- normal body cells have → 2 haploid set of chromosomes → having inherited one set from each parent.
- hence each chromosome is represented as a pair in each cell.
- have learnt that What Are Genes → are the hereditary factors found in chromosomes. they are responsible for the passing down of traits. each gene may exist in 2 or more diff (or alternative forms).
Definition
Different forms of the same gene are called alleles. Alleles occupy the same relative position on a pair of homologous chromosomes #BiologyDefinitions
e.g. eyelid gene → has 2 diff alleles.
- one allele gives you double eyelids and other gives you single eyelids.
if you have inherited 2 diff. alleles from your parents → you will have double eyelids.
- when person has 2 diff. alleles → one allele will express itself, while other will not.
in this case → double eyelid allele expressed itself → while single eyelid allele is not expressed.
If a person has 2 diff. alleles → allele that express itself is called dominant allele, while other is called recessive allele double eyelid is dominant.
Can Recessive allele express itself?
recessive allele can express itself in a person when the person has both recessive alleles in the gene. thus person with 2 alleles for single eyelids will have single eyelids.
let us use symbols to represent alleles; D = dominant allele. d = recessive allele.
Definition
The combination of alleles for a particular gene is called genotype The expressed trait or outward appearance is called phenotype #BiologyDefinitions
| Genotype | Phenotype |
|---|---|
| DD | double eyelids |
| Dd | double eyelids |
| dd | single eyelids |
Homozygous and Heterozygous
- terms used to describe allele combination (genotype) of the organism
Definition
Homozygous → refers to having 2 identical alleles of a particular gene (e.g. DD or dd) #BiologyDefinitions
- organism that is homozygous dominant → for a certain trait/gene → has 2 dominant alleles for the gene
- organism that is homozygous recessive → for certain trait/gene → has 2 recessive alleles for the gene
Definition
Heterozygous → refers to having 2 different alleles of a particular gene (e.g. Dd)
- hence ⇒ dominant allele expresses itself and gives same phenotype → in both homozygous and heterozygous condition.
- recessive allele → is only expressed in the homozygous condition.
- organism is pure bred → if it contains identical alleles (e.g. TT or tt).
- pure bred is a homozygous organism.