Welcome to Chromosomes and Genes
Have you ever wondered why you have your mother's eye colour, your father's smile, or why brothers and sisters look similar but not completely identical? The secret lies deep inside your cells!
In this chapter of Unit 1 Biology, we will explore the wonderful world of genetics. Don't worry if these terms seem strange or confusing at first. We will break everything down into simple, bite-sized steps so you can master this topic for your CCEA GCSE Single Award Science exam.
1. The Genetic Hierarchy: Cells, Chromosomes, and DNA
To understand genetics, it helps to think of your body as a library. Let's look at how the genetic information inside you is organized from largest to smallest:
• Cell: The basic building block of all living things.
• Nucleus: The control centre inside the cell that stores your genetic instructions.
• Chromosome: Long, thread-like structures found inside the nucleus. They are made of DNA wrapped tightly around proteins.
• Gene: A short section of DNA located on a chromosome. Each gene codes for a specific sequence of amino acids to make a particular protein.
• DNA (Deoxyribonucleic acid): The actual chemical molecule that carries the instructions. DNA is a polymer made of two strands twisted together into a double helix shape (like a twisted rope ladder).
• Genome: The entire genetic material or total DNA of an organism.
Chromosomes in Numbers
Every species has a characteristic number of chromosomes:
• Body cells (Somatic cells): Contain \(46\) chromosomes organized into \(23\) pairs. Because they have two copies of each chromosome, they are called diploid cells.
• Gametes (Sex cells – sperm and egg): Contain only \(23\) single chromosomes. They have half the normal number, so they are called haploid cells.
• When a sperm cell (\(23\) chromosomes) fertilises an egg cell (\(23\) chromosomes), the resulting embryo has a complete set of \(46\) chromosomes (\(23\) pairs).
Boy or Girl? Sex Determination
Out of your \(23\) pairs of chromosomes, the \(23\text{rd}\) pair determines your biological sex:
• Females have two \(X\) chromosomes: \(XX\)
• Males have one \(X\) and one \(Y\) chromosome: \(XY\)
Did you know? Because mothers only have \(X\) chromosomes to pass on in their eggs, it is always the father's sperm (carrying either an \(X\) or a \(Y\)) that determines whether a baby is genetically male or female!
Quick Review: The Size Hierarchy
Memory Trick: Remember the order from largest to smallest:
Cell \(\rightarrow\) Nucleus \(\rightarrow\) Chromosome \(\rightarrow\) Gene \(\rightarrow\) DNA
Key Takeaway: Chromosomes are found in the nucleus and are made of DNA. Body cells have \(46\) chromosomes (\(23\) pairs), while gametes have \(23\) single chromosomes. \(XX\) produces females, and \(XY\) produces males.
2. Genetic Language and Inheritance
To solve genetics questions, you need to know the specific vocabulary examiners look for.
Key Terms You Must Know
• Alleles: Different forms or variants of the same gene. For example, a gene might control eye colour, but one allele codes for brown eyes and another codes for blue eyes.
• Genotype: The combination of alleles an organism possesses (written using letters, such as \(BB\), \(Bb\), or \(bb\)).
• Phenotype: The observable physical characteristic or trait shown by the organism (for example, "brown eyes" or "blue eyes").
• Dominant Allele: An allele that is always expressed in the phenotype if at least one copy is present. It is represented by a capital letter (e.g., \(B\)).
• Recessive Allele: An allele that is only expressed in the phenotype when two copies are present. It is hidden if a dominant allele is around. It is represented by a lowercase letter (e.g., \(b\)).
• Homozygous: Having two identical alleles for a particular gene (e.g., \(BB\) is homozygous dominant, \(bb\) is homozygous recessive).
• Heterozygous: Having two different alleles for a particular gene (e.g., \(Bb\)).
Monohybrid Crosses and Punnett Squares
A Punnett square is a grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross.
Step-by-Step Example:
Let's cross two parents who are both heterozygous for brown eyes (\(Bb\)), where brown eyes (\(B\)) is dominant and blue eyes (\(b\)) is recessive.
1. Identify parental genotypes: Parent 1 = \(Bb\), Parent 2 = \(Bb\)
2. Separate alleles into gametes: Parent 1 produces \(B\) and \(b\); Parent 2 produces \(B\) and \(b\)
3. Fill in the \(2 \times 2\) Punnett square:
• Top-left: \(B \times B = BB\)
• Top-right: \(B \times b = Bb\)
• Bottom-left: \(b \times B = Bb\)
• Bottom-right: \(b \times b = bb\)
4. Determine the Genotype Ratio:
\(1\ BB : 2\ Bb : 1\ bb\) (or \(1:2:1\))
5. Determine the Phenotype Ratio:
• \(BB\) and \(Bb\) will have brown eyes (\(3\) out of \(4\), or \(75\%\))
• \(bb\) will have blue eyes (\(1\) out of \(4\), or \(25\%\))
• Phenotype ratio = \(3\ \text{brown eyes} : 1\ \text{blue eye}\) (or \(3:1\))
Predicting Sex: The \(XX \times XY\) Cross
If we cross a mother (\(XX\)) and a father (\(XY\)):
• Offspring possibilities: \(XX\), \(XX\), \(XY\), \(XY\)
• Probability of female (\(XX\)): \(2\) out of \(4 = 50\%\)
• Probability of male (\(XY\)): \(2\) out of \(4 = 50\%\)
• Ratio: \(1:1\)
Common Pitfalls to Avoid
• Letter Choice: Avoid using letters where uppercase and lowercase look almost identical (like \(C/c\) or \(S/s\)). If the exam does not specify a letter, choose clear letters like \(B/b\) or write your lowercase letters very clearly.
• Ratios vs Percentages: Read the question carefully! If the examiner asks for a ratio, write \(3:1\). If they ask for a percentage, write \(75\%\) and \(25\%\).
• Dominant does not mean "common": A dominant allele is simply one that masks a recessive allele; it does not automatically mean it is the most common allele in a population.
Key Takeaway: Genotype is the genetic makeup (letters like \(Bb\)), while phenotype is the physical appearance. A dominant allele only needs one copy to show, while a recessive allele requires two copies.
3. Mutations and Genetic Conditions
What is a Mutation?
A mutation is a spontaneous, random change in the structure of a gene or in the number/structure of chromosomes.
While mutations happen naturally by chance, their frequency can be greatly increased by environmental agents called mutagens:
• Ionising radiation: Ultraviolet (UV) light, X-rays, and gamma rays.
• Chemical mutagens: Carcinogenic chemicals (such as those found in tobacco smoke).
Down's Syndrome (A Chromosome Abnormality)
• Caused by an error during gamete formation where a child inherits an extra chromosome.
• Individuals with Down's syndrome have an extra copy of chromosome \(21\) (trisomy \(21\)).
• This means their body cells contain \(47\) chromosomes instead of the normal \(46\).
Cystic Fibrosis (An Inherited Recessive Condition)
• Cystic fibrosis is caused by a recessive allele (we can write this as \(c\)).
• A person needs two faulty recessive alleles (\(cc\)) to suffer from cystic fibrosis.
• A person with one normal dominant allele and one faulty recessive allele (\(Cc\)) is called a carrier. Carriers do not have the condition themselves, but they can pass the faulty allele on to their children.
Example Cross: If two carriers marry (\(Cc \times Cc\)):
• \(CC\) (\(25\%\)): Unaffected non-carrier
• \(Cc\) (\(50\%\)): Healthy carrier
• \(cc\) (\(25\%\)): Has cystic fibrosis
There is a \(1\ \text{in}\ 4\) (\(25\%\)) chance that their child will inherit cystic fibrosis.
Key Takeaway: Mutations are changes in genes or chromosomes. Down's syndrome is caused by having \(47\) chromosomes (an extra chromosome \(21\)). Cystic fibrosis is a recessive disorder, meaning two carrier parents (\(Cc\)) have a \(25\%\) chance of having an affected child (\(cc\)).
4. Genetic Screening and Genetic Engineering
Genetic Screening
Genetic screening involves testing individuals, populations, or embryos to detect the presence of faulty alleles or chromosome abnormalities.
Main Applications:
1. Pre-natal screening (e.g., Amniocentesis): Testing cells taken from amniotic fluid surrounding an unborn baby in the womb to check for conditions like Down's syndrome.
2. Adult carrier screening: Testing prospective parents to see if they carry recessive alleles for inherited conditions (such as cystic fibrosis).
Ethical and Social Considerations:
Genetic screening gives parents valuable information, but it also raises difficult decisions and risks:
• Risk of miscarriage: Procedures like amniocentesis carry a small risk of harming the pregnancy or causing a miscarriage.
• False results: Tests can sometimes give false positive or false negative results, causing unnecessary stress or false reassurance.
• Emotional stress: Receiving news of a genetic abnormality can cause severe anxiety and emotional distress.
• Termination decisions: Parents may face very difficult and controversial ethical choices regarding whether to terminate a pregnancy.
Genetic Engineering
Genetic engineering is the process of modifying the genome of an organism by introducing a gene from another organism to give it a desired characteristic.
Do not confuse genetic engineering with selective breeding! Selective breeding involves breeding parents with chosen traits over many generations. Genetic engineering involves directly taking a specific gene from one organism and inserting it into the DNA of another.
Example 1: Producing Human Insulin in Medicine
People with Type 1 diabetes need regular injections of insulin. Insulin used to be extracted from cows or pigs, but today it is made cheaply and safely using genetically engineered bacteria:
1. The human gene responsible for making insulin is identified and isolated.
2. The gene is inserted into a small loop of bacterial DNA called a plasmid (which acts as a vector).
3. The genetically modified plasmid is introduced into a bacterium.
4. These modified bacteria are grown in large industrial vats called fermenters, where they multiply rapidly and produce large quantities of human insulin.
5. The insulin is extracted, purified, and bottled for medical use.
Example 2: Genetically Modified (GM) Crops in Agriculture
Crops can be genetically modified to introduce beneficial traits:
• Pest resistance: Plants produce their own natural insect repellent, reducing the need for chemical pesticide sprays.
• Drought tolerance: Plants can survive with less water in dry climates.
• Increased crop yield: Producing more food per hectare to help feed growing populations.
Key Takeaway: Genetic screening detects genetic conditions but involves ethical dilemmas such as miscarriage risk and termination choices. Genetic engineering transfers a gene from one organism to another, such as placing the human insulin gene into bacteria or creating GM crops for pest resistance and higher yields.