- Unit 1: Cell Function and Inheritance
- 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
- Mrs Smith: Ch11 Monohybrid Cross.
You need to know these words - 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.
- 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
- 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.
- 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.
- 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.
- Mrs Smith: Ch11 Monohybrid Cross.
- Parent plant true breeding for round seeds
- Parent plant true breeding for wrinkled
- Second filial generation (F2) –
- 3 ROUND: 1 WRINKLED SEEDS
Phenotypes and genotypes - 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.
- Mrs Smith: Ch11 Monohybrid Cross.
Locus - spot on the chromosome where an allele (gene) is located. - 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.
- 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.
- Mrs Smith: Ch11 Monohybrid Cross.
Task: Torrance pg 83 Qu’s 1-4 - 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.
- Mrs Smith: Ch11 Monohybrid Cross.
- Genetics of tongue rolling
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
- 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.
- Mrs Smith: Ch11 Monohybrid Cross.
Agglutination of Red Blood Cells - 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-)
- Mrs Smith: Ch11 Monohybrid Cross.
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
- 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.
- 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)
- 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.
- 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
- Mrs Smith: Ch11 Monohybrid Cross.
Incomplete dominance – Example: Sickle cell anaemia. - 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.
- 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.
- 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).
- 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.
- 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
- 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.
- 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.
- 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)
Essay Question – Guide to H essays – pg 58 - Discuss monohybrid inheritance in humans. (15)
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