SUMMARY OF GENETIC CROSSES

Codominance
1. What does it mean?
  1. Both alleles of the heterozygote are expressed in the phenotype
2. Annotations to use
  1. C*C* *-Small letters which are different. Use big letters (same) and little letters (different)
3. Examples
  1. Roan coat colour in cattle
  2. Sickle Cell anemia
Linkage
1. What does it mean?
  1. Looking at 2 phenotypes, controlled by 2 genes but both genes are on the same chromosome
2. Examples
  1. could be any features
3. Annotations to use
  1. Make sure alleles for the different genes stay next to eachother when you form gametes
  2. RyrY into gametes: Ry and rY
  3. bGBG or Adad
4. Remember!
  1. crossing over during prophase 1 of meiosis could unlink the alleles of the genes. The further apart ehy are on the chromatid, the more likely it is that crossing over could happen.
Sex Linkage
1. What does it mean?
  1. Genes that are found on sex chromosomes
2. Annotations to use
  1. X*X* X*Y-
3. Examples
  1. Haemophilia. Toroiseshell colour in cats. Colour blindness.
5. Remember!
  1. Males can never be heterozygous as they only have one allele on he X chromosome and no allele on the Y chromosome
Dihybrid Inheritance
1. Remember!
  1. 9:3:3:1
2. What does it mean?
  1. Looking at 2 features controlled by 2 genes. The genes are on different chromosomes and will not stay together during random independent assortment in meiosis.
3. Annotations to use
  1. RrYy BbGG Aadd
  2. When gametes are formed all combinations are possible eg Ry Ry rY ry BG bG
4. Examples
  1. could be any features
Recessive Epistasis
1. What does it mean?
  1. The homozygous recessive allele combination (epistatic alleles) will prevent the expression of a different allele at a separate location (the hypostatic alleles)
2. Annotations to use
  1. Hypostatic alleles: Purple B Pink b Epistatic Alleles: A and a
    1. aa means no colour will be expressed
    2. Puple: AaBb AABb AABB AaBB Pink: AAbb Aabb White: aaBB aaBb aabb
    3. Treat as dihybrid cross when doing cross
3. Examples
  1. usually colour
4. Remember!
  1. If colour is not expressed they will be white
    1. How this might work
      1. Epistatic gene codes for a colourless pigment. Pigment is only made if A is present. aa= no pigment therefore no colour. B codes for an enzyme that converts the colourless pigment into a purple colour. b codes for an enzyme that converts the colourless pigment into a pink colour.
Dominant Epistasis
1. What does it mean?
  1. The presence of a dominant epistatic allele will mask the presence of a hypostatic allele
2. Annotations to use
  1. Hypostatic allele in squash: Yellow- E Green- e
  2. Epistatic allele D or d
    1. E and e can only be expressed if D is NOT present.
      1. White squash: DDEE DDEe DDee DdEE DdEe Ddee
      2. Yellow Squash: ddEe ddEE
      3. Green Squash: ddee
  3. Treat as dihybrid cross when doing a cross
3. Examples
  1. usually colours
4. Remember!
  1. If colour is not expressed they will be white
  2. How this might work
    1. Epistatic dominant allele codes for a repressor protein. This binds to the operator of the hypostatic allele and prevents transcription of gene. Therefore no colour. If there are 2 recessive epistatic alleles the repressor is not made. Therefore transcription can take place therefore colour is expressed.
Complementary Epistasis
1. What does it mean?
  1. Not hypostatic vs epistatic
  2. 2 alleles working together
  3. sweetpea colour. Purple and white. 2 gene loci are involved Cc and Rr. Both C and R need to be present for the purple colour to be expressed.
2. Annotations to use
  1. Purple: CcRr CCRR CCRr CcRR White: ccRr ccRR ccrr Ccrr CCrr
  2. Treat as dihybrid cross when doing a cross
3. Exmaples
  1. usually colours
4. Remember!
  1. If colour is not expressed they will be white
  2. How this might work
    1. Precursor--> intermediate --> pigment
      1. Allele C codes for a colourless intermediate pigment. Allele R codes for an enzyme that converts the colourlesSintermediate pigment to purple. If rr or cc allele combination is present the purple cant be expressed.

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