inheritance Flashcards
Autosomal dominant traits
single copy of an allele will dominate
- capital letter signifies dominance
dominant hypermorphic alleles
negative phenotypic consequences due to
a) the over-production of a normal protein OR
b) the production of a protein with increased activity levels
Neomorphic alleles
- negative phenotypic concequence due to
a) the presence of an altered protein that has a new function (enzyme binds to wrong substrate)
b) the altered protein interferes with the wildtype protein (dominant-negative allele)
Rare autosomal dominant traits
Most affected individuals are heterozygote (Bb) because if trait is rare in the population then matings between Bb and bb individuals would occur most often
incomplete dominance
BB, Bb, and bb all differ phenotypically with Bb being the intermediate between homozygous phenotypes
Complete dominance and recesiveness
the extremes of a range
Phenotype BB and Bb are the same while bb is different
Codominance
BB, Bb and bb all differ phenotypically but Bb exhibits phenotypes of both homozygotes
wildtype allele
the most common normal allele in the wild for that species - functional enzyme or other protein is produced
alleles in a population vs in an individual
Population: multiple alleles may exist
Individual (diploid): only 2 alleles coexist in each cell
dominance series / allelic series
describes the dominance hierarchy of multiple alleles
ABO blood type phenotype
- three alleles: I^a, I^b, i
- 4 blood types, A, B, AB, O
- I^a and I^b both dominate i, however the i allele is the most abundant in the human population (allele frequency)
what does the I^a blood type allele encode
- A transferase which adds Acetylgalactosamine to the surface of red blood cells
what does the I^b blood type allele encode
- A transferase which adds galactose to the surface of the red blood cell
- galactose acts as antigens
what does the i blood type allele encode
- a non-functional transferase
- has neither component on the cell surface
why are people with type O blood type universal donors
because the blood won’t get rejected by the body since they do not have antigens on the surface.
- individuals with type A and B could possibly carry the i allele
Type AB blood
- Rare
- universal receivers of blood since both antigens are present on the surface of red blood cells
- alleles are codominant - both are fully expressed in heterozygote
loss of function allele
an enzyme or other protein is no longer being produced, is produced at lower levels or is non functional
dominant vs recessive loss of function allele
dominant: loss of function produces a phenotype, needs both copies
recessive: loss of function can still work with only one copy
haplosufficiency
- one copy is enough to function
- half as much protein is synthesized yet is still sufficient to achieve the wildtype phenotype
- often wildtype allele is dominant over loss of function allele
true or false: the dominant allele is always normal and the recessive allele is a mutation
false - not always the case
recessive lethal alleles
- 2 copies of the dominant allele results in the death of an empbryo
- Yy x Yy gives phenotypic ratio 2:1
- Y allele is dominant for trait but recessive for lethality
recessive lethal allele example: Manx cat
mm = normal tail
Mm = no tail
MM = lethal (death)
- every cat that expresses this gene is heterozygote
Dominant vs recessive lethal genes
Dominant: can be expressed in both the heterozygote and homozygote (YY = lethal)
Recessive: only expressed in the homozygote (tsts = lethal)
true or false: your genotype for one gene can influence how the genotype contributes to another gene-phenotype
true - through penetrance and expressivity
Penetrance
the proportion of individual organisms having a particular genotype that express the expected phenotype - variation in population
- a yes or no
Expressivity
the degree to which a phenotype is expressed - variation in the individual
- a scale mild to severe (a yes to a degree)
complete vs incomplete penetrance
complete: identical known genotypes yield 100% expected phenotype
incomplete: identical known genotypes yield <100% expected phenotype
constant vs variable expressivity
constant: identical known genotypes with no expressivity effect yield 100% expected phenotype
variable: identical known genotypes with an expressivity effect yield a range of phenotypes
Example of incomplete penetrance: polydactyly
having extra fingers and toes
not fully penetrant
- if only half the genotype for polydactyly have extra digits, you would say it shows 50% penetrance
(there us another gene protecting other individuals from expressing this gene)
Piebaldism - variable expressivity
rare autosomal dominant disorder
- single gene where if you have the mutation you have an absence of melanocytes of different sizes and in different areas of the skin and hair
Huntington disease - variable expressivity
rare autosomal dominant disorder
- variable in the time onset of disease
- can cause loss of muscle coordination, cognitive decline and dementia - affected individuals may not exhibit all symptoms and to different severities
what causes incomplete penetrance and expressivity?
effects of other genes and environmental factors that suppress the effect of a particular gene
Environmental factors that can affect phenotype expression
- age
- sex
- temperatures
- chemicals
norm of reaction: range of phenotypes expressed by a single phenotype under different environmental conditions
Environmental effects on phenotype ex: the Himalayan rabbit
- Himilayan allele produces dark fur at tips of the body
- body of rabbit is white due to body heat being high enough and slightly denatures the enzyme
- dark pigment only develops at low temperatures
- if breeded at higher temperatures would be completely white
phenocopy
a change in phenotype arising from the environmental factors that mimic the effects of a mutation in a gene
genetic interaction
- many genes contribute to a single phenotype
- different combinations of alleles from two or more genes can result in different phenotypes, because of interactions between their products at the cellular or biochemical level
eye colour genotypes (theory)
A-B- = brown
A-bb = green
aaB- = grey
aabb = blue
- 2 genes confer a single phenotype (eye colour) but confer what colour
Biochemical level of gene interaction hypothetical example : eye colour
- humans start off with a baseline blue pigment
- if you have at least A-B-, convert from blue to green to brown
- alternative path - in the absence of green, the functional B enzyme can use blue to make grey
Complete dominance at two genes contributing to a single phenotype example: pigmentation
Dominant / dense allele = D, produces dense pigmentation
recessive / dilute allele = d, results in pigment clumping causing coat to dilute or lighten
- dilute effect is autosomal recessive, hence cat requires 2 copies of the d allele for the coat to dilute
complementation
occurs when two strains of an organism with different homozygous recessive mutations that produce the same phenotype produce offspring of the wildtype phenotype when mated or crossed
requirements for complementation
- will only occur if mutations are in different genes
- the other genome supplies the wild-type genome to “compliment” the mutated allele
- just need one of each dominant allele to exhibit trait
complement example: cats
2 white cats (ccAA and CCaa) mate and produce a black cat (CcAc)
- genes from family one complement genes from family 2 to receive the black phenotype
- homozygous recessive mutations in different genes for each family create heterozygote
complementation ratios
genotype = 9:3:3:1
phenotype = 9:7
heterogenous trait
a mutation in any one of a number of genes can give rise to the same phenotype
ex: about 50 genes can have recessive mutant alleles that can cause deafness in humans
epistasis
the masking of the expression of one gene by another, no new phenotypes are produced
- epistatic gene does the masking
- hypostatic gene is masked
epistasis ratios
dominant epistasis: 12:3:1
- A-B- and A-bb have the same phenotype
recessive epistasis: 9:3:4
- A-bb and aabb have same phenotype
dominant vs recessive epistasis
dominant: one dominant allele at one gene masks expression from the other gene
recessive: homozygous recessive at one gene pair mask expression from the other gene
pleiotropy
a single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects
hardy weinberg principle
predicting genotypes through allele frequencies in a population
p^2 + 2pq + q^2 = 1
p + q = 1
the hardy Weinberg principle is correct as long as there is no…
- nonrandom mating
- unequal survival
- population subdivision
- Migration
