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Theoretical genetics

Theoretical genetics. 4.3. Terminology. Genotype – The alleles of an organism (gene combination for a trait e.g. RR, Rr, rr) Phenotype - the physical characteristics of an organism resulting from a genotype (e.g. red, white). Terminology.

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Theoretical genetics

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  1. Theoretical genetics 4.3

  2. Terminology • Genotype – The alleles of an organism (gene combination for a trait e.g. RR, Rr, rr) • Phenotype - the physical characteristics of an organism resulting from a genotype (e.g. red, white)

  3. Terminology • Alleles - two forms of a gene (dominant & recessive) • Dominant - stronger of two genes expressed in the hybrid; represented byacapital letter (R) • Recessive - gene that shows up less often in a cross; represented by alowercase letter (r)

  4. Terminology • Codominant allele – pairs of alleles that both affect the phenotype when present in the heterozygote(e.g. AB blood type) • Carrier – an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this allele.

  5. Genotypes • Homozygousgenotype - gene combination involving 2 dominant or 2 recessive alleles (e.g. RR or rr); also calledpure  • Heterozygousgenotype - gene combination of one dominant & one recessive allele    (e.g. Rr); also calledhybrid

  6. Terminology • Test cross – testing a suspected heterozygote by crossing it with a known homozygous recessive.

  7. Test Cross • How could you determine if a tall plant is heterozygous (Tt) or homozygous dominant (TT)? • A test cross! • Unknown genotype is crossed with a homozygous recessive plant

  8. THE WORK OF MENDEL • Gregor Mendel was an Austrian monk who did important early work on inheritance and genetics • He was an excellent mathematician and this helped him understand what others could not • Mendel worked with common garden peas for much of his early work

  9. MENDEL • Mendel’s work on the genetics of peas began with observation of the peas to determine what traits were inherited • He noticed at least 7 characteristics that appeared to be inherited • We will work with 3 of these - plant height, pea color and pea shape

  10. PLANT HEIGHT • Mendel noticed that garden peas occur in at least two heights - Tall and Short • Since peas are self-pollinating, tall peas tend to produce tall peas and short produce short • Mendel’s first genetic cross involved tall peas cross-pollinated with short peas

  11. PLANT HEIGHT • Mendel noticed that tall peas crossed with short peas yielded all tall peas in the F1 generation (first group of offspring) TALL X SHORT ALL TALL

  12. PLANT HEIGHT • Mendel now took the F1 peas and crossed them with themselves to produce an F2 generation (2nd group of offspring) • This produced tall and short offspring in a 3 tall to 1 short ratio F1 TALL X F1 TALL 3 TALL : 1 SHORT

  13. LAW OF SEGREGATION The Law of Segregation • at the time of gamete formation, members of a pair of alleles segregate (separate) from one another and move into different gametes • Thus parents like this Tt and Ttmake these gametes TtTt

  14. STEPS For WORKING A GENETICS PROBLEM • Assign symbols for alleles • Determine the parents’ GENOTYPES • Determine the kinds of GAMETES • Look at all possible combinations of gametes: PUNNETT SQUARE • Determine the possible offspring PHENOTYPES

  15. Doing the Cross • Using these laws we can see how Mendel got the 3 tall to 1 short ratio among the peas: TtXTtparents T t T t gametes TT Tt Tttt offspring

  16. Punnett Squares • Punnett Squares are an organized way to show these same results in chart form • It is easier to see the Genotypes(allele combinations) and Phenotypes (appearances) T t T t T T T T t t t t

  17. Test Cross • If unknown is homozygous dominant (TT), then all offspring from test cross will be Tall • TT X tt • Can you do the Punnett Square?

  18. Test Cross • If unknown is heterozygous (Tt), then all offspring from test cross then half will be Short and half will be Tall • Tt X tt • Can you do the Punnett Square?

  19. NON-MENDELIAN GENETICS • This type of genetics was not discovered by Mendel so it is Non-Mendelian • It is not any harder than Mendelian, but there are some twists to it that make it seem stranger than it is ....

  20. MULTIPLE ALLELES • Genes may have more than two alleles in this type of Non-Mendelian Genetics • Example is Human Blood Type • There are three alleles of ABO blood type gene I or A promotes blood type A I or B promotes blood type B i or O promotes blood type O A B

  21. What Genotypes are possible from these 3 alleles? • IAIA or IAi = Type A blood • IBIB or IBi = Type B blood • IAIB = Type AB blood • ii = Type O blood

  22. Hypothetically? • If John has type A blood .... and Ruby has type B blood... Could Farqhar (their love child) be type O? • What are the odds of this happening? • 1 in 4 !

  23. The Square IAi X IBi then : IB i IA i ii IA IB i IB IA i

  24. Boy or Girl? The Y Chromosome Decides X - Chromosome Y - Chromosome

  25. 4.3.6 • Some genes are present on the X chromosome and absent from the shorter Y chromosome in humans. • X chromosome has ~ 900 to 1,400 genes • Y chromosome has ~ 70 to 200 genes

  26. 4.3.7 Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are X and Y • XX genotype for females • XY genotype for males • Many sex-linked traits carried on X chromosome

  27. fruit fly eye color XX chromosome - female Xy chromosome - male Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes

  28. Xr Xr XR Y Sex-linked Trait Problem • Example: Eye color in fruit flies • (red-eyed male) x (white-eyed female)XRY x XrXr • RR = red eyed • Rr = red eyed • rr = white eyed • XY = male • XX = female

  29. Xr Xr XR Xr XR Xr XR Y Xr Y Xr Y Sex-linked Trait Solution: 50% red eyedfemale 50% white eyed male

  30. 4.3.8 • Some traits are associated with the X chromosome • Color-blindness • Hemophilia • Example: color blindness • More males tend to be color-blind than females. Why? • XB – normal • Xb – color blind • For a woman to be color-blind, she must inheritXbXb A man need only XbY • Example: hemophilia • XH – normal (XH Xh) and (XH Y) • Xh – hemophilia (Xh Xh) and (Xh Y)

  31. 4.3.9 • A human female can be homozygous or heterozygous with respect to sex-linked genes (because she has 2 X chromosomes).

  32. 4.3.10 • Female carriers for an X-linked trait are heterozygous for X-linked recessive alleles • Can males be carriers for X-linked traits?

  33. 4.3.12 Pedigree Analysis • Predictions about genotypes and phenotypes can be made based upon patterns of inheritance • Carrier- Individuals that have one recessive copy ‘hidden’ Filled in = shows the trait Empty = does not show the trait

  34. Pedigree

  35. 8 I II III IV 1 2 5 3 4 6 7 Pedigree

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