Chapter 11: Monohybrid Cross



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Chapter 11: Monohybrid Cross

  • Higher Human Biology
  • Unit 1: Cell Function and Inheritance
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

You need to know these words

  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • ALSO REMEMBER: Dominant and co-dominant alleles should be represented by upper case letters and recessive alleles by lower case letters.

History

  • Gregor Mendel - The Father of Genetics
  • Monk who used science and maths to
  • establish patterns in how traits were inherited 2. 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
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Monohybrid cross.

  • A cross between two parents who possess different forms of a gene referred to as a MONOHYBRID INHERITANCE.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Mendel’s 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 --> they always displayed a single trait, the dominant one.
  • F2 (Second Filial): The offspring of the F1 generation, self fertilized --> always had a 3:1 ratio.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • Parent plant true breeding for round seeds
  • x
  • Parent plant true breeding for wrinkled
  • Self-pollination
  • Second filial generation (F2) –
  • 3 ROUND: 1 WRINKLED SEEDS
  • Cross-pollination

Phenotypes and genotypes

  • 08/07/17
  • Mrs Smith
  • 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
  • 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

Mendel’s Law of Segregation

  • States…The 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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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

  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Punnet squares

  • A punnet square is a representation of the law of segregation, showing how gametes separate and then come together during fertilization.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • ALSO REMEMBER: Dominant and co-dominant alleles should be represented by upper case letters and recessive alleles by lower case letters.

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 it’s 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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Task: Torrance pg 83 Qu’s 1-4

  • 08/07/17
  • Mrs Smith
  • CAN YOU ROLL YOUR TOUNGE?

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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • Genetics of tongue rolling
  • RR
  • Rr
  • Rr
  • rr
  • R
  • R
  • r
  • r

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
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Rhesus D Antigen Con’t

  • 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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Agglutination of Red Blood Cells

  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Presence of Antigen D is genetically dominant (D)

  • 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-)
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • Dd
  • Dd
  • Dd
  • Dd
  • D
  • d
  • D
  • d
  • Dd
  • dd
  • Dd
  • dd
  • D
  • d
  • d
  • d

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
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Huntingdon’s 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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • Example of a DOMINANT monohybrid inheritance in humans

Huntington’s 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% don’t survive (hh lives)
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Huntington’s Chorea – The genetics

  • H = allele for Huntington's, h = allele for normal condition
  • Most likely combination Hh (but doesn’t know yet: breeds with hh.......
  • Potentially tragic situation 1 in 2 inherit condition.
  • Hh x hh - offspring = Hh, Hh, hh, hh – 50% don’t survive (hh lives) – but no one will know till mid thirties.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Task: Torrance pg 85 Qu’s a-h

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
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

Incomplete dominance – Example: Sickle cell anaemia.

  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • 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.
  • Can see the cells have the typical sickle cell shape.

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, RBC’s fail to perform function well.
    • Causes shortage of oxygen, damage of internal organs and in many cases death.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • Picture shows blood containing only Haemoglobin wit the Sickle shape.

Heterozygous for the mutant allele: HS (H=normal S=sickle both uppercase because neither is dominant)

    • Do not suffer from Sickle Cell Anaemia,
    • Instead RBC’s contain both forms of Hb – giving a milder condition called SICKLE CELL TRAIT.
    • Causes slight anaemia, which does not prevent moderate activity.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.
  • Heterozygous for the mutant allele:
  • 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

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 RBC’s containing haemoglobin S.
  • People with the normal homozygous genotype HH are susceptible to malaria (and may die).
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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 RBC’s.
  • In addition to the ABO and Rhesus D-Antigen system, a further example is the MN blood group system.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

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, A&B, 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.
  • 08/07/17
  • Mrs Smith: Ch11 Monohybrid Cross.

TASK: Complete Torrance TYK questions on page 87

Essay Question Guide to H essays – pg 58

  • Discuss inheritance under the following headings
    • (a) Patterns of dominance (8)
    • (b) Multiple Alleles. (7)
  • 08/07/17
  • Mrs Smith

Essay Question – Guide to H essays – pg 58

  • Discuss monohybrid inheritance in humans. (15)
  • 08/07/17
  • Mrs Smith


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