Inheritance
An organism's characteristics are controlled by the genes it inherits from its parents.
A gene is a location on a chromosome where there is a DNA base sequence that codes for a protein that controls a particular characteristic.
An individual usually receives two copies of the gene for each characteristic. One copy is carried on a chromosome passed on by the male gamete and one on the matching chromosome passed on by the female gamete.
Different versions of a gene contain different sequences of DNA bases, that code for different proteins and produce different phenotypes. The different versions of a particular gene are called Different forms of the same gene..
There are two alleles of the gene that controls seed shape in a pea plant:
- Allele R produces round seeds.
- Allele r produces wrinkled seeds.
The The alleles that an organism has for a particular characteristic, usually written as letters. of an organism is a statement of the alleles it has for a characteristic.
If the two alleles of a particular gene are identical, the individual is described as being This describes a genotype in which the two alleles for the characteristic are identical. for that characteristic.
If the two alleles of a particular gene are different, the individual is described as being This describes a genotype in which the two alleles for a particular characteristic are different. for that characteristic.
A A genetics cross that shows the inheritance of one characteristic. is a genetics cross that shows the inheritance of one characteristic, such as pea seed shape.
The pea seed shape phenotypes of three plants with different genotypes are shown in the table below.
| Individual | Genotype | Phenotype |
| 1 Homozygous for allele r | rr | Wrinkled |
| 2 Homozygous for allele R | RR | Round |
| 3 Heterozygous | Rr | Round |
| Individual | 1 Homozygous for allele r |
|---|---|
| Genotype | rr |
| Phenotype | Wrinkled |
| Individual | 2 Homozygous for allele R |
|---|---|
| Genotype | RR |
| Phenotype | Round |
| Individual | 3 Heterozygous |
|---|---|
| Genotype | Rr |
| Phenotype | Round |
Plants 2 and 3 both have the same phenotype even though they have different genotypes. This is because the effect of allele R masks the effect of allele r.
Allele R is said to be An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself. and allele r is said to be recessive. The dominant allele of a gene is given a capital letter and the Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant. A recessive gene will remain dormant unless it is paired with another recessive gene. allele of a gene is given a lower case letter.
The round pea seed shape is the dominant characteristic and the wrinkled pea seed shape is the recessive characteristic.