Inherited Change

1. Inherited Change

2. Meiosis • Used in sexual reproduction to allow for variation. • Homologous chromosome – chromosomes that have the same genes but not the same alleles. One from mum and one from dad. • Bivalent – when the homologous chromosomes have replicated they join together by a chiasma and this forms a bivalent made of 4 chromatids. • Crossing over – the non-sister chromatids that lie next to each other in the bivalent may join temporarily and then break off swopping some of their genetic information.

5. Variation • Due to: • Crossing over of non-sister chromatids in the bivalent • Random assortment of bivalents and sister chromotids during metaphase I and II • Random fertilisation

6. Definitions • Gene – part of a chromosome that codes for a particular protein • Locus – position of a gene on a chromosome • Allele – different forms of a gene. Eg. The gene is eye colour but the allele is blue or brown

7. Only one allele can be expressed most of the time. Eg. Eye colour does not constantly change even if you inherit two different alleles from your parent. • This means that one is normally dominant over the other and is shown in the physical appearance of a person. • Recessive is the allele that does not show in the physical appearance but it may show in the next generation.

8. Codominant – when both alleles show in the physical appearance. Eg. Red flowers crossed with white flowers give pink flowers. • Homozygous – alleles are the same on both chromosomes. • Heterozygous – alleles are different on the homologous chromosomes • Phenotype – physical appearance • Genotype – genetic make-up

9. Monohybrid crosses • Genetic crosses that involve just one gene. • Eg. Eye colour (Make sure the outline of the cross is written out exactly as this gains the marks in an exam)

10. Let B represent brown eyes. (parent is homozygous) Let b represent blue eyes (parent is homozygous) Parent phenotypes? Parent genotypes? Gametes? F1 genotype? F1 phenotype? Ratio?

11. Dihybrid Crosses • Mendel • Looked at pea plants • Colour of seeds and round or wrinkled • Colour of flowers and tall or short • Dihybrid means showing two characteristics in one genetic cross.

12. Round seeds are dominant over wrinkled seeds • Yellow seeds are dominant over green seeds • Let the dominent allele for shape be? • Let the recessive allele for shape be? • Let the dominant allele for colour be? • Let the recessive allele for colour be?

13. Parent Phenotypes? • Parent Genotype (pure breeding)? • Gametes? (there are four Possible)

14. F1 genotype (see punnett square) • F1 phenotype

15. Try June 2006 paper 4

16. Monohybrid phenotype outcomes • 4 : 0 homozygous dominant crossed with homozygous recessive or homozygous dominant crossed with heterozygous • 3 : 1 heterozygous crossed with heterozygous • 1 : 1 homozygous recessive crossed with heterozygous • Codominance – heterozygous cross heterozygous 1 : 2 : 1

17. Codominance • If both alleles show in the phenotype in the heterozygous condition this is called codominance. • Eg. Red and white alleles make pink. • Still use one letter though. • RR is red, rr is white, Rr is pink • Main example is blood groups

18. Sex Linkage • When genes are located on the either the X chromosome or Y chromosome they are said to be sex linked. • Eg. Ability to see particular colours and blood clotting. • These are both found on the X chromosome. • Therefore a recessive allele will be more likely to show in a male than in a female. As there is no other X chromosome to mask it.

19. Haemophilia • When blood does not clot normally. • Sex-linked character caused by a recessive allele carried on the X chromosome. • If the male has the recessive allele then he has the disease. • The female only gets the disease if she inherits two recessive alleles. • If the female is heterozygous she is known as a carrier.

20. Let Xh represent haemophiliacLet XH represent normal clotting • Parent Phenotype • Parent Genotype • Gametes

21. Test cross • If we don’t know the genotype of an individual but we can find out by doing a test cross. • This is a cross with a homozygous recessive individual.

22. Let T represent tallLet t represent dwarf • Parent phenotype Tall Dwarf • Parent genotype TT tt • Gametes

23. Chi – squared Test • To see if there is a difference between numbers observed in an experiment and the numbers given in an hypothesis. • And if that difference is real or if it is due to chance or sampling error. • Chi-squared =

25. Null hypothesis – where we assume there is no difference between the expected and the observed. • Which means our hypothesis (genetic diagram) will be backed up by actual results if we did the experiment. • Eg. If we said there should be a 3:1 ratio then the actual result will be 9 white rabbits and 3 pink ones.

26. To calculate the chi-squared value • first determine the number expected in each category. • If the ratio is 3:1 and the total number of observed individuals is 880, then the expected numerical values should be 660 white and 220 pink. 

27. Say that the actual cross between two rabbits yields a population of 880 rabbitss, 639 with white fur and 241 with pink fur. • Now plug it into the equation:

28. The chi-squared value will be changed into a probability value using a chi-squared table. • The table contains degrees of freedom. • Degrees of freedom ≡ mathematical term relating to the number of free variables in the system. It is always (n – 1). • n is the number of categories or phenotypes in a given example. In our example 2 (white and pink) therefore degrees of freedom would be 2 – 1 = 1

30. Use the chi-square distribution table to determine significance of the value. a) Determine degrees of freedom and locate the value in the appropriate column. b) Locate the value closest to your calculated chi-squared value on that degrees of freedom row. • Move up the column to determine the p value.

31. Probability value means the probability that our observed will match up to our expected every time we do the practical. • State your conclusion in terms of your hypothesis. • If the p value is p > 0.05, accept your hypothesis. • 'The deviation is small enough that chance alone accounts for it. A p value of 0.6, for example, means that there is a 60% probability that any deviation from expected is due to chance only. This is within the range of acceptable deviation. • If the p value is p < 0.05, reject your hypothesis, • conclude that some factor other than chance is operating for the deviation to be so great. For example, a p value of 0.01 means that there is only a 1% chance that this deviation is due to chance alone. Therefore, other factors must be involved.

32. So bigger than 0.05 backs the null hypothesis. • Smaller than 0.05 means there is a significant difference between the hypothesis and the actual results so chuck your hypothesis out and start again. • Eg. Pg 233 and 235

33. Variation • Phenotype results from the interaction between the genotype and the environment. • The genotype determines the potential of an individual but the environment determines to what extent that potential is fulfilled. • Divided into • Discontinuous – usually genetic only with an either or outcome. Distinct groups. • Continuous – environmental effects come in to play. Groups merge in to one another.

34. Mutation and phenotype • Gene mutation is a change in the base sequence of DNA • Alters primary structure of proteins • Substitution - one base is swopped for another • Deletion – one base is removed • Insertion – one base is added • Duplication – one or more bases are repeated • Inversion – a sequence of bases is reversed

35. examples • Sickle-cell anaemia – base substitution • Normal haemoglobin is: • Val – His – Leu – Thr – Pro – Glu – Glu – Lys • Sickle-cell is: • Val – His - Leu – Thr – Pro – Val – Glu – Lys Valine is non-polar and hydrophobic In low oxygen concentrations haemoglobin becomes less soluble and crystallises Sticky fibres then distort the red blood cell shape.

36. Cystic fibrosis • Recessive mutation on chromosome 7 • Normal gene codes for a chloride channel protein called CFTR • In cystic fibrosis sufferers channel defective. • Result – • as chloride ions do not flow out • sodium ions rush in to balance charge and this prevents water leaving the cell • Mucus becomes thick and sticky. • Affects lungs, pancreas, liver • Mucus clogs ducts and passages

37. Down’s syndrome • Chromosome mutation on 21 • Sufferers have 47 chromosomes instead of 46 as there is an extra copy of 21. • Called trisomy • Symptoms • Mental retardation • Short stature • heart defects • Coarse hair

38. Enviromental effect on phenotype • Height • Weight • Dark tips on rabbit ears and tails. Areas exposed to cold temperatures cause a pigmentation to be formed in those areas.