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
All organisms have the potential to reproduce. Reproduction is one of the 7 characteristics of life (8 if you study EdExcel IGCSE Biology…..) and of course it means the ability to produce new individuals. But over the 4 billion or so years life has been around on the planet, evolution has developed a myriad ways of producing new individuals. So as biologists are simple folk (well all the ones I work with are….), it makes sense to group different reproductive strategies together to make it easier to understand.
The major distinction between different ways of reproducing is to divide them into asexual and sexual reproduction. This works fairly well, although it seems to be a subject many GCSE students don’t understand too well. So here goes….
The first big idea to dispel is that the number of parents involved determines whether reproduction is sexual or asexual. Too often I am told that asexual involves one parent, sexual two. But although this works in most cases, sexual reproduction can happen with only one parent, for example in flowering plants that self-pollinate. So we need a different way of deciding whether reproduction is sexual or asexual.
And in fact a clear distinction does exist and it is to do with genetics. If the offspring produced are genetically identical to the parent (i.e. a clone) then it is an asexual form of reproduction. If the offspring produced are genetically different to the parents, then it is sexual.
Often sexual reproduction involves the process of fertilisation. This allows two parents to each contribute half their genetic material to their offspring thus generating individuals with new and unique genetic make ups. These specialised cells that contain half the genetic material are called gametes and as you all know, they are made by a special type of cell division called meiosis. Meiosis is vital for sexual reproduction as it produces cells that are haploid (one member of each pair of chromosomes) and all genetically unique.
So this diagram shows two humans each producing gametes by meiosis. The parents on the left will have 23 pairs of chromosomes and so each haploid gamete will have 23 individual chromosomes. Fertilisation restores the diploid number. Mitosis is then used to turn this single cell, the zygote into a multicellular embryo and then indeed into a new individual. (You will remember mitosis is a type of cell division that always produces genetically identical daughter cells)
The examples of asexual reproduction on the left all involve only this second type of cell division, mitosis. There are no gametes, no fertilisation and hence no genetic variation. The simplest type of asexual reproduction is shown as (A) and this is called binary fission. A single-celled organism can divide in two to produce two genetically identical daughter cells. Hydra (B) are a simple type of animal and they reproduce by budding. A new individual just grows off the side and when it is big enough, it drops off….. And many plants can reproduce asexually using a technique called vegetative propagation. The sweet potato plant in (C) can produce several offspring plants from each potato but as they are all clones of each other, this is definitely asexual reproduction.
Students do get confused with this topic so please ask me a question using the comment feature below the post. Keep revising hard!
I am slightly wary about writing about the male and female reproductive systems. Not because I get embarrassed with this topic (5 terms of human dissection at medical school removed any squeamishness about body parts….) But rather that I worry that the school’s internet filters might start blocking my website if the wrong words appear. But you don’t know until you try, so here goes…..
Male Reproductive System
I will start with the male reproductive system as males are simpler than females in many, many ways… The male reproductive system has three functions:
- to produce the male gametes, sperm cells, at a prodigious rate
- to make the male sex hormone testosterone
- to act as a delivery system to ensure sperm cells are carried into the female reproductive tract in conditions that will allow them to fertilise an egg
The first two functions listed above happen in the testis. There are cells in the testis that secrete the hormone testosterone into the blood from puberty onwards. Testosterone switches on secondary sexual characteristics in the male (body hair growth, muscle development, change in pitch of voice etc.) after puberty. The main part of the testis is made up of very long coiled tubules called seminiferous tubules in which the sperm cells are made.
Humans have over a hundred meters of seminiferous tubules in total in both testes and this allows sperm cells to made at a very fast rate. Even though it takes around 75 days to make an individual sperm cell, the testes make them at a rate of around 85 million sperm cells per day. The epididymis is found next to the testis in the scrotum and is a coiled tube in which sperm cells continue to develop and mature. Sperm are stored here too in readiness for ejaculation.
Everyone knows that in humans the testes are found outside the body cavity in order to keep them cool. Sperm production happens at a maximal rate 3 degrees below core body temperature and having testes outside the body keeps them at this temperature.
The vas deferens is a tube lined with smooth muscle that carries sperm cells away from the testis for ejaculation. As you can see it loops around the back of the bladder, before joining up with the urethra just below the bladder. The urethra is the tube that carries urine away from the bladder but can also carry semen once the vas deferens has joined with it.
There are three accessory glands in the male system (prostate gland, seminal vesicle and the Cowpers‘ glands) These glands produce the fluid that when mixed with the sperm cells is called semen. Semen contains a sugar fructose to provide energy for the sperm cells to swim. It is slightly alkaline to neutralise the acidity in the vagina and also contains mucus to make the fluid easy to move along the tubes.
The sperm cells only acquire the ability to swim when in the epididymis and only become totally mature and able to fertilise the egg right next to the egg cell in the female tract.
The penis is an organ that contains erectile tissue that can fill with blood to allow the penis to fit into the female vagina for ejaculation.
Few things in life are certain, famously just death and taxes. Northampton Town flirting with relegation can perhaps be added to this list. But you can be pretty certain that tucked away somewhere in your iGCSE Biology exam there will be a genetics question that asks you to draw a genetic diagram. There are usually four or even five marks available and so learning how to ensure you get all these marks is vital in your quest for an A* grade.
GCSE candidates are terrible at doing genetic diagrams: they fill the space with messy scribbles, doodles, strange tables and lines and then confidently write 3:1 at the bottom… Not a recipe for success. So learn how to do it, be neat, take your time and you can guarantee full marks.
If the question doesn’t do it for you, you should start by defining what the letters you will use for the alleles. If one allele is dominant over the other, it is conventional to use the upper case letter for the dominant allele, the lower case letter for the recessive one. It will tell you in the question which allele is dominant.
Start your genetic diagram by writing the phenotype of the parents in the cross.
e.g. Parental Phenotype: Tall Tall
Underneath the phenotype, write the genotype of the parents.
Parental Genotype: Tt Tt
Then you need to think about which alleles are present in the gametes. Gametes are haploid and so will contain one of each pair of homologous chromosomes – in this example there can only be one allele in each gamete (as we are only looking at one gene)
Gametes: T t T t
Next show random fertilisation. I think it is much better to draw a Punnett square that has the male gametes down one side, the female gametes down the other and then carefully pair them up. This is a stage where mistakes can be made if you rush so however simple you think this process is, take your time…..
Finally you need to copy out the offspring genotypes from your Punnet square, like so
Offspring Genotypes: TT Tt Tt tt
And underneath each one, write the offspring phenotype
Offspring Phenotypes: Tall Tall Tall Dwarf
Finally, answer the question. If it asks for a probability, express your answer as either a percentage or a decimal or a fraction. So if I were asked what is the probability of a homozygous pea being produced, the answer is 50% or 0.5 or 1/2
Follow these rules and you will always score full marks – happy days……..