Unlinked genes segregate independently as a result of meiosis

• Unlinked genes?are genes that an organism carries on?separate chromosomes
  • Not on homologous copies of the same chromosome
• An example of a pair of unlinked genes in fruit flies (Drosophila melanogaster)?is
  • The gene for curly wings on?chromosome 2, and
  • The gene for mahogany eyes on?chromosome 3
• An example of a pair of unlinked genes in humans?is
  • The gene for?trypsin (a stomach enzyme) on chromosome?7,?and
  • The gene for human growth hormone on chromosome?17
• Assortment?of chromosomes refers to their?alignment in metaphase I of meiosis
  • Each bivalent assorts (aligns) itself independently of all the others
• Segregation?of chromosomes (ie. how they get separated) is governed by their pattern of assortment
  • Segregation just refers to?which pole of the cell?the whole chromosomes are pulled to in anaphase I
  • Segregation determines?which combinations of alleles?end up in which gamete cells by the end of meiosis II
• By contrast, linked genes (on the same chromosome) tend to be?inherited together

The loci of selected genes in the human genome

Trypsin and CFTR are linked genes (both on the same chromosome);

Human Growth Hormone and trypsin are unlinked genes (both on different chromosomes)

• Monohybrid crosses look at how the alleles of?one?gene transfer across generations
• Dihybrid crosses look at how the alleles of?two?genestransfer across generations
  • ie. dihybrid crosses can be used to show the?inheritance?of?two completely different characteristics?in an individual
• The genetic diagrams for both types of cross are very similar
• For dihybrid crosses, there are several more genotypes and phenotypes involved
• When writing out the different genotypes, write the?two alleles for one gene, followed immediately by the?two alleles for the other gene.
• Do not mix up the alleles from the different genes
  • For example, if there was a gene with alleles?Y?and?yand another gene with alleles?G?and?gan example genotype for an individual would be?YyGg
• Alleles are usually shown side by side in dihybrid crosses e.g.?TtBb

Dihybrid Cross Punnett Square Table

• Predicted ratio of phenotypes in offspring =
  • 3?black coat, brown eyes :
  • 3?chestnut coat, brown eyes :
  • 1?black coat, blue eyes :
  • 1?chestnut coat, blue eyes
• Predicted ratio of genotypes in offspring =?3?BbEE?: 3?bbEE?: 1?Bbee?: 1?bbee

• Predicted ratio of phenotypes in offspring =
  • 9?black coat, brown eyes :
  • 3?chestnut coat, brown eyes :
  • 3?black coat, blue eyes :
  • 1?chestnut coat, blue eyes