Chapter 11: Monohybrid Cross Higher Human Biology Unit 1: Cell Function and Inheritance 03/12/20151Mrs 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 03/12/20152Mrs Smith: Ch11 Monohybrid Cross.

You need to know these words 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 3

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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 4 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 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 03/12/20155Mrs Smith: Ch11 Monohybrid Cross.

Monohybrid cross. A cross between two parents who possess different forms of a gene referred to as a MONOHYBRID INHERITANCE. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 6

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. 03/12/20157Mrs Smith: Ch11 Monohybrid Cross.

Pea plant cross Since wrinkled seeds were absent in the F 1 and reappears in the F 2, ‘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. 03/12/2015 Mrs Smith: Ch11 Monohybrid Cross. 8 Parent plant true breeding for round seeds x Parent plant true breeding for wrinkled First filial generation (F 1 )– ALL ROUND SEEDS Self-pollination Second filial generation (F 2 ) – 3 ROUND: 1 WRINKLED SEEDS Cross-pollination

Phenotypes and genotypes 03/12/2015Mrs Smith9 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. 03/12/201510Mrs Smith: Ch11 Monohybrid Cross.

Locus - spot on the chromosome where an allele (gene) is located. 03/12/201511Mrs 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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 12 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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 13

Task: Torrance pg 83 Qu’s 1-4

03/12/2015Mrs Smith15 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. 03/12/ Mrs Smith: Ch11 Monohybrid Cross. Genetics of tongue rolling RRRr 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 03/12/ 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. 03/12/201518Mrs Smith: Ch11 Monohybrid Cross.

Agglutination of Red Blood Cells 03/12/201519Mrs Smith: Ch11 Monohybrid Cross.

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-) 03/12/201520Mrs Smith: Ch11 Monohybrid Cross. Dd D d D d dd Dddd 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 03/12/201521Mrs 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. 03/12/201522Mrs 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) 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 23

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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 24

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 03/12/201526Mrs Smith: Ch11 Monohybrid Cross.

Incomplete dominance – Example: Sickle cell anaemia. 03/12/201527Mrs 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 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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 28 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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 29 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). 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 30

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. 03/12/201531Mrs 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 03/12/201532Mrs 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. 03/12/2015Mrs Smith: Ch11 Monohybrid Cross. 33

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. 03/12/201534Mrs 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) 03/12/2015Mrs Smith36

Essay Question – Guide to H essays – pg 58 Discuss monohybrid inheritance in humans. (15) 03/12/2015Mrs Smith37