Video: Genetic Inheritance (Higher)

﻿Hello, and welcome to this Oak National Academy lesson. It's Mrs. Gibbs here. If you haven't done so already, please write the title, it's Genetic Inheritance. Please also note that this is a Higher Tier Lesson, so if you're sitting the Foundation Tier for your exams, please complete that version of the lesson. Now would also be a good time to remove any distractions. Make sure you in a room where you won't be disturbed, that your phone is on silent, and any tabs that are closed. Work your way through the activities right to the very end and once you're done complete the quiz. Now we're ready to start the lesson. The equipment you'll need for this lesson is a pen and something to write on. If you've not got that yet, please pause the lesson now, get a pen and some paper or a notepad, come back, get yourself settled and then resume the video. This lesson will begin with us looking at DNA, Chromosomes and Alleles Alleles is a new term in this lesson, and there's some other ones we'll be looking at as well. They are genotype, phenotype, heterozygous, and homozygous, and we'll be looking at how those link in with genetic inheritance. As there's quite a lot of new key terms in this lesson, we'll tie everything together with a review at the end. Let's begin with DNA, Chromosomes and Alleles. DNA and Chromosomes. The nucleus of a cell contains DNA made of chromosomes. Chromosomes are arranged in pairs. So if I was to draw out a nucleus, we've come across this idea before. So I'll draw cell first and then my nucleus, the chromosomes will be arranged in pairs. In a human cell, there were 46 chromosomes arranged in 23 pairs. The reason is that they were in pairs is 'cause during the process of meiosis, which we look to in an earlier lesson, gametes are produced, sperm and egg cells, that have half the number of chromosomes 23. When a sperm and egg fuse in fertilisation, the chromosomes pair up, and the full number of chromosomes 46 is restored. We're looking at a new term now, alleles. So I'm going to say it, and you're going to say it back, just so we remember, alleles, and again, but louder, alleles, excellent. Humans have two copies of each gene, that's because their chromosomes are in pairs, one from the mother one from the father. To first to draw the chromosome here, and let's say that this one is from the father, and this one is from the mother. Each chromosome contains hundreds of genes. Let's imagine on this father's chromosome, that carries a gene for eye colour, remember a gene is a length of DNA, so that's what I've got there, I'll fill that in, so we can see. Well, the mother's chromosome will also have a gene for eye colour on it. So there'll be another gene there, so humans have two copies of each gene. But for eye colour, there's different versions, and for lots of genes, there's different versions. A version of a gene is called an allele. So for example, there's an allele for blue eyes, brown eyes, green eyes, and so on and so forth. Let's do this quiz to check our knowledge. I'm going to read out the question and I want you to say the answer to me. First one then, chromosomes are arranged in? Three, two, one, pass, excellent, well done. Next one then, how many versions of each gene do humans have? Three, two, one, two, brilliant. A version of a gene is called an? Three, two, one, an allele. Well done, is that word we were saying out loud earlier. Let's learn a little bit more about alleles then. Some characteristics are caused by one single gene. An example is fur colour in mice. What that means is there is one gene that will determine whether a mouse has brown fur or black fur. But what if a mouse inherits a brown allele from its mother and a allele from his father? 'Cause remember we have two versions of every gene. What colour would it's fur be? That's what we're going to learn about now. All mice will have two alleles for fur colour, and alleles can be represented with letters. So for fur colour in this instance, I'm going to use the letter B. I'm going to use capital B to represent black fur. I'm going to use a lower case b to represent brown fur. I'll tell you in a moment why I've chosen a B for black to be capital. As we know, each mice will inherit two alleles, so what if the mice inherits two different alleles? Alleles can be dominant or recessive. If an allele is dominant, only one copy of it is needed for that characteristic to be expressed. So let's look at this black mouse again, it will have a dominant allele, that's why I drew a capital B, 'cause dominant allele are represented with capital letters, and black fur colour is dominant. It could have inherited a brown allele. Brown is recessive, so that's why I drew a lowercase letter earlier. So even though that mouse has inherited a brown allele, it will still have black fur, it won't have any brown in it's fur. Alternatively, it could just inherited two black alleles, and it would have two dominant alleles. For a recessive allele to be expressed, however, there must be two alleles present. So this brown mouse will have to recessive brown alleles. So we'll draw two lower case bs for that. If it had a dominant allele, if it had one black allele, it would have black fur. So for recessive alleles, two copies needed, sorry, the writings are covering another word. Let's do another quiz then, I want you to point to the correct answer this time. For a dominant allele to be expressed, how many copies of the allele are needed? Point to the correct answer, 10 seconds, five seconds, three, two, 1.2. Number one, excellent. For a recessive allele to be expressed, how many copies of the alleles are needed? 10 seconds, point to the correct answer, five, four, three, two, one. It's two, excellent, well done. So that would be shown with two lowercase letters. Dominant alleles are represented with a? Five seconds for this one, three, two, one, a capital letter, fantastic, well done. Now let's look at Genotype and Phenotype. Again, We're going to practise saying these words. So let's start with genotype, genotype, and again, genotype, and then let's go for phenotype, one more time phenotype,, excellent. A genotype is a combination of alleles that causes a characteristic. A phenotype is a characteristic that is actually expressed. Let's break those definitions down then. We'll go back to our mice example. We said for a black mouse to have black fur, it must have a dominant allele present. Now there's two possible combinations of alleles that would produce black fur. Remember the genotype is a combination of alleles. The genotype then for black fur could either be two dominant alleles or a dominant and recessive. No matter what that genotype are, the mouse will have black fur, that is the phenotype, a phenotype is a characteristic which has expressed. So the phenotype is black fur. The brown mice to have brown fur, they must have two recessive alleles. So there's only one possible genotype for brown mice, that's two lowercase letters, and the phenotype would be brown fur. Another quiz then, and you're going to point to the correct answers. The combination of alleles that causes a characteristic is called the? 10 seconds, five, four, three, two, one. It's the genotype, excellent. Next one, the characteristic which is expressed is called the? Five, four, three, two, one, phenotype , well done. We're now going to look at another two key words. They are Homozygous and Heterozygous. That's time, we're going to say them out loud. Homozygous, homozygous, heterozygous, one more time, heterozygous, excellent. I'd just like to draw your attention to the prefix of these two words, the prefix is the start of the word. First one, we have two prefix homo, homo means the same. The second one, we have the prefix hetero, hetero mean different. Homozygous alleles, mean the alleles are the same. Heterozygous alleles, mean the alleles in the genotype are different. So once again, back to our mice example. We've established that the black mouse, would either have the genotype of two dominant alleles or genotype with one of each. The one I'm circling, the alleles are the same, so the animals are homozygous. Can you point to another set of homozygous alleles on the screen? Fantastic, we should be pointing to the two brown alleles on the brown mouse. That leaves these alleles here as heterozygous, because they are different to one another. I'd now like you to complete an independent task. Please pause the lesson, attempt the questions, and resume once you are finished. How did you do? Again, pause the lesson, mark your answers, and resume once you are finished, We're now going to the Genetic Inheritance. We are able to predict what characteristics offspring will inherit based on the characteristics of their parents, as long as we know what alleles their parents have. Let's look at another example with mice. Let's pretend that this mouse here is a father and the mouse on the right is the mother. Let's give the father a heterozygous genotype, so one of each allele, remember he's got one dominant, so he will have black fur, the mother cause she's got brown fur, and brown fur is recessive, she must have two recessive alleles. So if they would have offspring, we could be asked to predict what's the probability that their offspring would have black fur. I'm going to show you how to do this, so if you put your pen down so you can give your screen full attention, what we use is something called a Punnett square, I'm just going to move that so we can see it clearly. Now what that does is show us the possible combinations of alleles that could arise following fertilisation. Because remember during meiosis, only half the chromosomes will enter the gametes, therefore, offspring will only inherit one allele from the mother, and one from the father. Let's imagine, let's look to the top left of this box, let's imagine if an offspring inherited this allele from the father and this allele from the mother, the offspring would inherit one of each. Let's move to the right, this time the offspring would inherit two recessive alleles. When we go to the bottom left, I'm joining up here, I'm joining up this one again, and this one, the offspring would inherit again, one of each. And the bottom right, it would get two recessive alleles. Now I asked at the start of this, what's the probability the offspring would have black fur? To show it, we circle any offspring with black fur, it's these two here 'cause they each have a dominant allele present. The probability, so it's not certain, but it's highly likely, the probability that offspring will have black fur as a percentage is 50%. We could also show it as a ratio or as a proportion. I'll come onto ratios in a bit, but to show it as a proportion, it would be 0.5, 1/2. Now because you'll sitting the Higher Tier exam paper, you would be expected to construct these Punnett squares from scratch, so let's have a look at how to do that. Two mice, both with the alleles capital B and lowercase b bred. What is the probability of the offspring inheriting black fur? I'd like you to pick up your pen or pencil and draw this Punnett square out with me at the same time. We can start by drawing up a father's alleles, so let's put father here, it matter which way round you put the father or the mother, you can write mum or dad. The father's alleles are one dominant and one recessive, remember the offspring will only inherit one of the father's alleles, it's the same with the mother as well, and the offspring will inherit one of the mother's alleles, it said both have these alleles, so she's also got one dominant and one recessive. We can put that into a Punnett square like so, and what we can say is, well, if it was these two alleles in the top left, it would be two capital Bs. We go to the top right, we've got one dominant, one recessive. Down to the bottom left, can you do that one? Write it in, excellent. So it's again, one dominant and one recessive, and the last one, can you write that in? Lovely two recessive alleles. Now, remember I said it's really important to circle any allele they ask about in the question. They want to know the probability of the offspring inheriting black fur. So we're going to circle all offspring with black fur. Let's look to the top left two dominant ones, so yes, that will have black fur. Top right, black or brown fur? It's black again well done, because it's got one dominant allele. Bottom left black or brown fur? Excellent, it's black again, because it's got one dominant allele. Bottom right, black or brown fur? Excellent, it's got brown fur so I'm not going to circle that one, because it's got two recessive alleles. So our probability is three out our four, as a percentage, we would say the probability is 75%, as a proportion, we can show that as 0.75. I'd now like you to complete this independent task, pause the video, answer the question, you will have to do a Punnett square, and resume once you are done. How did you do? Pause the video to mark your answers and resume once you are done. It's also possible to show probability as a ratio. Let's go back to our first example. We can see that the probability of offspring inheriting black fur is 50%. There were four possible outcomes for the alleles of the offspring. We come to write number four there. Two of the four would have black fur and two out of the four possible combinations would be for brown fur. So if I write that out, I can show it as a ratio and I can simplify that ratio to one to one, like so. I'd now like you to have another go at an independent task. So again, pause the video, attempt the questions, and resume once you are finished. How did you do? Use this slide to mark your answers, and resume the video once you are done. We've learned quite a lot of new key terms in this lesson and you've been absolutely brilliant, let's now review some of them. I'm going to show you a definition of a word and I would like you to point to the correct word. Let's go then, a version of a gene, 10 seconds, five seconds, point to it in three, two, one. It's an allele, well done. Only one copy of an allele is needed for it to be expressed. 10 seconds, point to it in five, four, three, two, one. It's dominant, excellent. Remember for it to be recessive, there's got to be two copies of the allele. Next one, two identical copies of an allele are present on a path of chromosomes, point to the correct answer in 10 seconds, five, four, three, two, one. It's homozygous, excellent. Remember if it was heterozygous, there'd be two different alleles. Last one, the characteristic which is expressed is called the? 10 seconds, five, four, three, two, one. It's the phenotype, excellent. If you said genotype, remember that is the combination of alleles which causes that characteristic to be expressed. We've covered loads in today's lesson, we've looked at so many new terms, such as alleles, genotype and phenotype, heterozygous and homozygous, as well as that, we've looked at genetic inheritance and how to construct a Punnett square, and we've reviewed our learning. You've been absolutely brilliant in this lesson. Don't forget to click next activity to complete the end of lesson quiz, and I'll see you in the next lesson.

Worksheet: Genetic Inheritance (Higher)

Exit Quiz: Genetic Inheritance (Higher)

Completed: Genetic Inheritance (Higher)

In this lesson we will learn key terms related to genetic inheritance and construct diagrams to predict the outcomes of genetic crosses.

Combined Science

Genetic Inheritance (Higher)

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