Chapter 11: Monohybrid Cross

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Chapter 11 Monohybrid Cross
Higher Human Biology
Unit 1 Cell Function and Inheritance
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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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You need to know these words
Recessive
Haploid
Gene
Co-dominant
Genotype
Dominant
Diploid
F1 Generation
F2 Generation
Allele
Phenotype
Incompletely dominant
Heterozygous
Homozygous
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Things you need to know

• Monohybrid inheritance
• i The pattern of inheritance of a pair of
• alleles where one is dominant and one is
  recessive.
• ii The effects of alleles exhibiting dominance,
  co-dominance and incomplete dominance.
• iii Possible combinations of multiple alleles.

ALSO REMEMBER Dominant and co-dominant alleles
should be represented by upper case letters and
recessive alleles by lower case letters.
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History

• Gregor Mendel - The Father of Genetics
• Monk who used science and maths to
• establish patterns in how traits were
  inherited2. Year 1857 carried out early
  monohybrid cross.3. He used the garden pea as
  his test subjects
• Some Vocabulary
• Character - a heritable feature (e.g. flower
  colour)
• Trait - a variant of each character (e.g. purple
  or white)
• Cross Pollination - one plant fertilizes a
  different plant
• Self Pollination - a plant fertilizes itself
• True-Breeding - plants that over several
  generations only produce plants like themselves

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Monohybrid cross.

• A cross between two parents who possess
  different forms of a gene referred to as a
  MONOHYBRID INHERITANCE.

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Mendels Experiments - Monohybrid Cross (pea
plant cross).

• Monohybrid 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 plants
• F1 (First Filial) The offspring of the P
  generation --gt they always displayed a single
  trait, the dominant one.
• F2 (Second Filial) The offspring of the F1
  generation, self fertilized --gt always had a 31
  ratio.

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Pea plant cross

• Since wrinkled seeds were absent in the F1 and
  reappears in the F2, something has to be
  transmitted undetected in the gametes from
  generation to generation. Today we call this a
  GENE. In this case it is a gene for seed shape,
  which has two alleles, round and wrinkled.
• Since the presence of round allele masks the
  presence of the wrinkled allele, round is said to
  be DOMINANT and wrinkled RESSESSIVE.

Parent plant true breeding for round seeds
x
Parent plant true breeding for wrinkled
Cross-pollination
First filial generation (F1 ) ALL ROUND SEEDS
Self-pollination
Second filial generation (F2) 3 ROUND 1
WRINKLED SEEDS
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Phenotypes and genotypes

• An organisms genotype is its genetic
  constitution (i.e. Alleles of genes) that is
  inherited from parents.
• These instructions are intimately involved with
  all aspects of the life of a cell or an organism

• An organisms phenotype is its appearance
  resulting from this inherited information
  (Genotype).
• This is anything that is part of the observable
  structure, function or behaviour of a living
  organism. e.g. Eye colour

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Mendels Law of Segregation

• StatesThe alleles of a gene exist in pairs but
  hen gametes are formed, the members if each pair
  pass into different gametes. Thus each gamete
  contains only one allele of each gene.
• For example a Tt parent can produce both T sperm,
  and t sperm.

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

• A punnet square is a representation of the law of
  segregation, showing how gametes separate and
  then come together during fertilization.

ALSO REMEMBER Dominant and co-dominant alleles
should be represented by upper case letters and
recessive alleles by lower case letters.
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Homozygous and Heterozygous

• When an individual possesses two similar alleles
  of a gene (e.g. R and R or r and r), its genotype
  is said to be HOMOZYGOUS (true-breeding) and all
  of its gametes are identical with respect to
  that characteristic.
• When an individual possesses two different
  alleles of a gene (e.g. R and r), its genotype is
  said to be HETEROZYGOUS. It produces two
  different types of gamete with respect to that
  characteristic.

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Task Torrance pg 83 Qus 1-4
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CAN YOU ROLL YOUR TOUNGE?
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Monohybrid Inheritance in Humans
Genetics of tongue rolling

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

R
r
RR Rr
Rr rr
R
r
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Monohybrid inheritance in humans Rhesus D Antigen

• In addition to the ABO system of antigens, most
  people have a further antigen on the surface of
  their red cells. This is called Antigen D.
• Most people are Rh (rhesus positive) as they
  posses this antigen
• A minority of people are Rh- (rhesus negative)
  they do not possess this antigen. But these
  people react to the presence of antigen D by
  forming anti-D antibodies

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Rhesus D Antigen Cont

• 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 (Rh) F1 Dd (Rh) and dd
  (Rh-)

D
D
D
d
d
Dd Dd
Dd Dd
Dd dd
Dd dd
d
d
d
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Examples RECESSIVE monohybrid inheritance in
humans

• 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
Example of a DOMINANT monohybrid inheritance in
humans

• 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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Huntingtons Chorea The genetics

• H allele for Huntington's, h allele for
  normal condition
• 5 combinations HH x HH, HH x Hh, Hh x Hh, HH x
  hh, hh x hh.
• HH x HH all offspring HH none survive
• HH x Hh offspring HH, HH, HH, Hh None survive
• Hh x Hh offspring HH, Hh, Hh, hh 75 dont
  survive (hh lives)

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Huntingtons Chorea The genetics

• H allele for Huntington's, h allele for
  normal condition
• Most likely combination Hh (but doesnt know yet
  breeds with hh.......
• Potentially tragic situation 1 in 2 inherit
  condition.
• Hh x hh - offspring Hh, Hh, hh, hh 50 dont
  survive (hh lives) but no one will know till
  mid thirties.

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Task Torrance pg 85 Qus a-h
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Incomplete Dominance

• Sometimes one allele is not completely dominant
  over the other,
• Occurs when the recessive allele has some effect
  on the heterozygote.
• Here the heterozygote exhibits a phenotype which
  is different from both of the hetrozygotes .
• e.g.
• Sickle Cell Anaemia
• Resistance to malaria

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Incomplete dominance Example Sickle cell
anaemia.
Can see the cells have the typical sickle cell
shape.

• An example of incomplete dominance is illustrated
  in the condition known as sickle cell anaemia.
• Here one of the genes which codes for haemoglobin
  (Hb) undergoes a mutation The Hb produced is an
  unusual type called Hb- which is an inefficient
  carrier of oxygen.

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Homozygous for the mutant allele SS

• Homozygous for the mutant allele SS

• Disastrous consequences, sufferers SICKLE CELLED
  ANAEMIA, they have the abnormally shaped sickle
  cell blood, RBCs fail to perform function well.
• Causes shortage of oxygen, damage of internal
  organs and in many cases death.

Picture shows blood containing only Haemoglobin
wit the Sickle shape.
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Heterozygous for the mutant allele HS (Hnormal
Ssickle both uppercase because neither is
dominant)

• Heterozygous for the mutant allele

• Do not suffer from Sickle Cell Anaemia,
• Instead RBCs contain both forms of Hb giving a
  milder condition called SICKLE CELL TRAIT.
• Causes slight anaemia, which does not prevent
  moderate activity.

Picture shows blood containing both forms of
Haemoglobin (although the mutant cells are not
completely sickle)
This in-between situation where the mutant
allele is partially expressed, neither allele is
completely dominant over the other
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Resistance to malaria (HS genotype)

• The S is rare in most populations.
• However, in some parts of Africa up to 40 of the
  population has the heterozygous genotype HS.
• This is because the parasite cannon make use of
  the RBCs containing haemoglobin S.
• People with the normal homozygous genotype HH
  are susceptible to malaria (and may die).

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Co-dominance

• Describes the situation where two alleles can be
  expressed in the heterozygote, neither
  suppressing the other, e.g. MN blood grouping.
• Blood groups are determined by the presence of
  antigens on the surface of RBCs.
• In addition to the ABO and Rhesus D-Antigen
  system, a further example is the MN blood group
  system.

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MN Blood Group

• Controlled by two alleles M and N which are
  co-dominant (both alleles expressed in the
  phenotype of the heterozygote).
• Heterozygous MN blood group have both M and N
  antigens on rbc
• Homozygous MM blood group have M antigens on rbc
• Homozygous NN blood group have N antigens on rbc

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Multiple Alleles

• Each of the genes considered so far has two
  alleles ( which display complete, incomplete or
  co-dominance).
• Some genes are found to possess 3 or more
  different alleles for a certain
  characteristic.... It has multiple alleles.
• If 3 alleles of a gene exist, and since a diploid
  individual has 1 or 2 of these alleles, then
  there are 6 genotype combinations possible.
• The phenotype depends on whether the alleles are
  complete, incomplete or co-dominant.

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ABO Blood Group

• Antigens coded by a gene that has three alleles
  A, B and O.
• 6 possible genotypes AA, AO, BB, BO, AB, OO
• 4 Phenotypes, A, B, AB, or Neither A or B...
• 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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TASK Complete Torrance TYK questions on page 87
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Essay Question Guide to H essays pg 58

• Discuss inheritance under the following headings
• (a) Patterns of dominance (8)
• (b) Multiple Alleles. (7)

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Essay Question Guide to H essays pg 58

• Discuss monohybrid inheritance in humans. (15)