Genetics & DNA
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Genetics?
- Genetics is the study of how characteristics are passed from parents to offspring through genes
- this helps explain why children resemble their parents, and why organisms vary
- DNA (deoxyribonucleic acid): the chemical molecule that carries genetic information in all living organisms
- Gene: a section of DNA that codes for a specific characteristic, e.g. eye colour, blood type
- Chromosome: a long strand of DNA containing many genes, found in the nucleus of cells
- Genome: the complete set of genetic material in an organism
DNA, Genes, and Chromosomes
- Genetic material is organised in a hierarchical structure, from small to large:
- DNA → genes → chromosomes → nucleus → cell
DNA Structure
- DNA has a double helix structure, shaped like a twisted ladder
- made of two strands wound around each other
- Contains coded instructions for making proteins
- The sequence of bases (A, T, C, G) determines characteristics
Chromosomes
- Found in the nucleus of cells
- made of tightly coiled DNA
- Humans have 46 chromosomes, arranged in 23 pairs
- each chromosome contains hundreds to thousands of genes
Genes
- Specific sections of DNA on chromosomes
- each gene codes for a specific characteristic
- Genes contribute to determining an organism's characteristics
- examples: eye colour, hair colour, blood type, height
The hierarchy from DNA to genes to chromosomes to the nucleus of a cell
- The genetic material follows a hierarchy: DNA makes up genes, genes are organised on chromosomes, and chromosomes are stored in the nucleus of every cell
How Genes Determine Characteristics
- Genes provide instructions for making proteins
- different proteins determine different characteristics in organisms
- this process is called gene expression — converting genetic information into observable traits
- Genes don't create characteristics directly
- they provide instructions for making proteins, which then produce the characteristics we observe
- pathway: DNA sequence → instructions → protein made → characteristic expressed
Examples of Gene-Protein Relationships
- Gene for melanin protein → determines skin, hair, and eye colour
- Gene for insulin protein → controls blood sugar levels
- Gene for haemoglobin protein → carries oxygen in the blood
Sex Inheritance in Humans
- Fertilisation is when gametes (sex cells) fuse to form a zygote
- the male gamete (sperm) and the female gamete (egg) join (fuse) together, combining genetic material — including a sex chromosome — from each parent
- Humans have 23 pairs of chromosomes, 46 in total
- 22 pairs are autosomes (body chromosomes), and one pair are sex chromosomes
- sex chromosomes determine biological sex
| Sex | Sex Chromosomes | Description |
|---|---|---|
| Female | XX | Two X chromosomes |
| Male | XY | One X and one Y chromosome |
How Sex is Inherited
- During reproduction, each parent contributes one sex chromosome to their offspring
- mother (XX): can only pass on an X chromosome, in the egg
- father (XY): can pass on either an X or a Y chromosome, in the sperm
- The possible combinations determine the sex of the child:
- egg (X) + sperm (X) = XX = female child
- egg (X) + sperm (Y) = XY = male child
- There is a 50% chance of having a female child (XX) and a 50% chance of having a male child (XY)
- the father's sperm determines the sex of the child, since only sperm can carry a Y chromosome
- The 50:50 ratio is only expected to show up clearly over a large number of births
- in a small sample — for example, only 5 or 6 births or trials — the results may not show exactly 50:50, simply because of chance (random variation) in which gametes happen to combine
- to get results closer to the true 50:50 probability, you need more results/repeats — a much larger sample size
Punnett square showing the 50:50 probability of inheriting XX or XY
Inheritance Patterns
- Alleles are different versions of the same gene
- for example, the gene for eye colour has different alleles for brown, blue, and green eyes
- You inherit two alleles for each gene, one from each parent
Dominant and Recessive Alleles
| Type | Description | Symbol |
|---|---|---|
| Dominant allele | Always expressed if present; masks recessive allele | Capital letter (e.g., B) |
| Recessive allele | Only expressed if two copies present | Lowercase letter (e.g., b) |
Genotype and Phenotype
| Term | Definition | Example |
|---|---|---|
| Genotype | The genetic makeup (alleles present) | BB, Bb, or bb |
| Phenotype | The physical appearance (observable characteristic) | Brown eyes or blue eyes |
Worked Example: Eye Colour Inheritance
- Muumbe is investigating eye colour, where B = brown eyes (dominant) and b = blue eyes (recessive)
- Step 1: list the possible genotypes: BB, Bb, bb
- Step 2: BB is homozygous dominant, with two dominant alleles, so the phenotype is brown eyes
- Step 3: Bb is heterozygous, with one of each allele, so the dominant allele shows and the phenotype is brown eyes
- Step 4: bb is homozygous recessive, with two recessive alleles, so the phenotype is blue eyes
- Answer: only individuals with genotype bb will have blue eyes — individuals with BB or Bb genotypes will have brown eyes, because the dominant allele masks the recessive allele
Genetic Variation
- Offspring are genetically different from their parents and from each other, except identical twins
- this genetic variation arises from several sources
Sources of Genetic Variation
- Sexual reproduction: offspring receive half their genes from each parent, creating new combinations
- Random fertilisation: any sperm can fertilise any egg, producing unique combinations
- Mutation: random changes in the DNA sequence create new alleles
- Every individual, except identical twins, has a unique genetic makeup due to the combination of these factors
Genetic Disorders
- Some genetic disorders are caused by faulty or mutated genes inherited from parents
- these disorders can be dominant or recessive
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