Transcript Theoretical genetics

Theoretical genetics
Ms. Jackie
Important definitions

Genotype- the alleles possesed by an
organism
 Phenotype- all the characteristics of an
organism.
 Dominant allele- an allele which has the same
effect on the phenotype whether it is present in
the homozoygous or heterozygous state
 Recessive allele- an allele which only has an
effect on the phenotype when present in the
homozygous state
Important definitions

Codominant alleles- alleles which only has an effect
on the phenotype when present in heterozygous but a
greater effect in homozygous individuals.
 Locus- The particular position of a gene on
homologous chromosomes.
 Homozygous- two identical alleles of a gene
 Heterozygous- having two different alleles of a gene
 Carrier- an individual that has a recessive allele of a
gene that does not have an effect on the phenotype
 Test cross- testing a suspected heterozygote by
crossing with a known homozygous recessive.
Construction of Punnett grids
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Way of finding he
expected ratio of the
offspring, giving
certain parental
phenotypes
Punnett grids
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Example using
characteristics used by
Mendel
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He study the size of
pea plants and found
that tall is dominant
over short
Lets start with two pure
breeding (homozygous)
plants with contrasting
traits (tall and short)
F1 generation will have
the dominant
phenotype tall but will
be heterozygous.
Phenotypes
Corresponding
genotypes
Tall
TT or Tt
Short
tt
T
t
t
T
Tt
Tt
Tt
Tt
Punnett grids
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IF the F1 generation
self fertilized we will
obtained F2:
¾ tall
¼ short
3:1
1TT:2Tt:1tt
T
T
t
t
TT
Tt
Tt
tt
Lets practice
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IF a tall garden pea is crossed with a
dwarf garden pea, the F1 are all tall.
Predict the result of self fertilization of
the F1, using a Punnett square.
Pedigree chart
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Used to show the inheritance of certain traits
over several generations of humans
 Example Queen Victoria hemophilic condition
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Males are normal or affected
Females normal or carriers
Hemophilia allele is homozygous lethal as well as
sex linked since it is carried on the X chromosome
Males can be XHY (normal) or XhY (affected)
Females can be XHXH (normal) or XHXh (carrier)
Queen Victoria and hemophilia
Affected male
Affected female
Normal
female
Normal male
Victoria
Victoria Edward
Alice
George V
Duke of
Windsor
George VI
Alfred
Helena
Albert
Louise
Arthur
Leopold
Beatrice
3
Irene
Frederick
Alexandra
NicholasII
3
Elizabeth
Margaret
Alexis
Multiple alleles
Some genes have more than two alleles
 Multiple alleles can be found in blood
groups
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ABO blood groups
Based on 4 different phenotypes (A;B;AB
and O)
 Caused by different combinations of 3
different alleles
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Phenotypes
Genotypes
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
0
ii
ABO blood groups
Using a Punnett Grid work out the
possible combinations of blood groups of
a male with blood group B and a female
with blood group
 Using a Punnett Grid explain why a
female with blood group O and a male
with blood group AB cannot have
children with either pf the parents´ blood
group.
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How sex chromosomes determine
gender
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Gender determination
Decided by sex chromosomes
 Females has two X chromosomes
 Males have one X and one Y chromosome
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Genes in X and Y chromosomes
The X chromosome is relatively
large
 Y chromosome is much smaller
 Several genes are located on the
X chromosome
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Sex linked examples
Colorblindness
 hemophilia
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Few genes on the Y
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Hairy ears
Define sex linkage and state two
examples
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Genes carried on the sex chromosomes, most often on the X
chromosome.
Examples
 Color blindness
 Certain colors cannot be distinguished, and is most commonly
due to an inherited condition. Red/Green color blindness is by
far the most common form, about 99%, and causes problems in
distinguishing reds and greens. Another color deficiency
Blue/Yellow also exists, but is rare and there is no commonly
available test for it.
 Depending on just which figures you believe, color blindness
seems to occur in about 8% - 12% of males of European origin
and about one-half of 1% of females. Total color blindness
(seeing in only shades of gray) is extremely rare.
 There is no treatment for color blindness, nor is it usually the
cause of any significant disability.
Define sex linkage and state two
examples
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Hemophilia
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Blood condition in which an essential clotting factor
is either partly or completely missing.
This causes a person with hemophilia to bleed for
longer than normal.
Cuts and grazes are not great problems as a little
pressure and a plaster are usually enough to stop
bleeding.
The main problem is internal bleeding into joints,
muscles and soft tissues.
Hemophilia is a lifelong inherited genetic condition,
which affects females as carriers and males who
inherit the condition.
Human females can be homozygous, hetero with respect to sex
linked genes
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Human males cannot be heterozygous
since they only have one copy of one
allele
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Example
Existing
alleles
XB for normal
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Colorblindness
vision
Xb for
colorblindness
A female can be
XBXB or XBXb
XbXb
A male can be
XBY
XbY
Human females can be homozygous, hetero with respect
to sex linked genes
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Hemophilia example
Existing alleles
XH for normal
Xh for
hemophilia
A female can be
XHXH or XHXh
XhXh (not exist)
A male can be
XHY
XhY
•Homozygous lethal
•Female carriers are heterozygous for X linked recessive
alleles
•They will not show the disease but can pass it on to the
next generation
Explain that female carriers are heterozygous for X-linked
recessive alleles
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Heterozygous females for sex linked
traits
Are said to be carriers
 They will not show the disease
 Can pass the disease to the next generation
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Calculate and predict the genotypic and
phenotypic ratios of monohybrid crosses
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Monohybrid crosses
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Involves the cross
between two individuals
to study the inheritance
of one gene
Example
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Character:
Tongue Rolling
Gene:
R
Alleles:
R, r
Genotypes:
RR, Rr, rr
Phenotypes: rollers,
non rollers
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Cross:
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Heterozygous X
Heterozygous
Rr X Rr
Gametes R, r X R, r
Punnett square
R
r
R
RR Rr
r
Rr
Ratio 3:1
rr
Calculate and predict the genotypic and
phenotypic ratios of monohybrid crosses
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In your notebooks calculate the
genotypic and phenotypic ratios of a
cross between a heterozygous tongue
roller and a recessive individual for
tongue rolling
Calculations and codominance
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Type pf inheritance in
which the two alleles of
the gene are equally
dominant and so they
are both expressed in
the heterozygous
genotype.
Alleles
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R red
W white
RR red
WW white
RW- pink
Calculations and codominance
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The two alleles are equally dominant
 This kind of inheritance produces more
variation.
 Example
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Phenotypes of parents Red X White
Genotypes of parents RR X WW
Gametes R and W
F1- RW all pink
Perform a self cross between the F1 generation
Deduce the genotypes or phenotypes of individuals in
pedigree charts
Go to page 434-435.
 Answer exercises 2 and 4
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