BIOLOGY NOTES FOR FORM FOUR – ALL TOPICS

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GENETICS

Concept of Genetics

The concept of Genetics

Explain the concept of the genetics

Genetics is a branch of biology which deals with the study of heredity and variations in organisms.

Heredity refers to the transmission of genetic information from one generation to another. That is, the passing on of features or characteristics from parents to offspring or progeny. In humans, for example, features such as hair colour and body shape of the parents can be passed on (inherited) to their children. In genetics, a feature that can be inherited by the offspring from the parent is known as a trait. Thus, features such as hair texture, hair colour, and skin colour are all traits.

The hereditary characteristics are passed on from parents to their offspring through distinct units called genes. Genes are hereditary materials or factors, which determine a specific characteristic or trait in an organism.

Variations are the observable differences in organisms of the same species. Living things arise from other living things of the same species through reproduction. However, organisms show a great number of variations. No two organisms are exactly the same. The variations may be due to mutations of genetic material (DNA) caused by x-rays, gamma rays, ultra radiations or radioactive elements. Variations may also occur during gamete formation and combination of gametes at fertilization.

Genetics, therefore, attempts to explain either how organisms do resemble their parents in certain features or differ from their parents in other features.

Common terms used in Genetics

State common terms used in genetics

In genetics, there are several terms that are often used to describe different genetic features, variations or phenomena. Defined below are some of the common terms used in genetics.

1. F1 generation: The first filial generation arising from the crossing of two pure breeds.
2. F2 generation: The generation obtained by crossing individuals of the F1 generation.
3. Genotype: The genetic makeup or constitution of an organism, which is responsible for certain physical traits.
4. Phenotype: The outward appearance of an organism determined by a gene.
5. Haploidy (n): Having one set of unpaired chromosomes in the nucleus.
6. Diploidy (2n): Having two sets of homologous (similar) chromosomes in the nucleus.
7. Alleles: Different forms of the same gene occupying the same position on a chromosome, for example T or t for height.
8. Homozygosity: The state of possessing two identical forms (alleles) of a particular gene, one inherited from each parent, for example tt, TT BB or bb.
9. Heterozygosity: The state of possessing two different alleles of a particular gene at one or more corresponding chromosomal loci, one inherited from each parent, for example Tt or Bb.
10. Dominance: A relationship between alleles of one gene, in which the effect on phenotype of one allele masks the contribution of a second allele at the same locus. The first allele is dominant and the second allele is recessive. For example, in height, both TT and Tt express tallness. Here, the contribution of “t” allele to the phenotype is masked by “T” allele.
11. Recessiveness: A condition where an allele can only express itself in a homozygous form, for example tt for dwarfness.
12. Mutation: A sudden, random change in the genetic make up of a cell, causing it and all cells derived from it to differ from normal cells.
13. Mutagen: An agent capable of increasing the rate of mutation in an organism, such as formaldehyde or nitrous acid.
14. Selfing: Crossing offspring of the same pair of parents. For example, the crossing of offspring of F1 generation to produce F2 (F1×F1 = F2) is called selfing.

Genetics Materials

The concept of Genetics Materials

Explain the concept of genetics Materials

Hereditory characteristics are passed from parents to their offspring through distinct units called genes.There are a lot of genes in an organism’s body. Genes are arranged in a linear manner, making chromosomes.

Chromosomes are thread like structures found in the nuclei of all body cells. Gene is made up of chemical substances called Nucleic acid.

There are two types of nucleic acids found in cells, these are:

1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)

These acids are made up of building blocks called nucleotides. Each nucleotide consists of three molecules linked together, that is a pentose sugar, phosphoric acid and organic base.

The structure and composition of Genetics Materials (Deoxyribonucleic Acid and Ribonucleic Acid )

Describe the structure and composition of genetics materials (Deoxyribonucleic Acid and Ribonucleic Acid )

DNA is called the “molecule of life”. This is because it determines the physical and behavioural characteristics of an organism. The DNA determines example the colour of your hair, eyes, skin, ears and nose, height, ability or inability to roll the tongue all.

he DNA is made up of many nucleotides that make the double stranded.

STRUCTURE OF DNA

DNA is a double stranded helical (spiral) molecular chain of a nucleic found within the nucleus of a cell.By “double stranded helical” it means that the DNA consists of two strands, which twist around each other in a spiral fashion.

The DNA is made up of many nucleotides forming a polynucleotide chain. Polynucleotide means many nucleotides

The polynucleotide chain runs in the opposite direction. Each chain is joined to the other by pairs of bases. There are four bases namely Guanine (G), Cytosine ©, Adenine (A) and Thymine (T)

Note:

Guanine and adenine are collectively called purines while cytosine and thymine are collectively called pyrimidine

Guanine pairs with cytosine and adenine pairs with thymine.DNA plays a key role in inheritance because it replicates itself during mitosis and meiosis. DNA also undergoes changes and has genetic information the characteristics of a species.

RNA

The chromosomes replicate during cell division. The replication occurs during mitotic and meiosis cell divisions

The chromosomes determine the type of protein synthesized. The genes determine the actual characteristics of the organisms. In protein synthesis deoxyribonucleic acid acts as a template for the formation of ribonucleic acid (RNA).

STRUCTURE OF RNA

RNA consists of only a single strand of polynucleotide. The polynucleotide is made up of many nucleotides. Each nucleotide consists of a nucleobase, ribose sugar and phosphate group.The RNA sugar is ribose and not deoxyribose. Its nucleotides contain only one of four bases that are:

1. Guanine (G)
2. Cytosine (C)
3. Adenine (A)
4. Uracyl (U)

NB:Uracyl replaces the thymine of DNA. So in this the adenine can pair with uracil while the Guanine pairs with thymine.

The RNA is involved in protein synthesis. There are various types of RNA:

1. Messenger (mRNA)
2. Transfer (tRNA)
3. Ribosomal (rRNA)

Difference between Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA)

Differentiate Deoxyribonucleic Acid (DNA) from Ribonucleic Acid (RNA)

DNA versus RNA comparison chart

Principle of Inheritance, Concept of Inheritance

The Concept of Inheritance

Explain the concept of inheritance

Gregor John Mendel advanced the principles of inheritance. In 1856 – 1863 Mendel grew and tested some 29,000-pea plants. From these studies, he formulated the law of segregation and the law of assortment.

After his work on peas, Mendel began to experiment with honeybees. However, he failed to produce a clear picture of their heredity because of difficulties in controlling the mating behaviour of queen bees. Mendes works was largely criticized and generally rejected during his lifetime. It was only after his death that his work gained broad recognition. He is now considered the father of modern genetics, Mendel died Mendel died on January 6^th, 1884.

Mendel chose the garden peas because of the following reasons:

• It is self-pollinating but can be cross-pollinated
• It matures very fast
• It produces many seeds and hence many off springs
• It has several physical properties

Some of the characteristics that be studies were:

• Height of the stem-tall or dwarfs
• Texture of the seed coat – smooth or wrinkled
• Colour of flowers – purple or white
• Colour of pods – green or yellow
• Position of flowers – axial or terminal

Mendelian Inheritance

Mendel’s First Law of Inheritance

State Mendel’s first law of inheritance

FIRST LAW

This law is also called Mendel’s first law of inheritance or law of segregation. The law states, “An organism’s characteristics are determined by internal factors which occur in pair”. Only one of the factors can be contained in a single gamete.

In modern terms this means that genes occurring in pairs control the characteristics of an organism but only one gene can be carried in a single gamete.

There are four main concepts in this law:

1. Genes can exist in more than one form
2. An organism inherits two alternative form of a gene for a particular trait, one from each parent
3. During the production of gametes pair of alleles separate. Thus each gamete has one allele for each trait.
4. When the two alleles in a pair are different one is dominant while the other is recessive. This condition is called complete dominance.

When inheritance of one pair of characteristics is studied at a time it is called Monohybrid inheritance.

Monohybrid Crosses and Interpretation of their Results of Crosses and Ratios

Illustrate monohybrid crosses and interpret their results of crosses and ratios

MONOHYBRID INHERITANCE

This is an inheritance of one pair of characteristic or trait at a time. 

All plants produced are normal and also are known as the first filial generation (F1).Hence he concluded that the tallness is said to be dominant which the gene for dwarfness is said to be recessive in the garden pea. Because the gene of dwarf are masked by the gene for tallness.Hybrid is an offspring of a cross-between parents showing unlike characteristics.Test Cross (Back cross) this is the cross that involves off springs of two different pure lines.

Interpretation of data from Monohybrid Experiments to Demonstrate Mendel’s First Law of Inheritance

Interpret data from monohybrid experiments to demonstrate mendel’s first law of inheritance

Some conditions in human follow Mendelian monohybrid inheritance. Example, a condition that is associated with a simple pair of alleles and are inherited in Mendelian fashion

Examples of such conditions are:

1. Albinism
2. Sickle Cell anemia
3. Rhesus blood group
4. Haemophilia
5. Achondroplasia

Patterns of Inheritance that Follow Mendel’s First Law

Illustrate patterns of inheritance that follow mendel’s first law

DIHYBRID INHERITANCE

Mendel continued to study the inheritance of two pairs of characteristics. This inheritance is known as dihybrid cross.Dihybrid Cross is the inheritance of two characteristics in which each is controlled by a different gene, different locus.

Examples of two characteristics to an organism:

1. Tall with purple flower
2. Dwarf with white flower

From the above experiment Mendel made the following conclusions:

1. Two phenotypes in the ration 9:3:3:1 resembled one or other of the parent
2. Two phenotypes did not resemble any of the parents’ phenotypes but instead had combined the characteristics of both parents
3. Ratio of tall to dwarf plants was 3:1 and that of purple flowered plant was 3:1

Non-Mendelism Inheritance

Concepts of Incomplete Dominance and Co-dominance

Explain concepts of incomplete dominance and Co-dominance

Not all inheritance follows Mendelian fashion. Mendel only considered characteristics that were determined by single genes with two alleles in which one is dominant and the other recessive. Later research showed that in some alleles neither one is dominant over the other. That condition is known as co-dominance or incomplete dominance.

Incomplete Dominance

This is the condition in which no allele is dominant or recessive compared to the otherExample when red and white flowered varies of the four o’clock plant are crossed, all the plant of the F1 generation produce pink flowers.

Complete dominance

Complete Dominance is a condition in which a dominant gene completely masks recessive gene. Example: Homozygous tall plant crossed with the homozygous short/dwarf plant. chart showing Tall x Dwarf.

Patterns of Inheritance that deviates from Mendel’s First Law of Inheritance

Illustrate patterns of inheritance that deviates from mendel’s first law of inheritance

NON-MENDEL INHERITANCE

Not all inheritance follows Mendelian fashion. Mendel only considered characteristics that were determined by single genes with two alleles in which one is dominant and the other recessive. Later research showed that in some alleles neither one is dominant over the other. That condition is known as co-dominance or incomplete dominance.

INCOMPLETE DOMINANCE

This is the condition in which no allele is dominant or recessive compared to the other

Example when red and white flowered varies of the four o’clock plant are crossed, all the plant of the F1 generation produce pink flowers.

Consider belowchart Red x White

INHERITANCE OF ABO BLOOD GROUPS

The entire human population falls under four main blood group that are –A, B, AB and O.Allele A and B are condomint white allele O is recessive to both A and B.

Example: parents with heterozygous blood group A and B have off spring with blood group A, B, AB and O as illustrated in the following cross.

Phenotype Blood group A x Blood group Bchart

Sex Determination and Inheritance

The Mechanism of Sex Determination and Inheritance

Describe the mechanism of sex determination and inheritance

Human beings have 46 chromosomes (23 pairs of homologous chromosome). In every body cell of these, two are sex chromosomes while 44 are referred to as Autosome.

By definition:

Sex determination refers to the interpretation between male sex and female sex.

A diagrammatical representation of human sex determination is shown below.chart showing cross Male x Female

The Concept of Sex Linked, Sex Limited and Sex Influenced Characters

Explain the concept of sex linked, sex Limited and sex influenced characters

This refers to the tendency in which one chromosome carries other genes.

Unlike other chromosomes in which each of the homologous chromosomes carries gene for the same characteristics, X and Y do not carry the same gene.

Consequences of Sex Preference and Sex Selection

Explain consequences of sex preference and sex selection

Sex preference and selection is the tendency of people to like one type of sex more than the other. This tendency is very common in African countries and some parts of Asia.

Some people in a family prefer having boys than girls while others prefer girls to boys. Those who prefer boys do so in a belief that boys will perpetuate the linage and take care of the parents when females are living far away with their husbands. Those who prefer girls argue that, girls are kind and merciful; therefore they can take care of their parents in old age.

The sex preference and selection is influenced by a number of socio-cultural factors. Some of the factors include the following:

1. Manpower Generation:Some societies prefer boys to girls because they generate wealth upon getting married. A family will get a lot of cattle or money as bridal price.
2. Generation and protection of wealth:Some societies prefer girls more than boys because girls will prefer to have more sons than girls so that they can somehow benefit indirectly through their son.
3. Land ownership In some societies a woman cannot own land thus prefers more sons than daughters because they can benefit from the sons.

Variation Among Organisms

The concept of Variation

Explain the concept of variation

Variation, in biology, refers to any difference between cells, individual organisms, or groups of organisms of any species caused either by genetic differences (genotypic variation) or by the effect of environmental factors on the expression of the genetic potentials (phenotypic variation). Variation may be shown in physical appearance, metabolism, fertility, mode of reproduction, behaviour, learning and mental ability, and other obvious or measurable characters. If you consider almost any characteristic, you will find differences between various people (or other animals or plants) in a population.

Variations among Organisms

Identify variations among organisms

Genetic variation describes naturally occurring genetic differences among individuals of the same species. This variation permits flexibility and survival of a population in the face of changing environmental circumstances. Consequently, genetic variation is often considered an advantage, as it is a form of preparation for the unexpected. Variation between different species is always greater than the variation within a species.

Genetic variations are caused by differences in number or structure of chromosomes or by differences in the genes carried by the chromosomes. Eye colour, body form, and disease resistance are genotypic variations. A variation cannot be identified as genotypic by observation of the organism. Breeding experiments must be performed under controlled environmental conditions to determine whether or not the alteration is inheritable.

Environmentally caused variations may result from one factor or the combined effects of several factors, such as climate, food supply, and actions of other organisms. These variations do not involve any hereditary alteration and in general are not transmitted to future generations.

The Meaning of Continuous and Discontinuous Variations

Give the meaning of continuous and discontinuous variations

Types of variation

Variations are classified either as continuous, or quantitative (smoothly grading between two extremes, with the majority of individuals at the centre, as height in human populations); or as discontinuous, or qualitative (composed of well-defined classes, as blood groups in man). A discontinuous variation with several classes, none of which is very small, is known as a polymorphic variation. The separation of most higher organisms into males and females and the occurrence of several forms of a butterfly of the same species, each coloured to blend with a different vegetation, are examples of polymorphic variation.

Continuous variation

This type of variation exhibits a wide range of differences for the same characteristics, from one extreme end to the other. Characteristics showing continuous variation vary in a general way, with a broad range, and many intermediate values between the extremes. As a matter of fact, if you consider a large enough sample from a population, perhaps plotting frequency as a histogram or as a frequency polygon, you will find that most of the values are close to the average (mean), and extreme values are actually rather rare. Examples of continuous variations in human beings include weight, height and complexion. Height is an example of continuous variation. People vary in height from very short to very tall, with many intermediate heights.

Discontinuous variation

Discontinuous variation is a type of variation that shows sharp differences among individuals of a species, with no intermediate forms.

Individuals fall into a number of distinct classes or categories. This is based on features that cannot be measured across a complete range. A person either has the characteristic or not. There is no intermediate condition.

The ability to roll the tongue (one is either tongue roller or non tongue roller), fingerprints, sex (one is either male or female) and the ABO blood group system where one can only have blood group A, B, AB or O. and blood groups. In plants, a pawpaw tree is either male or female. These characteristics can be explained much more easily by simple rules of genetics and are less likely to be affected by other factors. Discontinuous variations are unchangeable and unaffected by the external environment.

Difference between Continuous and Discontinuous Variation

Differentiate continuous from discontinuous variation

Some of the major differences between continuous and discontinuous variations in inheritance are as follows:

Continuous Variations:

1. The variations fluctuate around an average or mean of species.
2. Direction of continuous variations is predictable.
3. They are already present in the population.
4. Continuous variations are formed due to chance segregation of chromosomes during gamete formation, crossing over and chance pairing during fertilization.
5. They can increase adaptability of the race but cannot form new species.
6. Continuous variations are connected with the mean or average of the species by intermediate stages.
7. The continuous variations are also called fluctuations.
8. When represented graphically, continuous variations give a smooth bell shaped curve.
9. They are very common.
10. Continuous variations do not disturb the genetic system.

Discontinuous Variations:

1. A mean or average is absent in discontinuous variations.
2. The direction of discontinuous variations is unpredictable.
3. Discontinuous variations are new variations though similar variations might have occurred previously.
4. Discontinuous variations are produced by changes in genome or genes.
5. Discontinuous variations are the fountain head of continuous variations as well as evolution
6. These variations are not connected with the parental type by intermediate stages.
7. Discontinuous variations are also known as mutations or sports.
8. A curve is not produced when discontinuous variations are represented graphically.
9. These variations appear occasionally.
10. They disturb the genetic system of the organism.

Causes of Variation among Organisms

Explain causes of variation among organisms

Variation can be due to inheritance, and also to environmental factors such as climate and diet.

Genetic causes of variation (inherited variation)

Some variation within a species is inherited. Variation in a characteristic that is a result of genetic inheritance from the parents is called inherited variation. Each egg cell and each sperm cell contains half of the genetic information needed for an individual. When these join at fertilisation a new cell is formed with all the genetic information needed for an individual. Examples of inherited characters in humans include eye colour, hair colour, skin colour and lobed or lobeless ears.

Gender is inherited variation too, because whether you are male or female is a result of the genes you inherited from your parents.

Genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism’s offspring). Some of these variation causes are explained in detail below:

Independent assortment of homologous chromosomes

This occurs at the time of gamete formation. At the time of gamete formation during meiosis, the parental chromosomes separate at random hence forming different gametes with different chromosomes. This independent assortment gives a wide variety of different gametes and hence individuals.

Crossing-over

Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes during sexual reproduction. Crossing over and random segregation during meiosis can result in the production of new alleles or new combinations of alleles. Portions of paired chromosomes may be exchanged to form new chromosomal and gene combinations in gametes resulting into new trait combinations in offspring. 

Non-disjunction

Non-disjunction results into doubling of the chromosome number due to failure of chromosomes to segregate during meiosis. This leads to increase in cell size and subsequent increase in size of various parts of the organism, hence variation.

Random fertilization

Random fertilization that results during the fusion of the gametes also contributes to variation. Gametes are the egg and sperm, or pollen, produced by meiosis. Each gamete has a unique set of combination of genes. A male gamete can fertilize any of the female gametes. The fertilization between a male gamete and a female gamete occurs randomly in the fallopian tube. As a result, each zygote is unique and hence variation occurs due to the different combination of genes from the male and female gamete.

The random fusion of gametes is a source of genetic variation in offspring (with the same parents). For example, a litter of puppies or kittens sired (bred) by the same father will show variation between individuals as shown bellow. 

Random mating

Random mating involves individuals pairing by chance, not according to their genotypes or phenotypes. Random mating is a source of variation in a population. For example, a population in which mating only occur between organisms of similar phenotypes, such as red beetles mating with red beetles and yellow beetles mating with yellow beetles, will tend to show less variation than a population where crosses are random. For example, red beetles mating with yellow beetles.

Mutations

Mutations are sudden and permanent changes in the genes and chromosomes which are then passed on from cell to cell during mitosis. Such changed genes or chromosomes will produce offspring that differ from parents.

Environmental causes of variation

Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, water, temperature, light, diseases, degree of acidity, soils nutrients, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will grow taller as it tries to reach more light. Such variations are produced in the body (somatic) cells and not in the sex cells hence cannot be inherited.

Variation caused by the surroundings is called environmental variation. Here are some other examples of features that show environmental variation:

• Your language and religion
• Flower colour in hydrangeas – these plants produce blue flowers in acidic soil and pink flowers in alkaline soil

Genetic Disorders

The meaning of Genetic Disorders

Give the meaning of genetic disorders

This refers to an abnormality, which results from problems in the genes of an organism, and it is inherited. This means that the genetic disorder is caused by the change in the gene or chromosomes due to an error in the person’s genetic materials.

OR

Genetic disorders are malfunctioning of the body’s physiological mechanisms due to changes on gene or chromosomes; for example the change in number of chromosomes from 46 to 47 chromosomes or below that hence leading to genetic disorders.

Examples of Genetic Disorders

Cite examples of genetic disorders

Genetic disorders includes:

• Down’s Syndrome or Mongolism/Mongolia
• Turner Syndrome
• Super male and Super female
• Haemophilia
• Colour blindness

NOTE: The changes in the genes and chromosomes may be caused by different factors, which lead to genetic disorder. The sudden change in genetic materials of the cell that may cause it to differ from other cells is known as mutation.

The Causes and Effects of Genetic Disorders

Explain the causes and effects of genetic disorders

An organism is affected by mutation, which occurs naturally at a low rate. A number of factors may contribute to mutation. Such factors include various chemicals and radiation example X-rays. Mutation can be due to a change in gene itself i.e. this is called point mutation only or in the arrangement of gene chromosome. Other causes of mutation include addition or loss of chromosomes and duplication of genes.

There are two types of mutation including chromosomal mutation and gene mutation. Both leads to the genetic disorder is to lead to change in gene and chromosome. Chromosomal mutation leads to genetic disorder like Down’s syndrome, Turner Syndrome, Klinefelter’s Syndrome. It affects the appearance or the number of chromosomes. Gene mutation leads to genetic disorders like Haemophilia and Colour blindness, which affects the genes.

Down’s syndrome or Mongolism:

This is a chromosomal abnormality in which there are three copies of chromosome number 21 instead of the usual 2. The person with Down’s syndrome therefore has 47 chromosomes in his/her body cells. The affected individuals have a short broad face, slanted eyes, short fingers and weak muscles. Such individuals are usually mentally retarded.

The presence of the extra chromosome on chromosome 21 is known as trisomy. The extra chromosome on chromosome 21 is due to its failure to separate during meiosis. This is known as non-disjunction. This non-disjunction occurs when homologous chromosome fail to separate in meiosis II of the egg. Therefore after the fertilization, chromosome 21 will contain 3 (i.e. the 2 which failed to separate plus the one from the father) instead of normal two (i.e. each from one parent). In some few cases non-disjunction may occur in the father’s sperm.

Causes of Down’s syndrome

An extra chromosome on chromosome 21, thus making three instead of two, causes this disorder. The extra chromosome on chromosome 21 is caused by failure to separate during meiosis.

Effects of Down’s syndrome

People with down’s syndrome are very susceptible to diseases including heart diseases. They thus die young, mostly not more than 30 years. They may also suffer discrimination in those societies that consider it as a curse or something very unusual.

Turner’s syndrome (XO)

This is a genetic disorder of female (women) caused by absence of second sex chromosome. Such women are XO, rather than the normal XX chromosome. In this disorder there are only 45 chromosomes, the female lacks secondary sexual feature, small uterus, the internal genitals never mature and therefore she is sterile. This disorder is characterized by lack of ovaries and menstrual cycle.

Causes of Turner’s syndrome

This genetic disorder is caused by absence of X chromosome in normal XX chromosome. This occurs when the second X chromosome lacks i.e. make XO instead of normal XX chromosome.