Key Terms in Genetics (CIE A Level Biology)

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 I^A

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 (I^AI^A or I^AI^O) and allele B results in blood type B (I^BI^B or I^BI^O)
• If both allele A and allele B are present in a heterozygous individual they will have blood type AB (I^AI^B)
• Blood type O (I^OI^O) 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 chromosome
  • 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 X^GX^g. A males genotype would be written as X^GY

• 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