Genetic complexity and breeding Flashcards
(25 cards)
Variation in chromosome number can be due to variation in:
Number of sets of chromosomes
Polyploidy in mammals is lethal
Number of individual chromosomes in a set
Aneuploidy is
total number of chromosomes is not an exact multiple of a set
Monosomy is
one chromosome missing (2n-1)
Trisomy is
one extra chromosome (2n+1)
Aneuploidy results from
Usually results from nondisjunction during meiosis
A pair of chromosomes fail to separate during anaphase I
A pair of chromatids do not separate during anaphase II
Monosomy is caused by
Phenotypic changes due to monosomy depend on the chromosomes involved
Lacking a sex chromosome is not lethal
Example – Turner syndrome (XO)
Missing autosomes is usually lethal
Metabolic imbalance – only one unit of expression for one chromosome
Trisomy is caused by
Phenotypic outcome is generally not as severe as monosomy
Viability of trisomic animals varies
Related to size of chromosome
Larger sizes are usually lethal
Extra sex chromosomes is not lethal
Metabolic imbalances – three units of expression for one chromosome
Trisomy Examples
Autosomal trisomy
Canine trisomy 13 can cause malignant lymphoma
Trisomy 18 in cattle – extreme brachygnathia – calves die shortly after birth
Sex chromosome trisomy
XXY – Klinefelter syndrome - cats
Pleiotropy is
Pleiotropy – a mutation in a single gene can have multiple effects on an individual’s phenotype
Occurs because:
Expression of a gene can affect cell function in more than one way
Gene can be expressed in different cell types
Gene can be expressed at different stages of development
Pleiotropy Example 1 chickens
Chickens that have the frizzle gene have feathers that curl outward rather than lying flat
Also have trouble regulating body temperature, have higher metabolic and blood flow rates, greater digestive capacity and lay fewer eggs
Pleiotropy Example 2
cats
Approximately 40% of white cats with blue eyes are deaf
White cats with one blue eye and one yellow eye are often deaf only on the blue-eyed side
The white coat colour is produced by an autosomal dominant allele
This gene also affects iris colour and deafness, so the gene is called pleiotropic
Mechanism by which the gene affects hearing is being researched – may be related to a lack of ear canal fluid
Quantitative Traits
Quantitative traits also known as complex, continuous, polygenic, or multifactorial traits
Show continuous variation over a range of phenotypes
Several genes contribute to the outcome of the trait
Environment has a major impact
Majority of traits in animals are quantitative traits
Examples of quantitive traits
Milk production
Fleece weight
Body weight
Egg production
Speed
Behavioural traits
Inbreeding is
Inbreeding is breeding between related individuals
Examples:
First degree relatives
mother to son, daughter to father, siblings
Second degree relatives
half-siblings, grandparents to grand-offspring, uncles/aunts to nieces/nephews
Line Breeding is and results in
Similar to inbreeding, but more distant relations (not first or second degree relatives)
Results of inbreeding and line breeding:
Reduction in heterozygosity and increase in homozygosity
Increase in homozygosity reveals recessive genes
Increase in homozygosity fixes genes so animals are more likely to breed true
Reasons for Inbreeding
To fix a desirable characteristic
To determine whether an individual is a carrier of a recessive allele
Laboratory animals are inbred so lines are genetically identical
Inbreeding depression is
Inbreeding depression describes a general reduction in vitality and viability of inbred animals
Occurs because of increased homozygosity and expression of deleterious recessive genes
Affects fertility and progeny survival
Line Crossing / Out Crossing breeding is
Mating individuals of the same breed, but from different pedigrees
Best method of mating to preserve a breed
Cross breeding is and effects
Mating from different breeds
Provides best genetic diversity
Effects:
Increased heterozygosity
Masking of recessive genes
Reducing the likelihood of breeding true
Hybrid Vigour (Heterosis) is
Arises because of masking of recessive genes that depress viability when homozygous
Hybrids often have superior performance compared to their parents
Seen especially when two inbred lines of animals are mated
Selective Breeding is and used for
Selective breeding refers to programs and procedures designed to modify traits in domesticated species
human-driven
Breeder chooses the parents that possess traits desirable to humans
Selective breeding can dramatically modify traits
Example: dog breeds
Breeds all descended from a wild wolf-like ancestor
Methods of Selection for Eliminating Disease-Causing Alleles
Pedigree analysis
Molecular genetic screening
Pedigree analysis is
Can avoid breeding from a line that has shown a disease trait in their pedigree
Not all animals whose ancestors were affected will inherit the mutated gene, so may miss out on some beneficial traits as well
Molecular genetic screening is
Also known as DNA testing
Detecting a particular gene in a sample of DNA
Can detect carriers and homozygous individuals
Useful for recessive traits
Must know the DNA sequence for the disease-causing gene or must identify DNA markers linked to the disease-causing locus
Test for one breed may not be applicable to another
New mutations may occur
