Having spent the last day or two writing material about one of the hardest topics in the IGCSE Biology specification (DNA and Protein Synthesis), I am going to write today about something much simpler. You need to understand how the sex of a human is determined at the moment of fertilisation. But this is a topic which can confuse students so I am going to try to explain it for you as best I can.
The sex of a human (whether male or female) is determined by the 23rd pair of chromosomes. Please remember that just because humans determine their sex this way, this doesn’t mean that other species have to be the same. In fact other species use a variety of ways to ensure the correct proportion of male and offspring are born.
As you can see from the picture above, the 23rd pair of chromosomes in humans are called the sex chromosomes. The person whose chromosomes are shown above is male because he has one X and one Y chromosome in his 23rd pair. If we looked at a picture of a human female set of chromosomes, pairs 1 to 22 would be exactly as above, but the 23rd pair would be different. There would be two large X chromosomes rather than one large X and one tiny Y chromosome as shown above.
So a human female has XX as her 23rd pair of chromosomes, a human male has XY as his 23rd pair.
Gametes (Sperm and Egg cells) are made in a process called Meiosis. Remember that meiosis produces daughter cells that are haploid (this means they only have one member of each pair of chromosomes and so half the genetic material)
When a female cell undergoes meiosis in her ovary, the daughter cells produced (egg cells) will contain one of each of the 23 pairs of chromosomes. For the 23rd pair this will always be an X chromosome since both chromosomes in the 23rd pair are X chromosomes.
When a male cell undergoes meiosis in the testis, the daughter cells produced (sperm cells) will contain one of each of the 22 pairs of chromosomes exactly as above. But the 23 pair are different to each other and so half the sperm cells will contain an X chromosome as the 23rd chromosome and half the sperm cells will contain a Y chromosome as the 23rd chromosome.
If you understand the picture above, you understand sex determination in humans. You also need to be able to draw a genetic diagram to show this.
Phenotype: Mum Dad
23rd pair: XX XY
Gametes: X ½X ½Y
Offspring 23rd pair of chromosomes: ½ XX and ½ XY
Offspring phenotypes: ½ female and ½ male
You will have learned at KS3 about the basic structure of a “typical” animal cell. But our bodies are not made of cells that look like this “typical” cell. Humans have just over 200 different types of cell, each specialised to carry out a particular function. For example red blood cells are specialised for transporting oxygen, muscle cells are specialised for movement and sperm cells are specialised as the male gamete for delivering a haploid nucleus to the egg cell.
The diagram above shows examples of a few types of specialised cells from the human body.
These specialised cells are produced in the process of cell division.
Cells that are not yet specialised but that retain the ability to develop into a variety of different cell types are called stem cells. Many cells in the embryo are stem cells (as they have not yet specialised into a particular cell type) but we also have a few stem cells in the adult (for example the cells in the bone marrow that can develop into all the different cell types in blood).
The process by which stem cells develop into specialised cells is called differentiation. Luckily you don’t need to understand exactly how this works but basic idea is this: differentiation involves certain genes in the nucleus being switched on and off so that a specialised cell only makes a certain set of proteins. Remember that a gene is a section of our DNA that codes for a single protein. Nerve cells make the proteins needed to send nerve impulses, white blood cells make the proteins needed to combat infections. You get the idea…..
Stem cells play an important role in medicine but that’s for another post……… If you want to read more about stem cells, this website is a good place to start.
I haven’t had much time to work on my IGCSE Biology revision blog lately – life seems a bit more busy – but the summer holiday does give me time to sit in front of the computer and work on my blog.
I have been through the new specification and can see there are some “gaps” that I need to fill. So please look out for posts on the following specification points:
- Cell Differentiation and Stem Cells 2.5B, 2.6B
- Balanced Diets and Energy Requirements 2.25, 2.26
- Stomata, Leaves and Gas Exchange 2.42B, 2.43B, 2.44B
- Transpiration and the environmental factors that affect it 2.56B, 2.57B, 2.58B
- Nervous v Hormonal Communication comparison 2.86
- Hormones 2.94, 2.95B
- Hormones of the Menstrual Cycle 3.9, 3.10B, 3.13
- DNA, RNA and Protein Synthesis 3.16B, 3.17B, 3.18B
- Determination of Sex of Offspring 3.26, 3.27
- Fermenters and Industrial Culturing of Microorganisms 5.8
If these posts haven’t appeared by September, please contact me by leaving a comment below and give me a “nudge”!
In the meantime, happy holidays!!
In an earlier post, written for my own Y11 students at a previous school, I asked them to check that they knew the answers to 5 questions before attempting to answer any practice questions involving inheritance and genetics. I think that as this blog is now read by a much wider audience, it is probably time to answer the 5 questions myself! Here goes……
1) What is the difference in meaning between a gene and an allele?
People who I have taught know that there is just one thing that every A* student should memorize for IGCSE Biology and that is the definition of a gene.
“A gene is a sequence of DNA that codes for a single protein”
Genes are located on chromosomes and exist in alternate versions called alleles. So for example in pea plants there is a gene that codes for a protein that determines the height of the pea. This gene exists in two possible versions: a T version that makes the plant tall and a t version that makes the plant short. These alternate versions of a single gene are called alleles.
2). Why does the genotype of a person, plant or rabbit always contain two alleles for each gene?
In order to understand this, you need to understand something about chromosomes. Humans, plants and rabbits are all diploid organisms – this means that they have pairs of chromosomes one inherited from mum, one from dad. Because genes are found on chromosomes, this means that a genotype (combination of alleles) will also have two alleles. Alleles are versions of genes, genes are found on chromosomes and chromosomes come in pairs! Simple…
3) What is different about the genotype of a gamete compared with other body cells? Why are gametes different?
Gametes only contain one allele for each gene. This is because gametes are cells that do not contain pairs of chromosomes like every other cell in the body. Gametes are haploid – they only have one member of each pair of chromosomes. One chromosome per pair means only one version of each gene…. Gametes have to be different because they have a different fate or destiny to every other body cell. (Just typing the word destiny means I can hear the Star Wars theme as clear as anything in my head!). Luke’s destiny was to unite the light and dark sides of the force. A gamete’s destiny is less exciting but it is to fuse with another gamete in the act of fertilisation. If both gametes had pairs of chromosomes like every other body cell, then the act of fertilisation would result in a doubling of the chromosome number in every generation and that is clearly unlikely to do anyone any good!
4) How would you explain what is meant by a recessive allele?
A recessive allele is always given the lower case symbol, for example t. The best way to explain what is meant by a recessive allele is to say that recessive alleles only determine the phenotype (the appearance of the organism) if there are two of them. An individual with two identical alleles for a gene is described as being homozygous so a recessive allele will only determine the phenotype in a homozygous individual. In the example I have used so far, because the dwarf allele, t is recessive, the only genotype possible for a short pea plant is tt.
5) What does it mean if two alleles are codominant?
Codominant alleles are alleles that both contribute to the phenotype in a heterozygous individual. Heterozygous is the adjective used if the two alleles present are different to each other.
A good example is the genetics of the ABO blood group system in humans. There are three possible alleles present for this gene in the human population: IA, IB and IO.
The IA allele is dominant to the IO allele.
The IB allele is dominant to the IO allele.
But if you are heterozygous with the genotype IAIB then you have an intermediate blood group called AB. Both alleles are contributing to your blood group and so you are neither blood group A, nor blood group B but a different phenotype called AB. This is because the alleles IA and IB are codominant to each other.
This is a complex topic (not tested at all in the 2018 Paper 1B) so it is worth trying to get your head around all the jargon here over half term…….
If you have any questions at all, please ask me in the “Leave a Comment” box at the foot of this post.
Tomorrow is the first day of the 2018 Easter holidays and so it is definitely a good time for Y11 students to think about how they are going to make the most effective use of the time available for revision. I know that everyone works differently but I thought I would write a post to give you something to think about…. Please leave a comment at the end of this post if you find any of this useful.
Easter holidays are a critical time for Y11 students. The IGCSE and GCSE specifications contain so much content that the challenge for you is mostly one of being on top of so much material come exam day. And your exams arrive so early in the summer term that there won’t be much time when you get back to school after the holidays. So it has to be now! (The summer will be very long and there will be plenty of time for lazy days when nothing at all happens…. )
The hardest thing about revision is getting started. If you can build up some early momentum, you will be able to keep your energy and enthusiasm up right through the holidays. So why not start tomorrow…..?
How to organise a revision plan
- It is vital you have a plan. A little time this evening or tomorrow morning spent on getting organized will be time well spent! This would be how I would do it if I could rewind the clock to the summer of 1987….. U2 released The Joshua Tree that year and I thought at the time it was the greatest music ever made….
- Speak to your parents tonight and ask them to talk you through what’s coming up in the holidays. What family commitments do you have that will impact on your revision schedule?
- Count up how many days you will be able to work between now and when school starts again. For my current students, we have 25 days before term starts. But I think you need a few days off at times in the coming weeks. So let’s say, there may be 20 “work” days.
- This is how I suggest you organise a “work day”…. You always do work in the morning session (9am-11.30am) and then you choose either the afternoon slot (2-4.30pm) or the evening slot (7.30pm – 10pm). This is an ambitious schedule as 5 hours work in a day is quite a lot. But let’s aim high!
- Each session is divided into 5 periods of 30 minutes. You must work on a different subject in each period in a session. Have a plan before the session so you know exactly what you are going to achieve. The next bit is very important. At the start of the session, switch your phone off and put it in a different room. Start a kitchen timer (not a timer on your phone obviously!) and set it to 25 minutes. Work at your revision until the alarm goes off. Then you get a 5 minute gap (tea/check Snapchat) before the next slot starts.
- The PMG schedule has two huge advantages: firstly you have completed half your work for the day by 11.30am (which feels good, believe me) and secondly you can enjoy the time you are not working without feeling guilty…. This is a key component to a successful revision programme. Work when you are supposed to be working but then do other stuff, see friends, do some exercise , watch tv, relax. Revision isn’t effective if you are tired or bored so both must be avoided at all costs.
Many of you have 9 or 10 subjects to revise. If we assume 10 subjects to revise (which makes the Maths easier) 20 work days in the holidays and 10 periods per day (see schedule above), this means that you have 20 periods per subject in the holidays. Do the calculation with your numbers so you know how many revision tasks you need to plan per subject. My example gives you 8 hours 20 minutes per subject – your job is to make the very best use of this time so you gain the most from it. I wish you all the very best of luck and don’t forget to leave a comment below. Happy revising!
The last time I changed jobs was in July 1997 after three hectic but happy years teaching at the Perse School in Cambridge. But with a new position to take up in January at a different school in Cambridgeshire and having now passed the 20 year mark here, I feel like I am hurtling on my final lap towards a finish line in a couple of weeks.
Things now are very different from the heady days at the end of the Millenium. I was probably at least two stone lighter in 1997 for a start…. No Facebook, no Twitter, no Snapchat, dial up Internet but no wifi, no Sky Sports, no texting…. What did we do all day? But the last 20 years have been an amazing experience and I feel privileged to have worked with and indeed taught so many outstanding individuals. I have made memories in my schoolroom, on the sports pitches, at Queens Club, in Dubai and in tutorials that will last a lifetime and I know I have spent the last two decades in a very special institution and one I will continue to hold dear.
But my overriding thoughts at the moment are most certainly looking forwards, not back and I am massively excited at the thought of a fresh professional challenge in 2018. There are lots of people I will miss terribly in the New Year but I also know there is a vibrant, friendly and supportive community at my new school and I am looking forward to running a department. And moving back to live in Northampton full time after 25 years away does feel like coming home…… I can’t wait!!
Well done to everyone who has now finished their 2017 IGCSE Biology course. I hope paper 2 was to your liking (it seemed pretty typical to me) and all the hard work you have put in over two years has finally paid off. I guess that many Y11 students will be almost finished so it is nearly time for you to have a long, relaxing summer break. I wish you all the best of luck when the results come out in August.
Can I ask for one small favour before you switch off schoolwork completely?
If you have time, please can you leave a comment below with any tips you have as to things you have done in revision that really helped you. I imagine you have learned a great deal about how to motivate, organise and maximise your own learning over the past few months so why not leave a short comment to inspire/enthuse/help those that follow you……?
My page settings require me to approve any comment before it appears so don’t expect to see your comment straightaway as it may take an hour or two before it becomes visible.
But I hope you all have a brilliant summer and I wish you the best of luck!
Well we are nearly there….. 24 hours to go until the main IGCSE Biology exam (worth 120 of the total 180 marks). I hope you are all excited and looking forward with optimism to being able to show the examiner how much you understand from the extensive specification. What should you be doing in this final 24 hours?
The most important thing is that you all get a good night’s sleep tonight. Please do not stay up late cramming -it does not work! The evidence base for this is completely clear: you will perform better tomorrow with a normal night’s sleep tonight. So please stop work an hour before you intend to go to bed, relax for an hour watching TV or socialising and then go to bed……
This afternoon you should want to look over a paper or two to familiarise yourself with the kinds of questions and the mark schemes. I would have a final look again at the summer 2015 and summer 2016 paper 1B scripts. Look at your answers and the kind of ways you lost marks. In particular focus on the longer answer questions (for 4,5 or 6 marks) and look at the mark schemes. Often there are marks available for saying obvious things but only if the correct vocabulary is used correctly.
Remember the PMG list of banned words:
Amount (oh no, please don’t ever write this in an exam – think – do you mean mass/volume/concentration?)
Level (do not talk about the level of something, you always mean “concentration” and concentration is a noun that actually means something!)
Substance – what substance are you talking about? oxygen? glucose?
Gases – in questions on gas exchange or transport, please do not write about “gases” – say which gases you mean!
Nutrients – not really a banned word but one that needs very very careful use….. A nutrient is a food molecule, for example glucose, an amino acid or a lipid. Nutrients like these are transported dissolved in blood plasma in mammals but if asked about it, don’t use the word but actually state which molecule you are talking about. None of these nutrients are absorbed into the roots of a plant. Plants absorb mineral ions (nitrate, phosphate, magnesium, potassium etc) through their roots into root hair cells by active transport.
Look over your revision notes a few final times to familiarise yourself with the key words. Focus your final revision on the key areas that you know will definitely come up: there will be a genetics question, there will be an experimental design question, the chances are that fish farming and fermenters will be there as usual……..
If you have worked hard for several weeks (and I know many of you have) you have little to fear in the exam from a lack of knowledge. The thing to fear is losing marks due to rushing, due to not reading the question and due to not giving yourself time to think. None of you will be rushed for time I promise, so please
- read every word in every question
- give yourself time to think before answering – even easy marks can be lost by rushing!
- plan longer answer questions to make sure you cover all the key points using the correct jargon – think before you write anything, “what are the key terms in this topic?” – and then make sure you use them correctly in your answer
Good luck! By the end of tomorrow you will have completed two thirds of your GCSE exams in Biology and that should be a happy thought……
There are lots of you out there revising hard for your IGCSE Biology exams in May/June. I can see because of a graph like the one below showing page views on the blog in every month from August onwards. Let’s see if April and May 2017 can break records on PMGBiology and then with a little luck, your cohort of students can break the record for the highest proportion of A* grades ever awarded.
Many of the students I teach are facing end-of-term exams next week. And for my Y9 and Y10 students these are multiple choice papers which pose a different challenge to the more usual structured answer format. So here is a quick PMG guide to help you score the highest mark you can on this kind of paper. Good luck!
1) Don’t underestimate the opposition!
The biggest mistake students make with these exams is to be over-confident. The reason GCSE exams no longer contain any multiple choice questions is because it was felt that this type of question is too difficult. The examiner, by selecting four incorrect responses to every question, is trying to catch you out and the more plausible the incorrect responses, the better the question! So your understanding needs to be really good to not be tricky by these underhand tactics…. Get revising!
2) Read the question carefully.
This skill is tested even more fully in a multiple choice exam. You spend less time writing answers (obviously), so this allows more time for reading and thinking. Even with a “mark a minute” time frame, there is time for thinking and planning before deciding on a correct answer. Write on the question paper, use it for rough work and planning. And RTQ carefully – with many similar but incorrect answers on the page, it is so easy to decide too quickly what the correct response is and so miss out on marks you could obtain with a little more care.
3) Try to eliminate the obviously incorrect answers: this will mean you are only focussing your thinking on the possibles. It also means if you have to guess, you are maximising your odds of being lucky! But be careful….. This strategy has an obvious flaw so be careful in what you eliminate.
4) Your best chance of getting a question right is the first time you answer it. So I suggest you leave blank any answers you are unsure of on your first go through the paper. Don’t spend more than a minute on any question on this first pass through the paper. You can come back to look at the harder questions at the end of the exam. It is much harder in an exam to spot an error in a question you have already answered, so I would leave answers blank unless I am sure I am right….
5) Keep an eye on the time. After 20 minutes of the exam, check that you are ahead of the time. Most multiple choice exams are a “mark a minute” but I know that in our internal exams this isn’t always the case, so check.
6) Finally, make sure you answer every question. There is only positive marking in these exams so you should always guess (see point 3 above). The only way you can guarantee you won’t get a mark is if you leave the question blank!
Take a pencil and a rubber to these exams – both will be useful! And good luck……..