Gene Action Flashcards
what is gene action?
how the genotype of a particular trait affects the expression in the phenotype
list the 5 types of gene action
- non-additive
- sex-linked
- sex-limited
- sex-influenced
- additive
define non-additive gene action
effect of one gene won’t add to the effect of another
define sex-linked gene action
inheritance controlled by genes on sex chromosomes
define sex-limited gene action
expression of traits limited to one gender
define additive gene action
effect of one gene adds to the effect of another to determine the phenotype
what are the 3 kinds of non-additive gene action traits?
- complete dominance
- incomplete dominance
- epistasis
what is complete dominance?
one allele of a pair can mask the expression of the other (recessive), recessive expression only occurs in the homozygous condition
when is a gene dominant?
if only one allele is needed to control the phenotype
give an example of complete dominance non-additive gene action
coat color in Hereford and Angus cattle
what happens to the heterozygote in incomplete dominance?
it is a different phenotype from the dominant one
what are the genotypic and phenotypic ratios in incomplete dominance?
1:2:1
how does epistasis work in non-additive gene action?
interactions between non-allelic genes; the phenotypic effect of genes at one locus is dependent upon what alleles are present at another locus DISTINCT from the first locus
what are the 3 types of epistasis?
- complete dominance epistasis
- recessive epistasis
- no/incomplete dominance
give an example of complete dominance epistasis
wool pigmentation in sheep; alleles at one locus control pigmentation of wool fibers, but white is dominant, so if one W is present, the sheep will be white; if homozygous recessive (ww), color is then dependent on a different allele, see slide if need more info
give an example of recessive epistasis
coat color in rabbits, see slides for more info
what is the phenotypic ratio of recessive epistasis?
9:3:4
give an example of no/incomplete dominance epistasis
coat color in horses
what is the phenotype of CCB_ in horses?
bay
what is the phenotype of CcB_ in horses?
buckskin
what is the phenotype of cc_ _ in horses?
cremello
what is the phenotype of Ccbb in horses?
palomino
what is the phenotype of CCbb in horses?
chestnut
what is found on the Y chromosome?
very few loci identified as controlling traits unrelated to the sex of the individual
what kind of gene action is concerned with the inheritance of traits controlled by alleles on the X chromosome
sex-linked
give an example of a sex linked trait
coat color in cats
how does a tortoise shell cat happen?
codominance of black and orange alleles (both purebreeding) found on the X chromosome
why can’t male cats be tortoise shell?
they only get one allele for color because they only get one X chromosome
what is recessive sex-linked inheritance
unaffected parents produce normal daughters, but half the sons of heterozygous females will show recessive phenotype, females must be homozygous to show trait, so sire must have had it too
what does Klinefelter syndrome (XXY) do to male cats?
gives them calico coloration, because they inherit both X’s for heterozygous coloration, and the X chromosome deactivation leads to the patchy coat, with the white fur coming from the piebald allele
what are sex-influenced traits?
the inheritance of a trait appears to be different in the 2 sexes even when they had the same genotype, so expression differs for male and female in the heterozygote, like horned vs polled sheep
give an example of sex limited traits
milk yield, egg production, cryptorchidism
what is pleiotropy?
one gene affects more than one trait
what is variable expressivity?
the existence of more than one grade of a trait
what is overdominance?
interactions between gene alleles resulting in a heterozygote superior to either homozygote for a particular trait, also called extreme heterozygote
what is genetic correlation?
selecting for one trait can affect other traits as well, also called correlated response
what are the 2 reasons why we have genetic correlation?
pleiotropy and gene linkage
what is incomplete penetrance? give an example
not all animals with a particular genotype exhibit the phenotype; polydactyly in fowl, humans, dogs, cats
what are lethal genes?
cause embryonic death, or death shortly after birth
what are semi-lethal genes?
cause death before puberty, predictable
what are detrimental genes?
reduction in some measure of fitness or productivity
what phenotypic ration do lethal genes cause?
2:1, because one of the homozygous conditions will always cause death, so an absence of phenotype
what is gene linkage?
condition where non-allelic genes consistently travel together through meiosis and syngamy
how are two loci linked?
if they occur on the same chromosome
what is the exception to gene linkage?
crossing over
what does the probability of recombination due to crossing over depend on?
the distance between the loci, farther from center = more likely
what kind of traits are controlled by additive gene action?
quantitative traits, expressed over a range of values
what are major genes in additive gene action?
those that contribute more than others
what is heritability in a broad sense?
the ratio of genetic variance to phenotypic variance, can be misleading
what is heritability in the narrow sense?
a measure of the strength of the relationship between performance and breeding values for a trait in a population
what does heritability measure?
additive gene action
if a trait is highly heritable (>0.4), what does that mean?
the performance of offspring will match parents
if heritability is low or medium (0.21-0.4) or (0.1-0.2), what does that mean?
offspring performance will not match sire
