the work of gregor mendel biology honors 8.1-8.3
Post on 17-Dec-2015
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What is inheritance? Heredity:
Passing of characteristics from parents to offspring
Genetics Study of how those characteristics are passed on
200+ years ago People knew that
we resemble our ancestors Traits are passed
on from generation to generation
The question became – HOW?
Gregor Mendel Father of Genetics
1860’s Austrian monk Studied genetic traits
of peas and how traits are passed on (parent offspring)
Why study peas?!?1. Many varieties (traits):
7 traits of focus Traits showed complete dominance
2. Control reproduction: cross-pollinate/hybrid or self-pollinate/purebred
3. Short lifespan: Three generations in only three years P-generation, F1-generation, F2-generation
2. Control Reproduction Pea flowers have both male (stamen)
and female (carpel) parts Plant can self-pollinate
Its own stamen fertilizes its own carpel
Mendel controlled this by removing stamen
Plant can cross-pollinate Stamen from one plant fertilizes the
carpel of another Mendel used a paintbrush to control
which plant bred with which so he could follow specific traits
Mendel’s Experiment #1 (P generation F1 generation) Trait: plant height
Tall, short Self-pollinated pea plants
for many generations Phenotypes:
Purebred tall, purebred short Genotypes:
Purebred tall (TT), purebred short (tt)
Remember Alleles… Organisms contain 2 alleles for each trait
One from mom, one from dad (2n zygote) Only pass on 1 allele to offspring (1n gamete)
Tall x short All offspring
were tall Hybrids (Tt) Tall alleles are
dominant to short alleles
Mendel’s Experiment #1 (P generation F1 generation)
Mendel’s Experiment #2 (F1 generation F2 generation) Hybrid x Hybrid Tall (Tt) x Tall (Tt) Phenotype
3 tall 1 short
Genotype 1 TT 2 Tt 1 tt
Mendel’s Experiments Notice the ratios! F1 – genotype is always
100% heterozygous; phenotype is always 100% dominant trait
F2 – genotype is 1:2:1 homozygous : heterozygous : recessive; phenotype is always 3:1 dominant : recessive
Review: When Mendel crossed pure
(homozygous) plants with two different traits (ex: purple x white):
He always found the same pattern – Only one trait showed in the F1
generation BUT… Missing trait showed up again in
the F2 generation in a 3:1 ratio
Principle (Law) of Dominance Some alleles are
dominant, others are recessive
Dominant alleles are always expressed
Recessive alleles are “hidden” in the presence of a dominant allele
Principle (Law) of Segregation When F1 plants made
gametes, their alleles for purple and for white separated
When these gametes recombined to make the F2 generation, the recessive trait reappears in ¼ of the offspring
Principle (Law) of Independent Assortment
Alleles for different traits separate during meiosis independently (randomly)
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