Chapter 11: Monohybrid Cross Higher Human Biology Unit 1: Cell Function and Inheritance...

Chapter 11: Monohybrid Cross

Higher Human Biology

Unit 1: Cell Function and Inheritance

18/04/23 1Mrs 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


You need to know these words

18/04/23 Mrs Smith: Ch11 Monohybrid Cross.

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Things you need to knowMonohybrid 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.


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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


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.


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.


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


Punnet squares

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


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.


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

18/04/2317Mrs 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.


Agglutination of Red Blood Cells


• 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-)


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


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.


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


Incomplete dominance – Example: Sickle cell anaemia.


• 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.


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


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.


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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Chapter 11: Monohybrid Cross Higher Human Biology Unit 1: Cell Function and Inheritance...

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