13) Non-Mendalian Genetics Flashcards
What are the strict assumptions which Mendalian model makes?
1) Genes are on nuclear chromosomes
2) Only one gene locus determines a trait
3) Alleles are either dominant or recessive
What are the problems with these assumptions ?
1) Trait isn’t determined by nuclear DNA sequence= e.g. can be extranuclear DNA (i.e. mitochondrial or chloroplasts) OR something other than DNA sequence (epigenetics)
2) More than one gene can be involved= Gene interaction
3) Alleles can have wide range of effects= Incompletely dominant, co-dominant or can show varying degrees of expressivity
What is an allele?
DNA sequence that codes for a protein (old fashioned definition)
Often multiple alleles will exist each producing very slightly different phenotypes
What is incomplete dominance?
What is an example?
Phenotype is determined by a blend of the products of different alleles
Examples: Flower colour in snapdragons= Pigments produced by different alleles ‘blend’ together
Can have Red and White and pink= When RW
What is co-dominance?
What is an example?
Alleles products are co-expressed
Example: Blood groups
Blood cells express a surface antigen= Determined by genotype at I gene locus
Alleles: A, B or O
IA IB = Both A and B antigens= AB phenotype, it is not blending as it is a dual phenotype= A AND B
What is lethality? Why does it complicate Mendel’s ratio?
Homozygous recessive isn’t viable= lethal
Example: Agouti gene in mice
Agouti gene= Determines coat colour
2 alleles:
1) A= Recessive agouti allele= Brown if AA
2) AY= Yellow allele= Yellow= AAY or AYA
Can have: AYAY= Lethal and embryo does not develop
AY
Linkage: AY allele contains deletion in Agouti gene and deletion all knocks out gene involved in embryonic development
AY= Dominant for colour coat, but recessive= Embryonic development
What is genetic interaction?
When more than one gene is involved in determining a trait, as several genes take part in the metabolic pathway that determines the phenotype
What is epistasis?
What is an example?
2 genes working together
Effect of 1 gene is dependent on another gene
Functions of genes interact which determine the gene= 2 proteins do not meet, they interact within a pathway
Example: Blood groups
Also need gene for H factor synthesis= Required if the blood antigens are to be synthesised properly
Ratio is very different as hh homozygote cannot make any antigen therefore if you get IAIB hh= Becomes O
Recombination becomes of a big deal when start to look at traits whose phenotypes are determined by multiple genes= Recombination= New allele combinations= New phenotypes
What is complementation?
Genes= Multiple alleles
Different alleles= Can produce similar phenotypes
Some phenotypes= Result of multiple gene interactions
THEREFORE: Hard to tell whether 2 mutants which change a particular trait that produce the same phenotype are:
1) Mutants in the same gene= Different alleles, but give similar phenotype
2) Mutants in different, epistatic genes
Complement test: Cross pure-breeds of each mutant
IF in different genes: Offspring inherits one working copy of each gene= Normal phenotype is restored and mutant genes are said to complement each other
IF in same genes: Offspring inherits 2 broken copies of the gene= remains mutant phenotype
What is extranuclear inheritance? What are some examples?
Not all genes are in nucleus and on nuclear chromosomes
Example:
1) Mitochondrially-determined traits: Mitochondrial mutants= Cause phenotypes with diseases of energy generation, and are inherited through mother
2) Chloroplast inheritance:
Example: Leaf colour from 4 O’clock plant
Leaf colour is determined by chlorophyll content which is determined by a gene in the chloroplast genome
Leaf colour is inherited from mother as plants embryos get cytoplasm from mothers which is where the chloroplasts are
