How to do genetics crossings

1. How to do genetics crossings • 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: • The trait with the contrasting alleles • Which allele is dominant and which one is recessive • The genotype of the two parents (homozygous or heterozygous) • The phenotype of the two parents • 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. Trait: Contrasting alleles: Dominant: Recessive: Genotype of Grey parent: Genotype of black parent: Colour of bodies Grey and black Grey (G) black (g) heterozygous Gg Not given

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

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 • F1 ratio of genotypes and phenotype

6. General Marking guidelines √

7. Memo for question P1 Phenotype Grey body x Black body √ Genotype Gg x gg √ Meiosis Correct gametes √ F1 Phenotype: 3 Grey : 1 black √ Genotypes: 3 Gg : 1 gg √ F1 &P1 :√ Meiosis and fertilisation: √ max 7

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

9. Answering an incomplete dominancecrossing • 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 F1.

11. Memo for question P1 Phenotype Red flower x White flower √ Genotype RR x WW √ Meiosis Correct gametes √ F1 Phenotype: All pink flowers√ Genotypes: all heterozygous / RW√ F1 &P1 :√ Meiosis and fertilisation: √ max 7

12. Answering an co-dominancecrossing • 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 = IA, IB • 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 P1 Phenotype Blood group A x Blood group B √ Genotype IAIA x IBIB√ Meiosis Correct gametes √ F1 Phenotype: All blood group AB√Genotypes: all IAIB √ F1 &P1 :√ Meiosis and fertilisation: √ max 7

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 P1 Phenotype Red fur coat x white fur coat √ Genotype RR x WW√ Meiosis Correct gametes √ F1 Phenotype: All calves roan√ Genotypes: all RW√ F1 &P1 :√ Meiosis and fertilisation: √ max 7

17. Sex-linked crossing • 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 XH, Xh

19. Memo for question P1 Phenotype Unaffected male x heterozygous female √ Genotype XH Y x XH Xh √ Meiosis Correct gametes √ F1 Phenotype: 2 unaffected females, 1 unaffected male, 1 affected male√ Genotypes: 1 homozygous and 1 heterozygous female, 1 XH Y, 1 Xh Y male, √ F1 &P1 :√ Meiosis and fertilisation: √ max 7

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.