Unit 5: Continuity of life
Essential Biology notes · PTU syllabus (BMLS101-18)
On this page
- Unit summary
- Heredity and variation
- Mendel's laws of inheritance
- Chromosomal basis of inheritance
- Incomplete dominance, multiple alleles and quantitative inheritance
- The genome, linkage and crossing over
- Gene mapping and recombination
- DNA structure and replication
- RNA and protein synthesis
- Key terms
- Quick revision
- Important questions
Unit summary
Heredity passes traits from parents to offspring, and DNA carries the instructions. This unit covers heredity and variation, Mendel's laws, the chromosomal basis of inheritance, incomplete dominance, multiple alleles, quantitative inheritance, the genome, linkage and crossing over, gene mapping, recombination, DNA structure and replication, and RNA and protein synthesis.
After this unit you can
- Apply Mendel's laws to crosses
- Explain incomplete dominance, multiple alleles and quantitative inheritance
- Explain linkage, crossing over and gene mapping
- Describe DNA replication and protein synthesis
PTU syllabus topics
- Heredity and variation
- Mendel's laws of inheritance
- chromosomal basis of inheritance
- incomplete dominance
- multiple allelism
- quantitative inheritance
- genome
- linkage and crossing over
- gene mapping
- recombination
- DNA structure and replication
- RNA structure and role in protein synthesis
- 1DNA
Stores genetic information
- 2Replication
DNA copies itself
- 3Transcription
DNA to mRNA in the nucleus
- 4Translation
mRNA to protein on ribosomes
- 5Protein
Does the cell's work
Topic 1
Heredity and variation
- Heredity: transmission of traits from parents to offspring. Variation: differences among individuals, from mutations, crossing over and independent assortment.
Topic 2
Mendel's laws of inheritance
- Law of dominance
- In a heterozygote, one allele (dominant) masks the other (recessive)
- Law of segregation
- Two alleles separate during gamete formation; each gamete gets one
- Law of independent assortment
- Genes for different traits assort independently
Example
Monohybrid cross Tt × Tt (tall pea plants): genotypes TT : Tt : tt = 1 : 2 : 1; phenotypes tall : dwarf = 3 : 1. A dihybrid cross gives 9 : 3 : 3 : 1.
Topic 3
Chromosomal basis of inheritance
- Sutton and Boveri: genes are on chromosomes; chromosome behaviour in meiosis explains Mendel's laws. Humans have 46 chromosomes (22 pairs of autosomes + XX or XY).
Topic 4
Incomplete dominance, multiple alleles and quantitative inheritance
Incomplete dominance
Heterozygote is intermediate
Red × white snapdragon → pink; F2 ratio 1 : 2 : 1
Codominance
Both alleles fully expressed
Blood group AB
Multiple alleles
More than two alleles of a gene in a population
ABO blood groups: IA, IB, i
Quantitative (polygenic) inheritance
Many genes add small effects; continuous variation
Height, skin colour
Example
Parents with blood groups A (IAi) and B (IBi) can have children of groups AB, A, B and O.
Topic 5
The genome, linkage and crossing over
- Genome: the complete set of DNA of an organism — the human genome has about 3 billion base pairs and around 20,000 protein-coding genes.
- Linkage: genes close together on a chromosome tend to be inherited together. Crossing over separates linked genes, producing recombinants.
Topic 6
Gene mapping and recombination
- Recombination frequency = recombinant offspring ÷ total offspring × 100; 1% = 1 map unit (centimorgan). Greater distance means more crossing over.
Example
If genes A and B give 8% recombinants and B and C 3%, while A and C give 11%, the order is A – B – C.
Topic 7
DNA structure and replication
- Watson and Crick (1953): double helix of two antiparallel strands; sugar–phosphate backbone; bases pair A–T (2 hydrogen bonds) and G–C (3).
- 1
Helicase unwinds the helix at the origin
- 2
Primase lays RNA primers
- 3
DNA polymerase adds nucleotides 5′ to 3′
- 4
Leading strand made continuously; lagging strand as Okazaki fragments
- 5
Ligase joins fragments
- 6
Each new DNA has one old and one new strand
Topic 8
RNA and protein synthesis
- 1Transcription
DNA → mRNA in the nucleus by RNA polymerase
- 2Processing
Introns removed, cap and tail added
- 3Translation
mRNA read by ribosomes in codons of three bases
- 4tRNA
Brings amino acids matching each codon (anticodon)
- 5Polypeptide
Chain folds into a protein
- Genetic code: 64 codons; AUG = start (methionine); UAA, UAG, UGA = stop.
Key terms
- Allele
- Alternative form of a gene
- Heterozygous
- Having two different alleles
- Linkage
- Tendency of nearby genes to be inherited together
- Centimorgan
- Map unit equal to 1% recombination
- Codon
- Three-base mRNA sequence specifying an amino acid
Quick revision
- Dominance, segregation, independent assortment; 3 : 1 and 9 : 3 : 3 : 1.
- Incomplete dominance, codominance, multiple alleles, polygenic traits.
- Genome; linkage; crossing over; mapping.
- Double helix; semi-conservative replication.
- Transcription and translation; genetic code.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.State Mendel's law of segregation.
- Q2.Give the phenotypic ratio of a monohybrid cross.
- Q3.What is incomplete dominance?
- Q4.Why are ABO blood groups an example of multiple alleles?
- Q5.What is a centimorgan?
- Q6.Name the start codon.
Long-answer questions
- Q1.Explain Mendel's laws with crosses.
- Q2.Explain incomplete dominance, multiple alleles and quantitative inheritance.
- Q3.Explain linkage, crossing over and gene mapping.
- Q4.Describe DNA replication and protein synthesis.
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