Download Mendel`s peas - Seattle Central

Meiosis -> Inheritance
How do the events of Meiosis
predict patterns of heritable
variation?
Mendel’s peas
1. Genes determine appearance (phenotype)
2. Genes vary and they are inherited
3. Their “behavior” explains patterns of
variation
4. Genes lie on the chromosomes that
recombine and segregate during meiosis
• Genes go wherever chromosomes go;
genes are just along for the ride.
Mendel to the rescue
• 1822-1884; contemporary
of Darwin*
• Monk; St. Thomas Abbey;
pea plants
• Involved in Agricultural
Society*
– Animal & Plant breeders
knew that traits could be
inherited
– Mendel sought to describe
these patterns of
inheritance
Competing Hypotheses
• Offspring inherit traits that parents
acquire during their lifetime (acquired
inheritance)
– Jean-Baptiste Lamarck
– Body-builders will have muscle-bound offspring
• Offspring traits are an average of their
parent’s traits. (blending inheritance)
– Nägeli
– Black sheep + white sheep = gray sheep
Advantages of peas
• Inexpensive to keep &
propagate
• Short generation time
• Prolific
• Small (space economy)
• Tolerant of inbreeding
(mutants survive to produce
a phenotype)
• Simple phenotypic variation
• Matings (experiments) were
COMPLETELY within his
control – Testcrosses!
Easy to
pollinate
• Self
• Cross
• Generate truebreeding lines
Simple, informative traits
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Terminology
Phenotype
genotype
Diploid
haploid
gamete
Homologs/homologous
gene
Locus/loci
Allele
Homozygous
heterozygous
True-breeding
Hybrid
P, F1, F2 generation
Reciprocal cross
Dominant/recessive
Partial (incomplete) dominance
Particulate inheritance
Segregation of alleles
Independent assortment of loci
Recombination/Crossing over
Sister/non-sister chromatids
Linkage
sex-linked
Autosome sex-chromosome
What did Mendel DO?
• Created true- breeding
lines for various traits
Trait
determinants
– When bred within lines,
always produce same traits
– Round x round = all round
• Crossed pure wrinkled peas
with pure round peas
– Genotype = genetic
complement
– Phenotype = outward
appearance
M
F
Whoa! All round?
• The wrinkled phenotype “receded”
– Recessive (r)
• Oh, peas from male plant were round;
males dominant over females…right?
Reciprocal cross
Self-pollinate
1.
Variation (wrinkled) reappeared in F2;
round:wrinkled ~ 3:1
2. P generation phenotypes were unaltered
–
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Inheritance is particulate, not blending!
Trait determinants, or alleles, (R & r) segregate between
generations
Summary
• This same pattern recurred for all 7
traits:
– F1 disappearance, F2 reappearance, unaltered
parental phenotypes, 3:1 ratio
• Trait determinants are distinct,
unalterable units
• M & F contribute = to genotype
• More than 1 determinant per plant, and
they segregate from each other between
generations RR ---> R & R
What happened?
• Monohybrid cross
– Separation into
gametes
– Union of gametes
produces new
genotype
• Punnett Square
• Let all Round plants
self. Phenotype
ratio?
– 3:1!!
F1 generation
What about 2 traits?
• How will two different traits behave?
How will the alleles that produce them
segregate relative to each other?
– Will they segregate together?
– Will they segregate independently?
2 Traits
2 trait
dependent
• If alleles
segregate
together, we
should expect
only Parental
phenotypes in F2
• Same 3:1 ratio
as monohybrid
cross
2 Traits
2 trait
independent
• Dihybrid cross
• These were
Mendel’s results!
• 9:3:3:1
• Principle of
independent
assortment
Testcrosses confirm
predictions
• A testcross uses a parent with all recessive
alleles to reveal the unknown genotype of
another parent.
Mendel’s work helped explain
Charles Darwin’s postulates
1. Variability among traits must exist
2. Some of that variability must be heritable
3. Due to battles for resources, individuals
will vary in their ability to survive &
reproduce
4. Those that reproduce the most, are those
with the most favorable variations
(adaptations)
Darwin’s postulates require
that:
1. Traits vary among
individuals within a
population
2. Some of this variation
is heritable
•
For natural selection to
work there must be
heritable variation in
every generation
Where Darwin got stuck
• What produces variation?
• How is it propagated from one
generation to the next?
• He (indeed, anyone) could demonstrate
that traits are heritable, but not how.