# Predicting Inheritance: Dihybrid Crosses ## Introduction to dihybrid crosses - Monohybrid crosses examine how alleles of **one gene** transfer across generations - Dihybrid crosses examine how alleles of **two genes** transfer across generations - Used to show the inheritance of **two different characteristics** in an individual ## Similarities and differences with monohybrid crosses - Genetic diagrams for both types of crosses are very similar in structure - For dihybrid crosses, there are **several more genotypes and phenotypes** involved - This makes the Punnett squares larger and more complex

When writing genotypes, write the two alleles for one gene followed immediately by the two alleles for the other gene - do not mix up alleles from different genes

- For example, if there is a gene with alleles $R$ and $r$ and another gene with alleles $T$ and $t$, an example genotype would be **RrTt** - **Not** RtTr or any other mixed arrangement ## Setting up a dihybrid cross - Both genes must be inherited **independently of one another** - This occurs when the two genes are located on **different non-homologous chromosomes** - Such characteristics are said to be **unlinked** ## Worked example: Dihybrid genetic diagram ### The scenario - Pea plants have a gene for seed texture with two alleles: - **R** - a dominant allele produces round seeds - **r** - a recessive allele produces wrinkled seeds - Pea plants also have a gene for plant height: - **T** - a dominant allele produces tall plants - **e** - a recessive allele produces short plants - Each of these genes (consisting of a pair of alleles) are **inherited independently** of one another - The two genes are located on **different non-homologous chromosomes** ### The cross - In this example, a pea plant that is **heterozygous for both genes** is crossed with a plant that is **homozygous for one gene** and **heterozygous for the other** **Parental phenotypes:** round seeds, tall plant × wrinkled seeds, tall plant **Parental genotypes:** RrTt × rrTt **Parental gametes:** RT or Rt or rT or rt (from first parent) and rT or rt (from second parent) ### Punnett square | | **rT** | **rt** | |---|---|---| | **RT** | RrTT / round seeds, tall plant | RrTt / round seeds, tall plant | | **Rt** | RrTt / round seeds, tall plant | Rrtt / round seeds, short plant | | **rT** | rrTT / wrinkled seeds, tall plant | rrTt / wrinkled seeds, tall plant | | **rt** | rrTt / wrinkled seeds, tall plant | rrtt / wrinkled seeds, short plant | ### Results **Predicted ratio of phenotypes in offspring:** - 3 round seeds, tall plant : 3 wrinkled seeds, tall plant : 1 round seeds, short plant : 1 wrinkled seeds, short plant **Predicted ratio of genotypes in offspring:** - 1 RrTT : 3 RrTt : 1 Rrtt : 1 rrTT : 3 rrTt : 1 rrtt

The phenotypic ratio for a dihybrid cross depends on the genotypes of the parents - different parent combinations will produce different ratios

## Key points for dihybrid crosses - Always clearly identify the **parental phenotypes** first - Write out the **parental genotypes** using appropriate letter notation - Determine all possible **gamete combinations** for each parent - A heterozygous parent for both genes (e.g., RrTt) can produce **four different gamete types** - A homozygous parent for one gene (e.g., rrTt) can produce **two different gamete types** - Use a Punnett square to systematically show all possible offspring combinations - State both the **phenotypic** and **genotypic** ratios in your final answer

When both parents are heterozygous for both genes (RrTt × RrTt), the classic phenotypic ratio is 9:3:3:1

## Common mistakes to avoid - **Mixing up alleles** from different genes when writing genotypes - Always keep the alleles for each gene together - **Forgetting gamete types** - make sure you list all possible combinations - For RrTt there are **four** possible gametes: RT, Rt, rT, rt - **Not showing your working** - genetic diagrams require all steps to be shown: - Parental phenotypes - Parental genotypes - Parental gametes - Punnett square - Offspring genotypes and phenotypes - Final ratios