Chapter 8: Genetics Flashcards
Genetics
Study of the transfer of genes from parents to offspring
The nature in nature vs nurture
Genotype is an organism’s genetic makeup
Phenotype is how genes are expressed
Animal Breeding
Humans have understood for centuries that parents pass traits to offspring
Through domestication humans selectively bred individuals for desired traits
Docility, intelligence, strength, coat quality, fat content, milk production, appearance
Genetics: What are Genes?
Nucleus holds chromosomes
Chromosomes are made of DNA
Pairs of chromosomes
Genes are segments of DNA
Code for specific proteins, control biochemical processes
Blueprint of life
DNA
Deoxyribonucleic acid
Double strand of nucelotides= deoxyribose sugar, phosphate, nitrogenous base
Purines- adenine (A) and guanine (G)
Pyrimidines- thymine (T) and cytosine ©
A to T and G to C
Genes are made of sequence of base pairs
Genome
Genome is all the genes for an organism
Different species have different numbers of chromosomes, genes
DNA must be supercoiled to fit thousands of genes per chromosome
Somatic Cells
Somatic cells- body cells have 2 pairs, diploid (2n)
Humans- 46, 23 pairs
Gametes
reproductive cells 1 pair, haploid (n)
Sex Chromosomes
have genes related to sex characteristics, Autosomes- everything else
Sex Determination
In mammals males are XY (sperm), females are XX (egg)
In chickens females are ZW, males are ZZ
Crocodilians, turtles temperature dependent
Amphibians, fish lots of forms of determination
Chromosome Number
Expressed as n, Varies widely
Polyploidy
more than 2 copies of chromosomes, or whole genome
Mistake during cell division
Not common in vertebrates, widespread in plants, can cause low seed production
Aneuploidy
variation in number of individual chromosomes
Monosomic
one less (2n-1)
Trisomy
one more (2n+1)
Often lethal, sometimes not.
Down’s syndrome caused by Trisomy of chromosome 21
DNA Replication
Must be accurate to prevent mistakes in offspring
DNA molecule unzips, RNA primer attaches to DNA polymerase
Polymerase moves down original strand
Bonds new nucleotides together to original strand, polymerization
DNA Replication: When do we do it?
Anytime body cells (somatic cells) divide
Growth, injury, maintenance
Anytime sex cells (gametes; sperm, eggs) divide
Reproduction
Mitosis
Somatic cell division, produces two 2n cells- 2 copies of chromosomes
Meiosis
Reproductive cell division, produces 4 n cells-1 copy of chromosomes
Principle of Inheritance
1866 Gregor Mendel discovered principles of inheritance while studying pea plants
Alleles
forms of a given gene, each creates different proteins, so express traits differently, get one copy from each parent
Chromosomes in homologous pairs, genes for the same traits
Locus
is a certain spot on chromosome
Animals with matching alleles at a locus
Homozygous, (AA or aa) different alleles are Heterozygous (Aa)
Inheritance
is passing of alleles in gametes
gamete production
pairs separate
Law of Segregation
alleles separate so only one per gamete
Law of Independent Assortment
separation of a pair of genes is independent of separation of other pairs
Gene Expression: Complete Dominance
Dominant alleles mask recessive alleles and are expressed over them, signify dominant allele with capital letter
EE colored coat, ee white, Ee colored
EE is the genotype, colored is the phenotype
Punnett Square
Used to predict the results of a genetic cross
In a genetic cross, two parents (P generation) are crossed to produce offspring (F1 generation).
The simplest Punnett square follows one
character in a monohybrid cross.
Test Crossing
Can’t always tell genotype by looking at phenotype
Can test cross individual with homozygous recessive to see what offspring are
If some recessive, then parent is heterozygous
Codominance
neither allele masks the other, both are expressed
Red coat (RR) + white coat (WW) = roan (RW)
Incomplete dominance
heterozygous individual is intermediate
Epistasis
expression of one gene is influenced by the presence of another
Defies classical Mendelian genetics
Horse hair color black is E at E locus, but whether black all over or just points is controlled by another locus
Ea is black, EA is bay with black points
Multiple alleles
only 2 alleles on each locus, but population could have many alleles
A-B-O blood types in humans
Polygenic inheritance
is the effect of many genes on a single character.
In humans, height and skin color are each affected by several genes.
Size and growth rates common in animals
Sex-Linked Inhertiance
Some genes only found on X or Y, homologous in some regions, not in others
X-linked genes can be inherited by males and females
Y-linked only in males
Found on autosomes, phenotypes are not expressed the same way for the sexes
In male sheep horns (H) is dominant to polled (no horns(h)), in females h is dominant
So heterozygous male has horns, hetero female doesn’t
Sex-Limited Traits
Traits unique to a single sex
Milk production, egg production
Both have genes, but only one sexes expresses them
Gene frequency
proportion of a particular allele in a population
Genotypic frequency
how often a genotype occurs in a population
Phenotypic frequency
how often a phenotype occurs in a population
Animal breeding
uses pop genetics to improve livestock
Mutations
changes in the composition of a gene that alter DNA, producing new alleles
Mutations that affect phenotype are rare
Beneficial ones spread through natural selection
Genetic drift
change in gene frequency due to chance, affected by population size
Big problem for small populations
Migration
wild animals migrating or bringing new breeding stock into a population
Can make large changes in gene frequencies quickly this way
Works best when new sire is very different from herd
Selection
allowing some animals to breed more than others
Natural Selection
some individuals are better adapted and will therefore breed more
Happens in managed herds with multiple bulls
Wild populations
Artificial selection
breeding controlled by manager, culling less desirable animals
Affected by rate of reproduction and generation time
Hardy-Weinberg Law
In a large, randomly mating population without natural selection, migration, or mutation, gene frequencies will remain stable
Rare in the wild, common in managed herds
Once desired genes are achieved in a large herd, they will remain constant
Qualitative traits
phenotypes classified into groups
Coat color, horn presence
Quantitative traits
numerically measured, usually polygenic
Growth rate
Additive gene action- no dominance at locus, each gene increases the effect
A+ gene makes calves 2lbs heavier at birth, AA is norm, A+A+ is 4lbs heavier
Nature vs. Nurture
Genes determine your potential, but environment determines if live up to that potential
Climate, management practices, health
Heritability
How much phenotypic variability among individuals that is due to genetics and can be passed down
Ranges from 0-1
First calculate a selection differential- difference between the population and desired trait, multiply by heritability value
Need to be considered before breeding
Pig litter size- norm is 7, breed parents from litters of 11
Differential is 11-7= 4
Heritability value is .1, so 4x.1= .4
So litter will be 7.4 piglets larger
Some traits are more heritable than others
Relationship Coefficient
Measures how closely related individuals are, also important before breeding.
Ranges from 0-1
Parent-offspring- 0.5
Siblings- 0.5
Half-Siblings- 0.25
Grandparent-offspring- 0.25
Inbreeding
mating closely related individuals
Increases homozygosity, decreases genetic variation
Can increase incidence of detrimental recessive traits
Causes decline in quality called inbreeding depression
Linebreeding
inbreeding to concentrate genes of an outstanding ancestor
Genetic evaluations have eliminated need for this in livestock, but still practiced in companion animals
Outbreeding
mating less closely related individuals
Increases heterozygosity, genetic diversity
Increases vigor, known as heterosis
Lowly heritable traits show more heterosis
Crossbreeding
mating animals of different breeds
Most successful if parent genetics complement each other
Strengthen good traits, depress bad ones
Great for reproductive traits
Often breed crossbred mothers with purebred sires
Tend to be more vigorous, fertile, and healthy
Biotechnology
Tools that use living organisms to make improvements or modifications for specific uses
Genetic engineering
altering an animals genes
Transgenic Organisms
Bacteria that produce human insulin
Before used pig and cattle insulin, slightly different
Cows, sheep, goats can produce proteins in milk to treat disease
Can produce large quantities
Chickens can produce proteins in eggs
Animal models for human diseases
Animals to produce organs for human transplant
