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: I A I A, I A i = blood type A I B I B,I B i = blood type B I A I B = 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 I A, I B, i. I AB A and B are co-dominant

PhenotypeGroup A Group B X GenotypeIAIAIAIA IBIBIBIB Gametes IAIA IBIB IAIBIAIB F 1 genotype F 1 phenotypeGroup AB

PhenotypeGroup A X GenotypeIAiIAi Gametes IAIA IBIB F 1 genotype? F 1 phenotype? Group B IBiIBi i i IBIB i IAIA IAIBIAIB IAiIAi iIBiIBiii

How to test for blood types? All three genes (A, B and O) cause the production of glycoprotein (inside the membrane of RBC) I A adds acetyl-galactosamine to the glycoprotein. Only people with A blood have this protein, therefore if someone who does not have I A is exposed to the altered glycoprotein, they produce anti-A antibodies. I B adds galactose, Only people with B blood have this protein. therefore if someone who does not have I B is exposed to the altered glycoprotein, they produce anti-B antibodies. I A I B causes both proteins to be produced. As a results neither antibodies are produced when someone with I A I B 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 X b – recessive colour blindness X B – colour vision X h - haemophilia X H – blood clotting Construct a monohybrid cross for each Assume each genotype is heterozygous X B X B – phenotype, non-affected female X B X b – phenotype, carrier female X b X b – phenotype, affected female X B Y – phenotype, non-affect male X b Y – phenotype, affected male

X b 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 – X B X b Males cannot be carriers – X b Y 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 True breeding = homozygous for desired traits. R = round peas r = wrinkled peas Y = yellow peas y = green peas Complete the following table to show the genotypes: Parents phenotypesRound yellowGreen wrinkled Parents genotypesRRYYrryy Parent gametesRYry F 1 genotypesRrYy F 1 phenotypeRound yellow F 1 = first generation What would the offspring of the F 1 generation look like?

RYRyrYry RYRRYYRRYyRrYYRrYy RyRRYyRRyyRrYyRryy rYRrYYRrYyrrYYrrYy ryRrYyRryyrrYyrryy 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