| Representing genetic crosses | Genetic crosses are usually represented in a standard form of shorthand. Although you may occasionally come across variations to this scheme. Once you have practised a number of crosses, you may be tempted to miss out stages or explanations. Not only is this likely to lead to errors, it often makes your explanations difficult for others to follow. |
| Instructions | Questions usually give the symbols to be used, in which case always use the ones provided. Choose a single letter to represent each characteristic.
Choose the first letter of one of the contrasting features.
If possible, choose the letter in which the higher and lower case forms differ in shape as well as size.
If possible, choose the letter in which the higher and lower case forms differ in shape as well as size.
Let the higher case letter represent the dominant feature and the lower case letter the recessive one. Never use two different letters where one character is dominant.
Represent the parents with the appropriate pairs of letters. Label them clearly as 'parents' and state their phenotypes.
State the gametes produced by each parent. Label them clearly, and encircle them.
Use a type of chequerboard or matrix, called a Punnett square, to show the results of the random crossing of the gametes. Label male and female gametes even though this may not affect the results. |
| Questions usually give the symbols to be used, in which case always use the ones provided. Choose a single letter to represent each characteristic. | An easy form of short hand |
| Choose the first letter of one of the contrasting features.
If possible, choose the letter in which the higher and lower case forms differ in shape as well as size. | When's more than one character is considered at one time such a logical choice means it is easy to identify which letter refers to which character. |
| If possible, choose the letter in which the higher and lower case forms differ in shape as well as size. | If the higher and lower case forms differ it is almost impossible to confuse them, regardless of their size. |
| Let the higher case letter represent the dominant feature and the lower case letter the recessive one. Never use two different letters where one character is dominant. | The dominant and recessive feature can easily be identified. Do not use two different letters as this indicates codominance. |
| Represent the parents with the appropriate pairs of letters. Label them clearly as 'parents' and state their phenotypes. | This makes it clear to any reader what the symbols refer to. |
| State the gametes produced by each parent. Label them clearly, and encircle them. | Encircling them reinforces the idea that they are separate. |
| Use a type of chequerboard or matrix, called a Punnett square, to show the results of the random crossing of the gametes. Label male and female gametes even though this may not affect the results. | This method is less liable to error than drawing lines between the gametes and the offspring. Labelling the sexes is a good habit to acquire - it has considerable relevance in certain types of crosses, e.g., sex-linked crosses. |
| State the phenotypes of each different genotype and indicate the numbers of each type. Always put the higher case (dominant) letter first when writing out the genotype. | Always putting the dominant feature first can reduce errors in cases where it is not possible to avoid using symbols with the higher and lower case letters of the same shape. |
| Inheritance of pod colour in peas | Monohybrid inheritance is the inheritance of a single gene. To take a simple example we will look at one of the features Gregor Mendel studied-the colour of the pods of pea plants. Pea pods come in two basic colours-green and yellow.
If pea plants with green pods are bred repeatedly with each other so that they consistently give rise to plants with green pods, they are said to be pure-breeding for the character of green pods. Pure-breeding strains can be bred for almost any character. This means that the organisms are homozygous (that is, they have two alleles that are the same) for that particular gene.
If these pure-breeding green-pod plants are then crossed with pure-breeding yellow-pod plants, all the offspring, known as the first filial, or F,, generation, produce green pods. This means that the allele for green pods is dominant to the allele for yellow pods, which is therefore recessive.
These observed facts led to the formation of a basic law of genetics (the law of segregation). This states,
In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete. |
