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FIRST CLASS Lecture notes Genetics
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Mendel lecture notes
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MendelGregor mendel discovered the basic principles of
heredity
Mid 19th century there was much confusion about how traits were
inherited: conventional wisdom thought that inheritance was
“blending”
Whether by accident/intent, Mendel’s discovery was reliant on his
choice of organism (peas): they give plenty of progeny & there were
plenty of pure breeding varieties available to him. Peas are not a
model species now as they only give 1 generation per year (slow by
standards of today’s “model” species) – but Mendel had time.
And, crucially, his choice of traits: Mendel picked 7 traits that
showed discontinuous variation (no “medium”) – seed colour, seed
shape, seed coat colour, pod colour, pod shape, flower position &
stem length.
Monohybrid crosses reveal the principle of
segregation and the concept of dominance
Monohybrid cross = cross between 2 parents that differ in a single
characteristic (cross looking at 1 trait, crossing between lines that
wouldn’t normally be crossed)
- Conclusion 1: one character is encoded by 2 genetic factors
- Conclusion 2: 2 genetic factors (alleles) separate when gametes are
formed
, - Conclusion 3: the concept of dominant & recessive traits
- Conclusion 4: two alleles separate with equal probability into the
gametes
What Mendel did: he took pea plants that were true breeding (peas
usually are self-fertilising) – we now know these to be homozygotes
for the short stem allele. He forced plants different in 1 of his 7 traits
(but the same otherwise) to mate by artificially transferring pollen.
He then let the F1 self-
fertilise. This 2nd generation
of progeny is the F2
He noticed something odd:
for each of the 7 traits about
¾ of the progeny resembled
one of the original parents,
¼ resembling the other
This is now the classic
“Mendelian” proportions of
3:1
Mendel knows nothing about
meiosis, DNA or mutation,
but works out that:
a) Inheritance is not blending – there was no medium type
b) The pattern of inheritance supports the idea that inheritance is
“particulate”, not blending
c) Because the F2 has types resembling both parents, the F1 must
still contain the information for both, even though only 1 phenotype
is seen
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