genes and linkage

• the position of a gene on a chromosome is known as its locus (plural: loci)
• each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome
• each gene can exist in two or more different forms called alleles
• autosomal linkage only occurs on the autosomes (any chromosome that isn't a sex chromosome)
• two or more genes on the same autosome do not assort independently during meiosis

  instead these genes are linked and they stay together in the original parental combination

  these linked genes are passed on to offspring all together (through the gametes)
• two sex chromosomes: X and Y

  female: XX male: XY

  some genes are only present on one sex chromosome and not the other
• as the inheritance of these genes is dependent on the sex of the individual they are known as sex-linked genes

  • most often sex-linked genes are found on the longer X chromosome
• if the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two
• because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
• females having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele