Mendelian Inheritance Flashcards
What was Mendel’s success based on?
- Used a self-fertilising plant
- CHose traits that had clear cut alternative forms
- Started his experiments with pure-breeding lines
- Did reciprocal crosses to eliminate maternal effects
- Analysed a large number of plants
- All plants were planted under same conditions
The ratio for 2 genes
9:3:3:1
How many gametes are there for 3 genes?
2^3 gametes
What are the ultimate source of new alleles?
Mutations
What is incomplete dominance?
Situation where F1 phenotype is halfway between those of parents
Describe familial hypercholesterolemia
Causes very high levels of LDL at a young age: caused by a mutation in the gene that codes for LDL receptor.
Homozygous FH patients may have heart attacks in their 20s. Heterozygous individuals have 50% of the LDL recpetors
What are the phenotypic/genotypic ratios in incomplete dominance?
Genotypic and phenotypic: 2:1:1
What is the explanation of the round & wrinkled peas?
INCOMPLETE DOMINANCE
in round peas, the dominant allele is a wt for an enzyme that catalyses the conversion from unbranched to branched starch: high levels of glucose lets water in. Wrinkled peas the enzyme is not functional
What is codominance?
2 alleles at a locus produce different and detectable gene products in heterozygote
What is an example of codominance?
Human blood groups
What is overdominance?
Heterozygotes are more vigorous than both the corresponding homozygotes
What are examples of overdominance?
Sickle cell anemia: confers heterozygote advantage
What are lethal alleles?
Alleles that cause an organism to die only when present in the homozygous state: causes the ratio to be 2:1 instead of 3:1
What is epistasis?
Interaction of genes: effect of one gene pair masks or modifies the effect of another gene pair
How are the RBCs in heterozygotes for sickle cell anemia?
Are likely to rupture when infected by plasmodium preventing the propagation of the parasite
Examples of epistasis
- Agouti mice
- Bombay pedigree
What is the Bombay pedigree?
When there is no fucose transfer, there is no H substance: when the H substance is not present the A and B antigens can not be expressed so the phenotype will be O
What is pleiotrophy?
One gene has effects on multiple phenotypes
What are examples of pleiotrophy?
- Cystic fibrosis: multiple systems affected
- Marfan syndrome
- Parphyria variegata
What is penetrance?
the % of individuals that show at least some degree of expression of the genotype
What is expressivity?
The range of expression of the phenotype [can be the result or both genetic background differences or environmental effects]
What is incomplete penetrance with variable expressivity?
Identical known genotypes produce a broad range of phenotypes
What are modifier genes?
Gene interacting with non-allelic gene modifying the phenotype associated with the expression of alleles of the gene in a milder way
Sex-limited inheritance
Specific phenotype linked to one sex
Sex-influenced inheritance
Sex influences expression of phenotype but not limited to one sex or another i.e. hormones
How is pattern baldness controlled?
Sex-influenced: in BB women is reduced and occurs later in life, Bb women not affected men are affected
Sex-limited trait
expressed in one sex but not the other- due to anatomical and physiological limitations
