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Mechanisms of Heredity Complete Summary
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Complete summary covering the Mechanisms of Heredity module for MCB2020f including patterns of Mendelian Inheritance and deviations from it.
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Module 1Mechanisms of Heredity
WHAT IS GENETICS
• Classical genetics —> understanding the study of inheritance
• Origin of the study of genetics
• Molecular genetics —> a understanding the function of DNA and genes
• What genes are involved in what processes?
• Evolutionary geneticists —> look at genetic variation and how it changes over time
• Phylogenetics —> uses DNA sequence information to infer evolutionary relationships between
species and individuals
• Modern branches = molecular vs population/evo genetics
NB!! Mendelian genetics function without knowing about DNA/genes/choromosomes and how it
works
Genes = basic units of biological information
Heredity = ways genes transmit physiological, anatomical and behavioural traits from parents to
o spring
MISCONCEPTIONS PRIOR TO MENDELIAN GENETICS
1. One parent contributed more than the other to o spring’s appearance
1. Thought to be the male
• The homunculus —> drawing of the head of a sperm via a microscope seemed to point to the
fact that inheritance was 100% male and females were an incubator for the growth of a foetus
2. Inheritance was blended
• Parental traits become mixed and forever changed in the o spring they way pigments of
paint can mix
MENDELIAN GENETICS
• Mendel had 3 key questions
• What is inherited?
• How is it inherited?
• What is the role of chance?
MENDEL’S FIRST EXPERIMENTS
• Wanted to know:
• Is inheritance blended or discrete?
• Discrete = information from parents would be passed onto the next generation and
would be maintained separately in the individual
• Blended = the o spring of two individuals would exhibit traits that were irreversibly
mixed traits of the parent generation
• Popular belief at the time!
• What are the respective contributions of the male and female parents to inheritance?
• Series of monohybrid crosses = parents di ered in only one trait/phenotype
• Success in simplicity!!
• Mendel’s pea plants —>
• IDEAL!
• Relatively short generation time
• Therefore able to complete experiments in a relatively short period of time
• Produces lots of o spring
• More robust data than a small number of o spring (reduce sampling error)
ff ff ff ff ff ff ff
, • Able to self fertilise
• Variety of clearly de ned binary phenotypes
• Amenable to genetic analysis
• Eg. Yellow or Green —> no intermediates
• Used True Breeding plants
• = plant that always produces o spring of the same phenotype when self-fertilized; one
that is homozygous for the trait being followed
• Mendel realised that certain traits were not always true breeding varieties
• Needed to use TRUE BREEDING to
follow the crosses
• Set up reciprocal monohybrid crosses
• In order to to gure out how parent plants
were contributing to the characteristics
inherited by o spring
1. Removed anthers of one true breeding plant to
stop it from self fertilising (female plant)
2. Remove pollen from “male” plant with
paintbrush + transfer to “female” plant
3. Set up the same cross but where the traits are
swapped in the “male” vs “female”
4. Examined F1 and F2 generation
1. F2 generated by allowing F1 to self fertilise (can generate progeny without introducing new
DNA)
FINDINGS —>
• Single phenotype present in F1 generation
• F2 generation would show both of the
parental phenotypes
• 3:1 ratio observed in F2 generation
• Disproved the blending hypothesis!!!
• If it was true, the phenotype that
“disappeared” (recessive phenotype) in the
F1 gen would not reappear in the F2
generation
• Reciprocal crosses yielded identical results
• Each parent contributes equally to the
o spring’s phenotype
• Some phenotypes are dominant over others (those seen in the F1 generation)
• But the unit of inheritance that determines the other phenotype is still carried by the plant
showing the dominant trait —> the information carried is maintained
• Two units for inheritance exist in each individual —> one from each parent
MENDEL’S FIRST LAW = PRINCIPLE OF SEGREGATION
—> the two alleles for each trait segregate (separate) during gamete formation and then
unite randomly at fertilisation (one from each parent)
• Explains why:
• Reciprocal crosses yield identical results
• 3:1 ratio in the F2 generation
ff
fiff fi ff
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