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Genetics

Gregor Mendel. Genetics. GHB 2004. Gene:. The unit of inheritance. A. B. C. Genes. Chromosome:. Chromosome. A bundle of DNA containing many genes. B. b. Alleles:. Different versions of the same gene. Red flowers. Brown eyes. White flowers. Blue eyes. Phenotype:.

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Genetics

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  1. Gregor Mendel Genetics GHB 2004

  2. Gene: The unit of inheritance

  3. A B C Genes Chromosome: Chromosome A bundle of DNA containing many genes

  4. B b Alleles: Different versions of the same gene

  5. Red flowers Brown eyes White flowers Blue eyes Phenotype: Physical appearance

  6. B B B b b b Genotype: The combination of alleles which you carry for a particular gene Genotype: BB Genotype: Bb Genotype: bb

  7. B b Dominant allele: An allele which is always expressed when it is present… … it cannot be masked by another allele.

  8. b b Recessive allele: An allele which can be masked by another allele… … it is only expressed when two copies are present.

  9. If an individual has two different alleles of a gene on a pair of chromosomes the individual is… If an individual has two copies of same allele of a gene on a pair of chromosomes the individual is… B B B b b b …heterozygous …homozygous The prefix homo- means… The prefix hetero- means… ‘same’ ‘other’

  10. Normal body cells are diploid A cell which contains two copies of each chromosome is… diploid

  11. Sperm Plant egg cell Pollen Ovum A cell which contains one copy of each chromosome is… haploid Gametes (sex cells) are haploid

  12. Diploid parent cell The chromosomes in the nucleus are copied Four haploid daughter cells are produced Sperm cells are produced by … meiosis Each haploid daughter cell contains only one of the two alleles from the diploid parent cell

  13. Diploid parent cell Haploid Ova are also produced by meiosis

  14. Fertilisation: An ova and a sperm fuse to form a diploid zygote Ova Zygote Sperm The zygote contains one allele from each parent This process is random

  15. b b Blue Inheritance of eye colour Genotype is: bb Phenotype is:

  16. B B Brown Genotype is: BB Phenotype is:

  17. B b Brown Genotype is: Bb Phenotype is:

  18. B b Brown Genotype is: Bb Phenotype is:

  19. … pollen … the egg cell The same rules apply to inheritance in plants as apply to inheritance in animals. The male gamete or sex cell is … The female gamete is…

  20. Example: The dominant allele R produces … … a red flower R r The recessive allele r produces … … a white flower Plant genes can have dominant and recessive alleles

  21. RR R R rr r r A genetic cross in plants: Rr x Rr There is only one way to make a homozygous plant with two dominant genes… … and only one way to make a homozygous plant with two recessive genes

  22. R R Rr Rr r r In the genetic cross Rr x Rr there are two ways of making a heterozygous plant… OR … so, heterozygous offspring are twice as likely as either homozygous dominant or homozygous recessive offspring

  23. We can show the likeliness or probability of different genotypes in different ways: 1. As a percentage - Probability of RR is… 25% Probability of Rr is… 50% Probability of rr is… 25% ¼ 2. As a fraction - Probability of RR is… ½ Probability of Rr is… Probability of rr is… ¼ 3. As a ratio - RR : Rr : rr = 1 : 2 : 1

  24. RR R R Rr R r Rr r R There are three ways to make a plant with red flowers… … one produces a homozygous plant… … and two produce heterozygous plants…

  25. r r rr …but, there is only one way to produce a plant with white flowers… So, in the genetic cross, Rr x Rr… …red flowers are three times more likely than white flowers!!!

  26. We can show the likeliness or probability of different phenotypes in different ways: 1. As a percentage - Probability of red is… 75% Probability of white is… 25% ¾ 2. As a fraction - Probability of red is… ¼ Probability of white is… 3. As a ratio - red : white = 3 : 1

  27. The Punnett Square We can use the Punnett Square to work out the probability of the different genotypes and phenotypes in a genetic cross x Egg cells pollen = R + r = R + r Step 1: Write down the genotypes of the parents. = Rr x Rr Step 2: Write down the genotypes of the gametes that each parent produces.

  28. R r Egg cells R r Pollen …this shows you the possible genotypes of the offspring Genotypes of offspring Step 3: Draw your Punnett Square Step 4: Write the genotype of the different gametes into your Punnett Square R R R r Step 5: Write in the different ways in which the gametes can be combined… r R r r

  29. R r Egg cells R R R R r r r R r r Pollen = ½ 2 4 Probability of Genotypes There are four possible ways to combine egg cells and pollen Count up how many times each genotype appears in the Punnet Square – this gives you its probability: ¼ Probability of RR is… = 25 % Probability of Rr is… = 50 % The ratio of RR : Rr : rr is… Probability of rr is… ¼ = 25 % 1 : 2 : 1

  30. R r Egg cells R R R R r r r R r r Pollen Probability of Phenotypes Look at the genotypes of the offspring in your Punett Square… …decide which phenotype each one should produce. Count up how many times each phenotype appears in the Punnet Square – this gives you its probability: ¾ Probability of red is… = 75 % The ratio of red : whiteis… Probability of white is… ¼ = 25 % 3 : 1

  31. Test Cross • Which is the genotype of an organism showing the dominant characteristic in its phenotype?

  32. Inheritance of blood groups

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