Inheritance: Autosomal Linkage | A-level Biology | OCR, AQA, Edexcel

Inheritance: Autosomal Linkage in a Snap! Unlock the full A-level Biology course at http://bit.ly/2K1CRN9 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 Autosomal Linkage 2. Genetic Crosses and Autosomal Linkage 3. Example of Autosomal Linkage in the Drosophila Fruit Fly Introduction to Autosomal Linkage We know that there are 23 pairs of chromosomes in a human cell. One of these chromosome pairs are the sex chromosomes which determine whether an individual is male or female. The rest of the 22 pairs of chromosomes are homologous pairs and these 22 pairs are called autosomes. Autosomes are chromosomes which are not concerned with sex determination. Each chromosome can have many different genes located on it. Any genes which are located on the same chromosome are described as being linked. If two or more genes are located on the same autosome (non-sex chromosome) it is called autosomal linkage. Genetic Crosses and Autosomal Linkage Normally genes which are located on separate homologous chromosomes are independently separated into gametes during crossing over in meiosis. Consider a pea plant in which the gene for seed colour is on a different homologous pair to the gene for seed shape and which is heterozygous for both characteristics. When these homologous pairs cross over during meiosis, the alleles are independently separated from each other. This process is called the independent assortment of genes. This means that there are four possible combinations of alleles in the gametes. However, if two genes are closely located on the same chromosome then they are less likely to be separated during crossing over. This means that there will be just two possible combination of alleles in the gametes, because the alleles of the linked genes will be inherited together. Example of Autosomal Linkage in the Drosophila Fruit Fly In the Drosophila melanogaster fruit fly there is autosomal linkage between the gene for body colour and the gene for wing length. For the gene for body colour, the allele for a grey body is dominant to the allele for a black body. For the gene for wing length, the allele for normal wings is dominant over the allele for vestigial wings (tiny wings that don’t work). If two heterozygous Drosophila with grey bodies and normal wings are mated, we can work out the phenotype and genotype of the offspring using a Punnett Square. However, in this example we have to remember there is autosomal linkage between the gene for body colour and the gene for wing length. 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 in autosomal linkage, a dihybrid cross of two heterozygotes results in ¾ of the offspring having both dominant characteristics and ¼ having both recessive characteristics.