Chapter 11: Monohybrid Cross

1. Higher Human Biology Unit 1: Cell Function and Inheritance Chapter 11: Monohybrid Cross Mrs Smith: Ch11 Monohybrid Cross.

2. 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.

3. You need to know these words Recessive Haploid Gene Co-dominant Genotype Dominant Diploid F1 Generation F2 Generation Allele Phenotype Incompletely dominant Heterozygous Homozygous Mrs Smith: Ch11 Monohybrid Cross.

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

5. History Gregor Mendel - The Father of Genetics • 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 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.

6. 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.

7. 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.

8. Pea plant cross Parent plant true breeding for round seeds x Parent plant true breeding for wrinkled • 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. Cross-pollination First filial generation (F1 )– ALL ROUND SEEDS Self-pollination Second filial generation (F2) – 3 ROUND: 1 WRINKLED SEEDS Mrs Smith: Ch11 Monohybrid Cross.

9. Phenotypes and genotypes • 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 • 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 Mrs Smith

10. 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.

11. Locus - spot on the chromosome where an allele (gene) is located. Mrs Smith: Ch11 Monohybrid Cross.

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

13. 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.

14. Task: Torrance pg 83 Qu’s 1-4

15. CAN YOU ROLL YOUR TOUNGE? Mrs Smith

16. Monohybrid Inheritance in Humans Genetics of tongue rolling • Tongue rolling is inherited as a simple Mendelian trait. • R is the allele for roller • r is the allele for non-roller. R r R r Mrs Smith: Ch11 Monohybrid Cross.

17. 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.

18. 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.

19. Agglutination of Red Blood Cells Mrs Smith: Ch11 Monohybrid Cross.

20. 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-) D D D d d d d d Mrs Smith: Ch11 Monohybrid Cross.

21. 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.

22. Huntingdon’s Chorea Example of a DOMINANT monohybrid inheritance in humans • 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.

23. 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.

24. 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.

25. Task: Torrance pg 85 Qu’s a-h

26. 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.

27. Incomplete dominance – Example: Sickle cell anaemia. Can see the cells have the typical sickle cell shape. • 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. Mrs Smith: Ch11 Monohybrid Cross.

28. 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. Picture shows blood containing only Haemoglobin wit the Sickle shape. Mrs Smith: Ch11 Monohybrid Cross.

29. Heterozygous for the mutant allele: HS (H=normal S=sickle both uppercase because neither is dominant) Heterozygous for the mutant allele: • 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. 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 Mrs Smith: Ch11 Monohybrid Cross.

30. 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.

31. 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.

32. 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.

33. 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.

34. 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.

35. TASK: Complete Torrance TYK questions on page 87

36. Essay Question Guide to H essays – pg 58 • Discuss inheritance under the following headings • (a) Patterns of dominance (8) • (b) Multiple Alleles. (7) Mrs Smith

37. Essay Question – Guide to H essays – pg 58 • Discuss monohybrid inheritance in humans. (15) Mrs Smith