1. How to do genetics crossings 1 SIMPLE MONOHYBRID CROSSES INVOLVING A SINGLE PAIR OF CONTRASTING ALLELES OF A SPECIFIC TRAIT DOMINANT AND RECESSIVE INCOMPLETE DOMINANCE CO-DOMINANCE

2. Introduction (dominance & recessive characteristics) Study the information given in the question. Look for the following aspects: 1. The trait with the contrasting alleles 2. Which allele is dominant and which one is recessive 3. The genotype of the two parents (homozygous or heterozygous) 4. The phenotype of the two parents 5. What is the extra question? (ratio, probability, numbers)

3. QUESTION Flies with heterozygous grey bodies were crossed with flies with black bodies. Grey bodies were dominant over black bodies. Use the letters G and g to represent a genetic cross to show the F1 genotypes and phenotypes. 1. Trait: 2. Contrasting alleles: 3. Dominant: 4. Recessive: 5. Genotype of Grey parent: 6. Genotype of black parent: Colour of bodies Grey and black Grey (G) black (g) heterozygous Not given Gg

4. When is a recessive characteristic expressed? Can always be deducted: There is only one combination of recessive alleles that will make the recessive characteristic to be visible/functional/expressed … the homozygous recessive alleles (two recessive alleles) In this case: gg 4

5. Using a template to do a crossing Have to provide the examiner the following:  Phenotype and Genotype of the two parents  meiosis and the gametes formed  Fertilisation and the punnett square  F 1 ratio of genotypes and phenotype 5

6. General Marking guidelines GametesGG gGg g gg 6 √

7. Memo for question P 1 Phenotype Grey body x Black body √ Genotype Gg x gg √ Meiosis Correct gametes √ F 1 Phenotype: 3 Grey : 1 black √ Genotypes: 3 Gg : 1 gg √ F 1 & P 1 : √ Meiosis and fertilisation: √ max 7 7 gametesGg gGg g gg

8. Some important notes   In answering the questions about a genetic crossing always use this format/template described in the previous slide.  Use a punnett square to do the crossings of the gametes – less confusing  Use the generations (P1 and F1) as well as the processes (meiosis and fertilisation) in the correct places – can obtain two marks even if your crossing is incorrect.  This format/template is applicable for all crossings – also for dihybrid crossings 8

9. 9 Answering an incomplete dominance crossing  No dominance or recessive alleles.  The homozygous offspring or two different parents have two contrasting characteristics.  The heterozygous offspring or parents have an characteristic which is in between (intermediate) to the two contrasting characteristics of the different homozygous organisms  Any information about an intermediate/in between value refers to incomplete dominance.  Use for both contrasting alleles capitals e.g. R = Red and W = white. RW = pink (intermediate colour).  Use the same template/format for the crossing.

10. Incomplete dominance question A red flower snapdragon is crossed with a white flower snapdragon. Use the symbols R for red flowers and W for white flowers. The heterozygous for this trait is a pink flower. Use a genetic crossing to illustrate the possible genotype and phenotype ratio of the F 1.

11. Memo for question P 1 Phenotype Red flower x White flower √ Genotype RR x WW √ Meiosis Correct gametes √ F 1 Phenotype: All pink flowers√ Genotypes: all heterozygous / RW√ F 1 & P 1 : √ Meiosis and fertilisation: √ max 7 11 gametesRR WRW W

12. 12 Answering an co-dominance crossing  Both the contrasting alleles for a trait are equally expressed.  The homozygous offspring or two different parents have two contrasting characteristics.  In the heterozygous offspring or parents both contrasting characteristics appear separately / are equally expressed.  For example: Inheritance of blood groups A and B  Genotype for blood groups A and B = I A, I B  Note: Blood group O (genotype = ii) is recessive to both blood group A and B.  Another example: cattle with red fur coat crosses white fur coat, produce calves that are roan.

13. Co-dominance question A man with a homozygous blood group A is crossed with a woman with a homozygous blood group B. Use a genetic crossing to illustrate the possible ratio of the phenotype and genotype of the offspring.

14. Memo for question P 1 Phenotype Blood group A x Blood group B √ Genotype I A I A x I B I B √ Meiosis Correct gametes √ F 1 Phenotype: All blood group AB√ Genotypes: all I A I B √ F 1 & P 1 : √ Meiosis and fertilisation: √ max 7 14 gametes IBIB IBIB IAIA IAIBIAIB IAIBIAIB IAIA IAIBIAIB IAIBIAIB

15. Co-dominance question A bull with a red fur coat (R ) is crossed with a cow with a white fur coat (W ) and produces calves that are roan. Use a genetic crossing to illustrate the possible ratio of the phenotype and genotype of the offspring.

16. Memo for question P 1 Phenotype Red fur coat x white fur coat √ Genotype RR x WW√ Meiosis Correct gametes √ F 1 Phenotype: All calves roan√ Genotypes: all RW √ F 1 & P 1 : √ Meiosis and fertilisation: √ max 7 16 gametes WW RRW R

17. Sex-linked crossing 17 Introduction – sex chromosomes Females have two X chromosomes If a woman has a rare recessive allele on the one X chromosome, she is likely to have a dominant allele on the second chromosome Males have a X and a Y chromosome The Y chromosome is much shorter and carries a different set of genes If a man has a rare recessive allele on his X chromosome, his phenotype will show it because there is no second X chromosome to carry a dominant allele that could hide it.

18. Sex-linked crossing question Two examples prescribed, namely, haemophilia and red-green colour blindness. Both are recessive sex-linked genes carried on the X chromosomes. Haemophilia An unaffected father is married to a heterozygous mother for haemophilia. Use a genetic cross to illustrate the possible genotype and phenotype ratio of the offspring. Use the symbols X H, X h

19. Memo for question P 1 Phenotype Unaffected male x heterozygous female √ Genotype X H Y x X H X h √ Meiosis Correct gametes √ F 1 Phenotype: 2 unaffected females, 1 unaffected male, 1 affected male√ Genotypes: 1 homozygous and 1 heterozygous female, 1 X H Y, 1 X h Y male, √ F 1 & P 1 : √ Meiosis and fertilisation: √ max 7 19 gametes XHXH XhXh XHXH XHXHXHXH XHXhXHXh YX H YX h Y

20. Some notes Males are affected or unaffected. Only one X chromosome Females can be a carrier. Have heterozygous alleles on the two X chromosomes. Females very seldom have the recessive genes on both X chromosomes. 20