A-level Biology OCR [New Spec] - Inheritance: Dihybrid Crosses | A-level Biology | OCR, AQA, Edexcel

Inheritance: Dihybrid Crosses in a Snap! Unlock the full A-level Biology course at http://bit.ly/2K1CRwD created by Adam Tildesley, Biology expert at SnapRevise and graduate of Cambridge University. SnapRevise is the UK’s leading A-level and GCSE revision & exam preparation resource offering comprehensive video courses created by A* Oxbridge tutors. Our courses are designed around the OCR, AQA, SNAB, Edexcel B, WJEC, CIE and IAL exam boards, concisely covering all the important concepts required by each specification. In addition to all the content videos, our courses include hundreds of exam question videos, where we show you how to tackle questions and walk you through step by step how to score full marks. Sign up today and together, let’s make A-level Biology a walk in the park! The key points covered in this video include: 1. Introduction to Dihybrid Crosses 2. Dihybrid Crosses with Pure Breeding Seeds 3. Dihybrid Crosses with the F1 Generation Introduction to Dihybrid Crosses We previously saw that monohybrid inheritance consists of a single gene being passed from one generation to the next. Dihybrid inheritance involves the inheritance of two different characteristics, determined by two different genes located on different chromosomes. Dihybrid crosses are therefore used to investigate the simultaneous inheritance of two different characteristics such as eye colour and height. An example of two different characteristics that can be inherited simultaneously are seed colour and seed shape in pea plants. In this example, the allele coding for a yellow seed colour is dominant to the allele coding for a green seed colour. The allele coding for a round seed shape is dominant to the allele coding for wrinkled seed shape. Dihybrid Crosses with Pure Breeding Seeds A dihybrid cross looking at the colour and shape of seeds of pure breeding pea plants was first done by Mendel. In this dihybrid cross, the two types of pure breeding seeds were: The yellow and round seeds (both characteristics encoded by dominant alleles), The green and wrinkled seeds (both characteristics encoded by recessive alleles). The gene for seed shape and gene for seed colour are on different homologous pairs of chromosomes. Therefore any one of the two alleles for seed colour can combine with any one of the two genes for seed shape. When the pure breeding seeds are crossed, we can draw out a Punnett square to work out the genotype and phenotypes of the offspring. Step 1: Work out the parental genotypes. Step 2: Write out the parental gametes. Step 3: Work out the offspring genotypes. Step 4: Work out the offspring phenotypes. Dihybrid Crosses with the F1 Generation A dihybrid cross of pure breeding seeds will result in the F1 generation all being heterozygous for both seed colour and seed shape. This means that all of the F1 generation will express the dominant alleles in their phenotype. A dihybrid cross can then be done with two seeds from the F1 generation to give rise to the F2 generation. Step 1: Work out the parental genotypes. Step 2: Write out the parental gametes. Step 3: Work out the offspring genotypes. Step 4: Work out the offspring phenotypes. Step 5: Calculate the phenotype ratios. Therefore, the resultant F2 generation from a dihybrid cross always contains. Summary Dihybrid inheritance is when two different characteristics encoded by different genes are simultaneously inherited Dihybrid crosses involve mating organisms with different alleles for two different genes In a dihybrid cross, pure breeding organisms either have all dominant alleles or all recessive alleles When a dihybrid cross is done with pure breeding organisms, the resultant F1 generation are all heterozygous for both characteristics inherited The F1 generation can then be crossed with each other to give rise to an F2 generation In a dihybrid F1 generation cross the phenotypic ratio for the F2 generation is always 9:3:3:1