Genes & alleles

• The DNA contained within chromosomes is essential for cell survival
• Every?chromosome?consists of a long DNA molecule which?codes for several 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 its?locus?(plural: loci)
• 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

Chromosomes showing genes, alleles and loci

Example of alleles

• One of the genes for coat colour in horses is Agouti
• This gene for coat colour is found on the same position on the same chromosome for all horses
• Hypothetically there are two different forms (alleles) of that gene found in horses:?A?and?a
• Each allele can produce a different coat colour:
  • Allele?A?→ black coat
  • Allele?a?→ chestnut coat

Genotype & phenotype

• The chromosomes of eukaryotic cells occur in?homologous pairs?(there are two copies of each chromosome)
• As a result cells have?two copies of every gene
• As there are two copies of a gene present in an individual that means there can be?different allele combinations within an individual
• The?genotype?of an organism refers to the?alleles of a gene?possessed by that individual. The different alleles can be represented by letters
• When the two allele copies are identical in an individual they are said to be?homozygous
• When the two allele copies are different in an individual they are said to be?heterozygous
• The genotype of an individual affects their phenotype
• A phenotype is the?observable characteristics?of an organism

Example of genotype & phenotype

• Every horse has two copies of the coat colour gene in all of their cells
• A horse that has two black coat alleles?A?has the genotype?AA?and is homozygous. The phenotype of this horse would be a black coat
• In contrast a horse that has one black coat allele?A?and one chestnut coat allele?a?would have the genotype?Aa?and is heterozygous

Dominance

• Not all alleles affect the phenotype in the same way
• Some alleles are?dominant: they are?always expressed?in the phenotype
  • This means they are expressed in both heterozygous and homozygous individuals
• Others are?recessive: they are only?expressed?in the phenotype?if no dominant allele?is present
  • This means that it is only expressed when present in a homozygous individual

Example of dominance

• If for horses the allele?A?for a black coat is dominant and the allele?a?for a chestnut coat is recessive the following genotypes and phenotypes occur:
  • Genotype?AA?→ black coat
  • Genotype?Aa?→ black coat
  • Genotype?aa →?chestnut coat

Codominance

• Sometimes?both alleles can be expressed?in the phenotype at the same time
• This is known as codominance
• When an individual is heterozygous they will express both alleles in their phenotype
• When writing the genotype for codominance the gene is symbolised as the capital letter and the alleles are represented by different superscript letters, for example?IA

Example of codominance

• A good example of codominance can be seen in human blood types
• The gene for blood types is represented in the genotype by?I?and the three alleles for human blood types are represented by?A,?B?and?O
• Allele?A?results in blood type?A?(IAIA?or IAIO) and allele?B?results in blood type?B?(IBIB?or IBIO)
• If both allele?A?and allele?B?are present in a heterozygous individual they will have blood type?AB?(IAIB)
• Blood type?O?(IOIO) is recessive to both group?A?and group?B?alleles

F1, F2?& test crosses

• When a?homozygous dominant?individual is crossed with a?homozygous recessive?individual the offspring are called the?F1?generation
  • All of the F1?generation are?heterozygous
• If two individuals from the F1?generation are then crossed, the offspring they produce are called the F2?generation
• A?test cross?can be used to try and?deduce the genotype of an unknown individual?that is expressing a dominant phenotype
  • The individual in question is crossed with an individual that is expressing the?recessive phenotype
  • The resulting phenotypes of the offspring provide sufficient information to suggest the genotype of the unknown individual
  • If there are any offspring expressing the recessive phenotype then the unknown individual must have a heterozygous genotype

Linkage

• There are two types of linkage in genetics:?sex linkage?and?autosomal linkage
• Sex linkage:
  • There are two sex chromosomes: X and Y
  • Women have two copies of the X chromosome (XX) whereas men have one X chromosome and one shorter Y chromosome (XY)
  • Some?genes are found on a region of a sex chromosome that is not present on the other?sex chromosome
  • As the?inheritance of these genes is dependent on the sex?of the individual they are called sex-linked genes
  • Most often sex-linked genes are found on the longer X chromsome
  • Haemophilia is well known example of a sex-linked disease
  • Sex-linked genes are represented in the genotype by writing the alleles as superscript next to the sex chromosome. For example a particular gene that is found only on the X chromosome has two alleles?G?and?g. The genotype of a heterozygous female would be written as XGXg. A males genotype would be written as XGY
• Autosomal linkage:
  • This occurs on the autosomes (any chromosome that isn’t a sex chromosome)
  • Two or more genes on the same chromosome?do not assort independently?during meiosis
  • These genes are linked and they stay together in the original parental combination