the factors which determine an organisms phenotype

the factors which determine an organisms phenotype

1. sex-linkage

   • when the certain allele is located on a sex chromosome
   • females have two X chromosomes and males have one X and one Y chromosome
     • the Y chromosome is smaller so fewer genes are carried on it, so most genes are carried on the X chromosome
     • if a gene is carried on a Y chromosome then a female will not inherit the gene, therefore it will not be expressed in her phenotype
   • males only have one X chromosome so they often only have one allele for sex-linkage, so they often express the characteristic in thei phenotype even if it is recessive so males are more likely than females to show recessive phentoypes for genes that are sex-linked e.g. colour blindness

2. dominant, recessive and codominant alleles

   • dominant alleles
     • an allele whose characteristic appears in the phenotype even when there is only one copy
   • recessive alleles
     • an allele whose characteristic only appears in the phenotype if tow copies are present
   • codominant alleles
     • alleles that are both expressed in the phenotype

3. epistasis

   • many different genes can control the same characteristics - they interact to form the phenotype
     • if genes are epistatic they can mask or be masked.

4. mutations

   • mutagenic agents
     • increase the rate of mutation
     • mutations occur randomly
     • Ultraviolet radiation, ionising radiation, chemicals and viruses
     • some chemicals called base analogs can substitute for a base during replication, changing the base sequence in the new DNA
     • some chemicals can delete or alter bases, changing the structure
     • some types of rediation can change the structure of DNA, which causes problems during DNA replication
   • substitution - base/s swapped for another
   • deletion - base/s removed
   • addition - base/s added
   • duplication - base/s repeated
   • inversion - a sequence of bases reversed
   • translocation - a sequence of bases moved from one location to another on the genome

5. monohybrid or dyhibrid genes

   • monohybrid inheritance
     • the inheritance of a characteristic controlled by a single gene
   • dihybrid inheritance
     • the genes that determine the two characteristics are located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.

6. autosomal linkage

   • genes on the same autosome are linked
     • they're on the same autosome so will stay together during independant segregation of chromosomes in meiosis 1
       • the alleles will be passed on the the offspring together
     • crossing over occurs in meiosis 1, and the closer the genes are, the less likely crossing over will split them up