genes-and-linkage

Locations of Genes on a Chromosome

• Every chromosome consists of a long DNA molecule that contains several hundred or even thousands of different genes coding for different proteins

  • A length of DNA that codes for a single polypeptide or protein is called a gene

• The position of a gene on a chromosome is known as its locus (plural: loci)

  • Through experiments and genetic mapping techniques, scientists have been able to work out the specific physical locations of the genes on different chromosomes

  • 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

• Different alleles of a gene have slightly different nucleotide sequences but they still occupy the same position (locus) on the chromosome

Five different genes found at five different loci

Autosomal Linkage

• As its name implies, 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)

Sex Linkage

• There are two sex chromosomes: X and Y

• Females have two copies of the X chromosome (XX), whereas males have one X chromosome and one shorter Y chromosome (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