Variation 2

1. 1 Variation 2 Sources of variation

2. Sources of heritable variation 2 There are two principal ways in which heritable variations can be acquired Independent assortment of genes Mutation

3. Independent assortment of genes 3 Independent assortment arises as a result of meiosis and fertilisation The separation of parental chromosomes at meiosis and their recombination at fertilisation introduces the possibility of new combinations of genes In the example which follows, the parental types have each inherited one set of chromosomes from the father (blue) and one set from the mother (red)

4. Example 1 b b B b C c c c b b b B C c c c 4 Genotype:Bbcc Genotype:bbCc Phenotype: brown eyes,straight hair Phenotype: blue eyes, curly hair Male sperm mother cell Female ovum mother cell X Meiosis; the homologous chromosomes are separated in the gametes

5. Recombinations 5 bC bc Bc bc Possible recombinations at fertilisation ova sperms Bb cc Bb Cc bb cc bb Cc Bbcc Brown eyes, straight hair – like father bbCc Blue eyes, curly hair – like mother BbCc Brown eyes, curly hair – new variation Bbcc Blue eyes, straight hair – new variation

6. Example 2 (cattle) 6 A black (B) uniformly coloured (U) bull is crossed with a red (b) spotted (u) cow X bbuu BBUU The alleles B and U are dominant so all the calves areuniformly black BbUu BbUu BbUu BbUu F1 offspring

7. 7 F2 offspring An F1 bull is mated with an F1 cow X BbUu BbUu Possible offspring Parental type New variety Parental type New variety

8. 8 The next slide offers a blank Punnett square which can be printed out and filled in to show the genotypes and expected number of phenotypes from a cross between the F1 black, uniform, cattle

9. Punnett square 9 BbUu x BbUu Enter female gametes here Enter male gametes here phenotypes Red uniform Black uniform Black spotted Red spotted genotypes

10. Breeding true 10 If you worked out the genotypes from the Punnett square, you will realise that the only cattle which will breed true are the ones homozygous for both characteristics BBUUBBuu bbUUbbuu Other crosses are likely to result in some offspring which do not resemble either parent For this reason, all possible variations will keep cropping up in a breeding population If, however, either of the homozygotes BBUU or bbuu were to be more successful* than the others, they might become established as a stable population

11. 11 Linkage B b C c If the genes for eye colour and hair curliness occurred on the same chromosome, they would be said to be linked At meiosis you would expect the linked genes to remain together in the gametes. In this case you would expect that most people with brown eyes would also have curly hair, and most people with blue eyes would have straight hair.

12. Crossing over 12 A B A B a b a b A B a b In fact, linked genes may be separated at meiosis by a process known as crossing over The genes AB and ab are linked but when the homologous chromosomes paired up at meiosis, breaks occurred in adjacent chromatids The chromatids rejoin but with their opposite partner

13. A b B a A B A b a B a b 13 The chromosomes separate, carrying the exchanged portions with them The linkage is broken When gametes are formed during meiosis, some will have Ab and aB and some will have AB and ab

14. 14 Mutations A mutation is a spontaneous change in a gene or chromosome Gene mutations may arise when a gene fails to make an exact copy of itself during replication prior to meiosis or mitosis Chromosome mutations can result from: Damage to, or loss of a chromosome Incomplete separation of chromosomes at meiosis leading to extra chromosomes in one gamete Part of a chromosome becoming attached to another chromosome (translocation) Doubling the whole set of chromosomes

15. 15 Translocation 2 3 Y X 4 Part of chromosome 3 is replicated and becomes attached to the Y chromosome Chromosomes Fruit fly (Drosophila)

16. 16 2 Y X 3 ru b 4 ru b

17. 17 Leading to mis-shapen eyes dark patterned thorax imperfect cross veins broad wings incurved hind legs

18. 18 Radish x cabbage radish and cabbage are cross pollinated X 18 chromosomes 18 chromosomes Seed pod of radish Seed pod of cabbage

19. 19 9 radish 9 cabbage chromosomes hybrid seed pod The hybrid is sterile (i.e. produces no seeds) because the chromosomes cannot pair up properly at meiosis

20. Polyploidy 20 18 radish 18 cabbage chromosomes If, however, the chromosome set doubles the hybrid is fertile and produces seeds because the homologous chromosomes can pair up at meiosis The occurrence of multiple sets of chromosomes is called polyploidy The hybrid is a tetraploid variety

21. Wheat a b c d e 21 Primitive wheat (a) crossed with wild grass (b) to produce an infertile hybrid. Chromosome doubling produces a fertile hybrid (c) which is crossed with wild grass (d) to produce an infertile hybrid Chromosome doubling results in fertile hybrid (e) This hybrid is a cultivated wheat used for flour production Many of our crop plants result from doubling of chromosome number

22. 22 Chromosome mutations in humans Chromosome mutations in humans usually result in spontaneous abortion of the foetus But a proportion survive e.g. Downs syndrome. The affected person has one extra chromosome in their genome (i.e. 47 instead of 46 chromosomes) This results in characteristic facial features, varying degrees of mental impairment and, usually, a very cheerful disposition Klinefelters syndrome. The affected male has an extra X chromosome (XXY) The person appears to be a normal male but he is infertile

23. Gene mutations 23 Gene mutations often arise as a result of faulty replication of DNA If a nucleotide is not copied accurately, the triplet which contains the fault will not code for the correct amino acid A protein with an incorrect amino acid will not function properly If the protein is an enzyme, this means that the enzyme will not work The cell chemistry will be disrupted if an essential enzyme fails to function normally This means that most gene mutations have a damaging effect on the cells and the whole organism

24. 24 Sickle cell anaemia The amino acid sequence in part of the haemoglobin molecule is Val- His- Leu- Thr- Pro- Glu-* The triplet code for Glutamic acid is either CTT or CTC+ Sometimes (rarely) in the course of replication, the middle thymine (T) is replaced by adenine (A) So the triplet becomes CAT or CAC But CAT or CAC code for Valine, not glutamic acid The amino acid sequence therefore becomes Val - His-Leu-Thr- Pro-Val- This leads to the production of faulty haemoglobin molecules which become distorted in low oxygen concentrations and cause sickle cell anaemia

25. Mutations 25 A mutated gene which is dominant, or inherited as a homozygous recessive, will affect the whole organism A mutated gene which does not result in abortion or early death of an organism will be inherited by the offspring Very rarely, a mutation will be beneficial and will be inherited by the offspring Gene mutations in humans may result in: Albinism Sickle cell anaemia Haemophilia Cystic fibrosis Dwarfism Colour blindness

26. 26 If a gene mutation takes place in a gamete which contributes to a zygote, the mutation will affect the whole organism A gene mutation in a body cell will affect only that cell and any cells derived from it (See next slide) It is mutations in body cells which give rise to cancers These mutations usually affect the genes which control cell division The control on cell division is reduced so that the cell divides repeatedly, giving rise to a tumour

27. 27 A mutation in a cell at an early stage in flower formation produced a parti-coloured blossom in this chrysanthemum ©Brookhaven National Laboratory

28. Mutation in bacteria 28 antibiotic bacterium reproducing these bacteria are killed by antibiotic mutation (resistance to antibiotic) the resistant mutants survive

29. 29 Mutation rate Mutations may occur as infrequently as once in 100,000 replications Nevertheless there are many replications involved in gamete production The human ejaculate of about 500 million sperms is bound to carry some sperms with mutations

30. Mutagens 30 Some environmental effects may increase the mutation rate These include radiation (X-rays, ultra-violet, radioactive materials) and carcinogenic chemicals such as those in tobacco smoke These are all known as mutagens Mutagens which affect body cells may produce cancers. Mutagens affecting the reproductive organs may result in defective offspring

31. 31 Question 1 Independent assortment of genes arises from the processes of … (a) meiosis and fertilisation (b) mitosis and cell division (c) replication of DNA (d) pairing of homologous chromosomes

32. 32 Question 2 From the genotype AaBb which of the following combinations could occur in the gametes ? (a)AB ab Ab aB (b) Aa Bb AB ab (c) Ab aB Bb Aa (d) Ab aB AB Ab

33. 33 Question 3 Genes are said to be linked if … (a) they occur together in a genome (b) they appear in the same gamete (c) they occur on the same chromosome (d) they are carried on homologous chromosomes

34. 34 Question 4 Crossing over results from … • translocation of portions of chromosomes (b) new combinations of genes in gametes (c ) exchange of portions of homologous chromatids (d) chromosome mutations

35. 35 Question 5 Chromosome mutations can occur as a result of… (a) gain of an extra chromosome (b) doubling of the entire set of chromosomes (c) faulty replication of DNA (d) breaking of a chromosome

36. 36 Question 6 Gene mutations can occur during DNA replication prior to … • mitosis (b) meiosis

37. 37 Question 7 Sickle cell anaemia results from … • a change in a nucleotide triplet (b) a change in a single nucleotide (c) a change in a single amino acid (d) a chromosome mutation

38. 38 Question 8 Which of these conditions result from a gene mutation ? (a) pneumonia (b) haemophilia (c) cystic fibrosis (d) tuberculosis

39. 39 Answer Correct

40. 40 Answer Incorrect