InheritanceInheritance is the transmission of a gene from parent to child. By observing both a parent and their child, notable similarities can be distinguished between them. These characteristics are called traits and can include hair colour, eye colour, skin colour and much more. In Humans, there are 46 chromosomes of which are arranged in pairs called homologous chromosomes. Homologous pairs are comprised of one chromosome from the father and one from the mother.
These pairs of chromosomes are similar but not identical. Each pair carries the same genes in the same order but may have different alleles for the same trait. In humans, characteristics have been categorized into dominant or recessive traits. Genes have different forms called alleles. It is these alleles that can either be either dominant or recessive
|
A gene is a factor that determines an inherited chacteristic
An allele is an alternate form of a gene e.g. the gene for seed colour in peas (figure 1) has two alleles, one for yellow seed and one for green seed A homozygous is when an individual has the same alleles for a particular characteristic - also referred to as a pure breed A genotype is the genetic make-up of an individual as determined by the alleles for a particular characteristic A heterozygous is when an individual possesses different alleles for a specific trait, commonly referred to as a hybrid A genotype refers to the genetic makeup of an individual as determined by the alleles for the specific trait being looked at A phenotype is the physical expression of a particular genotype The sex chromosome us the 23rd pair of chromosomes that determine the sex of an individual |
Dominant Recessive Dominant Recessive
See Some Examples
Monohybrid CrossesA monohybrid cross is a mating between two individuals with different alleles. For example, tongue rolling and non tongue rolling in humans.
In the study of inheritance, each gene, one from the father and one from the mother, is represented though the use of letters. If the gene is dominant it is represented by a capital letter (A) whilst a recessive gene is represented by a lower case letter (a). For example, in humans, non tongue rolling is a dominant trait while tongue rolling is a recessive trait. Therefore the trait non tongue rolling can be represented by an uppercase "T" and tongue rolling a "t". By observing figure 1, we are able to state that pure breeding yellow seed plants are "AA". Therefore, pure breeding green seed pea plants are represented "aa". Using this information, we can determine that hybrid pea plants would contain a gene for both green and yellow seed colour and therefore would be "Aa". The different forms for seed colour, in this case "A" and "a", are termed alleles. Using the above information, all possible combinations using the alleles for seed colour can be constructed: As depicted above, there are three possible combinations: AA, aa or Aa. The official term for these sequences of possible combinations are genotypes.
|
As evident in figure 2, two of the three possible genotypes for seed colour are the same (AA and aa) these are called homozygous. In contrast, the hybrid, Aa, is called a heterozygous because it has two different alleles for the same characteristics (seed colour)
The three genotypes listed can produce two possible outcomes depending on whether they contain dominant or recessive traits. Pea plants with AA or Aa will produce yellow seeds because the dominant allele (A) masks the affects of the recessive allele (a). Therefore pea plants with the genotype aa will produce green seeds.
|