2. Key Terms

2.1 Chromatin thread vs Chromosome

Above: Structure of the chromatin thread and chromosome.

In the nuclei of cells, the long and thin threads in the nuclei are known as the chromatin thread.

During interphase, there is (1) DNA replication (2) further coiling of chromatin thread to form a highly-coiled X-shaped structure known as the chromosome. The chromosome consists of two sister chromatids (the two DNA strands - original and replicated DNA) held by the centromere.

2.2 Homologous Chromosomes

For simplicity, we will show each chromosome as single-stranded.

Characteristics of homologous chromosomes:

1. Both chromosomes have the same arm length - ie the chromosomes are of the same length and the position of the centromere is the same for both chromosomes.

2. One chromosome is inherited from the father and one chromosome is inherited from the mother.

3. Both chromosomes have the same gene loci - ie same genes are found at the same positions on both chromosomes.

For traits that are controlled by a single gene, each individual would inherit two alleles of the same gene. The two alleles may be identical or different.

2.3 Genotype

• Genotype is the genetic make-up of an individual, which consists of the two alleles inherited for a particular gene.

• Assuming that the rat coat colour is determined by a pair of contrasting alleles - ie albino coat colour (white) vs brown coat colour, letters are used to represent each allele. The same letter is used for the same gene but the different alleles are differentiated by capital and small letters.

• Capital letter represents the dominant allele while small letter represents the recessive allele.

• B represent the allele for brown coat colour and b for albino coat colour.

• The possible genotypes for rat coat colour are BB, Bb and bb. What do the genotypes mean?

1. BB - B allele is inherited from each of both parents.

2. Bb - B allele is inherited from one parent and b allele is inherited from the other parent.

3. bb - b allele is inherited from each of both parents.

2.3.1 Homozygous vs Heterozygous

Homozygous describes the genotype whereby both alleles are identical (NB homo: same) while heterozygous describes the genotype whereby both alleles are different (NB hetero: different).

BB is described as homozygous dominant while bb is described as homozygous recessive.

Bb is described as heterozygous dominant because the B allele is a dominant trait.

2.4 Phenotype

Phenotype is the expressed characteristics of an individual.

For eg. A rat that has BB or Bb genotypes would have brown coat colour as its phenotype while a rat with bb genotype would have albino coat colour.

• Phenotype is controlled by genotype but some traits may be affected by environmental factors as well.

• E.g. ABO blood type of an individual is only controlled by genotype but mass of an individual may be controlled by genotype and environmental condition, eg. amount and type of food consumed.