INIT IV. INHERITANCE Text 4.1. Variation
■ Essential targets:
By the end of this text you should be able to:
define the following genetic terms: allele; homozygous; heterozygous; dominant; recessive; polygenic
distinguish between genotype and phenotype
distinguish between continuous variation and discontinuous variation
explain how mutations contribute to variation.
Pre-reading
■ Discuss these questions with your partner. Then scan the text quickly to find the answers.
What is variation?
What is mutation?
Are mutations harmful or beneficial?
How do X-rays influence the mutation rate?
Exercise A. Match the words with their definitions:
1.
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ultimate
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A
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being the only one of its kind
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2.
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unique
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B
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a process by which two or more things have an effect on each other and work together
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3.
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distinguish
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C
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the final and the most important one
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4.
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source
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D
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the air, water and land in which people, animals and plants live
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5.
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feature
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E
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someone`s child or children; animal`s baby or babies
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6.
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spontaneous
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F
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how tall someone is
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7.
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twin
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G
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a thing, place, activity that you get something from
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8.
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offspring
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H
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a part of something that you notice because it seems important, interesting, or typical
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9.
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identical
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I
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to be able to recognize and understand the difference between two similar things or people
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10.
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environment
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J
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happening or done without being planned or organized
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11.
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interaction
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K
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one of two children born at the same to the same mother
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12.
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height
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L
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exactly the same
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■ Read and translate the given text and make your essential assignments:
The Earth is inhabited by billions of organisms, every one of which is unique. Individuals belonging to different species are usually easy to distinguish; members of the same species may differ only in small ways; but even clones (such as identical twins) show some subtle differences. The differences between individuals of the same species are called variation. These differences may be the result of genetic differences, the influence of the environment, or a combination of genetic and environmental influences.
Genetic variation
G enetic differences reflect the genotype of an organism, that is, its genetic make-up. A diploid organism has two sets of chromosomes and two forms (alleles) of each particular gene. These alleles may be the same (the organism is homozygous for that gene) or different (the organism is heterozygous for that gene). If different, one of the alleles (the dominant allele) may mask the other allele (the recessive allele). The dominant allele is therefore expressed in either the heterozygous or the homozygous condition, whereas the recessive allele is expressed only in the homozygous condition. If an organism is haploid (that is, it has only one set of chromosomes), all its alleles will be expressed and will be reflected in its observable or measurable characters (the features or traits transmitted from parent to offspring).
Phenotypic variation: continuous and discontinuous
The measurable physical and biochemical characteristics of an organism, whether observable or not, make up its phenotype. The phenotype results from the interaction of the genotype and the environment. The genotype determines the potential of an organism, whereas the environmental factors to which it is exposed determine to what extent this potential is fulfilled. For example, in humans the potential height of a person is genetically determined, but a person cannot reach this height without an adequate diet. Phenotypic variation (commonly referred to simply as variation) is of two main types: continuous and discontinuous.
In continuous variation, differences are slight and grade into each other. Characteristics such as human height and weight show continuous variation, and are usually determined by a large number of genes (they are polygenic) and/or considerable environmental influence.
In discontinuous variation, the differences are discrete (separate) and clear cut: they do not merge into each other. Discontinuous variations are generally caused by different alleles of one, two, or only a few genes.
Continuous variations are usually quantitative (they can be measured) whereas discontinuous variations are qualitative (they tend to be defined subjectively in descriptive terms). Thus height in humans is a continuous variation given a value in metres, whereas height in sweet peas is a discontinuous variation described as 'tall' or 'dwarf.
Mutations: more variation
Genetic variation arises partly from sexual reproduction by a combination of independent assortment, crossing over, and random fertilisation. However, these processes merely shuffle the existing pack of genes so that new combinations are made. The ultimate source of inherited variations is mutations.
A mutation is a change in the amount or the chemical structure of DNA. If the information contained within the mutated DNA is expressed (that is, transcribed into mRNA and translated into a specific polypeptide chain) it can cause a change in the characteristics of an individual cell or an organism. Mutations in the gametes of multicellular organisms can be inherited by offspring. Mutations of the body cells of multicellular organisms (somatic mutations) are confined to the body cells derived from the mutated cell; they are not inherited.
Mutations can happen spontaneously as a result of errors in DNA replication or errors during cell division, or they can be induced by various environmental factors (such as certain chemicals, X-rays, and viral infection). Factors that induce mutations are called mutagens.
Chromosome mutations and gene mutations
Alterations in the number or structure of chromosomes are called chromosome mutations. Chromosome mutations can happen during mitosis and meiosis when chromosomes are being condensed and pulled apart. Homologous chromosomes may fail to separate, resulting in non-disjunction. Chromosome mutations also occur during interphase when DNA replicates, and during crossing over when sections of chromosomes are exchanged.
Gene mutations are changes in the nucleotide base sequence in a cistron (the portion of DNA that makes up a single gene). A change of a single nucleotide base pair is called a point mutation. There are a number of types of point mutation, including:
▪ substitution - the replacement of one nucleotide with another containing a different base
▪ deletion - the loss of a nucleotide
▪ insertion or addition - addition of an extra nucleotide.
Sickle-cell anaemia is an example of an inherited condition that results from a substitution. Gene mutations may also result from duplication (repetition of a portion of a nucleotide sequence within a cistron) and inversion (reversal of the portion of the nucleotide sequence in the cistron).
Most mutations, if expressed, are harmful. Note, however, that in diploid organisms such as ourselves, mutations usually result in recessive alleles. These are expressed only in the homozygous condition unless the mutation is on the X chromosome. Many mutations result in a change in the shape of a protein so that the protein cannot function properly (for example, the mutation that causes sickle-cell anaemia). Mutations that affect large sections of a gene, and chromosome mutations are often lethal. However, some mutations have no effect: a mutation may occur in a non-coding part of DNA; it may produce a different codon for the same amino acid; or the altered amino acid sequence may not affect the protein's shape or function. Occasionally, a mutation is beneficial, changing the phenotype so that an organism has a better chance of surviving and reproducing. Although beneficial mutations are very rare events, they are bound to happen sooner or later if there is a large number of individuals in a population. These mutations are of immense importance because they are the ultimate source of all variation: the raw material for the evolution of new species by natural selection.
■ Glossary of essential terms for you to know
№
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English term
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Russian equivalent
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1.
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variation
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генетическая изменчивость, мутация
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2.
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source
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источник, причинa
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3.
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ultimate
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конечный, предельный, основной
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4.
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to inherit
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наследовать
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5.
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to arise
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возникать
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6.
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spontaneously
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спонтанно, хаотично
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7.
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to induce
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вызывать, побуждать, причинять
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8.
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mutagen
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мутаген
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9.
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mustard gas
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иприт, горчичный газ
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10.
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unique
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уникальный
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11.
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to distinguish
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различать
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12.
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twins
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близнецы
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13.
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subtle
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незаметный
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14.
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make-up
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состав, конституция
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15.
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to mask
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маскировать, скрывать
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16.
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feature
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черта, качество
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17.
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trait
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свойство, особенность
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18.
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offspring
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потомство
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19.
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to fulfill
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реализовывать
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20.
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to grade into еach other
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постепенно переходить одному в другой
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21.
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considerable
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значительный
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22.
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to merge
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сливаться
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23.
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quantitative
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количественный
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24.
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qualitative
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качественный
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25.
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subjective
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субъективный, индивидуальный
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26.
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descriptive
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описательный
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27.
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dwarf
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карликовый
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28.
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discontinuous
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прерывистый, прерывающийся
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29.
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assortment
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набор, выбор, ассортимент
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30.
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to cross over
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переходить, перекрещиваться
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31.
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random
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случайный, произвольный, беспорядочный
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32.
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fertilization
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оплодотворение, опыление
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33.
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shuffle
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перемешивать, перетасовывать
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34.
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to affect
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негативно воздействовать, поражать
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35.
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to transcribe
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записывать, воспроизводить
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36.
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to confine
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заточить, держать взаперти, ограничивать
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37.
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to derive
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наследовать, происходить, получать
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38.
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viral
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вирусный
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39.
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alteration
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изменение
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40.
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to condense
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конденсироваться
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41.
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to put apart
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разрывать на части (куски), раздирать
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42.
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disjunction
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разъединение, отделение
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43.
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to replicate
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реплицировать, воспроизводиться путем клеточного деления
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44.
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substitution
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замещение
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45.
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deletion
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делеция (удаление или утрата части генетического материала хромосомы), стирание
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46.
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sequence
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последовательность
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47.
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cistron
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цистрон
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48.
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inversion
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инверсия (генов), (взаимная перестановка)
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49.
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reversal
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полная перестановка
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50.
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lethal
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смертельный, летальный
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51.
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codon
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кодон
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52.
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to be bound to happen
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обязательно случаться, происходить
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53.
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immense
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огромный, колоссальный
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54.
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insertion
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инсерция (встраивание генетическогоэлемента в хромосому)
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55.
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sickle-cell anemia
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серповидноклеточная анемия (наследственная болезнь, преимущественно афро-американцев)
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■ Your Essential Assignments
I.Quick check
1. What is a mutagen? Give one example.
2. Distinguish between the genotype and the phenotype of an organism.
3. If a diploid organism has two different alleles for the same gene, is it homozygous or heterozygous?
4. Is weight in humans an example of continuous variation or discontinuous variation?
II. Using a monolingual English dictionary define the following genetic terms:
allele; homozygous; heterozygous; dominant; recessive; polygenic.
III. Find Russian equivalents to the following word combinations:
№
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English term
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Russian equivalent
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1.
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inherited variation
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2.
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spontaneous mutations
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3.
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recessive form
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4.
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to be easy to distinguish
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5.
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subtle differences
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6.
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genetic make-up
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7.
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observable or measurable traits
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8.
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human height and weight
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9.
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differences grade into each other
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10.
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considerable environmental influence
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11.
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to arise partly
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12.
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multicellular organisms
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13.
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to induce mutations
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14.
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base sequence
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15.
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to result from
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16.
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to result in
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17.
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beneficial mutation
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18.
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natural selection
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19.
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to be of immense importance
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IV. Fill in the gaps with the words and expressions from the text:
Mutations can either … spontaneously or … by agents called mutagens.
Mutations are usually thought of as … and they often are.
Occasionally, a mutation is …, changing the phenotype so that an organism has a better chance of … and ….
Genetic differences reflect… of an organism, that is, its genetic ….
Mutations that affect large sections of a gene and chromosome mutations are often ….
Although beneficial mutations are …, they are found to happen sooner or later if there is a large … in a population.
A mutation is … in the amount or the chemical structure of DNA.
The genotype determines the … of an organism, whereas the environmental factors to which it is exposed determine to ….
In continuous variations, differences are ….
10. In discontinuous variations, the differences are ….
V. Find English equivalents to the following word combinations:
№
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Russian term
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English equivalent
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1.
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возникать спонтанно
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2.
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уровень спонтанных мутаций варьирует
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3.
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оказывать негативное воздействие
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4.
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доза радиации
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5.
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члены одного вида
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6.
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влияние окружающей среды
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7.
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видимые или измеряемые характеристики
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8.
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взаимодействие генотипа и окружающей среды
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9.
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существующий набор генов
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10.
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быть результатом замещения
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11.
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поражать большие наборы генов
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12.
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нуклеотидная последовательность
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13.
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функционировать нужным образом
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14.
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незначительные различия
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VI. Answer the following questions. Use all information given before.
If a diploid organism has two different alleles for the same gene, is it homozygous or heterozygous?
What is the difference between the genotype and the phenotype of an organism?
Is weight in humans an example of continuous variations or discontinuous variations?
What is a mutagen?
Are mutations harmful or beneficial?
What is variation and what does it result from?
Could you give an example of gene mutations?
Why are beneficial mutations of immense importance?
When do chromosome mutations happen?
What is a haploid organism?
VII. Read and translate the short text without any dictionary:
Fact of life:
Mutations (changes in DNA) are the ultimate source of inherited variation. They can either arise spontaneously or be induced by agents called mutagens (such as X-rays, mustard gas, or ultraviolet radiation). The rate of spontaneous mutations varies for different genes and in different organisms. Each human gene has about a one in 100 000 chance of mutating. Mutations are usually thought of as harmful, and they often are. However, because we have so many genes, even the healthiest of us probably have at least a few spontaneously mutated genes hidden in the recessive form which do not affect us. X-rays and other mutagens increase the mutation rate, and the higher the dosage of radiation, the higher the rate of mutation.
VIII. Food for thought:
Twins (pairs of children born at the same time) may be dizygotic or monozygotic.
Each dizygotic or non-identical twin develops from a different egg and may be of a different sex. Monozygotic twins or identical twins develop from one egg and contain identical genetic information; they are always of the same sex. Suggest how the study of twins may be used to distinguish between the effects of inheritance and environmental factors on the variations of an individual character.
IX.Translate into English using all the active possible:
Мутации – это основная причина наследственных генетических изменений.
Каждый человеческий ген имеет шанс мутировать как 1 к 100000.
Рентгеновские лучи и другие мутагены увеличивают уровень мутирования и чем выше доза радиации, тем выше уровень мутирования.
Большинство выраженных мутаций вредны.
Однако, случайно мутации бывают полезными, меняя фенотип таким образом, что организм имеет больше шансов выживать и воспроизводиться.
Различия между особями одного вида называются генетической изменчивостью.
Измеряемые физические и биохимические характеристики организма, будучи видимыми или нет, составляют его фенотип.
Характеристики, такие как человеческий рост и вес, показывают непрерывную изменчивость, и они обычно определяются большим набором генов и ощутимым (заметным) влиянием окружающей среды.
Генотип определяет потенциал организма, в то время как факторы окружающей среды, в которой он существует, определяют, до какой степени этот потенциал будет реализован.
Мутация – это изменение в количестве или структуре ДНК.
Мутации могут происходить спонтанно или быть вызваны факторами окружающей среды (химические препараты, рентгеновские лучи, вирусная инфекция).
Многие мутации выражаются в изменении формы белка, что приводит к его неправильному функционированию.
Мутации, которые поражают большие секторы генов, и хромосомальные мутации часто являются смертельными.
Однако, полезные мутации являются сырьевым материалом для эволюции новых видов путём естественной селекции (естественного отбора).
X. Meet essential targets reflecting the following issues:
Define the following genetic terms: allele; homozygous; heterozygous; dominant; recessive; polygenic
Distinguish between genotype and phenotype
Distinguish between continuous variation and discontinuous variation
Explain how mutations contribute to variation.
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