Theoretical genetics

1
Theoretical genetics

• Ms. Jackie

2
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

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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.

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Construction of Punnett grids

• Way of finding he expected ratio of the
  offspring, giving certain parental phenotypes

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Punnett grids

• Example using characteristics used by Mendel
• 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
Tt Tt
Tt Tt
t t
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Punnett grids
T t

• IF the F1 generation self fertilized we will
  obtained F2
• ¾ tall
• ¼ short
• 31
• 1TT2Tt1tt

TT Tt
Tt tt
T t
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Lets practice

• 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.

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Pedigree chart

• Used to show the inheritance of certain traits
  over several generations of humans
• Example Queen Victoria hemophilic condition
• 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)

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Queen Victoria and hemophilia
Affected male
Affected female
Normal female
Normal male
Victoria Albert
Victoria Edward Alice Alfred
Helena Louise Arthur Leopold
Beatrice
George V
3
Irene Frederick Alexandra
NicholasII
George VI
Duke of Windsor
3
Alexis
Elizabeth Margaret
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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 (ABAB and O)
• Caused by different combinations of 3 different
  alleles

Phenotypes Genotypes
A IAIA or IAi
B IBIB or IBi
AB IAIB
0 ii
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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

• 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
• Sex linked examples
• Colorblindness
• hemophilia
• Few genes on the Y
• Hairy ears

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Define sex linkage and state two examples

• 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.

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Define sex linkage and state two examples

• Hemophilia
• 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.

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Human females can be homozygous, hetero with
respect to sex linked genes

• Human males cannot be heterozygous since they
  only have one copy of one allele
• Example
• Colorblindness

Existing alleles XB for normal vision Xb for colorblindness
A female can be XBXB or XBXb XbXb
A male can be XBY XbY
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Human females can be homozygous, hetero with
respect to sex linked genes

• 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

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Explain that female carriers are heterozygous for
X-linked recessive alleles

• 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

• Monohybrid crosses
• Involves the cross between two individuals to
  study the inheritance of one gene
• Example
• Character Tongue Rolling
• Gene R
• Alleles R, r
• Genotypes RR, Rr, rr
• Phenotypes rollers, non rollers

• Cross
• Heterozygous X Heterozygous
• Rr X Rr
• Gametes R, r X R, r
• Punnett square

R r
RR Rr
Rr rr
R r
Ratio 31
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Calculate and predict the genotypic and
phenotypic ratios of monohybrid crosses

• In your notebooks calculate the genotypic and
  phenotypic ratios of a cross between a
  heterozygous tongue roller and a recessive
  individual for tongue rolling

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Calculations and codominance

• 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
• R red
• W white
• RR red
• WW white
• RW- pink

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Calculations and codominance

• The two alleles are equally dominant
• This kind of inheritance produces more variation.
  
• Example
• 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

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Deduce the genotypes or phenotypes of individuals
in pedigree charts

• Go to page 434-435.
• Answer exercises 2 and 4