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Lecture Notes on Mechanisms of Hereditary (MCB2020F)
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Includes: - Mendelian genetics - Mendels 1st law - Mendels 2nd law - Causes on non-mendelian inheritance - Human pedigree analysis
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Mechanisms of HereditaryMECHANISMS OF HEREDITARY...................................................................................................................... 1
GENETICS:..................................................................................................................................................... 2
MENDELIAN GENETICS:..............................................................................................................................................2
Mendel’s first experiments: Pea plants...........................................................................................................2
Results:...................................................................................................................................................................... 2
Conclusion:................................................................................................................................................................ 2
MENDEL’S 1ST LAW – THE PRINCIPLE OF SEGREGATION:.................................................................................................3
Probability:......................................................................................................................................................3
Test Crosses:....................................................................................................................................................3
Dihybrid Crosses:.............................................................................................................................................3
MENDEL’S 2ND LAW – INDEPENDENT ASSORTMENT:.....................................................................................................3
Chi-Square Test:..............................................................................................................................................3
Degrees of Freedom:.......................................................................................................................................4
CAUSES OF NON-MENDELIAN INHERITANCE..................................................................................................................4
1. Dominance relationship between pairs of alleles is not complete.............................................................4
2. Lethality: Some allele combinations are unfavorable.................................................................................4
3. Interaction between genes – Phenotype is not determined by a single gene............................................4
4. Sex-linkage – gene carried on sex chromosome.........................................................................................5
5. Linkage & Recombination – 2 genes on same chromosome......................................................................5
HUMAN PEDIGREE ANALYSIS........................................................................................................................ 5
DOSAGE COMPENSATION...........................................................................................................................................6
SEX DETERMINATION................................................................................................................................................6
1
, Mechanisms of Hereditary
Genetics:
- Classical Genetics: Study of inheritance
- Now better defined as study of biological information
o Molecular Genetics
o Population/evolution Genetics
Mendelian genetics:
- Wanted to answer 3 key questions:
o What is inherited?
o How is it inherited?
o What is the role of chance?
Mendel’s first experiments: Pea plants
1. Is inheritance blended (info from parents irreversibly linked, no way for it to
separate out again) or discrete (info from parents passed down and maintained
separately in individual, such that it could then separate out again later))?
2. What are the respective contributions of the male & female parents to inheritance.
o To answer this he set up Monohybrid crosses:
Parents differ only in 1 trait (phenotype)
o At the time (1850’s) most people thought inheritance was mainly from the
father.
- Pro’s of using pea plants:
o Short generation time. 1 year per generation
o Individuals produce lots of offspring.
o Hermaphrodites – i.e. can self fertilize
o Clear-cut binary qualitative traits. E.g. yellow OR green etc.
- True breeders: (Homozygous dominant) their phenotype is always expressed when
self-fertilized. He used only true-breeding parents.
- He used reciprocal crosses – same monohybrid cross, but opposite way around (i.e. if
male was purple in first cross, female would be purple in second).
- Result of parent cross = F (filial) 1 generation
o Self-fertilization of F1 gen = F2 gen
- Results:
- In the F2 gen, sometimes 1 of the parental phenotypes that disappeared would
reappear
- IN a ratio of ±1:3.
- * disproves blended inheritance
- Conclusion:
- Some phenotypes are clearly more dominant over others
2
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