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Punnet squares Steps to remember… 1.Choose a letter 2.Identify parents genotypes 3.Identify gametes parents could produce 4.Draw punnet square 5.Calculate chances These are all mono-hybrid crosses. They show one trait. A = dominant allele for albinism a = recessive allele for fewer pigments/albinism What is the probability of the offspring developing albinism? The six genotypes are: IAIA, IAi = blood type A IBIB,IBi = blood type B IAIB = blood type AB ii = blood type O Multiple alleles With two alleles, three different genotypes are possible producing two different phenotypes. This is the simplified version. In blood types, there are three alleles for the same gene – ABO There are four possible Human blood types – A, B, AB, O. These can produce six genotypes The gene for blood type is I (or Ii) The four blood types are written IA, IB, i. IAB A and B are co-dominant Phenotype Group A Genotype I AI A Gametes X Group B IBIB IA IB F1 genotype IA IB F1 phenotype Group AB Phenotype Group A Genotype I Ai IA Gametes F1 genotype? F1 phenotype? Group B X IBi i IB IB i IA I AI B I Ai i IBi ii i How to test for blood types? All three genes (A, B and O) cause the production of glycoprotein (inside the membrane of RBC) IA adds acetyl-galactosamine to the glycoprotein. Only people with A blood have this protein, therefore if someone who does not have IA is exposed to the altered glycoprotein, they produce anti-A antibodies. IB adds galactose, Only people with B blood have this protein. therefore if someone who does not have IB is exposed to the altered glycoprotein, they produce anti-B antibodies. IAIB causes both proteins to be produced. As a results neither antibodies are produced when someone with IAIB blood is exposed to either protein (their bodies recognise the proteins). i is recessive, basic glycoprotein is produced. Sex chromosomes XX or XY X is longer than Y. All female eggs contain one X chromosome. In sperm, 50% contain X, 50% contain Y. Alleles which have their loci on the X or Y chromosomes are sex linked. E.g. colour blindness & haemophilia What are the main effects of these two conditions? Alleles & genotypes Xb – recessive colour blindness XB – colour vision Xh - haemophilia XH – blood clotting Construct a monohybrid cross for each Assume each genotype is heterozygous XBXB – phenotype, non-affected female XBXb – phenotype, carrier female XbXb – phenotype, affected female XBY – phenotype, non-affect male XbY – phenotype, affected male Xb alleles are rare in human population, the likely hood of having two recessive alleles is very small. This is why colour blindness mainly affects males. Females can carry the allele – XBXb Males cannot be carriers – XbY This is the same for haemophillia – it affects the X chromosome Queen Victoria’s Pedigree Diagram Analysis of pedigree charts Dihybrid crosses & gene linkage Mendel (yes again!) experimented with these traits in his pea experiments: - Seed shape - Seed colour F1 = first generation True breeding = homozygous for desired traits. R = round peas r = wrinkled peas Y = yellow peas y = green peas What would the offspring of the F1 generation look like? Complete the following table to show the genotypes: Parents phenotypes Round yellow Green wrinkled Parents genotypes RRYY rryy Parent gametes RY ry F1 genotypes RrYy F1 phenotype Round yellow RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy Round yellow peas x9 Round green peas x3 Wrinkled yellow peas x3 Wrinkled green peas x1 Mendels conclusions from his pea experiments were very close to these theoretical probabilities. This is unusual. Statisticians are convinced that Mendel’s results are too close to exact ratios to be genuine. We shall never know how this came about, but it offers an opportunity to discuss the need for scientists to be truthful about their results, whether it is right to discard results that do not fit a theory as Louis Pasteur is known to have done, and the danger of publishing results only when they show statistically significant differences. TOK: Reasons for Mendel’s theories not being accepted by the scientific community for a long time could be considered. Other cases of paradigm shifts taking a long time to be accepted could be considered. Ways in which individual scientists are most likely to be able to convince the scientific community could be considered, and also the need always to consider the evidence rather than the views of individual scientists, however distinguished. All chromosome which are NOT sex chromosomes are called Autosomes Humans have 22 pairs of autosomes and one pair of sex chromosomes Some genes can be linked (found on the same chromosome). Groups of inherited genes are called linkage groups. Applies to autosomes & sex chromosomes. Linked genes are the exception to the law of independent assortment. Linked groups can give a much wider variety of ratios - not just 9:3:3:1