A dihybrid cross examines genes for two traits in two heterozygous organisms. Dominant genes are expressed unless both genes for a trait are recessive. Punnett squares can predict offspring traits. An example is given using dog hair and eye color. Out of 16 possible combinations, 9 will have dominant traits, 1 will have recessive traits, and 6 will have one dominant and one recessive trait.
In the field of genetics, a dihybrid cross is a crossing of two organisms, where the genes for two traits are examined. Specifically, both organisms are heterozygous for both traits, meaning they carry both dominant and recessive genes for that trait. Because of this, the dominant gene is expressed in both parents, but seven out of 16 children will have at least one recessive trait.
An expressed gene is one that manifests itself in the organism. The simplest example is a visible trait, such as eye or hair color. In the basic form of gene expression, an individual carries two genes for each trait, but only one gene is expressed or appears.
For each trait, there are dominant and recessive genes. A dominant gene is always expressed, while a recessive gene is only expressed if both of the genes the organism carries for that trait are recessive. Thus, an individual with two dominant genes or a dominant and a recessive gene will express the dominant gene. In other words, the recessive gene will be expressed only if the organism carries two recessive genes.
Genetics is a very complex science, but many genetic predictions, including those for a dihybrid cross, can be simplified through the use of a tool called a Punnett square. This is a grid set up with possible gene combinations from one parent listed at the top and possible gene combinations for the other parent listed below. The grid squares are used to calculate all possible gene combinations that could be produced by crossing the two parent organisms. This can allow a user to calculate the probability that an offspring has a certain trait or combination of traits.
The hair and eye color of dogs can be used as an example of a dihybrid cross. In this example, the dominant hair color is black and is represented by H, while the recessive hair color is white and is represented by h. The dominant eye color is brown, represented by E, and the recessive eye color is blue, represented by e. Since both parents are heterozygous in a dihybrid cross, both have a set of genes represented by HhEe. This means that the possible combinations of genes from each parent listed in the Punnett square are HE, He, hE and he.
To continue, four possible combinations of hair color genes can be produced by crossing this set of parents: HH, Hh, hH, and hh. By the same logic, the possible combinations of eye color genes are EE, Ee, eE, and ee. Since there are four possible combinations for each gene type, the total number of possible gene combinations is 16.
Of the 16 possible combinations, nine of the theoretical offspring will possess the dominant gene for both traits and, therefore, have black hair and brown eyes, like the parent dogs. Only one in 16 dogs produced from this cross would have both the recessive traits of white fur and blue eyes. The remaining six offspring will each display one dominant trait and one recessive trait, so three of the dogs would have black hair and blue eyes, while three would have white hair and brown eyes.
Protect your devices with Threat Protection by NordVPN
