SUMMARY OF GENETIC CROSSES

SUMMARY OF GENETIC CROSSES

1. Codominance

   • What does it mean?
     • Both alleles of the heterozygote are expressed in the phenotype
   • Annotations to use
     • C*C* *-Small letters which are different. Use big letters (same) and little letters (different)
   • Examples
     • Roan coat colour in cattle
     • Sickle Cell anemia

2. Linkage

   • What does it mean?
     • Looking at 2 phenotypes, controlled by 2 genes but both genes are on the same chromosome
   • Examples
     • could be any features
   • Annotations to use
     • Make sure alleles for the different genes stay next to eachother when you form gametes
     • RyrY into gametes: Ry and rY
     • bGBG or Adad
   • Remember!
     • 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.

3. Sex Linkage

   • What does it mean?
     • Genes that are found on sex chromosomes
   • Annotations to use
     • X*X* X*Y-
   • Examples
     • Haemophilia. Toroiseshell colour in cats. Colour blindness.
   • Remember!
     • Males can never be heterozygous as they only have one allele on he X chromosome and no allele on the Y chromosome

4. Dihybrid Inheritance

   • Remember!
     • 9:3:3:1
   • What does it mean?
     • 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.
   • Annotations to use
     • RrYy BbGG Aadd
     • When gametes are formed all combinations are possible eg Ry Ry rY ry BG bG
   • Examples
     • could be any features

5. Recessive Epistasis

   • What does it mean?
     • The homozygous recessive allele combination (epistatic alleles) will prevent the expression of a different allele at a separate location (the hypostatic alleles)
   • Annotations to use
     • Hypostatic alleles: Purple B Pink b Epistatic Alleles: A and a
       • aa means no colour will be expressed
       • Puple: AaBb AABb AABB AaBB Pink: AAbb Aabb White: aaBB aaBb aabb
       • Treat as dihybrid cross when doing cross
   • Examples
     • usually colour
   • Remember!
     • If colour is not expressed they will be white
       • How this might work
         • 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.

6. Dominant Epistasis

   • What does it mean?
     • The presence of a dominant epistatic allele will mask the presence of a hypostatic allele
   • Annotations to use
     • Hypostatic allele in squash: Yellow- E Green- e
     • Epistatic allele D or d
       • E and e can only be expressed if D is NOT present.
         • White squash: DDEE DDEe DDee DdEE DdEe Ddee
         • Yellow Squash: ddEe ddEE
         • Green Squash: ddee
     • Treat as dihybrid cross when doing a cross
   • Examples
     • usually colours
   • Remember!
     • If colour is not expressed they will be white
     • How this might work
       • 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.

7. Complementary Epistasis

   • What does it mean?
     • Not hypostatic vs epistatic
     • 2 alleles working together
     • 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.
   • Annotations to use
     • Purple: CcRr CCRR CCRr CcRR White: ccRr ccRR ccrr Ccrr CCrr
     • Treat as dihybrid cross when doing a cross
   • Exmaples
     • usually colours
   • Remember!
     • If colour is not expressed they will be white
     • How this might work
       • Precursor--> intermediate --> pigment
         • 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.