Inheritance: Monohybrid Crosses | A-level Biology | OCR, AQA, Edexcel

Inheritance: Monohybrid Crosses in a Snap! Unlock the full A-level Biology course at http://bit.ly/2K2bcvy 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. A Monohybrid Cross With Pure Breeding Pea Pods 2. A Monohybrid Cross With the F1 Generation 3. The Test Cross A Monohybrid Cross With Pure Breeding Pea Pods In pea pods, the dominant allele for colour is the green allele (G) and the recessive allele is the yellow allele (g). Pure breeding pea pods are always homozygous - pure breeding green pods will have a GG genotype and pure breeding yellow pods will have a gg phenotype. When a pure breeding green pea pod is crossed with a pure breeding yellow pea pod, the offspring are called the first filial (F1) generation. We can use a Punnett Square to work out what the genotypes and phenotypes of the F1 generation will be 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. A Monohybrid Cross With the F1 Generation When two different pure breeding pods are crossed the resultant F1 generation are all heterozygous. When two green pea pods from the F1 generation are crossed with each other, the offspring are called the F2 generation. We can use a Punnett Square again to work out the genotypes and phenotypes of the F2 generation. Step 1: Work out the parental genotypes. Write out the parental gametes. Step 3: Work out the offspring genotypes. Step 4: Work out the offspring phenotypes. Step 5: Work out the ratio of the offspring phenotypes. Therefore, in the resultant F2 generation there are two heterozygous green pods, one homozygous green pod and on homozygous yellow pod. The Test Cross The test cross is a method used to work out the unknown genotypes of individual organisms. For example, if a pea pod has a green phenotype, it may have a homozygous dominant genotype (GG) or a heterozygous genotype (Gg). The test cross allows us to work out which of these genotypes the green pea pod has. In the test cross the green pea pod with the unknown genotype is crossed with a homozygous recessive yellow pea pod which has a gg genotype. If all the offspring from this test cross have a green phenotype, then we know that the unknown genotype was GG. If half of the offspring from this test cross have a yellow phenotype, then we know that the unkown genotype was Gg. Summary A monohybrid cross is a mating between two individuals who only have different alleles of a single gene When organisms with the same characteristics are repeatedly mated with each other they are called pure breeding organisms and they are always homozygous In monohybrid crosses the same letter is used to represent the dominant and recessive allele: an uppercase letter is used to represent the dominant allele and a lowercase letter is used to represent the recessive allele When two sets of pure breeding organisms with different phenotypes are mated, their offspring is called the F1 generation All offspring in the F1 generation are heterozygous When an F1 generation organism is mated with a puure breeding organisms the offspring is called the F2 generation The genotypes and phenotypes of the offspring generated from monohybrid crosses can be worked out using a Punnett Square In an F1 generation monohybrid cross the phenotypic ratio of the F2 generation is always 3:1 The test cross is a method used to work out the unknown genotype of an organism