Monohybrid Cross

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Monohybrid Cross

• Higher Human Biology

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Lesson Aims

• To revise and consolidate understanding of
  monohybrid crosses
• To examine Rhesus and Rhesus- blood groups
• To learn about different conditions caused by
  genetic mutations
• To find out the difference between incomplete
  dominance and co-dominance

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History

• Gregor Mendel - The Father of Genetics
• 1. Monk who used science and maths to establish
  patterns in how traits were inherited2. Year
  18573. He used the garden pea as his test
  subjects
• Some Vocabulary
• Character - a heritable feature (eg flower
  colour)Trait - a variant of each character (eg
  purple or white)Cross Pollination - one plant
  fertilizes a different plantSelf Pollination - a
  plant fertilizes itselfTrue-Breeding - plants
  that over several generations only produce plants
  like themselves

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• Mendels Experiments - a Monohybrid Cross
• cross involved plants that differed for a single
  character tall x short, purple flower x white
  flower, round seed x wrinkled seed
• P (Parental Generation) True breeding plantsF1
  (First Filial) The offspring of the P generation
  --gt they always displayed a single trait, the
  dominant oneF2 (Second Filial) The offspring of
  the F1 generation, self fertilized --gt always had
  a 31 ratio

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Mendels Law of Segregation

• The Law of Segregation two alleles separate
  during gamete formation (a Tt parent can produce
  both T sperm, and t sperm)
• A punnet square is a representation of this law,
  showing how gametes separate and then come
  together during fertilization

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• Locus - spot on the chromosome where an allele
  (gene) is located.

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Monohybrid Inheritance in Humans

• Tongue rolling is inherited as a simple Mendelian
  trait
• R is the allele for roller
• r is the allele for non-roller

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Genetics of Tongue Rolling
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Rhesus D Antigen

• Antigen D is found on the surface of some
  peoples red blood cells
• Rh (rhesus positive) posses this antigen
• Rh- (rhesus negative) do not possess this antigen

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• If a Rh- person is given Rh red blood cells
  during a transfusion the persons immune system
  responds by producing anti-D antibodies. This
  leaves the person sensitised.
• If this person receives more Rh red blood cells
  they suffer from severe or fatal agglutination.

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Agglutination of Red Blood Cells
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• Presence of Antigen D is genetically dominant (D)
• Lack of antigen D is due to a recessive allele
  (d)
• P DD x dd or P dd x Dd
• (Rh-) (Rh) (Rh-) (Rh)
• F1 all Dd F1 Dd and dd
• (Rh) (Rh) (Rh-)

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• Albinism - inability of the body to make melanin
  - inherited as simple Mendelian recessive trait
• Cystic Fibrosis - disorder of the mucus secreting
  glands - simple Mendelian recessive trait
• PKU inborn error of metabolism simple
  Mendelain recessive trait

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Huntingdons Chorea

• Degeneration of the nervous system which leads to
  premature death
• Determined by dominant allele
• Allele not expressed in phenotype until about 38
  years of age when sufferer will probably have had
  a family and passed on the allele

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Incomplete Dominance

• Occurs when the recessive allele has some effect
  on the heterozygote.
• An example of incomplete dominance is illustrated
  in the condition known as sickle cell anaemia.
• The haemoglobin produced is an unusual type
  called haemoglobin S which is an inefficient
  carrier of oxygen

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• HH x HH all offspring HH - survive
• HH x HS offspring HH, HH, HH, HS - survive
• HS x HS offspring HH, HS, HS, SS 75 survive
  (SS dies)

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Co-dominance

• Describes the situation where two alleles can be
  expressed in the heterozygote, neither
  suppressing the other.
• This is seen in the MN blood grouping.

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MN Blood Group

• M and N alleles are co-dominant (both alleles
  expressed in the phenotype of the heterozygote
• MM blood group have M antigens on rbc
• NN blood group have N antigens on rbc
• MN blood group have both M and N antigens on rbc

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ABO Blood Group

• Antigens coded by a gene that has three alleles
  A, B and O.
• Allele A produces antigen A Allele B produces
  antigen B Allele O produces no antigens
• Alleles A and B are co-dominant to one another
  and completely dominant over allele O

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The Facts You Need To Know

• p6
• from monohybrid inheritance depends on
• to if an individual has the genotype AO