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Monday, 3 November 2014

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Kim Foglia & Kelly Reidell

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The Work of Mendel
Gregor Mendel
* Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas
* used experimental method
* used quantitative analysis
* collected data & counted them
* excellent example of scientific method
Mendel’s work
* Bred pea plants
* cross-pollinate 
true breeding parents (P)
* P = parental
* raised seed & then 
observed traits
* F = filial
* allowed offspring 
to self-pollinate 
& observed next 
generation (F2)

Looking closer at Mendel’s work
What did Mendel’s findings mean?
* Traits come in alternative versions
* purple vs. white flower color
* alleles
* different alleles vary in the sequence of nucleotides at the specific locus of a gene
* some difference in sequence of A, T, C, G
Traits are inherited as discrete units
* For each characteristic, an organism inherits 2 alleles, 1 from each parent
* diploid organism 
* inherits 2 sets of chromosomes, 
1 from each parent
* homologous chromosomes
* like having 2 editions of encyclopedia
* Encyclopedia Britannica 
* Encyclopedia Americana
What did Mendel’s findings mean?
* Some traits mask others 
* purple & white flower colors are separate traits that do not blend 
* purple x white ? light purple
* purple masked white
* dominant allele 
* functional protein
* masks other alleles 
* recessive allele 
* allele makes a 
malfunctioning protein
Genotype vs. phenotype
* Difference between how an organism “looks” & its genetics
* phenotype 
* description of an organism’s trait
* the “physical”
* genotype 
* description of an organism’s genetic makeup
Making crosses
* Can represent alleles as letters
* flower color alleles ? P or p
* true-breeding purple-flower peas ? PP
* true-breeding white-flower peas ? pp
Looking closer at Mendel’s work
Punnett squares
Pp   x   Pp
* Homozygous = same alleles = PP, pp
* Heterozygous = different alleles = Pp
Phenotype vs. genotype
* 2 organisms can have the same phenotype but have different genotypes
Test cross
* Breed the dominant phenotype —
the unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele
How does a Test cross work?
Mendel’s 1st law of heredity
* Law of segregation 
* during meiosis, alleles segregate
* homologous chromosomes separate
* each allele for a trait is packaged into a separate gamete
Law of Segregation
* Which stage of  meiosis creates the 
law of segregation?
Monohybrid cross
* Some of Mendel’s experiments followed the inheritance of single characters 
* flower color
* seed color 
* monohybrid crosses
Dihybrid cross
* Other of Mendel’s experiments followed the inheritance of 2 different characters 
* seed color and 
seed shape
* dihybrid crosses
Dihybrid cross
What’s going on here?
* If genes are on different chromosomes…
* how do they assort in the gametes?
* together or independently?
Is this the way it works?
Dihybrid cross
Mendel’s 2nd law of heredity
* Law of independent assortment
* different loci (genes) separate into gametes independently 
* non-homologous chromosomes align independently
* classes of gametes produced in equal amounts
* YR = Yr = yR = yr
* only true for genes on separate chromosomes or 
on same chromosome but so far apart that crossing over happens frequently
Law of Independent Assortment

The chromosomal 
basis of Mendel’s 

Trace the genetic 
events through 
meiosis, gamete 
formation & 
fertilization to offspring
Review: Mendel’s laws of heredity 
* Law of segregation
* monohybrid cross 
* single trait
* each allele segregates into separate gametes
* established by Metaphase 1
* Law of independent assortment
* dihybrid (or more) cross
* 2 or more traits 
* genes on separate chromosomes 
assort into gametes independently
* established by Metaphase 1
Mendel chose peas wisely
* Pea plants are good for genetic research
* available in many varieties with distinct heritable features with different variations
* flower color, seed color, seed shape, etc.
* Mendel had strict control over 
which plants mated with which
* each pea plant has male & female 
* pea plants can self-fertilize
* Mendel could also cross-pollinate 
plants: moving pollen from one plant 
to another
Mendel chose peas luckily
* Pea plants are good for genetic research
* relatively simple genetically
* most characters are controlled by a single gene with each gene having only 2 alleles, 
* one completely dominant over 
the other


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