Inheritance

Monohybrid Crosses

When two parents that differ in only one characteristic breed, the process is called a monohybrid cross. Monohybrid crosses allow the genotype of offspring to be predicted.

Parental genotype

The first step in constructing a monohybrid cross involves identifying the parental genotypes.
- E.g. Two true-breeding pea plants have yellow or green peas.
  - The dominant seed colour is green so the parental genotype is GG for green pea plants and gg for yellow pea plants.

Gamete alleles

Gametes are haploid, so only one allele from each parent is found in the gametes.
All possible combinations of the parental alleles should be identified. This represents the meiotic segregation into haploid gametes. In our pea plant example:
- 100% of the gametes of green pea plants will have G alleles.
- 100% of the gametes of yellow pea plants will have g alleles.

Illustrative background for F<sub>1</sub> offspring

Illustrative background for F<sub>1</sub> offspring ?? "content

F₁ offspring

F₁ offspring are the first generation of offspring.
A monohybrid cross produces four different combinations of possible offspring.
- For the pea plants, both parents are homozygous. This means all the F₁ offspring produced have a Gg genotype.

Gamete alleles

The F₁ pea plants have two different alleles. They are heterozygous.
The gametes for an individual F₁ offspring may contain either the G allele or the g allele.
- 50% of an organism's gametes will contain the G allele.
- 50% of an organism's gametes will contain the g allele.

Illustrative background for F<sub>2</sub> offspring

Illustrative background for F<sub>2</sub> offspring ?? "content

F₂ offspring

F₂ offspring are the second generation of offspring.
When the F₁ pea plants breed, there are three possible genotypic combinations:
- GG
- Gg
- gg

Predicting genotypic ratios

Monohybrid crosses allow predictions to be made about the genotypic and phenotypic ratios of offspring.
- In the pea plant example, the ratio of yellow peas to green peas is 3:1. A monohybrid cross between two heterozygotes will always produce this ratio.
Monohybrid crosses can be drawn in two ways:
- Genetic diagrams.
- Punnett squares.

Dihybrid Crosses

When two parents that differ in two characteristics breed, the process is called a dihybrid cross.

Independent assortment

Mendel proved that genes do not influence each other with regard to the sorting of alleles into gametes. This is called the law of independent assortment.
The law of independent assortment means that genes separate independently of each other when gametes are made.
The combination of alleles can be shown in a dihybrid cross.

Dihybrid gamete alleles

In a dihybrid cross between two homozygotes, there is one possible gamete allele combination for each homozygote.
E.g. two pea plants differ in two characteristics: seed colour and seed texture. One plant has green, wrinkled seeds (yyrr) and one plant has yellow, round seeds (YYRR).
- 100% of the gametes of the green/wrinkled plant are yr.
- 100% of the gametes of the yellow/round plant are YR.

F₁ offspring

When two homozygotes breed, all the F₁ offspring have the same genotype.
- E.g. The offspring of the pea plants all have a YyRr genotype.

Illustrative background for F<sub>1</sub> gamete alleles

Illustrative background for F<sub>1</sub> gamete alleles ?? "content

F₁ gamete alleles

The law of segregation predicts that each gamete in F₁ generation has an equal probability of receiving any allele (e.g. R, r, Y or y).
This means there are four possible combinations of gametes produced by the F₁ offspring. For example:
- YR.
- Yr.
- yR.
- yr.

F₂ offspring

When the F₁ offspring breed, the four possible gametes from one individual can combine with any of the four possible gametes from the other individual.
The total possible combinations in the F₂ generation is 16.

Predicting phenotypic ratios

Dihybrid crosses can be used in this way to predict genotypic ratios of the F₂ offspring.
In the pea plant example, the ratio of offspring is:
- Nine round/yellow.
- Three round/green.
- Three wrinkled/yellow.
- One wrinkled/green.
When two dihybrid heterozygotes breed, the ratio is always expected to be 9:3:3:1.

Codominance

Codominance is where both alleles for the same characteristic are simultaneously expressed in the heterozygote. This can influence the outcome of monohybrid and dihybrid crosses.

Codominance

Codominant alleles are both expressed in a heterozygote.
Neither of the alleles are recessive.
Codominance influences the phenotypic ratios of monohybrid and dihybrid crosses.

E.g. sickle-cell anaemia

An example of codominance is sickle-cell anaemia.
There are two alleles for sickle-cell anaemia:
- H^N - normal haemoglobin.
- H^S - sickle haemoglobin.

Sickle-cell phenotypes

People who have two copies of the H^N allele (homozygotes) do not have sickle-cell anaemia.
People who have two copies of the H^S alleles (homozygotes) do have sickle-cell anaemia.
People who have one H^N allele and H^S allele (heterozygotes) have both normal haemoglobin and sickled haemoglobin.
- H^N and H^S are codominant.

Phenotypic ratios

Codominance affects the phenotypic ratios of monohybrid and dihybrid crosses.
E.g:
- If two heterozygous (H^NH^S) breed, the ratio becomes 1:2:1 instead of the normal 3:1 that is expected in a monohybrid cross.

1Cell Structure

1.1Cell Structure

1.1.1Studying Cells - Microscopes

1.1.2Introduction to Eukaryotic & Prokaryotic Cells

1.1.3Ultrastructure of Eukaryotic Cells

1.1.4Ultrastructure of Eukaryotic Cells 2

1.1.5Ultrastructure of Eukaryotic Cells 3

1.1.6Prokaryotic Cells

1.1.7Viruses

1.1.8End of Topic Test - Cell Structure

1.1.9Exam-Style Question - Microscopes

1.1.10A-A* (AO2/3) - Cell Structure

2Biological Molecules

2.1Testing for Biological Modules

2.1.1Tests for Sugars & Starch

2.1.2Tests for Lipids & Proteins

2.2Carbohydrates & Lipids

2.2.1Monomers & Polymers

2.2.2Carbohydrates

2.2.3Examples of Carbohydrates

2.2.4Triglycerides & Phospholipids

2.2.5Types of Fatty Acids

2.2.6Exam-Style Question - Carbohydrates

2.2.7End of Topic Test - Carbohydrates & Lipids

2.3Proteins

2.3.1Amino Acids & Overview of Protein Structure

2.3.2Primary & Secondary Structure

2.3.3Tertiary & Quaternary Structure

2.3.4Globular & Fibrous Proteins

2.3.5Haemoglobin

2.3.6Collagen

2.3.7End of Topic Test - Proteins

2.4Water

2.4.1Water Structure & Function

2.4.2End of Topic Test - Water

3Enzymes

3.1Enzymes

3.1.1Enzyme Action

3.1.2Factors Affecting Enzyme Activity

3.1.3Factors Affecting Enzyme Activity 2

3.1.4A-A* (AO2/3) - Enzymes

4Cell Membranes & Transport

4.1Biological Membranes

4.1.1Fluid Mosaic Model

4.1.2Cell Signalling

4.1.3End of Topic Test - Biological Membranes

4.1.4Exam-Style Question - Membranes

4.2Movement Into & Out of Cells

4.2.1Passive Movement Across the Membrane

4.2.2Active Movement Across the Membrane

4.2.3Osmosis

4.2.4Osmosis & Plant Cells

4.2.5Surface Area to Volume Ratio

4.2.6Adaptions to Surface Area to Volume Ratio

4.2.7Calculations - Volume

4.2.8Calculations - Surface Area

5The Mitotic Cell Cycle

5.1Replication & Division of Nuclei

5.1.1Chromosomes

5.1.2The Cell Cycle

5.1.3Mitosis

5.1.4Role of Mitosis

5.1.5Stem Cells

5.1.6Mitosis & Cancer

5.1.7End of Topic Test - Replication & Division

5.1.8A-A* (AO2/3) - Cell Cycle & Transport

6Nucleic Acids & Protein Synthesis

6.1Nucleic Acids

6.1.1Introduction to DNA & RNA

6.1.2Nucleotides

6.1.3Polynucleotides

6.1.4ADP & ATP

6.1.5DNA

6.1.6DNA Replication

6.1.7RNA

6.1.8End of Topic Test - Nucleic Acids & ATP

6.2Protein Synthesis

6.2.1The Genetic Code

6.2.2Polypeptide Synthesis

6.2.3RNA Processing in Eukaryotes

6.2.4Mutations

6.2.5Exam-Style Question - Protein Synthesis

6.2.6A-A* (AO2/3) - Coronavirus Translation

7Transport in Plants

7.1Transport in Plants

7.1.1Plant Vessels

7.1.2Xylem

7.1.3Investigating Transpiration

7.1.4Adaptations to Water Availability

7.1.5Phloem

7.1.6End of Topic Test - Transport in Animals & Plants

7.1.7A-A* (AO2/3) - Transport in Animals & Plants

8Transport in Mammals

8.1Circulatory System

8.1.1Double Circulation

8.1.2Blood Vessels

8.1.3Important Blood Vessels

8.1.4Blood Cells

8.1.5Tissue Fluid

8.2Transport of Oxygen & Carbon Dioxide

8.2.1Haemoglobin

8.2.2Oxygen Dissociation

8.3The Heart

8.3.1Structure of the Heart

8.3.2The Cardiac Cycle

8.3.3Coordination of Heart Action

9Gas Exchange

9.1Gas Exchange System

9.1.1Mammalian Gas Exchange

9.1.2Smoking

10Infectious Diseases

10.1Infectious Diseases

10.1.1Transmission of Disease

10.1.2Tuberculosis

10.1.3HIV

10.1.4Malaria

10.1.5Cholera

10.1.6Measles

10.2Antibiotics

10.2.1Antibiotics

11Immunity

11.1The Immune System

11.1.1Phagocytes

11.1.2Antigens

11.1.3Lymphocytes

11.1.4Primary & Secondary Response

11.2Antibodies & Vaccination

11.2.1Antibodies

11.2.2Monoclonal Antibodies

11.2.3Immunity

11.2.4Vaccines

11.2.5Exam-Style Question - Immune System

11.2.6A-A* (AO2/3) - Disease & Immune System

12Energy & Respiration (A2 Only)

12.1Energy

12.1.1Energy & ATP

12.1.2Respiratory Substrates & Quotient

12.1.3Respirometers

12.2Respiration

12.2.1Introduction to Respiration

12.2.2Anaerobic Respiration

12.2.3Aerobic Respiration

12.2.4Chemiosmosis & Coenzymes

12.2.5Mitochondria

12.2.6Respiration Experiments

12.2.7End of Topic Test - Respiration

12.2.8A-A* (AO3/4) - Respiration

13Photosynthesis (A2 Only)

13.1Photosynthesis

13.1.1Introduction to Photosynthesis

13.1.2Light-Dependent Reactions

13.1.3Light-Independent Reactions

13.1.4Limiting Factors

13.1.5Investigating Photosynthesis

13.1.6End of Topic Test - Photosynthesis

13.1.7A-A* (AO3/4) - Photosynthesis

14Homeostasis (A2 Only)

14.1Homeostasis

14.1.1Homeostasis

14.1.2Negative Feedback

14.2The Kidney

14.2.1Kidneys & Osmoregulation

14.3Cell Signalling

14.3.1Messengers & cAMP

14.4Blood Glucose Concentration

14.4.1Blood Glucose Concentration

14.4.2Controlling Blood Glucose

14.4.3Urinalysis

14.5Homeostasis in Plants

14.5.1Stomata & Guard Cells

15Control & Coordination (A2 Only)

15.1Control & Coordination in Mammals

15.1.1Neurones

15.1.2Receptors

15.1.3Taste

15.1.4Reflexes

15.1.5Action Potentials

15.1.6Saltatory Conduction

15.1.7Synapses

15.1.8Cholinergic Synnapses

15.1.9Neuromuscular Junction

15.1.10Skeletal Muscle

15.1.11Sliding Filament Theory Contraction

15.1.12Sliding Filament Theory Contraction 2

15.1.13Menstruation

15.1.14Contraceptive Pill

15.2Control & Co-Ordination in Plants

15.2.1Venus Flytrap

15.2.2Auixns

15.2.3Gibberellins

16Inherited Change (A2 Only)

16.1Passage of Information to Offspring

16.1.1Meiosis

16.1.2Gametogenesis

16.1.3Meiosis & Genetic Diversity

16.2Genes & Phenotype

16.2.1Key Terms in Genetics

16.2.2Inheritance

16.2.3Linkage

16.2.4Multiple Alleles

16.2.5Chi-Squared Test

16.2.6Mutations

16.2.7Specific Mutations

16.2.8End of Topic Test - Genetics

16.2.9A-A* (AO2/3) - Genetics

16.3Gene Control

16.3.1lac Operon

16.3.2Transcription Factors

16.3.3Gibberellins & DELLA Proteins

17Selection & Evolution (A2 Only)

17.1Variation

17.1.1Phenotypic Variation

17.1.2Continuous & Discontinuous Variation

17.1.3t-Tests

17.2Natural & Artificial Selection

17.2.1Natural Selection & Evolution

17.2.2Types of Selection

17.2.3Types of Selection Summary

17.2.4Genetic Drift & Founder Effect

17.2.5Populations

17.2.6Hardy-Weinberg Principle

17.2.7Artificial Selection

17.3Evolution

17.3.1Evolution by Natural Selection

17.3.2Evidence for Evolution

17.3.3Speciation

17.3.4Extinction

18Classification & Conservation (A2 Only)

18.1Biodiversity

18.1.1Key Terms

18.1.2Biodiversity

18.1.3Sampling Methods

18.1.4Correlation

18.1.5Spearman’s Rank

18.1.6Measuring Biodiversity

18.2Classification

18.2.13-Domain Classification System

18.2.2Linnaean Classification System

18.3Conservation

18.3.1Importance of Biodiversity

18.3.2Conservation

18.3.3Zoos, Seedbanks, & Other Conservation Methods

18.3.4Assisted Reproduction & Contraception

18.3.5Non-Governmental Organisations

19Genetic Technology (A2 Only)

19.1Manipulating Genomes

19.1.1Recombinant DNA & Genetic Engineering

19.1.2Restriction Endonucleases

19.1.3Gel Electrophoresis

19.1.4DNA Probes & Microarrays

19.1.5End of Topic Test - Manipulating Genomes

19.1.6A-A* (AO2/3) - Manipulating Genomes

19.2Genetic Technology Applied to Medicine

19.2.1Bioinformatics

19.2.2Applications of Recombinant DNA

19.2.3Genetic Screening

19.2.4Gene Therapy

19.2.5Ethics

19.2.6DNA Profiling & Forensics

19.3Genetically Modified Organisms in Agriculture

19.3.1GM Crops & Livestock

19.3.2Ethics

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