Lecture 5: Single gene inheritance pattern variations Flashcards
Protein Function Explains Dominance by
Mutant alleles are often defective in expressing a functional protein
In simple dominant-recessive relationship, 50% or more functional protein is enough to provide the dominant phenotype
Multiple Allelism is
A gene may have more than two alleles
Multiple allelism
Each individual can have no more than two alleles of one gene
Many more alleles of that gene can be present in the members of a population
The main gene determining fur colour in rabbits has four alleles
Incomplete dominance is
Complete dominance / recessive relationships do not always exist
Incomplete dominance can occur
Heterozygote has a phenotype intermediate between the corresponding homozygous individuals
Always have a C before the letter for the alleles
Explaining Incomplete Dominance
Only one of the two alleles produces a functional product
CYCY homozygote 100% functional
CYCW heterozygote 50% functional
CWCW homozygote 0 functional
Y allele codes for a crucial enzyme in a pathway to produce pigment
W allele codes for an inactive enzyme
Codominance is
In codominance, a heterozygote expresses both alleles simultaneously
Does not show an intermediate phenotype
Both alleles make an equal contribution to the phenotype
The heterozygote shows distinctive features of both homozygous traits
Examples:
Human ABO blood groups
Codominant, multiple-allele
Roan coat colour in cattle
Red haired parent CRCR
White haired parent CWCW
Roan progeny – mix of red & white CRCW
ABO blood types in genetics is
The ABO blood groups result from a variation in two different polysaccharides present on the surface of the red blood cells
A and B
The ABO blood group gene encodes an enzyme needed for synthesis of these polysaccharides
Polysaccharide A is synthesized by an enzyme encoded by allele IA
Polysaccharide B – allele IB
No functional enzyme – allele Ii
Example of:
Multiple allelism
Codominance
Dominant / recessive
Lethal alleles are
Certain gene products are essential to life
Any of these coding for a faulty product will be lethal in the homozygous state
An individual heterozygous for a lethal allele generally shows an affected, but less severe, phenotype
Manx cat
All tail-less are heterozygotes - Mm
Homozygous (MM) is believed to cause extreme spinal defects – lethal
mm gives a tailed cat
Might expect a 3:1 ratio as a result of a cross of two Manx tail-less cats (Mm x Mm)
But the result is usually a 2:1 ratio of tail-less and tailed
One resulting genotype is lethal, so the expected 3:1 ratio is altered, giving 2:1
Hairlessness in dogs is
Hairless allele is a duplication in FOX13 gene
Autosomal
All hairless dogs are heterozygous
Homozygous for the hairless allele – die as embryos
Are lethal alleles recessive or dominant?
Depends on context
The M allele in the Manx cat example is dominant with respect to its effect on tail length
But recessive in terms of viability
Such alleles, lethal when homozygous, but not when heterozygous, are often called recessive lethal alleles
Are examples of dominant lethal alleles
They exert their effects in both heterozygotes and homozygotes
Can only be recognized and studied if death occurs after the individual has reached reproductive age
Huntington’s disease
Pedigrees are
Results of specific crossings may not be possible
Family size may be small
Presents difficulty in scoring for phenotypic ratios
A family tree or pedigree can be constructed
The segregation pattern of a trait through several generations of related individuals often yields clues to its mode of inheritance
Characteristic segregation pattern shows that the trait:
Affects both sexes
Is not observed in each generation
Has a low frequency of expression
Two unaffected individuals can produce affected offspring
The recessive allele can be passed ‘silently’ for many generations
Heterozygotes are often silent carriers of a recessive trait
Autosomal Dominant Trait Pedigree is
Characteristic segregation pattern shows that the trait:
Affects both sexes
Every affected individual has an affected parent
Affects about half of all children
Affected individuals are heterozygous, Hh
Non-affected are homozygous, hh
In the case of Huntington’s disease:
Homozygous, HH, is inhibitory to development
It is ultimately lethal, so is a dominant lethal allele
The product rule is
Used to predict the likelihood of producing multiple offspring with particular genotypes or phenotypes
Probability that two or more independent events occur is equal to the product of their individual probabilities
Each fertilization is an independent event with a given probability
The probability of heterozygous parents having a homozygous recessive child is:
¼
The probability of having three successive children that are all homozygous recessive is:
¼ x ¼ x ¼ = 1/64
Genotype for type ab blood
IaIb
Genotype for type a blood
IaIa or IaIi
