Heredity and Genetics Study Guide

Genetics and Breeding

Genotype- an animal's genetic makeup

Phenotype- expression of the genotype into traits

Influence of genes vs. environment (nature vs. nurture)

We can modify our animals with understanding the relation of genotype and phenotype

Heredity- transmission of traits from mated animals to their progenies

BIOLOGY

1. Chromosomes- Rod like structures within the nucleus which contain the genetic material
1. Composed of DNA
2. Paired
3. Look at how many pairs of chromosomes an animal has
2. Genes- Sequences of DNA along the chromosomes
1. Locus- position of genes along chromosome
2. Allele- Specific form of the gene
3. DNA and RNA
1. DNA- composed of repeating nucleotide molecules
1. Nucleotide- consists of deoxyribose, a phosphate, and a base (A,T,C,G)
2. Gene is a specific sequence of nucleotides
3. Sugar-phosphate backbone, base attaches strands with hydrogen bonds to create double helix
2. DNA Replication/ Transcription
1. Bases always match the same way (A=T, C=G)
2. Unzip strand (Hydrogen bonds)
3. Duplicate new strand inside old helix
1. Complimentary mRNA
4. Creates two copies of old strand
3. Translation
1. mRNA copy leaves nucleus  to take info from DNA (copy of strand) to translate genetic code
2. Takes place in the ribosome
3. Manufactures a chain of amino acids from info found in DNA sequence

Inheritance

1. Chromosomes
1. Homologous- have genes that affect the same trait; carry the same type of information
2. Alleles- form of a gene which affects a trait
1. Matching alleles at a given locus- homozygous
1. Both alleles carry the same genes
2. Different alleles- heterozygous
3. Inheritance- the way alleles are passed on

Number of chromosomes

1. Species specific
2. Chromosomes occur in pairs in somatic
3. Somatic cells- regular body cells
1. 2n chromosomes (pair)
4. Germ cells- sperm and egg, contain haploid
1. N chromosomes

Sex Chromosomes

1. Homogametic sex: XX, female
2. Heterozygametic sex: XY, male
1. This way in mammals, not all animals

Cell Replication

1. Mitosis- the process of somatic cell division
1. Replication of cells
1. Diploid cell undergoes division
2. Production of two diploid (2n) cells
2. Meiosis/Gametogenesis- development of sex cells (sperm/oocyte)
1. Reductional process (2n to n)
2. Two different divisional procedures
1. First division- one diploid into two haploid cels
2. Replication of two haploid cells to produce four haploids
3. Spermatogenesis
1. Occurs in seminiferous tubules
2. Forms 4 haploid sperm cells
3. Continuous
4. May be seasonal
5. Temperature dependent
1. Vascular system/muscular system help control temperature in testes
4. Oogenesis
1. Occurs in ovary
2. Each diploid cell forms ONE haploid cell
3. First step usually completed during embryonic and fetal development
1. Female is  born with all potential ova
3. Fertilization
1. Union of haploid sperm and ovum
2. Sample of genetic information from each parent is recombined

Gene Expression

1. Recombined alleles

1. Dominant B
2. Recessive b
3. Co-dominance- both alleles traits are expressed simultaneously
1. Ex: Brown and white cow= white/brown cow
4. Incomplete dominance- expresses both but neither fully
1. Ex: Red and white flower=pink flower
5. Epistasis- One gene pair influences another
1. When the action of one gene is modified or controlled by one or several other genes
2. Ex: Albinism hides gene controlling color of person's hair

Qualitative Traits

1. Phenotypes classified into groups
2. Usually controlled by one gene pair
3. Coat color, horns
4. Simple
5. Little environmental effect

Quantitative Traits

1. Phenotypes can be measured
2. Usually controlled by many gene pairs
3. Influenced by the environment
4. Ex: Growth, Egg production, etc.
5. Economically Important traits

Heritability- the measure of the proportion of the phenotypic variation that can be passed down from parent to offspring

1. Differences due to genetics vs. environment
2. The part of "what you see" that is due to genetics
3. Range of values for heritability from zero to one
1. 0 is all environment, 1 is all genes
2. On average the percentage of individual differences that we observe attributable to genetic individual difference
4. Known for important quantitative traits
5. More controlled by genetics = more genetic progress by selection
6. When influenced by environment- varies with food, exposure to sun, etc.
7. Low heritability (ex: reproductive traits) (<.2)
8. Medium heritability (ex: growth traits) (.2-.4)
9. High heritability (ex: carcass) (.4<)

Animal Breeding

1. Selection of the "best"
2. Selection differential
1. How much better the best are than the rest (mathematical term)
2. Multiply by heritability to get progress
3. Difference between average for a trait in the animals selected for breeding and the average of the group from which they came
3. Heterosis/ hybrid vigor
1. Offspring outperform average of parents
2. Inversely related to heritability
4. Animal Model
1. Mathematical model that predicts breeding value
1. How an animal will perform based on the heritability/etc. of a trait
2. Livestock producers purchase based on this
3. Calculates EPDs
4. Put into a database
5. Breeding Value- estimate of transmitting ability, ability to pass on genes to next generation
6. EPD- (Expected Progeny Difference) half the breeding value (half of the genes of an animal)
1. Based on accumulation of data
2. Predicts how offspring will be different from parents

Beef Cattle Breeding

1. EPDs
1. For particular trait
2. Associated with ACC
1. Accuracy value
2. From 0-1 (closer to one, means EPD is a better estimate)
3. A better estimate has more data contributing to it
3. Ancestor data- looking at existing progeny data
4. Focused on bulls
1. In a herd, a bull makes more offspring
5. Examples
1. Birth, weaning, yearling weight
2. Maternal Milk
3. Carcass traits
4. Scrotal circumference
2. Genetic Improvement
1. Accuracy- the measure of reliability associated with an EPD and is expressed as a value between 0 and 1
2. Sire summaries- include EPDs, accuracies, graphs of the average change in EPD for the particular breed, breed average EPDs, possible change values, and other useful materials
3. Genetic Evaluation
1. Contemporary Group= A group in which animals of a given sec and age having similar treatment are given an equal opportunity to perform

Systems of Mating

1. Inbreeding
1. Breeding closely related individuals
2. Looking for a specific, qualitative trait
3. Decreases variation
4. Problem: Create more homozygous individuals
1. Detrimental homozygous recessive genes more common
5. More common in dogs and cats; rare to never in livestock
2. Outbreeding
1. Breeding- Individuals less closely related than average population
2. Creates more heterozygous individuals
1. Shows dominant genes more often
3. Crossbreeding
1. Breeding different breeds
2. Hybrid vigor

Dairy Cattle Genetic Improvement

1. Balance traits of economic importance to address goals
1. Milk yield levels
2. Milk composition
3. Generate profitable replacement animals
4. Sustain and improve cow longevity
2. DHI System
1. Dairy Herd Improvement system
2. Compares sire daughters with contemporaries in the same herd
3. PTA (basically EPD) (predicted transmitting ability)- one half of breeding value
4. REL (same as ACC) (Reliability)- measure of accuracy of amount of information in an evalutation
3. Animal Model
1. Mathematical prediction based on data
2. Produces predictions of breeding (genetic) value of an animal
3. Compiles lactation yield data for economically important milk

Swine Breeding

1. Extensive records
1. Within herds
2. Pigs produce huge number of offspring; large amounts of data
3. Sophisticated ON FARM software
4. Short generation interval/rapid genetic progress
5. Economic traits
1. Reproduction
1. Pigs per sow per years
2. Pregnancy percentage
3. Weaning rate percentage
4. Etc….
2. Growth rate and feed efficiency
3. Carcass merits
1. Backfat thickness, carcass fat depth, loin eye area, etc.
6. STAGES (Swine Testing and Genetic Evaluation System)
1. Incorporates performance info to generate breeding values
2. National evaluation run for specific herds to generate across-herd EPDs

Sheep Breeding

1. Small industry in the USA
2. Two traits
1. Carcass- lamb/mutton
2. Wool
3. Breed groups
1. Ewe- good wool and reproductive traits
2. Ram- good carcass traits
3. Dual-purpose- pretty good at all traits, not the best at any
4. NSIP (National Sheep Improvement Program)
1. Useful to both purebred and commercial producers
2. Evaluates maternal, growth, wool, carcass traits
5. EPDs available since 1986, not very useful
6. A.I. rarely used in sheep