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

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Things you need to knowMonohybrid inheritancei 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.

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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 1. 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 subjectsSome 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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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.

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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.

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Parent plant true breeding for round seeds x

Parent plant true breeding for

wrinkled

First filial generation (F1 )–

ALL ROUND SEEDS

Self-pollination

Second filial generation (F2) – 3 ROUND: 1 WRINKLED SEEDS

Cross-pollination

Phenotypes and genotypes

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• 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.

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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.

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12ALSO 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.

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Task: Torrance pg 83 Qu’s 1-4

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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.

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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

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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.

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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-)

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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

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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.

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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)

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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.

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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

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Incomplete dominance – Example: Sickle cell anaemia.

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• 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.

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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.

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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).

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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 RBC’s.

• 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 GroupAntigens 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.

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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)

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Essay Question – Guide to H essays – pg 58

• Discuss monohybrid inheritance in humans. (15)

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