• SS3 BIOLOGY SECOND TERM: HEREDITY (GENETICS)
• SS3 BIOLOGY SECOND TERM: CHROMOSOMES
• SS3 BIOLOGY SECOND TERM: VARIATION AND EVOLUTION
• DIFFERENT CASTES OF TERMITES AND THEIR ROLES
• SS3 BIOLOGY SECOND TERM: EVOLUTION
• THEORIES OF EVOLUTION
• PROCESS OF DEVELOPMENT OF ZYGOTE IN FLOWERING PLANTS (CONTINUATION)
• THE FRUIT
• THE FRUIT (CONTINUATION)
• ADAPTATION FOR SURVIVAL

SS3 Biology Second Term: Heredity (Genetics)

Biology SS 3, Second Term, Week: 1

Topic: Genetics: The Science of Heredity

Introduction

All living organisms reproduce. Reproduction results in the formation of offspring of the same kind. A pea plant produces only pea plants each time it reproduces. A rat produces only rats. Humans produce only humans. However, the resulting offspring need not and most often do not totally resemble the parent. Several characteristic differences may occur between individuals belonging to the same species. The similarities and differences among the members of a species are not coincidental. Both the similarities and differences have been received from their parents. The mechanism of transmission of characters, resemblances as well as differences, from the parental generation to the offspring, is called as heredity. The differences shown by individuals within the same species and in the offspring are described as variations. The scientific study of heredity, variations and the environmental factors responsible for these, is known as genetics.

What is Genetics?

Genetics is a branch of biology which studies heredity and variation. In other words, Genetics is the study of how genes bring about characteristics, or traits, in living things and how those characteristics are inherited.  Scientists those study heredity, variation are called geneticists. The term genetics was coined by W. Bateson. Genes are portions of DNA molecules that determine characteristics of living things. Through the processes of meiosis and reproduction, genes are transmitted from one generation to the next.

What is Heredity?

Heredity is the transfer of characters from parents to offspring generation after generation and hence responsible for biological similarity between them.

It is the heredity because of which progeny is similar in various characters to either of its parent. This phenomenon by which progeny retains characters from parents is named as inheritance.

What is Variation?

Variation is the difference among the parents and offspring of these parents.

Variation observed, can be because of two main reasons. So it is of two types based on these reasons.

·Hereditary variation

·Environmental variation

Only hereditary variations are transferred to progeny. Environmental variations belong to that generation only. These are not carried to the next generation.

Transmission and Expression of Characters in Organisms

Different characters (traits) are transmitted from parents to offspring (Progeny) and from generation to generation.

Hereditary Variation

Hereditary variation refers to differences among individuals which can be passed from parents to their offspring (progeny). Hereditary variation arises because, apart from the case of identical twins, no two offspring inherit exactly the same set of characteristics from their parents. Each offspring inherits a different combination of characteristics from parents plants and animals have transmittable characters.

Transmittable Characters in human Beings

Transmittable characters in human beings include:

• Body stature or shape
• Shape of head, nose and ear
• Size of nose, head and ear
• Colour of skin, hair and eye
• Characteristic of voice or speech
• Intelligence
• Height of human
• Blood grouping
• Baldness
• Tongue rolling
• Sickle cell anaemia
• Haemophilia
• Colour blindness
• Fingerprint
• Ability to taste (BTC)

Transmittable Characters in Plants

Transmittable characters in plants include:

• Height of plant
• Size or weight of fruit
• Size of leaf
• Taste of fruit
• Food content of fruit
• Colour of leaf, flower, fruits or seeds
• Resistance to environmental factors like diseases, pests and drought
• Shape of leaf, fruit and flower
• Leaf texture
• Life span or habit of growth

Non-Transmittable characters

They are characters that are not transmitted from one generation to another e.g. the loss of ones leg as a result of an accident, the development of big muscles by a boxer, etc.

Some Important Terms Used In Genetics

Allele: One alternative of a pair or group of genes that could occupy a specific position on a chromosome.

Chromosome: A linear strand of DNA harboring many genes.

DNA: Deoxyribonucleic acid; the molecule in which genetic information is encoded.

Dominant: An allele producing the same phenotypic effect whether inherited heterozygously or homozygously; an allele that “masks” a recessive allele.

Gene: A unit of genetic information that occupies a specific position on a chromosome and comes in multiple versions called alleles.

Genotype: The genetic constitution of an organism.

Heterozygous: Having a genotype with two different and distinct alleles for the same trait.

Homozygous: Having a genotype with two of the same alleles for a trait.

Phenotype: The physical or observable characteristics of an organism.

Recessive: An allele producing no phenotypic effect when inherited heterozygously and only affecting the phenotype when inherited homozygously; an allele “masked” by a dominant allele

Haploid: Haploid is when an organism has one set of chromosomes in the gamete. It is represented by small letter (n)

Diploid: Diploid is when an organism has two sets of chromosomes in the body cell. The bodies of animals and plants are diploids. It is represented by (2n)

Mutation: Mutation is a change in the genetic make-up of an organism resulting in a new characteristic that inheritable.

Backcross: The cross of an F₁ hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd.

Testcross: The cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous

Monohybrid cross: A cross between parents that differ at a single gene pair (usually AA x aa)

Dihybrid cross: Involves a study of inheritance patterns for organisms differing in two traits.

Mendel’s Work in Genetics

Gregor Mendel (1822-1884) was an Austrian monk, known as the father of genetics. He made first systematic approach for the investigation of the mechanism of inheritance. He did a number of experiments on inheritance in pea plants.

Mendel described the basic patterns of inheritance before genes was discovered. He called “factors” to what we now call genes. Presented the results of his experiments under name “Experiments on Plant Hybridization” before the Natural History Society of Brunn 1865 and published paper in 1866. No one realized the importance of his work until 1900.

Mendel’s brilliant and systematic work laid the foundation of a new branch of biology known as “genetics“.

Rediscovery of Mendel’s work

Mendel’s work was rediscovered by Hugo de Vries in Holland and Carl Correns in Germany. They independently obtained the same results as those obtained by Mendel.

Why Mendel selected pea as experimental material?

1. Mendel observed that pea plant has various contrasting characters among its different varieties. He selected seven traits that are easily recognized and apparently only occur in one of two forms.

– Seed form is round or wrinkled

– Cotyledon color is yellow or green

– Seed coat color is grey or white

– Pod from is inflated or constricted

– Pod color is green or yellow

– Flower position is axial or terminal

– Stem length is tall or dwarf

2. Pea plant has perfect flowers.

3. Ordinarily self fertilized, but when cross pollination required can be easily crossed.

4. Pea plant is annual i.e. has short life cycle.

5. Can be grown and maintained in small space, with little expenditure.

Reasons for Mendel’s Success

1. Proper maintenance of records i.e. observations of various characters in different generations.

2. Study of individual character so that systematic analysis is possible, no confusion.

3. Choice of material was right, reasons for this are as we have seen above.

4. Maintenance of purity, he used pure breeding parents.

5. Knowledge of shortfalls of earlier workers.

6. His mathematical background helped him a lot to understand and explain segregation of characters in F₂ and F₃ generations.

Practice Questions

1. ______ is the transfer of characters from parents to offspring generation after generation and hence responsible for biological similarity between them.
   a. Heredity
   b. Variation
   c. Genetics
   d. Mutation
2. ____ means having a genotype with two of the same alleles for a trait
3. An allele producing no phenotypic effect when inherited heterozygously and only affecting the phenotype when inherited homozygously is called a _____ allele
4. ____ is a change in the genetic make-up of an organism resulting in a new characteristic that inheritable.
   a. Heredity
   b. Variation
   c. Genetics
   d. Mutation
5. _____  is the study of how genes bring about characteristics, or traits, in living things and how those characteristics are inherited
   a. Heredity
   b. Variation
   c. Genetics
   d. Mutation
6. ____ means having a genotype with two different and distinct alleles for the same trait.
7. ____ is when an organism has two sets of chromosomes in the body cell. It is represented by (2n)
8. ____ cross is the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous
   a. Dihybrid 
   b. Test
   c. Monohybrid
   d. Back 
9. ____ is  a unit of genetic information that occupies a specific position on a chromosome and comes in multiple versions called alleles
10. A ____ allele  is an allele producing the same phenotypic effect whether inherited heterozygously or homozygously.

Answers

1. A
2. Homozygous
3. Recessive
4. D
5. C
6. Heterozygous
7. Diploid
8. B
9. Gene
10. Dominant

Theories of Evolution

Biology SS 3 Week: 6

Topic: Theories of Evolution

Theories of Evolution

The theories of evolution are attempts to explain how evolution have taken place. Two theories are explained below

Lamarck’s Theory of Evolution

Jean Lamarck a french zoologist was the first to suggest that orgnisms undergo evolution. His theory which was propounded in 1801 is also called the theory of use and disuse.

Lamarck’s contributions or postulates are as follows:

1. that great changes in environment result in corresponding changes in the species
2. that these changes cause the organism to form new structures or habits to adjust to the new prevailing environment
3. that organisms then deveop speciialised characters by use and disuse of organs
4. that frequently used organs become well developed while the ones not used degenerate and become atrophied or useless
5. that the well developed or dominant acquired characters are inheritable

Lamarck used the long neck of giraffe to give example of this theory. The longe neck arose from te need to browse on tree tops and subsequent generations have been inheriting this character. Lamarck’s theory is not accepted by modern scientists.

Darwin’s Theory of Evolution

Charles Darwin, a british naturalist in 1859 also propounded another theory of evolution. His theory is known as Darwin’s Theory of Natural Selection.

Darwin’s contributions or postulates are as follows:

1. that species have the ability to produce large number of offspring into the environment with limited resources
2. that this process then leads to competition by these offspring
3. that thhe survivors must have inherited the useful traits
4. then they reproduce and pass on these good traits to their offspring
5. those that could not survive the struggle die off or are eliminated
6. this process leads to survival of the fittest or natural selection
7. that there is great variability with population as this process continues through many generations
8. that the population became better adapted to the environment leading to the origin of new species

It sould be noted that Darwin’s theory is upheld by many scientists even till today.

Differences between Darwinism and Lamarckisim

Darwin’s theory of natural selection	Lamarck’s theory of use and disuse
Variations appear by themselves in a population	Variations appear because organisms are trying to satisfy needs which exist in the environment
The environment selects from members of a population only those with favourable adaptations or variations	The environment causes variations to be maintained, variation that actually arose from members of the population trying to satisfy the needs of the environment
Heredity variations are transmitted to offsprings	Characteristics acquired through use and disuse are transmitted to offsprings
Species that are fittest contribute more offspring to the next needs of the generation	Offspring inherit characteristics acquired by their parents through striving to satisfy the environment

Modern Evolutionary Theory

Modern evolutionary theory is based on the combination of natural selection and genetic origins of variation

This is summarised in  the statement below:

1. There exist variations in a species population
2. Some of the variations have special survival advantage
3. Individuals with favourable variations are more adapted to the environment than others
4. The individuals have to struggle for existence in te environment
5. The fittest contribute more offspring to the next generation than the unfit ones. This brings about a gradual shift in the features of the population
6. The main cause of variations are mutations and recombination of genes

Mendel’s Experiments

Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated.  Pea plants have both male and female reproductive organs.  As a result, they can either self-pollinate themselves or cross-pollinate with another plant.  In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.  This was the basis for his conclusions about the nature of genetic inheritance.

In cross-pollinating plants that either produce yellow or green pea seeds exclusively, Mendel found that the first offspring generation (f1) always has yellow seeds.   However, the following generation (f2) consistently has a 3:1 ratio of yellow to green.

He came to three important conclusions from these experimental results:

1.    That the inheritance of each trait is determined by “units” or “factors” that are passed on to descendants unchanged   (these units are now called genes)

2.    That the inheritance of each trait is determined by “units” or “factors” that are passed on to descendants unchanged   (these units are now called genes)

3.    That a trait may not show up in an individual but can still be passed on to the next generation

Mendel’s Laws

While working on pea plant Mendel proposed two laws.

1.    Law of segregation

2.    Law of independent assortment

Mendel’s First Law

The law of segregation state that during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete.

Using symbols we can depict the cross of tall and short pea plants in the following manner:

The F₂ generation was created by selfing the F₁ plants. This can be depicted graphically in a Punnett square. From these results Mendel coined several other terms and formulated his first law. First the Punnett Square is shown.

Union of Gametes At Random		D	D
	D	DD (Tall)	Dd (Tall)
	d	Dd (Tall)	dd (Short)

The Punnett Square allows us to determine specific genetic ratios.

Genotypic ratio of F₂: 1 DD : 2 Dd : 1 dd (or 3 D_ : 1 dd)

Phenotypic ratio of F₂: 3 tall : 1 dwarf

Mendel’s Second Law of Inheritance

In 1865, Gregor Mendel performed dihybrid crosses on pea plants and discovered a fundamental law of genetics called the Law of Independent Assortment. Mendel began his experiments by first crossing two homozygous parental organisms that differed with respect to two traits. An organism that is homozygous for a specific trait carries two identical alleles at a particular genetic locus.

Mendel chose to cross a pea plant that was homozygous and dominant for round (RR), yellow (YY) seeds with a pea plant that was homozygous and recessive for wrinkled (rr), green (yy) seeds, represented by the following notation:

RRYY    x    rryy

Organisms in this initial cross are called the parental, or P generation. The offspring of the RRYY x rryy cross, which is called the F1 generation, were all heterozygous plants with round, yellow seeds and the genotype RrYy.

Next, Mendel crossed two plants from the F1 generation. This step is the dihybrid cross, and it is represented as:

RrYy x RrYy

Gametes	YR	Yr	yR	Yr
YR	YYRR	YYRr	YyRR	YyRr
Yr	YYRr	YYrr	YyRr	Yyrr
yR	YyRR	YyRr	yyRR	yyRr
Yr	YyRr	Yyrr	yyRr	Yyrr

Mendel observed that the F2 progeny of his dihybrid cross had a 9:3:3:1 ratio and produced nine plants with round, yellow seeds, three plants with round, green seeds, three plants with wrinkled, yellow seeds and one plant with wrinkled, green seeds. From his experiment, Mendel observed that the pairs of traits in the parental generation sorted independently from one another, from one generation to the next.

Law of Independent assortment states that each character behaves as a separate unit and is inherited independently of any other character.

Incomplete dominance

In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait and where one allele was always dominant to the other. Although the rules that Mendel derived from his experiments explain many inheritance patterns, the rules do not explain them all. There are in fact exceptions to Mendel’s rules, and these exceptions usually have something to do with the dominant allele. One exception to Mendel’s rules is incomplete dominance.

Incomplete dominance is a condition when neither allele is dominant over the other. The condition is recognized by the heterozygotes expressing an intermediate phenotype relative to the parental phenotypes. If a red flowered plant is crossed with a white flowered one, the progeny will all be pink. When pink is crossed with pink, the progeny are 1 red, 2 pink, and 1 white.

ASSESSMENT

1. The use and disuse principal of evolution or theory of inheritance accquired characters was proposed by
   (a) Hugo de Vries
   (b) Lamarck
   (c) Weismann
   (d) Darwin
2. The present giraffe has a long neck as compared to its ancestors. Lamerck believed it could be due to
   (a) Natural selection
   (b) Isolation
   (c) Inheritance of accquired characters
   (d) Speciation
3. A species inhabiting different geographical areas is known as
   (a) Sympatric
   (b) Allopatric
   (c) Siblings
   (d) Biospecies
4. The evolution of a species is based upon sum total of adaptive changes preserved by
   (a) Natural selection
   (b) Isolation
   (c) Speciation
   (d) Human conservation
5. Which of the following concept is attributed to Charles Darwin?
   (a) Use and disuse of organs is of great importance in evolution.
   (b) Every cell must come from a pre-existing cell
   (c) In the struggle for existence, the fittest would survive
   (d) The gametes will carry only one of a pair of contrasting characters.

ANSWERS

1. b
2. c
3. b
4. b
5. c

SS3 Biology Second Term: Chromosomes

Biology, SS 3, Second Term, Week: 2

Topic: Basis Of Heredity 

Introduction

In a human, the normal chromosomes complement is 46, 44 of which are autosomes while 2 distinct chromosomes are deemed sex chromosomes, which determine the sex of an organism and various sex linked characteristics.

In most animals, those who possess XX chromosomes are female while male animals possess an X and a Y chromosome. However, this is not true of all organisms, as it can be reversed in some species. A humans’ sex is predetermined in the sperm gamete. 

The egg gamete mother cell is said to be homogametic, because all its cell possess the XX sex chromosomes. Sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome.

In light of this, there are two possibilities that can occur during fertilization between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilizes the egg) they are responsible for determining sex.

Chromosome

Our bodies are composed of millions of cells.  Within the center of each cell are rod-like structures known as chromosomes.  Typically, there are 46 chromosomes in each cell.  They are grouped into 23 pairs, one member of each pair coming from our mother and the other from our father at the time of conception.  The first 22 pairs of chromosomes are the same in both men and women and are number 1 through 22. The last two determine our sex and are call X and Y.  Women have two X chromosomes and men have one X chromosome and one Y chromosome. Our chromosomes carry our genes, the basic units of heredity. Our genes are made up of DNA. There are approximately 30,000 genes that influence our growth and development. 

Structure of a Chromosome

A chromosome is a condensed form of DNA that ensures it will not be cut during cell division. The long strands of DNA wrap around a core that consists of proteins called histones. 8 of these histone proteins are arranged in a very specific way so the DNA can wind smoothly around it. Once the DNA is wrapped around the histone core, it is now a chromosome.

Chromosomes have two parts called sister chromatids. Sister chromatids are identical to each other and were formed during DNA replication. They are held together by the centromere. Each sister chromatid will go into a different cell during cell division. This ensures each cell is an exact copy of the original cell and that it will function properly.

In most sexually reproducing organisms, chromosomes are arranged in homologous pairs. One of the chromosomes in the pair came from the organism’s mother and one came from its father. These chromosomes hold the information for the same traits and the combination of both determines what phenotype the offspring will show.

Chromosomes and Genes

A gene is a specific sequence of DNA that codes for a trait. DNA is made up of four chemical bases which are represented by letters: adenine (A), guanine (G), cytosine (C), and thymine (T). Each chromosome contains many genes. Not all genes that are coded for are expressed. There are some that need environmental or other triggers in order to be expressed. Some traits are actually a combination of information from several genes on many different chromosomes.

Probability in Genetics

Probability is a branch of mathematics which can be applied to those events that depend entirely on chance. The probability that an event will occur in genetics is greater than zero and less or equal to 1.

Two basic rules of probability are helpful in solving genetics problems: the rule of multiplication (or the rule of and) and the rule of addition (or the rule of or).

– Rule of multiplication is that the probability that independent events will occur simultaneously is the product of their individual probabilities. For example:

Question:

In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?

Answer:

Probability that an egg from the F₁ (Pp) will receive a p allele = 1/2.

Probability that a sperm from the F₁ will receive a p allele = 1/2.

The overall probability that two recessive alleles will unite, one from the egg and one from the sperm, simultaneously, at fertilization is: 1/2 X 1/2 = 1/4.

– Rule of addition is that the probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.

Question:

In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being a heterozygote?

Answer:

There are two ways in which a heterozygote may be produced: the dominant allele (P) may be in the egg and the recessive allele (p) in the sperm, or the dominant allele may be in the sperm and the recessive in the egg. Consequently, the probability that the offspring will be heterozygous is the sum of the probabilities of those two possible ways:

Probability that the dominant allele will be in the egg with the recessive in the sperm is 1/2 X 1/2 = 1/4.

Probability that the dominant allele will be in the sperm and the recessive in the egg is 1/2 X 1/2 = 1/4.

Therefore, the probability that a heterozygous offspring will be produced is 1/4 + 1/4 = 1/2.

Sex Linkage in Human-Beings

Characteristics whose genes are carried on the X chromosomes of the sex chromosomes are said to be sex-linked. Genes on the same chromosome are said to be linked because they tend to be inherited together. A sex-linked gene is a gene located on the x chromosome. Such genes are inherited along with such x chromosomes.

Examples of sex-link characteristics are:

1.    Colour blindness: Colour blind persons cannot distinguish near colours. It is an abnormality of the gene that controls the production of cone cells (light receptors) in the retina of the eye.

2. Haemophilia (Bleeding disease): Haemophilia is also an abnormality controlled by a recessive gene located on the x chromosome. Bleeding from a puncture or an open wound takes an abnormally long time to shop or fails to stop because clotting of blood wound not occur. Small injuries like puncture, extraction of tooth etc. can cause such persons to bleed to death.

3.  Baldness: Baldness is equally an abnormality controlled by a recessive gene. It is a situation which results in the inability of the hair to grow on the upper (dorsal) part of the head. The recessive gene causes the hair to pull off prematurely. This is more common with male human beings.

4.  Sickle cell anaemia: The sickle cell anaemia is controlled by a recessive gene which causes some of the red blood cells of some people to be sickle-shaped. The haemoglobin of the affected red blood cells is abnormally shaped and is inefficient in transporting oxygen. In a condition of low oxygen concentration, the haemoglobin breaks down causing the cells to become sickle-shaped. Such red blood cells block the cavities of small blood vessels in the body thereby hindering the free-flow of blood in them. Any part of the body affected receives insufficient blood, oxygen and nutrients. At such periods, the victim goes into crisis which is characterized by pains in the bones and joints, decrease in the level of haemoglobin, low oxygen concentration and a drastic fall in the level of blood fluid. This condition is called sickle cell anaemia and such sufferers are called sicklers.

5.  Albinism: Albinism is the condition in which the skin of an animal is non-pigmented because of lack of the pigment called melanin. The expression of this trait is controlled by a recessive gene.

The Pedigree

A pedigree is a drawing of a family tree. The pedigree is used by genetic counselors and other medical professionals to assess families and try to spot patterns or indications which may be helpful in diagnosing or managing an individual’s health. Pedigree uses specific symbols and “rules” so no matter who draws it, anyone can read and understand it.

Example of pedigree analysis

Application of the Principles of Heredity

Genetics is useful in many fields of human endeavour. Among its applications are:

Application of Genetics in Agriculture

The knowledge of the principles of heredity (genetics) is used in animal and crop husbandry to produce desirable breeds of animals and varieties of crops. The application is as follows:

• To increase yield: The varieties of crops and breeds of animals so developed by breeders are capable of giving high yield in crops and in animal products, e.g. meat, eggs or milk.
• To improve quality of product
• Development of early maturing varieties
• Development of diseases resistant varieties
• To obtain uniformity of plants
• To produce crops and animals that can adapt to climatic conditions

Application of Genetics Medicine

Genetics has contributed immensely in various field of medicine. These include:

• Determination of the paternity of a child
• Blood transfusion
• Marriage counseling
• Diagnosis of diseases
• Crime detection
• Development of test tube babies
• Choosing the sex of a baby
• Knowing the sex of a baby

Practice Questions

1. The normal human chromosomes are ____
   a. 44
   b. 46
   c. 44
   d. 42
2. Out of the 46 chromosomes, 44 are ___
   a. autosomes
   b. sex chromosomes
   c. genes
   d. DNA strands
3. ____ is also an abnormality controlled by a recessive gene located on the x chromosome and it is also called the bleeding disease
   a. Leukemia
   b. Anaemia
   c. Haemophilia
   d. Sickle cell
4. Albinism is the condition in which the skin of an animal is non-pigmented because of lack of the pigment called ___
   a. Keratin
   b. Serotonin
   c. Melanin
   d. Albinin
5. _____ is an abnormality of the gene that controls the production of cone cells (light receptors) in the retina of the eye
   a. Night blindness
   b. Scurvy
   c. Beri-beri
   d. Colour blindness
6. A(n) _____ is a specific sequence of DNA that codes for a trait
   a. Chromosome
   b. Gene
   c. Autosome
   d. Sex chromosome

Answers

1. B
2. A
3. C
4. C
5. D
6. B