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Chapter 2

HMB265H1 Chapter 2: HMB265 Lecture 2 Readings - Mendelian Genetics

Human Biology
Course Code
Maria Papaconstantinou

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HMB265 Lecture 2 Readings - Mendelian Genetics (pg 16-22, 24-31)
2.1 Background: The Historical Puzzle of Inheritance
Artificial selection was the first applied genetic technique
- AS important for hunter-gatherers
Desirable traits sometimes disappear and reappear
- People produced plants/animals w/ desired characteristics (AS) for long time
- Didn’t understand why valued trait sometimes disappear, then reappear in only some
- EX: Merino sheep breeding:
- Selective breeding to make sheep that have lots of wool
Mendel devised a new experimental approach
- Before Mendel misconceptions about heredity:
[1] One parent contributes most to offspring’s inherited features
[2] Blended inheritance = parental traits mixed + forever changed in offspring
- EX: Blue parent + yellow parent = green baby
- Might explain why kids look like combo of parents
- But doesn’t explain why siblings look diff
- Questions Mendel answered:
- What is inherited?
- How is it inherited?
- What is the role of chance in heredity?
Mendel used garden pea :
- Male + Female organs in same flower = normal self-fertilizing
- But can also cross-fertilize flowers:
- Remove anthers of one flower 1
- Brush pollen from anthers of flower 2 onto flower 1
- For each generation, can get lots of individuals in short growing season
- EX: If worked with sheep, only a few offspring
Mendel looked at inheritance of discrete traits
- EX: Purple vs white flowers & Yellow vs green peas
- Mendel called constant but mutually exclusive alternative traits “antagonistic
- Yellow vs Green (seed colour and unripe pod colour)
- Round vs Wrinkled (seed shape)
- Purple vs White (flowers)
- Round vs Pinched (pod shape)
- Long vs Short (stem length)
- Along stem vs Tip of stem (flower position)
- = unambiguously trace transmission of traits
- No intermediate forms (like in continuous traits)
Mendel collected/perpetuated lines of peas that bred true
- Used pure-breeding lines

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- = offspring have parental traits that remain constant over generations
- Plants with white flowers always produce offspring with white flowers
- = pure-breeding stocks for each member of pair
- Also cross-fertilized pairs to make hybrids
- = offspring have genetically dissimilar parents
Mendel controlled his matings
- Make sure didn’t get contamination from foreign pollen
- = observed progeny really resulted from intended fertilizations
- Controlled whether trait transmitted via egg cell (from egg) or via sperm cell (from pollen)
- EX: Use pollen from purple flower to fertilize eggs of white flower vs Use pollen from
white flower to fertilize egg of purple flower
- But hybrid progeny from reciprocal cross-ferts always have same characteristics
- EX: In both cases get purple flower
Mendel had a big sample size, used numerical analysis, compared results w/ model preds
2.2 Genetic Analysis According to Mendel
- 1865 Mendel publishes paper:
- Papers goal: Is there a generally applicable law governing formation of hybrids
Monohybrid crosses reveal units of inheritance and the law of segregation
- Mend first isolate pure-breeding lines for several sets of characteristics
- Then did matings btwn individuals that only differ in one trait (antagonistic pairs)
[1] Parental (P) generation:
Pure-green mom + Pure-yellow dad
Pure-yellow mom + Pure-green dad (reciprocal)
[2] First filial (F1) generation
Progeny of P generation all yellow (but green hidden?)
[3] Second filial (F2) generation
Plant F1 plants and allow them to self-fertilize = monohybrid cross
3:1 ratio yellow to green
Reappearance of the recessive trait
- Since got green back in F2 means that blending didn’t happen
- If did blend, then info to make green peas would be lost in F1
- Conclusion:
- 2 types of yellow peas in F1 generation:
[1] Breed true like yellow peas of P generation
[2] Contains latent info for green peas
- Dominant trait: Trait that appears in all the F1 hybrids
- Recessive trait: Antagonistic trait that hidden in F1 hybrids and reappears in F2
Genes: Discrete units of inheritance
- Every plant has 2 copies of genes (one from mom, one from dad) for each trait
- Gene comes in alternative forms
- Combinations of alternative forms = diff characteristics = alleles
- EX: Gene for pea colour has yellow and green alleles
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