 Mendel proposed that you inherit a gene either from your mother or from your father, but not a mixture of both. In our genetic crosses video, we looked at what happens when yellow pea pod and green pea pod parents are crossed and represented the offspring in genetic crosses. In this video we are going to look at mono hybrid crosses using punnett squares. A mono hybrid cross is a study of the inheritance of one characteristic such as the pea pod color. A punnett square is a diagram that is used to predict the outcome of a particular cross. It is used by biologists to determine the probability of an offspring having a particular genotype. The genotype represents the genes of the organism and is shown by two letters. A capital letter represents the dominant form of a gene or allele and a lower case letter represents the recessive form of the gene. So the dominant yellow pea pod is shown by capital Y and the recessive green pea pod by little y. Now batch our punnett squares. We start with an entry grid like this. If we crossed a heterozygous yy and a recessive yy parent together, we enter the first parent's genotype here. Split them and enter one y in the first column and the other y in the second column. And the second parent, yeah, doing the same thing. One y in the top row and the second y in the bottom row. It doesn't matter which parent you could wear, both are correct. So these are the parent's genotypes and these are going to be the possible offspring genotypes. Let's fill in the gaps. I'll do the first one for you. Dominant y and recessive y gives yy. Can you complete the rest of the punnett square? Pause the video, have a think and click play when you're ready to check. Did you get it right? Make sure you always have two letters in each of the four boxes. We have two yy genotypes which means 50% of these offspring would have the dominant yellow p-pods and we have two yy genotypes. So 50% of these offspring would have the recessive green p-pods. We could write this as one-to-one yellow green phenotypic ratio. Let's look at another example. In the Cystic Vibrosis video we've discovered that Cystic Vibrosis is caused by a mutation on a recessive gene. You can be a carrier but not suffer from the condition. Let's have a look at the probability of offspring being Cystic Vibrosis sufferers when two carrier parents with the CC genotype mate. Can you complete the Punnett square? Pause the video, work it out and click play when you're ready. Did you get it right? So there is a 25% chance that the offspring of two carrier parents will be sufferers of Cystic Vibrosis. We can represent this in a one-to-one ratio. See how easy Punnett squares are for showing probability of offspring genotypes? But remember these are just probabilities. Recall these the expected results. So whilst we could expect a 25% of offspring of two Cystic Vibrosis carriers to suffer from the disease it is a probability. The actual results may be different. Two carrier parents could potentially have four children that are all non-carrier CC genotypes. Before we finish maybe some of you have been wondering why we are allowed to just split up the parental genotype into two separate letters. This is because during meiosis the cell division that produces sperm and egg cells the chromosomes are separated into two halves. So the dominant C is pulled one way to form one sperm or egg cell and the recessive C of the other chromosome is pulled the other way to form a different sperm or egg cell. So half the sex cells will have the dominant C and the other half will have the recessive C. So this C represents half of the sex cells and this C represents the other half of the sex cells. So that is Punnett squares very useful for looking at the probability of genotypes of offspring. We will look at co-dominants and sex determination in different videos.