Heredity

This is elaborating a bit on Genes and Inheritance, with more info about the key words to further understanding.


Chromosome
This is a rod-like structure visible in the nucleus during cell division. It is made up of the molecule, deoxyribonucleic acid (DNA). Chromosomes carry the information for making new animal or plant bodies. This information is carried in DNA. Each chromosome may carry many genes along its length.

Gene
A gene is a unit of inheritance. It is a small segment of DNA in a chromosome where specific genetic (hereditary) information is stored. Each gene has a specific function. E.g. there is a gene which determines the height of the pea plant, another determines the colour of their flowers, and another for the shape of the seeds. The place on the chromosome where the gene is located is called the gene locus (plural: loci).

Allele
Each gene can have different forms. Different forms of the same gene are called alleles. E.g. the gene for the height of the pea plant has 2 alleles: short and tall. The gene for flower colour in pea plants also has 2 alleles: purple and white. The alleles occupy the same relative positions on a pair of homologous chromosomes. (i.e. chromosomes that have the same genes at the same loci but possibly different alleles.) Letters are usually used to represent alleles. E.g. the dominant allele for tallness in pea plants may be designated T and the recessive allele for dwarfness, t.

Homologous chromosomes
Homologous chromosomes:

• exist in pairs. One chromosome in the pair comes from the male parent and the other from the female parent.
• are similar in shape and size (with the exception of the sex chromosomes X and Y)
• have exactly the same order or sequence of gene loci. The alleles in these gene loci may not be the same. 

Phenotype

This refers to a trait which can be seen, e.g. the outward appearance or visible trait of an organism. Therefore, tallness in pea plants is a phenotype. 

Genotype

The genetic makeup of an organism. (Basically the combination of genes in an organism.)

• An organism is said to be homozygous for a trait if the 2 alleles controlling the trait are the same, e.g. TT (homozygous dominant) or tt (homozygous recessive) 
• An organism is heterozygous for a trait if the 2 alleles controlling the trait are different, e.g. Tt.

Dominant allele

This expresses itself and gives the same phenotype in both the homozygous and heterozygous conditions, e.g. tall plants have the TT or Tt genotype (phenotype: tall)

Recessive allele

Does not express itself in the heterozygous conditions. Only expressed in the homozygous condition (tt). Thus pea plants will only be dwarf if they have the 'tt' genotype whereas a tall pea plant may have either the TT or Tt genotype.