Lecture Thirteen - Patterns of inheritance Flashcards
What were some of the early theories of inheritance?
Selective breeding in agriculture lead to various theories:
Pangenesis.
Homunculus - there is a little person in the sperm.
Blending theory.
Mendel discovered the laws of inheritance in the 1860s.
What did Mendel discover?
Autosomal inheritance.
He used a god model organism which bred quickly and had lots of off spring.
He observed simple traits (controlled by one gene) that expressed one of only two variations.
What did Morgan discover?
X linked inheritance.
He found different results from reciprocal crosses - crosses where phenotypes of male and female parents reversed.
What is the chromosomal theory?
Genes are on chromosomes.
Supported by:
1) Behaviour of chromosomes at meiosis - parallels Mendel’s law.
Meiosis is the cellular basis of Mendel’s laws.
The two alleles for a gene are carried on the two different homologous chromosomes.
2) Sex linked genes - inheritance of a trait follows inheritance of a particular chromosome.
What are some complexities in inheritance patterns?
For traits controlled by a single gene:
- Alleles not completely dominant/recessive.
- More than two alleles.
- Single gene controls multiple phenotypes.
- Extranuclear genes
For traits controlled by more than one genes:
- Gene interaction.
- Many genes (polygenetic).
Effect on environment.
What is complete dominant, incomplete dominant and co-dominance?
Complete dominant: in heterozygotes, only one allele is expressed. Due to 50% of the gene product being enough for the normal phenotype to be expressed.
Incomplete dominance: heterozygote has the phenotype intermediate between the two homozygotes. I.e. Pure breeding red flower crossed with a pure breeding white flower has pink offspring.
Genotypic and phenotypic ratios of F2 coincide 1:2:1
Co-dominance: heterozygote exhibits both the phenotypes of the two homozygotes. I.e. a black and white off spring from a black father and white mother.
What happens when a single gene controls multiple alleles?
E.g blood types.
What is penitence and expressivity?
Penetrance is the extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype. E.g. a person may carry the mutated gene, but it may not be expressed as a phenotype.
Expressivity is the way a particular phenotype is expressed. e.g. A father may have six fingers and its offspring may have four toes. Controlled by the same gene.
What are extranuclear genes?
Mitochondrial DNA or chloroplast DNA containing genes.
Mitochondrial DNA is only passed to the offspring from the mother.
These genes do not obey Mendel’s laws.
What happens when two genes control one phenotype?
If two genes control a trait they will interact.
Many ways, one is epistasis - where on gene is required for the expression of another.
Polygenetic traits.
Many traits are quantitative - vary in population along a continuum.
How does the environment affect the expression of genes?
The same genotype can express different phenotypes depending on the environmental pressures.
