Variations, Heredity & Cell Division
Complete Notes & Free Quiz

Human beings belong to the species called Homo sapiens. All humans, despite their differences, belong to one type or species of animals. Scientists call this species Homo sapiens.

Variations are the small differences between individual members of the same species. Every species has characteristic features of shape and structure by which it can be recognized, but individual members show slight differences. These differences are called variations. People, like all living things, inherit characteristics from both parents, some from the mother and some from the father, which is why variations exist.

Variations are very important because they have helped Homo sapiens in particular to evolve over millions of years into a very successful species. Variations allow some individuals to survive better in changing environments and pass their successful traits to their offspring.

People, like all living things, inherit their characteristics from their parents. Some characteristics come from the mother and some come from the father. This inheritance from both parents is why there are variations in every species of plants and animals.

The world's tallest known human being measured 272 cm tall. The shortest ever human being measured a height of just 54.6 cm. These extremes demonstrate continuous variation in height among Homo sapiens.

People belong to the same species so they share the same general body shape and similar facial features. However, they look different because they inherit different characteristics from their parents. These small differences are called variations.

Continuous variation is a type of variation in which characteristics show a smooth range of values between two limits, without clear categories. Features like height and weight are examples because they can take many values within a range and change gradually.

Discontinuous variation is a type of variation in which characteristics fall into clear, separate categories with no intermediate (middle) values. A person either has a trait or does not. Examples include gender, tongue rolling, eye color, earlobe type, and freckles. It is usually controlled by genes and shows no gradual change, unlike continuous variation which has a range of values.

Acquired variation is variation that develops because of what an organism does or experiences during its lifetime, it is not inherited from parents. For example, height and weight can be affected by diet. Exposure to sunlight affects skin colour regardless of its original colour. This is called acquired variation because people acquire these characteristics as a result of what they do during their lives.

A normal distribution curve is a bell-shaped graph that shows how a continuously varying feature (like height) is distributed in a large population. The more people you measure, the better the graph will be. The average height is shown in the middle of the curve, most people have a height close to this average. Only a few people are really short or really tall, giving the graph its bell shape.

Tongue rolling is the ability to roll the tongue into a tube shape. It is an inherited discontinuous variation; you are either a tongue roller or a non-roller. Tongue rolling is called a dominant characteristic. There are more tongue rollers than non-tongue rollers in the human population because the allele for tongue rolling is dominant; it is expressed even when only one copy is present.

Continuous variation: Weight (any value on a scale), Height (any value in a range).
Discontinuous variation: Tongue rolling (either you can or cannot), Eye colour (distinct categories), Ear lobes (either present or absent), Freckles (either you have them or do not).
Key test: If the feature can have any value in a range, it is continuous; if it falls into distinct groups, it is discontinuous.

In plants, the amount of sunlight, water, and suitable temperature can affect the size of fruit. This is acquired variation, the variation results from environmental conditions, not from genes. Two plants with identical genes can produce different sizes of fruit if they grow in different environmental conditions.

Saying there is variation in a class means students are not all identical because they show differences in characteristics such as height, hair colour, eye colour, shoe size, handedness, and presence of freckles or ear lobes. These differences exist because each student has a unique combination of genes inherited from their parents. This variation is the same kind found in all species.

Our surroundings or activities can affect variation. Height and weight can be affected by what sort of food we eat or how much we eat. Exposure to sunlight affects the colour of our skin no matter what colour it is to begin with. This is acquired variation because individuals acquire these characteristics through their experiences, not through inheritance.

A dominant characteristic is one that appears more frequently in a population because the allele for it is dominant. Tongue rolling is called a dominant characteristic. There are more tongue rollers than non-tongue rollers in the human population because the dominant allele for tongue rolling is expressed even when only one copy is present.

An adaptation is a physical or behavioural characteristic that makes an organism better suited to the environment it lives in. Every living thing is adapted to live in a certain way. Adaptations help organisms survive, find food, and reproduce successfully in their particular environment.

Lions have strong teeth for tearing flesh and crushing bones, an adaptation for eating meat.
Butterflies have long tubular mouth parts for sucking nectar from flowers, an adaptation for feeding on nectar.
Most flowers have color and scent to attract insects for pollination, an adaptation for reproduction.

Arctic conditions are extremely cold, especially in winter. The polar bear lives in arctic conditions and needs many special features to survive. Some living things need extra special adaptations to survive in very harsh environments; the polar bear is a prime example of an animal adapted to extreme cold.

Polar bear adaptations:
Thick blubber under skin: Insulates the body and provides a food store.
Round body shape: Small surface area, volume ratio, so less body heat is lost. Thick, greasy fur: Keeps warm and stays dry when swimming in icy water.
Small ears: Reduce surface area, so less heat is lost. (5) White fur — camouflage; matches snow, protects young bears from predators. (6) Strong legs — enable swimming and running fast after prey. (7) Big feet — spread the bear's weight on snow and ice, stopping it from sinking.

Blubber is a thick layer of fat found under the skin of the polar bear. It serves two important functions: (1) It insulates the animal, keeping body heat in during extremely cold conditions. (2) It provides a food store that the bear can use when food is scarce.

The polar bear has small ears to reduce heat loss. Ears have a large surface area relative to their volume, meaning heat escapes through them. By having small ears, the polar bear reduces the surface area through which heat is lost, helping it stay warm in freezing arctic conditions.

The polar bear has white fur which matches its snowy surroundings. This is called camouflage. Young bears need camouflage for protection against predators — if they blend into the snow, predators cannot easily see them. White fur also helps polar bears sneak up on their prey.

Small animals have a large surface area compared to their volume. This means they lose heat rapidly. In arctic conditions, maintaining body temperature would require enormous amounts of food energy; more than available in winter. Therefore small animals hibernate (enter deep sleep, lowering body temperature and metabolism) to conserve energy and survive the winter.

Cactus adaptations:
Water stored in the fleshy stem; water supplies during dry periods.
Stem covered in a waxy cuticle; stops water loss.
Leaves reduced to spines; cuts surface area, so less water is lost.
Spreading root system that stretches far from the plant; can take in dew that forms on the ground.
These allow cacti to survive in deserts with very little water.

The spines on a cactus are reduced leaves. Normal leaves have a large surface area which would cause a lot of water to be lost through evaporation. By reducing leaves to spines, the cactus significantly cuts surface area so much less water is lost. Spines also protect the cactus from animals that might eat it.

Camel adaptations:
Big feet: Spread weight on sand so the camel does not sink.
Long eyelashes: Protect eyes from blowing sand.
Bald tummy: Allows heat to escape when resting on cool ground.
A hump: Stores fat as an energy reserve (not water).
These allow camels to survive extreme heat and water scarcity in the desert.

Fish that live in the arctic produce a special antifreeze chemical in their blood. This is an adaptation because the water they live in is ice-cold, cold enough to normally freeze blood. By producing antifreeze, these fish can survive in icy water where other fish could not. This is a special biochemical adaptation to an extremely harsh environment.

The polar bear has a round shape which keeps its surface area small compared to its volume. A smaller surface area relative to volume means less body heat is lost to the environment. A round (spherical) shape has the smallest possible surface area for a given volume, this is why arctic animals tend to be rounder and more compact than animals in warm environments.

A variation is a slight difference in an inherited characteristic between members of the same species. An adaptation is a physical or behavioural characteristic that makes an organism better suited to its environment. The key difference: a variation is just a difference; an adaptation is a variation that has been naturally selected because it gives the organism a clear survival advantage in its specific environment.

Tortoises on a remote Pacific island showed variation, not all had the same neck length. When food at ground level disappeared, only long-necked tortoises could reach leaves on branches and survive. These tortoises bred and passed their characteristics to offspring. After several generations, all tortoises had long necks. Long necks, which began as a variation, became an adaptation, a feature selected by the environment because it gave the survivors an advantage.

For humans, having a long neck is still a variation, a slight difference between individuals but it is not an adaptation because it does not give a significant survival advantage in the human environment. A long neck may be useful when watching cricket but has no real effect on survival otherwise. For something to become an adaptation, it must provide a clear survival benefit allowing individuals with that feature to survive and reproduce more successfully.

When a variation helps an organism survive better and reproduce more than others, those organisms breed more and pass the characteristic to offspring. Over several generations, more and more individuals have this beneficial variation. Eventually the variation becomes an adaptation, a characteristic the whole species has because of environmental selection. This process is called natural selection.

Heredity is the transfer of information that controls the structure, characteristics, and behaviour of offspring from their parents. Every type of animal and plant produces offspring which are not exactly the same as the parents but do share some common features. Heredity explains why offspring resemble their parents.

Genetics is the study of how heredity works. It helps to explain how offspring get some of their parent's characteristics but not others. Genetics studies genes, chromosomes, and the mechanisms by which traits are passed from one generation to the next.

Gregor Mendel (1822-1884) founded the science of genetics. He is called the first geneticist. He identified many of the rules of heredity by doing breeding experiments on garden peas. His experiments showed how characteristics are passed from parents to offspring and established the basic principles of genetic inheritance.

Genes are the basic units of heredity. They carry information that determines characteristics such as the colour of your hair, how tall you are, and even if you are left-handed or right-handed. Each gene controls one or more characteristics inherited from your parents. Genes are made of DNA and are found on chromosomes.

A child inherits one allele for each characteristic from the mother and one from the father. The combination of alleles from both parents determines the child's characteristics. Because each parent contributes different alleles, the child is a unique combination — not an exact copy of either parent. This is why offspring share features with parents but are not identical to them.

Every type of animal and plant produces offspring which are not exactly the same as the parents but do share some common features such as shape. Offspring get some but not all of their parent's characteristics because genes come from both parents — half from the mother and half from the father. The specific combination of genes determines which characteristics the offspring will show.

Chromosomes are thread-like structures found inside the nucleus of every cell. They are the fine threads that carry bits of information (genes) about your characteristics. Chromosomes are made of DNA. Genes are parts of chromosomes; each chromosome contains many genes. There may be up to 4000 genes on one chromosome.

Alleles are different forms of the same gene. The genes in a pair may carry the same message, but sometimes one carries a different message from the other, these are called gene alleles. For example, a person has two alleles for hair colour: one allele says "have blond hair" and the other says "have black hair." These are two different alleles for the same characteristic.

A dominant allele is a gene variant that shows its effect even when paired with a recessive allele. In a heterozygous condition, the dominant allele determines the visible trait while the recessive allele is hidden. For example, the allele for black hair is dominant, so it appears even if a blond allele is present.

A recessive allele is the weaker allele; it does NOT produce the final characteristic when paired with a dominant allele. For example, the allele for blond hair is recessive. It only expresses itself when a person has two recessive alleles for the same characteristic (no dominant allele present). A recessive allele is only expressed in the complete absence of a dominant allele.

A homozygous organism (also called pure bred) is one that has two identical alleles for a particular trait. Because both alleles are the same, it produces a stable and consistent characteristic. For example, BB (dominant alleles) and
Bb (recessive alleles) are homozygous conditions, showing the same trait.

A person with two different alleles (one dominant and one recessive) for a characteristic is hybrid or heterozygous for that characteristic. For example, one allele for black hair (dominant) and one for blond hair (recessive) makes the person heterozygous for hair colour. Because black hair is dominant, this person will have black hair even though they carry the recessive blond allele.

A person with black hair could be:
Homozygous having two dominant alleles for black hair (BB), will always pass on black hair gene.
Heterozygous having one dominant black allele and one recessive blond allele (Bb) appears black-haired but carries the recessive blond allele.
In both cases the person has black hair, but their genetic makeup is different. Only a person with two recessive alleles (bb) will have blond hair.

Examples of dominant characteristics in humans include: dark wavy hair, brown eyes, straight nose, projecting chin, and lobed ears. These features appear even when only one dominant allele is present, because dominant alleles mask (hide) recessive ones.

DNA stands for deoxyribonucleic acid.

DNA is the chemical that makes up chromosomes and genes. Every chromosome contains one long DNA molecule. Genes are short lengths of DNA. DNA is a sort of plan that determines how the body is made up, every cell carries a complete copy of it. DNA contains all the instructions for building, running, and maintaining an organism.

A DNA molecule is shaped like a twisted ladder. This shape is called a double helix. The "sides" of the ladder are made of alternating deoxyribose sugar and phosphate groups. The "rungs" of the ladder are pairs of organic bases. The double helix shape allows DNA to store a huge amount of information in a very compact form.

There are four organic bases in DNA: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). They are represented by the letters A, T, C, and G. The sequence of these bases forms the genetic code — like letters in an alphabet, these four bases are arranged in millions of combinations to code for all of an organism's characteristics.

A DNA molecule is made of structures called nucleotides. A nucleotide is made up of three parts:
A five-carbon sugar called deoxyribose
A Phosphate Group
An organic base.
Nucleotides link together to form the long DNA molecule.

About 99.9% of the DNA of every human being on Earth is the same. It is the remaining 0.1% that makes each person unique. This tiny difference in DNA is what causes all the visible differences (variations) between individual human beings showing how information-dense DNA is.

There may be up to 4000 genes on one chromosome. This large number is necessary because organisms have thousands of different characteristics that need to be controlled. The compact double helix structure of DNA allows many genes to be stored efficiently on one chromosome.

DNA is described as a “plan” or blueprint because it contains all the instructions needed to build and control a living organism. It determines how the body is formed, how cells function, and how traits appear. Just like a blueprint guides the construction of a building, DNA guides the formation of proteins, cells, and organs, ensuring proper growth, development, and maintenance of life.

Living things constantly make new cells for three main reasons:
To replace cells that die: Cells do not live forever.
To enable the organism to grow: More cells are needed as the organism grows.
To make sex cells (gametes) for reproduction: Cells divide when reproductive cells are made.

Phrasing 7: Define both mitosis and meiosis.

Produces four different daughter cells)
Reduces to half chromosome number (n)
Occurs in reproductive cells
Forms gametes
Involving two divisions

Mitosis is a process where two new cells are produced which are exact copies of the original cell. During cell division, chromosomes must split into two. This means the DNA must divide and make an exact copy of itself, this is called replication. Replication ensures each daughter cell gets a complete and identical set of genetic information.

Interphase is the longest stage of the cell cycle where the cell prepares for division. It is not a "resting" phase in terms of activity; instead, the cell is highly active, growing, and duplicating its organelles. It consists of three parts: G1 (growth), S (DNA synthesis), and G2 (final preparation). A cell is said to be in Interphase whenever it is not actively undergoing Mitosis.

DNA Replication is the biological process where one original DNA molecule produces two identical replicas, taking place during the S-phase of Interphase. Crucially, this duplication guarantees that upon cell division, each daughter cell receives a complete and exact set of genetic instructions, vital for their functioning and survival.

Mitosis has four stages:
Prophase
Metaphase
Anaphase
Telophase.
A cell that is not dividing is said to be in interphase.

During Prophase: The chromosomes are copied as the DNA replicates and becomes visible. Two chromatids now make up each chromosome — they are joined together. The membrane around the nucleus starts to break down.

During Metaphase: The membrane round the nucleus has completely gone. A structure called the spindle forms and the chromatids line up along the middle of it.

During Anaphase: The spindle fibres shorten and pull the chromatids towards the ends of the cell. The chromatids are pulled apart to opposite ends of the cell.

During Telophase: The new chromosomes are at each end of the cell. A nuclear membrane forms round each set of chromosomes. The cytoplasm of the cell starts to divide. The result is two daughter cells that are identical in every way to the parent cell, each with the same number of chromosomes (2n).

Meiosis is a process in which a single cell divides two times to produce four cells that contain half the original amount of genetic information. These are reproductive cells (gametes). Meiosis is essential for sexual reproduction because it produces sex cells with half the normal chromosome number so that when two gametes fuse at fertilisation, the offspring has the correct chromosome number.

During meiosis, the chromosome number is halved. This must happen because when two gametes fuse during fertilisation, the resulting cell must have the correct chromosome number for that species. If meiosis did not halve the chromosome number, each new generation would have double the chromosomes of the previous one which would be genetically catastrophic.

Meiosis has two rounds of division: First division — Prophase I: chromosomes copied, two chromatids form, nuclear membrane breaks down. Metaphase I: spindle forms, chromatids line up. Anaphase I: chromatids pulled to ends. Telophase I: nuclear membrane reforms, cytoplasm divides — two cells formed. Second division — Prophase II: NO replication, nuclear membrane breaks down. Metaphase II: spindle forms. Anaphase II: chromatids pulled to opposite ends. Telophase II: four gamete cells result, each with half the chromosome number (n).

In Prophase I, chromosomes are copied (DNA replicates). In Prophase II, there is NO REPLICATION of chromosomes. The chromosomes were already copied in Prophase I. The second division simply separates the chromatids to produce four gamete cells, each with half the chromosome number. If replication happened again in Prophase II, cells would have the wrong chromosome number.

Gametes are reproductive cells (sex cells) produced by meiosis. They contain half the chromosome number (n) of the parent cell. Examples: sperm cells (male gametes) and egg cells/ova (female gametes). When two gametes fuse during fertilisation, the resulting cell (zygote) has the full chromosome number (2n).

"2n" (diploid) refers to the full normal chromosome number in body cells (e.g., 46 in humans). "n" (haploid) refers to half the normal number found in gametes (e.g., 23 in human eggs or sperm). Body cells are 2n because they contain two sets of chromosomes (one from each parent). Gametes are n because meiosis halves the chromosome number — when two gametes fuse, 2n is restored in the offspring.

Organisms produced by asexual reproduction are identical because they are derived from a single parent through mitosis. Mitosis produces exact genetic copies with identical DNA. Since there is no mixing of genetic material from two parents, all offspring are genetically identical — they are clones of the parent. This is why organisms produced by asexual reproduction are identical to each other.

A mutation is a change in the genetic information (DNA) of a cell. Sometimes mistakes are made in the copying of DNA when chromosomes separate during cell division. This changes the instructions carried by a gene, which is how mutations happen. Mutations are significant because they create new variations and can affect how an organism develops and functions.

There are two main types of mutations: (1) Single gene mutation — affects a single gene, changing the instructions carried by that one gene. This changes one specific characteristic. (2) Chromosome mutation — affects the structure of one or more chromosomes. This is a larger-scale change that can affect many genes and characteristics at once. Chromosome mutations are generally more severe.

A mutation might affect an organism by: (1) Magnifying its specific characteristics — making a feature more extreme. (2) Reducing or eliminating a characteristic. (3) Creating an entirely new characteristic. Some mutations can be beneficial — if a mutation produces a variation that helps an organism survive better, it may become an adaptation over generations through natural selection. Many mutations are harmful or neutral.

Mutations are the original source of all new genetic variations. When a mutation occurs in a gene, it creates a new allele — a new version of that gene. If passed to offspring, it introduces new variation into the population. Over millions of years, beneficial mutations are selected for by natural selection, driving evolution. Without mutations, there would be no new variation and species could not adapt to changing environments.