Chapter 17 Heredity
Mendel’s Experiments With Peas
Mendel started his experiments with pure breeding lines. Pure
breeding lines are individuals that always produce a parental trait.
e.g. Height
Tall X Tall → always Tall
Short X short → always short
Then:
He crossed
pure Tall X pure short → all Tall
This generation is called the F1 or First Filial Generation
He then crossed
F1 Tall X F1 Tall → 3 Tall : 1 short
Mendel’s Conclusions
Mendel showed that the hypothesis was correct that elements are
passed through the parents’ gametes to offspring and these
elements determine the offspring’s traits.
However he also concluded that although trait carrying-elements
from both parents are indeed passed, one of the traits may be
masked and the other exhibited. The masked trait is said to be
Recessive and the trait that is ‘masking’ is called Dominant.
To understand this we must understand that all organisms have
chromosomes (along with genes) that are paired (2 of ‘em)
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Alleles
Genes controlling a trait come in different forms. These forms are
called alleles
For example these two homologous chromosomes each have 4 genes for the same traits A stands for
abnormal toe nails; B for blue blood; C for crinkles and D for dog-breath. A “+” means that it has this
condition…no superscript means that the condition is absent. The orange chromosome has abnormal toe
nails and crinkles; The purple chromosome has blue blood . Neither chromosome has + dog breath.
We normally represent genes with a letter. This helps keep things
‘tidy’.
E.g. height is expressed as a “t”
T = Tall
t = short
• Pure breeding Tall will be – “TT”
(one T for each chromosome/gene in the homologous pair)
• Pure breeding short will be – “tt”
(one t for each chromosome/gene in the homologous pair)
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3
Punet Square
F1 X F1
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Dihybrid Crosses
• When you compare two traits at the same time.
For example: suppose you were to breed two hybrid tall/ hybrid
green (TtGg) pea plants. What would you get?
TtGg (Male) X TtGg (Female)
Possible gametes:
Male: TG, Tg, tG, tg Female: TG, Tg, tG, tg
TG
Tg
tG
tg
TG
Tg
tG
tg
Complete the Punet Square
Describe genotype and phenotype
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Codominance
When an individual which is heterozygous for a trait expresses BOTH at the
same time.
E.g. Blood Types
There are 3 possible alleles in a population but each individual only carries 2.
IA
IB
Dominant
i
Dominant
A
What do you get if father is I
Recessive
/ i and mother is IB / i
IA
i
IB
i
Complete the Punet Square
Describe genotype and phenotype
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Gene Interactions
• When more than one gene controls a single trait.
One gene is dominant over another gene (Think a minute!)
E.g. Labrador retrievers come in three flavors
• Gold
• Black
• Brown
There are two genes involved Gene – E and Gene – B
Gene E is dominant over Gene B.
•
•
•
•
Allele E codes for gold
Allele e codes for no effect
Allele B codes for black
Allele b codes for brown
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Incomplete Dominance
• When the phenotype for a single trait is a mix of both parental
traits.
E.g. Carnations
RR
Red
X
rr
White
F1
F2
Rr X Rr
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Sex Linkage
• When a certain gene for a traits is located on the “X”
chromosome.
• Traits are usually exhibited by the male and only carried by a
female.
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