6. Two descriptions of an organisms characteristics we need to know are: Genotype and Phenotype Genotype: The alleles an organism carries for a particular characterisitic. e.g RR Rr rr Phenotype: The outward appearance of a characteristic described in words. e.g Tongue roller Tongue roller Non-tongue roller Phenotype is dependant upon genotype !
7. Two more new words: Heterozygous and Homozygous Hetero : difference between two Homo : same as A homozygous organism has two identical alleles of a gene. e.g. RR or rr TT or tt A heterozygous organism has two different alleles of a gene. e.g. Rr Tt
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9. Animals that always pass on the same characteristics to their offspring from generation to generation are know as True-breeding. This means that the offsping of true breeding black mice are always black and the offspring of true breeding white mice are always white.
10. X P (parents) X F 1 (first generation) F 2 (second generation)
11. Lets look at this in terms of genotype: Black coat colour is dominant (B) White coat colour is recessive (b) All offspring (F 1 ) BB BB BB Parents x BB BB x Second generation (F 2 ) BB Gametes all B B Gametes all B B
12. X P (parents) X F 1 (first generation) F 2 (second generation)
13. Lets look at this in terms of genotype: Black coat colour is dominant (B) White coat colour is recessive (b) All offspring (F 1 ) bb bb bb Parents x bb bb x Second generation (F 2 ) bb Gametes all b b Gametes all b b
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15. Where did the white mouse come from? P bb x BB Bb x Bb Gametes: (F 2 ) Bb BB Bb Bb bb Punnett Square = Gives a ratio of 3 : 1 black white (F 1 ) All Bb BB Bb bb B b B b Gametes all b B B b B b
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21. How can we tell if an organism is true breeding? If a black coat in mice is dominant(B) it’s genotype could be: BB or Bb True breeding Not True breeding A test cross is used to identify the genotype of an individual by crossing it with a homozygous recessive individual…
22. All offspring: black Ratio= Black: white 1 : 1 BB or Bb? B B b b Bb Bb Bb Bb B b b b Bb bb Bb bb X BB bb X Bb bb
25. It is possible for two different pairs of alleles to be equal in dominance. Offspring from a cross between two true-breeding parents have a phenotype in between each parent. This is known as codominance. This is seen in animal coat colours and feathers, and in flowers.
26. Consider a horse’s coat colour: Black Coat BB White Coat WW Roan Coat (grey) BW In codominance neither allele is recessive , so neither symbol is a small letter. The black and white phenotypes are truebreeding homozygous. The Roan phenotype is heterozygous. With codominance what you see is what you get. There is no need for test crosses.
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28. Variations between individuals fall into two categories: 1) Discontinuous variation: -Characteristics fall into distinct categories -Controlled by alleles of a single gene. -Single gene inheritance Discontinuous or Continuous
29. Examples of discontinuous variation: Other species: -Smooth or wrinkled pea coat -White or red eyes in fruit flies -flower colour in sweet peas Ear lobes unattached attached Blood Groups O B A AB Tongue rolling Roller Non-roller
30. 1) Continuous variation: - Characteristics which do not show clear cut differences (show a range of differences). - Controlled by alleles of two or more genes. - Polygenic inheritance Number of petals on a daisy Heart rate Height
31. Discontinuous data Continuous data Distinct data can be presented as a bar graph. Ranges of data can be presented as a histogram or line graph. No. of pupils Tongue rolling ability Roller Non-roller Pea seed shoot lengths (mm) No. of seeds 1-5 6-10 11-15 16-20 21-25
32. Polygenic Inheritance (continuous) When a characteristic is controlled by two genes, there will be four alleles working together. e.g. Maize kernal colour: R 1 =Red R 2 =Red r 1 =White r 2 =white R 1 R 1 R 2 R 2 r 1 r 1 r 2 r 2 R 1 r 1 R 2 r 2 R 1 r 1 R 2 r 2 R 1 r 1 R 2 r 2 X
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