[Music] genetic diagrams or punnett squares can be used to show monohybrid inheritance where characteristics controlled by a single gene are passed on to offspring let us look at how you draw a punnett square and then look at some exam examples firstly identify the parent's genome if the question has not given you a letter to use you may use any letter you like remembering that a dominant gene is always represented by a capital letter and a recessive gene by a lowercase letter draw a table as shown here splitting the alleles from each parent and then adding them into your punnett square so if we look at the father in this example he has two dominant alleles on his chromosomes and so each box represent a gammy or sex cell and we know that each gamete contains half the genetic information therefore each of the two boxes we place one version of the gene from one chromosome into one box and the other gene from the other chromosome into the other box we do the same thing for the mother but this time she has one dominant and one recessive allele so her eggs represented by the boxes will either contain the dominant allele or recessive allele finally complete the punnett square by inserting the different genetic combinations in the offspring as shown in the table remembering when you have a dominant and a recessive you should write the dominant letter first let's look at an exam style question in this example cystic fibrosis is an inherited disorder that mainly affects the lungs and the pancreas it is caused by a recessive allele complete the punnett square and write down the probability a child is born with cystic fibrosis so from this question we can establish the following firstly cystic fibrosis is caused by a recessive allele which means that the lower case f must be the cystic fibrosis gene for recessive genes to be expressed in the phenotype there must be two versions of it in box one it would be capital f capital f box two capital f capital f box three capital f lowercase f and box four capital f lowercase f in this case there's zero chance that any child that this couple will have will be born with cystic fibrosis we can use family pedigrees to see how genetic disorders are inherited in a family it also allows us to work out the chance that someone in a family will inherit a condition look at this example what is the chance that sue has the disease the chance is 50 sue's mother steph has cystic fibrosis but her father larry does not however larry must be a carrier because sue's sister paula has the disease and is caused by a recessive allele so if we were to look at this as a punnett square sue's mother would have two recessive alleles as she has the disease larry would have one dominant and one recessive allele completing the punnett square it would look something like this we can see that there's a 50 chance that a child being born to steph and larry would have cystic fibrosis we can also use genetic diagrams to predict the probability of being born a male or female we know that a male has x and y chromosomes and a female has two x chromosomes if we now complete the punnett square we can see that there's a 50 probability that a child being born is either male or female hi my name is mr science aka saleem if you're new to the channel please remember to like and subscribe and for more teaching resources you can visit my website at www.mrscience.com [Music]