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Summary Genetics lecture notes
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This is a summary of all the lectures for Genetics (second year Radboud).
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GeneticsLecture 1
Mendel’s laws
- Law of uniformity: when 2 pure lines that differ for one characteristic are crossed, the
appearance of the F1 progeny will be uniform
- Law of equal segregation: when the individuals from the first uniform f1-generation are
crossed, the features of the parents and f1 generation will appear in a ratio of 3:1 or 1:2:1
(single gene inheritance)
- Law of independent assortment: genes for different traits are sorted separately from one
another so that the inheritance pf one trait is not dependent on the heritance of another
- Genes are on chromosomes that undergo meiosis
- Genetic variation caused by meiosis and fertilization
- 22 autosomes (all but sex chromosomes) in haploid situation (x2 for multicellular diploid)
- Chromatin: DNA strands with chromosomal proteins/histones
Diploid organisms
- 2n-2c two homologous chromosomes that are not replicated
- 2n-4c two homologous chromosomes that are replicated: each chromosome consists of 2
sister chromatids
- Mitosis: 2n 2n + 2n
- Meiosis: 4 x n
- In meiosis: sister chromosomes pairs up; homologous chromosomes split up = haploid in
telophase 1 i.o. diploid in telophase
- Dihybrid AaBb: 2^n possibilities for gametes
Genetic variation
- Independent assortment
- Crossing over
- Random fertilization
3:1 phenotype, 1:2:1 genotype: complete dominance
1:2:1 phenotype: incomplete dominance
Testcross: 1:1
In case of a dominant trait, a testcross is used to determine an organisms genotype: use
unknown x recessive genotype
1
,9:3:3:1 – 1:1:1:1: two genes autosomal inheritance
Sex linked inheritance
Woman: homogametic, male: heterogametic
Males: genes in differential regions = hemizygous (not heterozygous)
Cytoplasmic (or extranuclear) inheritance
- DNA also in mitochondria and chloroplasts
- Circular chromosomes
- Not autonomous
- Female contains most cytoplasm
Lecture 2
Gene interactions; deviations from normal segregation ratios
- Chi square
- X2= sum ((O-E)2/E)
- Observed and expected
- Set up a null hypothesis
- Find the probability that the observed data set could have resulted from random
fluctuations (chance)
Deducing gene interactions by genetic analysis
Two categories of gene interaction
1. Interaction between alleles of one locus (variations on dominance)
2. Interactions between two or more loci (number/type of genes playing a role in process
studied)
Strategy
1. Crosses: single gene inheritance? Dominant/recessive?
2. Set of genes: check for allelism: genes of one locus or genes on different loci?
- Complementation test (only for recessive mutations): cross mutant individuals (comparable
phenotypes) that are homozygous for a recessive mutation and observe phenotype of
progeny
- Mutant x mutant WT = on different locus (?)
- Allelic series: mutation one of the same gene, but different location may not be so severe
- Children from albino parents may not express albinism: different loci, so may be
heterozygous all together
3. Make double mutants: do genes interact? Modified 9:3:3:1 ratio (eg. 9:7? – interact with
each other)
Haplosufficient: one copy (dose) of the gene is sufficient for expression
Haploinsufficient: opposite of Haplosufficient + mutations are dominant
Incomplete dominance = intermediate phenotype
Codominance = expression both alleles of a heterozygote
2
, Lethal alleles: essential genes: 2:1
- Eg. 2 alleles may cause the offspring to not be viable (=1:2:1 ratio 1:2)
- Most lethal alleles are silent in the heterozygote
- 25% nonviable
- Make double mutants to detect interaction
Penetrance and expressivity
- Penetrance: % individuals with a given allele who exhibit the phenotype associated with
that allele – phenotype has to be triggered
1. Environment
2. Other interaction genes (in rest of genome): eg. Other genes expressed more
3. Subtlety of the mutant phenotype
- Expressivity: degree to which a given allele is expressed at the phenotypic level (intensity of
phenotype): reasons one and two
Population genetics
Population:
- Interaction amongst individuals
- Genetic exchange
- Often characterized by genetic and phenotypic variation: polymorphism
Importance of genetic variation:
- Potential to change genetic population structure:
- response to environmental change
*Nature protection, response to evolving pathogen populations
- migration and colonization
- population divergence: local adaptation
Variation in population:
Gene pool (allele frequencies)
- Sum of all alleles in a population
- Microevolution: change in gene pool
Hardy Weinberg equilibrium assumptions:
- Allele frequency remains constant
- Preconditions: random mating, large number of individuals (no random fluctuations)
- No selection
- No mutation
- No migration
P = dominant
3
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