Inheritance

Introduction

Read the following passage and listen to the story about Gregor Mendel’s life. Find six false facts in the story you hear.

Before settling down as a monk in the monastery of St. Thomas in Brünn (now Brno, in the Czech Republic), Gregor Mendel tried his hand at several pursuits, including health care and teaching. To earn his teaching certificate, Mendel attended the University of Vienna for 2 years, where he studied botany and mathematics, among other subjects. This training proved crucial to his later experiments, which were the foundation for the modern science of genetics. At St. Thomas in the mid-1800s, Mendel carried out both his monastic duties and a groundbreaking series of experiments on inheritance in the common edible pea. Although Mendel worked without knowledge of genes or chromosomes, we can more easily follow his experiments after a brief look at some modern genetic concepts.

Reading

I. Read the extract from a textbook and write out all the terms relevant to the topic of the unit and their definitions contained in the text.

A gene’s specific physical location on a chromosome is called a locus (plural, ‘loci’). Homologous chromosomes carry the same genes, located at the same loci. Although the nucleotide sequence at a given gene locus is always similar on homologous chromosomes, it may not be identical. These differences allow different nucleotide sequences at the same gene locus on two homologous chromosomes to produce alternate forms of the gene, called alleles. Human A, B, and O blood types, for example, are produced by three alleles of the same gene.

If both homologous chromosomes in an organism have the same allele at a given gene locus, the organism is said to be homozygous at that gene locus. (“Homozygous” comes from Greek words meaning “same pair”.) If two homologous chromosomes have different alleles at a given gene locus the organism is heterozygous (“different pair”) at that locus and is called a hybrid. During meiosis, homologous chromosomes are separated, so each gamete receives one member of each pair of homologous chromosomes. Therefore, all the gametes produced by an organism that is homozygous at a particular gene locus will contain the same allele. Gametes produced by an organism that is heterozygous at the same gene locus are of two kinds: half of the gametes contain one allele, and half contain the other. (…)

Mendel’s choice of the edible pea as an experimental subject was critical to the success of his experiments. In plants, a male gamete, which for simplicity we’ll call the sperm, is contained in each pollen grain. The structure of the pea flower normally prevents another flower’s pollen from entering. Instead, each pea flower normally supplies its own pollen, so the egg cells in each flower are fertilized by sperm from the pollen of the same flower. This process is called self-fertilization. Even in Mendel’s time, commercial seed dealers sold many types of peas that were true-breeding. In true-breeding plants, all the offspring produced through self-fertilization are homozygous for a given trait and are essentially identical to the parent plant.

Although peas normally self-fertilize, plant breeders can also mate plants by hand, a process called cross-fertilization. Breeders pull apart the petals and remove the stamens, preventing self-fertilization. By dusting the carpels with pollen they have selected, breeders can control cross-fertilization. In this way, two true-breeding plants can be mated to see what types of offspring they produce.

In contrast to earlier scientists, Mendel chose to study traits – heritable characteristics – that are unmistakably different forms, such as white flowers versus purple flowers, and he worked with one trait at a time. These factors allowed Mendel to see through to the underlying principles of inheritance. Equally important was the fact that Mendel counted the numbers of offspring with each type of trait and analyzed the numbers. The use of statistics as a tool to verify the validity of results has since become an extremely important practice in biology.

II. Add to the list of terms the following and write their definitions in English.

first-generation offspring

second-generation offspring

parental generation

Mendel’s law of segregation

dominant allele

recessive allele

genotype

phenotype

the Punnett square method (by R.C.Punnett)

test cross

linkage

crossing over

III. In certain cattle, hair colour can be red (homozygous RR), white (homozygous R’R’), or roan (a mixture of red and white hairs, heterozygous RR’).

a. When a red bull is mated to a white cow, what genotypes and phenotypes of offspring could be obtained?

b. If one of the offspring in (a) were mated to a white cow, what genotypes and phenotypes of offspring could be produced? In what proportion?

Explain the solution of the problems in English.

Language focus 1

May and Might: Possibility and Degrees of Certainty

1 May and might are used mainly to express the uncertainty of something happening, and to ask for and give permission.

I may see you tomorrow. (Perhaps a 50% chance)

Peter might phone. (Perhaps, a 30% chance). If he does, ask him to ring later.

Do you think I might borrow your typewriter? (Permission, very polite)

In the majority of cases might is not the past form of may!

I felt very hot and tired. Perhaps I was ill. (NOT …I might be ill.)

2 Uncertainty in the past – use might have done:

What was that noise? – It might have been a cat (we don’t know and just guess)

Chances in the present or future with by:

I’ll try phoning him, but he may have gone out by now. (present)

By the end of this year I might have saved some money. (future)

3 Something was possible but did not happen – use might have done or could have done:

If she hadn’t been so bad-tempered, I might/could have married her .

4 In scientific and academic language, may is often used to talk about typical occurrences – things that can happen in certain situations.

A female crocodile may/can lay 30-40 eggs. (can is less formal)

The flowers may have five or six petals, pink or red on colour.

With this meaning might can be used to talk about the past:

In those days, a man might/could be hanged for stealing a sheep. (could is less formal)

May and Might: Degrees of Certainty

He may (might) (not) be at home. Может быть (возможно), он (не) дома.

(might is more uncertain)

He may (might) come soon. Может быть (возможно), он скоро придет.

He may (might) be reading now. Может быть (возможно), он сейчас читает.

He may (might) have come already. Может быть (возможно), он уже пришел.

He may (might) have been waiting for us for an hour. Может быть (возможно), он ждет нас уже час.

I. Say what the meaning of may (might) is in the following sentences:

1. I have found a screwdriver! That may work!

2. Might I possibly see the director?

3. You may not be late for the meeting.

4. You might have come home earlier. We were waiting for you to have dinner.

5. He might be killed on the street if he had taken part in the demonstration of protest.

6. They might need your help. Ask them if you could help with anything.

7. You might listen to me more attentively.

8. May I use your telephone, please?

9. I may go to Austria on holidays.

10. They were told they might see the boy if they wore special clothes.

II. Fill in the gaps with the most appropriate form of the infinitive.

1. The students might ……………… (wait) for half an hour already for the lecture to begin, but the professor is still talking to another professor who does not have a lecture at the moment.

2. Look! He may …………….. (come) already. There is no one in the hall.

3. It may …………….(rain) today. The clouds are so dark.

4. She might …………….. (be) at home, so call her and ask. I will give you her telephone number.

5. I might ……………. (not lose) my temper when I speak to her, I don’t know.

6. They may ………………. (tell) him everything about me. What will he think of me?!

7. I don’t let her know my secret because she may ……………… (tell) everybody.

8. Look! It may ………………….. (rain) for several hours. The roads are so wet.

9. Mum might ………………….. (talk) to her friend. She’s been in the other room for an hour. Or she might ………………… (go) out.

III. Translate into English.

1. Возможно, вчера еще были лекции, но я остался дома.

2. Может быть, он пошел к врачу. Я сегодня не видела его на занятиях.

3. Может, он гуляет сейчас где-то со своими знакомыми, но я не уверена.

4. Он может уехать в любой день, так что свяжись с ним сегодня.

5. Возможно, я не приду завтра. Что-то я плохо себя чувствую.

6. Он, возможно, уже два часа спит. Надо его будить.

7. Я, может, и соглашусь, но только когда узнаю все условия.

8. Мы могли бы подождать тебя. Иди и поговори с ним, пока он здесь.

9. Он, возможно, ждет твоего звонка. Позвони ему!

10. Быть может, он не сможет сегодня навестить тебя. Не расстраивайся.

IV. Make up situations of your own using one or several of the following sentences:

1. May I see your hand?

2. You might have been cleverer!

3. I may close my eyes and jump if you want.

4. They might have chosen the wrong one, if not for me!

5. Might I possibly see the one you’re holding?

Language focus 2

Permission, Offer

1 Several forms are used to ask for and give permission to do things. These forms differ in formality, or politeness.

Most used and not very formal:

Can I borrow your car, Dad? – Sorry, you can’t. I am going to need it soon.

Could I ask for something? – Yes, of course, you can.

Could I possibly ask for something? – Sure, please do.

Do you think I could borrow your bike? – OK, but be careful.

More formal/polite:

May I take this book home, please? – No, you may not. It is for library use only.

Might I possibly borrow your pen for a moment, Professor? – Please do.

Do you mind if I smoke in here? – Sorry, it is a non-smoking area.

I wonder if I could possibly have something to eat. – Make yourself at home!

2 When you offer to do things, you may sound more or less definite.

I’ll get you a taxi. – Thanks! (more definite)

Can I carry your bag? – Oh, thank you.

Would you like to have a cup of coffee? – Yes, that would be very nice.

I could mend your bicycle for you, if that would help. (less definite)

I. Fill in the gaps with a suitable word or expression.

a) - Could I ……………… borrow your textbook till tomorrow?

- Sorry, you ……………. . I’ve not yet done the home task.

b) …………………………. if I take a look at your notes?

- Please …………….. . Always welcome.

c) (on the phone) - May I speak to Professor Snow, ………………?

- Sorry, he is now at the meeting. Will you leave a message for him?

d) Do ……………………. I could go to that party with you?

- Well, I think everybody would be just happy if you did.

e) - …………………… if we could go out tonight.

- Oh, I think it is a good idea.

II. Choose the characters and situation and prepare a dialogue using at least two of the expressions from exercise I.

A father and a daughter A customer and a shop assistant A customs officer and a smuggler A police officer and a hippie A secretary and a visitor A doctor and a patient

III. Think of situations where the following sentences could be said. Act out the situations

1. Would you like a brown or a yellow one?

2. I think I could handle it for you.

3. I’ll send you one.

4. Can I help you out?

Words, Words, Words

I. First, fill in the gaps in the extract with appropriate words. Second, after checking the words formulate with your partner five true statements and five false statements based on the text. Read them to another pair so that they determine which ones are true/false.

Sex-Linked Genes are Found Only on the X or Only on the Y Chromosome

Genes that are on one _______ chromosome but not on the other are said to be sex-linked, or X-linked. In many animals, the Y chromosome carries relatively few genes other than those that determine _______, whereas the X chromosome bears many _______ that have nothing to do with specifically female traits. The human X-chromosome, for example, contains genes for colour vision, _______ clotting, and certain structural proteins in muscles that have no counterpart on the Y chromosome. Therefore, because they have two X-chromosomes, _______ can be either homozygous or _______ for genes on the X chromosome. Normal _______ versus recessive relationships among alleles will be expressed. _______, in contrast, most fully express all alleles they have on their single X chromosome, whether those _______ are dominant or recessive. For this reason, in humans, most cases of colour _______, hemophilia, and certain types of muscular dystrophy occur in males.

How does the sex linkage affect inheritance? Let’s look at the first example of sex linkage to be discovered, the inheritance of eye color in the fruit _______ Drosophila. Because these flies are small, have a rapid reproductive rate, and have few chromosomes, _______ have been favoured subjects for genetic studies for more than a century. Normally, Drosophila have red eyes. In the early 1900s, researchers in the laboratory of Thomas Hunt Morgan at Columbia University discovered a male fly with _______ eyes. This white-eyed male was mated to a virgin, true-_______ red-eyed female. All the resulting offspring were red-eyed flies, suggesting that white colour ( r ) is _______ to red ( R ). The second generation, however, was a surprise: there were nearly equal numbers of red-eyed males and white-eyed males but no females with white eyes! A test cross of the first-generation red-eyed females and the original white-eyed male yielded roughly equal numbers of _______ and white-eyed males and females.

II. From the data above, could you explain the case?

Language focus 3

May and Can

1 these two verbs can both express possibility due to circumstances:

He may find the book at the library = He can find the book at the library. (present or future)

But in questions and negatives only can is used.

Might in this meaning does not express past. It is found only in reported speech.

He said you might find the book at the library.

Could, however, expresses a past action.

He could find the book at the library yesterday.

2 asking for permission, we use both can and may but may is more formal:

May (might = more formal) I speak to you for a moment, professor?

Can (could) I have a cup of tea, Mum?

3 for uncertainty in affirmative sentences, use only may (might = more uncertain):

He might have gone home, if he’s not here.

4 for uncertainty in questions, use only can (could = more uncertain):

Could he have said that?

5 to say that you don’t believe something is true, use can’t or couldn’t (more uncertain):

He simply can’t have said that!

I. Add one sentence to each of the ones given here to clarify the situation they are used in.

1. You might have come earlier!

2. They could have called me.

3. He can sing and dance.

4. I may go to the Urals on holiday.

5. Can it be true?

6. Could he have said that?

7. You may go.

8. You might decide now!

9. He might be at home.

10. They could have been waiting for us for half an hour.

11. They can’t have done that!

12. Could I come tomorrow?

13. You could be more attentive.

14. How could you say that?

15. He may have been here for two hours, waiting.

II. Translate into English.

1. Может быть, наша группа и собиралась вчера, но я об этом не знаю.

2. Мы могли бы увидеться завтра, часов в пять.

3. Не может быть, чтобы ты это написал!

4. Можно, я возьму свои книги?

5. Он может и сообщить заведующему кафедрой обо всем.

6. Разве сейчас у другой группы идет лекция?

7. Он смог достать эту книгу в одном маленьком магазинчике.

8. Неужели вы уже все написали!

9. Может быть, мы всю жизнь его ждали.

10. Не может быть, чтобы мы всю жизнь ждали его.

11. Он мог там быть в двенадцать. Я не знаю, был ли он там.

12. Ты сможешь зайти ко мне завтра?

13. Разве она сможет сделать это?

Words, Words, Words

I. Fill in the gaps with appropriate modal verbs.

A DNA Probe ___ be Used to Locate the Gene within the DNA Library

Researchers find genes of interest by using DNA probes, sequences of nucleotides that are complementary to those genes. DNA probes are generated in specific ways. For example, if you know the amino acid sequence of the protein encoded by a gene, you ___ work backward through the genetic code to determine the corresponding DNA nucleotide sequences. Researchers synthesize a complementary copy of the nucleotide sequence for all or part of the gene, incorporating a marker such as radioactivity. This synthetic sequence ___ then be used to identify bacteria from the DNA library that contain a plasmid with the gene in question.

Messenger RNA ___ also be used as a DNA probe. Immature red blood cells, for example, synthesize lots of hemoglobin, so mRNA for hemoglobin ___ easily be extracted from them. This mRNA is complementary to the DNA of the hemoglobin gene and ___ be labeled and used as a DNA probe to locate the plasmid-containing bacteria. The amino acid sequences for many human hormones are known; these sequences ___ be used to create DNA probes for the genes for human hormones.

Once mRNA for the desired gene is obtained, it ___ also be used to generate the corresponding complementary strand of DNA (cDNA) by using a viral enzyme called reverse transcriptase. Geneticists ___ then use the cDNA as a DNA probe to identify the bacterium that contains the appropriate plasmid. The cDNA ___ instead serve as a template for generating double-stranded DNA, which ___ then be copied (…).

After checking the modal verbs look through parts of sentences below describing the synthetic sequence that can be used to identify bacteria from the DNA library. Match parts of the sentences and translate the resulting text into both Russian and English.

1 Bacteria from the DNA library are sparsely distributed over a culture… 2 Each bacterium multiplies into a visible bacterial population (called a colony)… 3 A sheet of special filter paper is pressed… 4 It picks up a few bacteria from each colony,… 5 The original culture dish… 6 The filter paper is placed in a basic solution, breaking open the bacteria, freeing the plasmids, and… 7 The paper is bathed in solution of neutral pH that contains a DNA probe (yellow),… 8 The probe hydrogen-bonds only to plasmid DNA that is complementary to the nucleotide sequence of the probe and… 9 The locations of radioactivity on the paper are matched to the… 10 Colonies in the same position consist of… 11 Samples of these bacteria are now cultured in new…

A …последовательность нуклеотидов, комплементарных нужному гену. B …на чашку для культивирования. C …чашках, воспроизводя нужный ген. D …местоположением бактериальных колоний на первоначальной чашке для культивирования. E …содержащей питательную среду. F …содержащую единичный тип комбинации «плазмида – ДНК человека». G …бактерий, содержащих плазмиды с нужным геном. H …разделяя двунитевую ДНК плазмид на одиночные нити. I …следовательно, содержит человеческий ген. J …остается в сохранности. K …сохраняя позиции колонии.

II. Choose the correct modal verb.

Most Human Genetic Disorders are Caused by Recessive Alleles

The human body depends on the integrated actions of thousands of enzymes and other proteins. A mutation in the gene coding for one of these enzymes almost always impairs or destroys enzyme function. However, the presence of one normal allele (could/may) generate enough functional enzyme or other protein that heterozygotes with a defective allele are phenotypically indistinguishable from homozygous normals. Therefore, most normal alleles are inherited as dominant traits and mutant alleles as recessive traits. In other words, both alleles (must/can) be defective for the problem to show up in the offspring.

Heterozygous carriers are phenotypically normal but (can/may) pass on their defective recessive allele to their offspring. Although many people are carriers of a serious genetic defect, an unrelated man and woman who marry are unlikely to possess the same defective allele and to produce a homozygous child with the disorder. Related couples, however (especially first cousins or closer), have inherited some of their genes from recent common ancestors. Therefore, they are much more likely to carry the same defective allele and, if they bear children, to pass on the defect.

III. Eugenics is a term applied to the notion that the human condition might be improved by improving the human genome. Do you think there are both good and bad sides to eugenics? What examples can you think of to back up your stand? What would a eugenicist think of the medical advances that have ameliorated the problems of hemophilia?

IV. Using the data that follow, give a brief description of some genetic diseases.