a slight reduction if you have two children in the nursery. It’d be £26 a day each. I know it’s not much but it all helps. Anyway, shall we take a little tour and then we’ll go over any details once you’ve decided what you think. I’ll make us a cup of …
014
twins. With identical twins, this doesn’t seem to be the case at all. There are no known causes for identical twinning. So, are identical twins the same in every way?
Unit 2 Listening 2
C Listen and complete the notes a student made. Voice: Well, we’ve been looking at various aspects of genetics, and this morning I want to talk specifically about twins. Some of you were asking about twins last week and particularly about how environment or nurture affected their development.
First of all, I want to establish the fact that there
are two and possibly even three types of twins. I’m sure everyone has heard the term identical twins. The scientific term is actually monozygotic twins. Now, people assume that with identical twins it’s all about appearance – that they look exactly the same. Now, that might be the case, but the important thing is actually how they form. Identical twins form when a single fertilized egg splits into two. Non-identical twins form when two eggs are fertilized separately. Twins that are not identical are usually called fraternal twins, though the scientific term here is actually dizygotic. Around a third of twins are identical – two-thirds
are fraternal. So, fraternal, or non-identical twins, are quite a lot more common than identical twins. Now, I mentioned a possible third category, but not very much is known about this yet. Some twins might be hybrid or half-identical. This may happen when an unfertilized egg splits into two and the two parts are then fertilized separately. I’m not going to say much about this today – I don’t know much! – but there’s a very good website where you can find out more. That’s
www.humangenetics.com. Human genetics is all one word. Take a look.
OK, some facts about the two types of twins.
Firstly, identical twins are always the same gender – two boys or two girls, and they have the same blood group. Fraternal twins can be the same gender or a boy and a girl. They often have the same blood group, but not necessarily. Now, perhaps most importantly, identical twins share 100% of their genetic markers – fraternal twins share around 50%. So, it’s clear that identical twins are more likely to be the same in many ways – not simply the way they look. Now, before I talk about how twins can be very similar or what might cause them to be different, I’m going to mention another difference between the two types of twins – the reason that they are born as twins in the first place. We know that fraternal twins are more common when women have babies later in life – perhaps after 35 years of age. We also know that fertility treatment can increase the likelihood of
No, they’re not. They’re probably very similar, but it’s a question of nature and nurture. There are all sorts of factors and influences that determine how a person grows up and it’s no different for twins. Even before birth, in the womb, twins can develop differently. It is common that one twin has a better connection to the placenta and this will mean he or she will grow bigger. Identical twins can often be slightly different heights and weights. Recent research suggests that although identical twins are born with many identical genetic characteristics, as they age and spend more time apart, these similarities diverge. Twins develop their own personalities and follow different interests. It is likely that identical twins even deliberately try to establish individual identities as they grow up. It is not known yet whether certain identical genes actually disappear as twins age, or whether external influences play a bigger part in determining individuality. Now, this research has been carried out at a
cancer research centre and this is important. It might be that the genetic make-up which makes identical twins so similar is also related to the development of disease like cancer and conditions such as autism. But, to get back to similarities and differences – I like one comparison I’ve seen that compares twins to cakes cooked from the same or different recipes. Two cakes cooked from the same recipe are more likely to be very similar than two cakes cooked from two different recipes. The two cakes from the same recipe will be very similar, but not exactly the same. Various external influences will mean that each cake has something slightly different about it. Another way to look at this is to think about your own face. Both sides of your face come from the same DNA, but they are not exactly the same – they’re not a mirror image of each other. All sorts of factors and experiences in life mean that one side of your face is almost certainly a little different, and in some cases very different, from the other side.
015 Unit 2 Writing 2
C Listen to some British students discussing the Writing task and answer the questions below. Male: What do you think about this composition then, Denise?
Female: Mm, I think it’s easier to do because we’ve been discussing the topic this week. There’s quite a lot to say about it.
Male: Really? I think it’s quite difficult. I’m not a scientist, so I don’t know whether nature or life experience is the bigger influence.
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