Genome, chromosomes, DNA and genetics

Welcome to Genetics: The Blueprint of Life!

Have you ever wondered why you have the same eye colour as one of your parents, or why siblings can look so similar yet so different? The answer lies in your genetics. In this chapter, we will explore the microscopic instruction manual that makes you who you are: your DNA, genes, and chromosomes.

Don't worry if this topic feels a bit intimidating at first! We are going to break it down step-by-step using clear everyday examples, memory tricks, and simple diagrams so you can master every concept for your CCEA GCSE Biology exam.

1. The Big Picture: Cell, Nucleus, Chromosome, Gene, and Genome

To understand genetics, let's start by zooming in from the whole body down to the molecular level:

• Cell: The basic building block of all living things.
• Nucleus: The control centre of the cell that contains all your genetic information.
• Chromosome: Long, thread-like structures found inside the nucleus, made of tightly coiled strands of DNA.
• Gene: A short section of DNA located on a chromosome that codes for a specific protein (which determines a particular characteristic, such as eye colour or blood group).
• Genome: The entire genetic material of an organism (the complete set of all its DNA).

The Recipe Book Analogy

A great way to remember how these terms link together is the library analogy:

• The nucleus is the library.
• A chromosome is a recipe book on the shelf.
• A gene is a single recipe inside the book.
• DNA is the alphabet/ink used to write the recipe.
• The genome is the complete collection of all the recipe books in the library.

Chromosome Numbers in Humans

Human body cells (such as skin or muscle cells) are diploid, meaning they contain two sets of chromosomes. In total, a normal human body cell has \(46\) chromosomes (or \(23\) pairs). You inherit \(23\) chromosomes from your biological mother (via the egg) and \(23\) chromosomes from your biological father (via the sperm).

The Human Genome Project (HGP)

The Human Genome Project was a massive international scientific research project that successfully mapped and sequenced all the genes found in human DNA.

Why is the Human Genome Project important?
• Improved diagnosis and treatment: Helps doctors identify genes linked to inherited diseases.
• Personalised medicine: Allows the development of targeted drugs tailored to an individual's genetic make-up.
• Understanding human evolution: Helps scientists trace human migration and evolutionary history over thousands of years.

Key Takeaway

Your genome is the total genetic code in your body. It is stored on \(46\) chromosomes inside the nucleus, which are divided into short instructions called genes.

2. The Structure of DNA and the Genetic Code

DNA stands for deoxyribonucleic acid. It is the chemical molecule that stores all your genetic instructions.

The Double Helix and Nucleotides

DNA has a shape called a double helix. You can picture this as a flexible ladder that has been twisted into a spiral.

DNA is a polymer made up of repeating units called nucleotides. Each nucleotide contains three parts:

1. A phosphate group
2. A sugar molecule (deoxyribose)
3. A nitrogenous base

The sugars and phosphates link together alternately to create the two strong side rails of the ladder, known as the sugar-phosphate backbone. The bases stick out into the middle and join together to form the rungs of the ladder.

Complementary Base Pairing

There are four different bases found in DNA:

• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)

These bases pair up in a very specific way using weak hydrogen bonds. This is called complementary base pairing:

• Adenine (A) always pairs with Thymine (T)
• Cytosine (C) always pairs with Guanine (G)

Memory Trick for Base Pairing

• Apples on Trees (\(A\) with \(T\))
• Cars in Garages (\(C\) with \(G\))

How DNA Codes for Proteins (The Base Triplet Code)

Proteins are made of long chains of smaller molecules called amino acids. The order of bases in a gene determines the order of amino acids in a protein.

• A group of three bases in a row is called a triplet code (or base triplet).
• Each triplet codes for one specific amino acid.
• For example, if a gene contains \(12\) bases, it will code for \(12 \div 3 = 4\) amino acids.
• The sequence of amino acids folds up into a specific 3D shape to make a working protein (such as an enzyme, hormone, or structural protein like keratin).

Quick Review

• DNA is a double helix.
• Base rules: \(A\) pairs with \(T\), and \(C\) pairs with \(G\).
• \(3\) bases \(= 1\) amino acid.

3. Genetic Profiling (DNA Fingerprinting)

Except for identical twins, every person has a completely unique DNA sequence. Genetic profiling is a technique used to analyse a person's DNA and produce a visual pattern of bands (a DNA profile).

Steps in Genetic Profiling

1. Extraction: A sample of cells containing DNA is collected (e.g., from blood, saliva, hair roots, or semen) and the DNA is extracted.
2. Fragmentation: The DNA is cut into small fragments using special biological scissors called restriction enzymes.
3. Separation: The fragments are separated by size using an electrical current in a gel (a process called electrophoresis). Shorter fragments travel further and faster than longer ones.
4. Analysis: The resulting band pattern is stained and visualised, producing a distinct pattern of dark bands that can be compared against other samples.

Uses of Genetic Profiling

• Forensic Science: Matching DNA found at a crime scene to a suspect.
• Paternity Testing: Confirming the biological father of a child. (A child gets half their DNA bands from their biological mother and the other half from their biological father).

Ethical Issues and Concerns

• Privacy: Who owns the genetic data? Should insurance companies or employers be allowed to see it?
• Data security: Storing genetic profiles on national databases raises risks of hacking or misuse.
• False convictions: Contamination of samples at a crime scene could lead to an innocent person being falsely accused.

Key Takeaway

Genetic profiling produces a unique pattern of bands from an individual's DNA, useful in forensic investigations and paternity tests, but raises important ethical questions regarding privacy.

4. Essential Genetics Vocabulary

Before working out genetic crosses, you need to be comfortable with these key terms. Take your time to review them!

• Allele: An alternative or different version of the same gene. For example, the gene for eye colour might have a blue allele and a brown allele.
• Dominant allele: An allele that is always expressed in the phenotype, even if only one copy is present. Represented by a capital letter (e.g., \(B\)).
• Recessive allele: An allele that is only expressed if two copies are present (no dominant allele is around). Represented by a lowercase letter (e.g., \(b\)).
• Homozygous: Having two identical alleles for a particular gene (e.g., \(BB\) or \(bb\)).
• Heterozygous: Having two different alleles for a particular gene (e.g., \(Bb\)).
• Genotype: The combination of alleles an organism possesses (e.g., \(BB\), \(Bb\), or \(bb\)).
• Phenotype: The visible physical characteristic or trait shown by the organism (e.g., brown eyes or blue eyes).

Common Mistake to Avoid

Do not mix up genotype and phenotype! Remember: Genotype \(= \) Genetic code (the letters), whereas Phenotype \(= \) Physical appearance.

5. Monohybrid Genetic Crosses and Punnett Squares

A monohybrid cross looks at the inheritance of a single characteristic controlled by one gene.

Step-by-Step Guide to Solving a Genetic Cross

Let's look at an example: In pea plants, the allele for tall stems (\(T\)) is dominant over the allele for short stems (\(t\)). What happens if we cross two heterozygous (\(Tt\)) tall pea plants?

Step 1: Write down the parental phenotypes:
Tall plant \(\times\) Tall plant

Step 2: Write down the parental genotypes:
\(Tt \times Tt\)

Step 3: State the gametes (sex cells) produced by each parent:
Parent 1 gametes: \(T\) or \(t\)
Parent 2 gametes: \(T\) or \(t\)

Step 4: Draw a Punnett Square:

• Top row (Parent 1): \(T\) and \(t\)
• Left column (Parent 2): \(T\) and \(t\)

Filling in the boxes gives four possible offspring combinations:
• Box 1 (Top-Left): \(T + T = TT\)
• Box 2 (Top-Right): \(T + t = Tt\)
• Box 3 (Bottom-Left): \(t + T = Tt\)
• Box 4 (Bottom-Right): \(t + t = tt\)

Step 5: Determine the offspring genotypes and phenotypes:
• \(1 \times TT\) (Homozygous dominant \(\rightarrow\) Tall)
• \(2 \times Tt\) (Heterozygous \(\rightarrow\) Tall)
• \(1 \times tt\) (Homozygous recessive \(\rightarrow\) Short)

Step 6: State the ratios and probabilities:
• Genotype ratio: \(1 TT : 2 Tt : 1 tt\)
• Phenotype ratio: \(3 \text{ Tall} : 1 \text{ Short}\)
• Probability of a tall plant: \(\frac{3}{4}\) or \(75\%\)
• Probability of a short plant: \(\frac{1}{4}\) or \(25\%\)

Pedigree Diagrams (Family Trees)

A pedigree chart shows how a genetic trait or condition is passed down through generations in a family.

• Squares represent males.
• Circles represent females.
• Horizontal lines between a circle and square show mating/parents.
• Vertical lines branch down to their offspring.
• Shaded shapes usually represent individuals who have the genetic condition.

Key Takeaway

When crossing two heterozygous parents (\(Tt \times Tt\)), the resulting phenotypic ratio is always expected to be \(3:1\) (dominant : recessive).

6. Sex Determination in Humans

Of the \(23\) pairs of chromosomes in human cells, one pair consists of the sex chromosomes, which determine biological sex:

• Females have two identical \(X\) chromosomes: \(XX\)
• Males have one \(X\) and one smaller \(Y\) chromosome: \(XY\)

Inheritance of Biological Sex

All human egg cells produced by a female carry one \(X\) chromosome. Human sperm cells produced by a male carry either an \(X\) chromosome or a \(Y\) chromosome (\(50\%\) chance of each).

Let's look at the cross: Mother (\(XX\)) \(\times\) Father (\(XY\))

• Female gametes: \(X\) and \(X\)
• Male gametes: \(X\) and \(Y\)

Offspring genotypes:
• \(X + X = XX\) (Female)
• \(X + Y = XY\) (Male)
• \(X + X = XX\) (Female)
• \(X + Y = XY\) (Male)

This results in an exact \(1:1\) ratio (or \(50\%\) chance) of having a boy or a girl with each pregnancy.

Did You Know?

Because the mother always contributes an \(X\) chromosome, it is the father's sperm (whether it carries an \(X\) or a \(Y\)) that determines the biological sex of the child!

7. Mutations and Inherited Conditions

A mutation is a random, spontaneous change in the DNA base sequence or in chromosome number.

• Mutations can occur naturally during DNA replication.
• The rate of mutation is increased by mutagens, including ionising radiation (such as X-rays and UV rays) and certain chemical carcinogens (such as chemicals in tobacco smoke).

Examples of Inherited Conditions

1. Cystic Fibrosis (Recessive Condition)

• Cause: Caused by a recessive allele (let's denote it as \(c\)).
• Genotypes:
- \(CC\): Normal (unaffected)
- \(Cc\): Carrier (healthy, does not have symptoms, but carries the recessive allele)
- \(cc\): Affected by Cystic Fibrosis
• Symptoms: Production of abnormally thick, sticky mucus in the lungs and digestive system, leading to breathing difficulties and digestive problems.

2. Huntington's Disease (Dominant Condition)

• Cause: Caused by a dominant allele (let's denote it as \(H\)).
• Genotypes:
- \(hh\): Normal (unaffected)
- \(Hh\) or \(HH\): Affected by Huntington's Disease
• Note: There are no carriers for Huntington's because anyone who inherits even one dominant allele (\(H\)) will develop the disease.
• Symptoms: Progressive damage to nerve cells in the brain, leading to involuntary movements and cognitive decline. Symptoms typically appear later in life (usually after age \(35\)-\(40\)).

3. Down's Syndrome (Chromosome Number Mutation)

• Cause: Caused by the presence of an extra chromosome. Individuals have \(47\) chromosomes in each cell instead of the normal \(46\) (specifically, three copies of chromosome \(21\) instead of two).
• This happens due to an error during gamete formation where chromosome pairs fail to separate properly.
• Characteristics: Distinct facial features, learning difficulties, and increased risk of certain health conditions.

8. Genetic Screening and Testing

Genetic screening involves testing individuals or embryos to detect the presence of alleles linked to genetic disorders.

Amniocentesis

A common method of prenatal screening is amniocentesis:

• How it works: A needle is inserted through the mother's abdomen into the amniotic sac (guided by ultrasound) to collect a small sample of amniotic fluid containing fetal cells.
• The fetal cells are cultured, and their chromosomes/genes are examined for abnormalities (such as Down's syndrome or Cystic Fibrosis).
• Risk: Carries a small risk (around \(0.5\%\) to \(1\%\)) of causing a miscarriage.

Ethical and Social Considerations of Screening

• Benefits: Allows parents to make informed choices, prepare emotionally and financially for a child with special needs, or begin medical treatments early.
• Dilemmas: May present parents with difficult decisions regarding termination of pregnancy, and can cause significant emotional anxiety.

Quick Summary Checklist for Revision

Before your exam, make sure you can:

• Define genome, chromosome, gene, allele, dominant, recessive, homozygous, and heterozygous.
• State the complementary base pairing rules (\(A-T\) and \(C-G\)) and explain the base triplet code.
• Outline the steps and ethical issues of genetic profiling.
• Draw and interpret a Punnett square for a monohybrid cross, calculating expected ratios and percentages.
• Explain that sex is determined by \(XX\) (female) and \(XY\) (male), giving a \(1:1\) (\(50\%\)) probability.
• Identify the causes and inheritance patterns of Cystic Fibrosis (\(cc\)), Huntington's disease (\(H\_\)), and Down's syndrome (\(47\) chromosomes).
• Discuss the purpose, risks (e.g., miscarriage), and ethics of prenatal screening like amniocentesis.