Download Mendel`s Genetics

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Mendel’s First Law: The Law of Segregation
Two alleles of one gene segregate and recombine independently during the cross.
Mendel used 2 purebred organisms: tall and dwarf pea plants.
P1:
F1:
F2:
tall x dwarf

cross-pollination

all tall

self-pollination

¾ tall, ¼ dwarf
Mendel made a conclusion that tall was dominant, while dwarf was recessive.
Letters are used to represent different alleles. A purebred tall plant: TT. A purebred dwarf
plant: tt.
Homozygous organism: when it has two identical alleles for a given trait. Eg: TT or tt
Heterozygous organism: when it has two different alleles for a given trait. Eg: Tt. The tall allele
is dominant and is therefore expressed.
Dominant allele can be both homozygous (TT) and heterozygous (Tt). ATT: Recessive allele
can be only homozygous (tt)!!!
Genotype: the genetic make-up of an organism. Phenotype: appearance of the trait in an
organism.
PUNNETT SQUARE
- explains Mendel’s laws
- allows predicting the results of a cross.
- Was designed for F2 generation, but can be used for other ones.
In Punnett Square:
- Each off spring will receive one allele from each parent.
- Each allele represents a gamete.
- Each gamete should have one member of a pair.
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Using the Punnett Square, you can determine both the genotypes and phenotypes of the
offspring.
E.g. 1:
PxP
YY x yy (Yellow x Green)


Y
y
Yy (Yellow)
Gametes:
F1:
F2:
Y
y
Y
YY
yellow
Yy
yellow
y
Yy
yellow
Yy
green
This is a monohybrid cross: cross between two heterozygous individuals that differ in one trait.
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