Dihybrid Crosses & Unlinked Genes (HL) (HL IB Biology)

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 (i.e. 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
• We call this Mendel's Law of Independent Assortment which states that
  • alleles of different genes are inherited independently of one another; in other words inheriting a particular allele for one gene doesn't affect the ability to inherit any other allele for another gene
• By contrast, linked genes (on the same chromosome) tend to be inherited together

Linked and unlinked genes diagram

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 genes transfer across generations
  • i.e. dihybrid crosses can be used to show the inheritance of two completely different characteristics in an individual, for example unlinked genes
• 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 y and another gene with alleles G and g an 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

Dihybrid Cross Punnett Square Table 2

• 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

Dihybrid Cross Punnett Square Table 3

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

NOS: 9:3:3:1 and 1:1:1:1 ratios for dihybrid crosses are based on what has been called Mendel's second law

• Mendel's second law, also known as the law of independent assortment, states that a pair of traits segregates independently of another pair during meiosis when the sperm and egg cells are created during meiosis
  • As the different genes undergo independent assortment, different traits get the same opportunity to be expressed together 
• When Mendel was carrying out his experiments and coming up with his laws it was at a time when DNA and the chemical basis of inheritance was unknown
• Since then it has been discovered that the law is not universally true, but requires certain conditions in order to occur
  • For example, linked genes on the same chromosome do not follow the same observed ratios of phenotypes than would be expected if Mendel's second law was always correct
• Make sure that you remember that there are exceptions to all biological “laws” under certain conditions