshire boar × HY female, followed by Hampshire boar × YHY female, continuing the rotation.

What is an advantage of a two-way rotation?

It generates replacement gilts and uses maternal heterosis.

How much paternal heterosis does a purebred boar have in a rotational system?

0% paternal heterosis.

What is an example of a three-way terminal cross?

Landrace boar × Yorkshire sow = LY female; then Hampshire boar × LY female = HLY terminal offspring.

What traits are concentrated in the sow line of a three-way terminal system?

Maternal traits.