heredity n.
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  1. Heredity Heredity

  2. Genes control the characteristics of living organisms Genes are carried on the chromosomes Chromosomes are in pairs, one from each parent Genes are in pairs Genes controlling the same characteristics occupy identical positions on corresponding chromosomes

  3. Dominance The gene pairs control one characteristic But they do not always control it in the same way Of the gene pair which help determine coat color in mice, one might try to produce black fur and its partner might try to produce brown fur The gene for black fur is dominant to the gene for brown fur

  4. Symbols The genes are represented by letters The gene for black fur is given the letter B The gene for brown fur is given the letter b BB bb The genes must have the same letter but the dominant gene is always in capitals

  5. Alleles The genes of a corresponding pair are called alleles This means alternative forms of the same gene B and b are alleles of the gene for coat color B is the dominant allele b is the recessive allele

  6. F1 A black male mouse (BB) is mated (crossed) with a female brown mouse (bb) In gamete production by meiosis, the alleles are separated Sperms will carry one copy of the Ballele Ova will carry one copy of the b allele When the sperm fertilizes the ovum, the alleles B and b come together in the zygote

  7. B B B B B b b b b b All offspring will be black (Bb) meiosis fertilization sperm mother cell ovum mother cell zygote meiosis

  8. The offspring from this cross are called the F1 (First Filial) generation They are all black because the allele for black coat coloris dominant to the allele for brown coat color These Bb mice are called heterozygotes. Because the B and b alleles have different effects; producing either black or brown coat colorThe mice are heterozygous for coat color The BB mice are called homozygotes because the two alleles produce the same effect. Both alleles produce black coats. The bb mice are also homozygous for coat color. Both alleles produce a brown coat color The next slide shows what happens when the two heterozygotes are mated and produce young

  9. B B b b B B B B B b b b b B b b Fertilization Possible combinations sperms BB sperm mother cell Bb meiosis ovum mother cell Bb bb ova zygotes

  10. Punnett square B b B b A neater way of working out the possible combinations is to use a Punnett Square* Draw a grid Enter the alleles in the gametes Enter the possible combinations female gametes BB Bb male gametes These are the F2 generation Bb bb

  11. 3:1 ratio The offspring are in the ratio of 3 black to 1 brown Although the BBand Bb mice look identical, the Bb mice will not breed true. When mated together there is a chancethat 1 in 4 of their offspring will be brown This is only a chance because sperms and ova meet at random A litter of 5, may contain no brown mice; in a litter of 12, you might expect 3 brown mice but you would not be surprised at anything between 2 and 5. The total offspring from successive matings of the heterozygotes would be expected to produce in something close to the 3:1 ratio For example, 6 successive litters might produce 35 black and 13 brown mice. This is a ratio of 2.7:1, near enough to 3:1

  12. Some terminology The offspring of the heterozygotes are the F2 generation The genetic constitution of an organism is called its genotype The visible or physiological characteristics of an organism are called its phenotype The phenotype of this mouse is black. Its genotype is BB BB The phenotype of this mouse is also black, but its genotype is Bb Bb The phenotype of this mouse is brown. Its genotype is bb bb

  13. These tobacco seedlings are the F2 generation from a cross Between heterozygous (Cc) parents. C is the gene for chlorophyll. cc plants can make no chlorophyll. There are 75 green seedlings present. What is the ratio of green to white seedlings? What ratio would you expect?

  14. Cc CC Cc There are 21 white seedlings. This is a ratio of 75:21 or 3.57:1 C c You would expect the cross to produce 72 green to 24 white seedlings (3:1) C c cc 1 CC 2 Ccand 1 cc, a ratio of 3 green to 1 white seedling Is 3.57:1 near enough to 3:1 ?*

  15. Sex chromosomes In most populations of animals there are approximately equal numbers of males and females This is the result of a pair of chromosomes; the sex chromosomes called theX and Y chromosomes The X and Y chromosomes are a homologous pair but in many animals the Y chromosome is smaller than the X Females have two X chromosomes in their cells. Males have one X and one Y in their cells At meiosis, the sex chromosomes are separated so the the gametes receive only one: either an X or a Y.

  16. Sex ratio X X X X Y Y Y X X X X X X Y X X fertilization meiosis female male sperm mother cell female male ovum mother cell

  17. Single gene effects Very few human characteristics are controlled by a single gene Characteristics such as height or skin colorare controlled by several genes acting together Those characteristics which are controlled by a single gene are usually responsible for inherited defects (see slide 19)

  18. ABO blood groups An exception is the inheritance of the ABO blood group The IA allele produces group A The IB allele produces group B The IOallele produces group O IOis recessive to IA and IB The group A phenotype can result from genotypes IAIA or IAIO The group B phenotype can result from genotypes IBIB or IBIO The group O phenotype can result only from genotype IOIO The AB phenotype results from the genotype IAIB The alleles IA and IB are equally dominant (co-dominant)

  19. Genetic defects Cystic fibrosis (recessive) Glands of the alimentary canal produce a thick mucus which affects breathing, digestion and susceptibility to chest infection Achondroplastic dwarfism (dominant)The head and trunk grow normally but the limbs remain short Albinism (recessive) Albinos cannot produce pigment in their skin, hair or iris Polydactyly (dominant*) an extra digit may be produced on the hands or feet Sickle cell anaemia (recessive)The red blood cells become distorted if the oxygen concentration falls. They tend to block small blood vessels in the joints

  20. Genetic counselling D d d Dd dd d Dd dd (Genetic defects) If the genotypes of the parents are known, it is possible to calculate the probability of their having an affected child (i.e. one with the defect) For example if a male achondroplastic dwarf marries a normal woman, what are their chances of having an affected child? The father’s genotype must be Dd. (DD is not viable) The mother must be dd since she is not a dwarf There is a 50% probability of their having an affected child What are the probabilities if both parents are affected?

  21. 21 Cystic fibrosis N n N n (recessive) If two normal parents have an affected child, they must both be heterozygous (Nn) for the recessive allele n A nn parent would have cystic fibrosis A NN parent would produce only normal children NN Nn Nn nn Since the parents are now known to be heterozygous it can be predicted that their next child has a I in 4 chance of inheriting the disease This chance applies to all subsequent children*

  22. Sickle cell anemia (recessive) Hb = haemoglobin HbA is the allele for normal hemoglobin HbS is the allele for sickle cell hemoglobin A person with the genotype HbSHbS will suffer from sickle cell anemia A person with the genotype HbAHbA is normal The genotype HbAHbSproduces sickle cell ‘trait’ because HbA is incompletely dominant to HbS The heterozygote HbAHbS has few symptoms but is a ‘carrier’ for the disease

  23. Carriers Heterozygous recessive individuals do not usually exhibit any disease symptoms but because their offspring may inherit the disease, the heterozygotes are called ‘carriers’ carriers HbA HbS HbA HbAHbA HbAHbS HbAHbS HbSHbS HbS Similarly, individuals with the genotype Nn are carriers for cystic fibrosis

  24. Family trees = normal female = affected female = normal male = affected male It is sometimes possible to work out the genotypes of parents and to track the inheritance of an allele by studying family trees Parents have normal phenotypes but produce an affected child For this to happen, both parents must have heterozygous genotypes (Nn) for the characteristic

  25. AA Aa aa Aa Aa If one of the parents is homozygous for a dominant allele, all the children will be affected If one parent is heterozygous for a dominant allele and the other is homozygous recessive, there is a chance that half their children will be affected If both parents are heterozygous for a recessive allele, there is a chance that one in four of their children will be affected

  26. grandparents marriage marriage parents children cystic fibrosis What can you deduce about the genotypes of the grandparents from this family tree?

  27. Cystic fibrosis is caused by a recessive gene An affected person must therefore have the genotype nn Since neither of the grandparents is affected, they must be either NN or Nn genotypes If they were both NN, none of their children or grandchildren could be affected If one was Nn and the other NN, then there is a chance that 50% of their children could be carriers Nn If one of the carriers marries another carrier, there is a 1 in 4 chance of their having an affected child The genotypes of the grand parents must be either both Nn or one NN and the other Nn

  28. d D D DD Dd d Dd dd If both parents have the Dd genotype there is a 75% chance of their having affected children, but the DD individual is unlikely to survive

  29. Question 1 Which of the following are heterozygous genotypes? (a) Aa (b) bb (c) nn (d) Bb

  30. Question 2 A B C A b c Which of these genes are alleles? chromosomes (a) A and A (b) A and B (c) B and C (d) B and b

  31. Question 3 Which of the following processes separates homologous chromosomes ? (a) mitosis (b) cell division (c) meiosis (d) fertilization

  32. Question 4 Which of the following terms correctly describes the genotype bb ? (a) homozygous dominant (b) heterozygous dominant (c) homozygous recessive (d) heterozygous recessive

  33. Question 5 What is the likely ratio of affected children born to parents both of whom are heterozygous for cystic fibrosis ? • 1 affected: 3 normal (b) 3 affected: 1 normal (c) 2 affected: 2 normal (d) all affected

  34. Question 6 Which of the following phenotypes corresponds to the Genotype IAIO ? • Blood group A (b) Blood group B (c) Blood group O (d) Blood group AB

  35. Question 7 What is the expected ratio of offspring from a black rabbit Bb and a white rabbit bb ? (a) 3 black: 1 white (b) 1 black: 3 white (c) 50% white; 50% black (d) all black

  36. Question 8 a a a a A A A A A A A a a a a Which of these Punnett squares correctly represents a cross between two heterozygous individuals ? (a) (b) aa AA AA Aa aa AA Aa aa (c) (d) a Aa AA Aa Aa Aa Aa aa aa

  37. Question 9 A married couple has a family of 6 boys. What are the chances that the next child will be a girl ? (a) 6:1 (b) 1:6 (c) 3:1 (d) 1:1

  38. Question 10 Which of the following is a ‘carrier’ genotype for a disease caused by a recessive gene ? (a) nn (b) NN (c) Nn

  39. Question 11 If normal parents have a child with cystic fibrosis (a) one of them must be heterozygous (b) both of them must be heterozygous • one of them must be homozygous (d) both of them must be homozygous

  40. Answer Correct

  41. Answer Incorrect