كيف يحسب الرقم الهيدروجيني pH للمحاليل؟ How to calculate pH and pOH?

يحسب الرقم الهيدروجيني بالعلاقة: pH = -log[H⁺]، وللمحاليل المائية عند 25°C: pH + pOH = 14. إذا كان pH 7 فالمحلول قاعدي. [الصيغة الرمزية الموزونة: pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴]

D3.2Theme D — Continuity and change

Inheritance — IB Biology SL Revision Notes & Topical Questions

10 min read 13 parts Lesson 8 of 14 2 quizzes

📚 IB Biology SL Hub / Topical Revision

Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution

pH = -log[H⁺]  |  pOH = -log[OH⁻]  |  pH + pOH = 14  |  [H⁺][OH⁻] = 10⁻¹⁴

Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14

Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.

0%read

Lesson summary

D3.2 Inheritance — IB BIOLOGY SL notes: Carry out and interpret genetic crosses, and explain dominance, multiple alleles, sex linkage, pedigrees and continuous variation.Gametes — haploid cells made by meiosis, each with one chromosome of each pair and so one allele of each geneT…

Fertilisation
Units of inheritance
Flower types
Monohybrid cross
Punnett grid
Probability

Section 1 of 13Gametes, fertilisation and genotype terms

Gametes, fertilisation and genotype terms

Gametes

Gametes haploid cells made by meiosis, each with one chromosome of each pair and so one allele of each gene
The egg is large and stores food for the early embryo.
The sperm is small with many mitochondria that supply ATP for swimming.
Human gametes have 23 single chromosomes and body cells have 23 pairs (46).

Fertilisation

Fertilisation the fusion of gamete nuclei gives a diploid zygote with two alleles of each gene
One allele comes from each parent, so offspring are varied.

Gene

Gene a short length of DNA on a chromosome that codes for a characteristic

Allele

Allele a variant of the same gene, found at the same locus

Gene and allele must not be used interchangeably.

Genotype

Genotype the combination of alleles an individual inherits

Homozygoustwo identical alleles, either homozygous dominant or homozygous recessive.
Heterozygoustwo different alleles.

Phenotype

Phenotype the observable characteristics of an organism

Section 2 of 13Mendel and plant breeding

Mendel

Mendel the Austrian monk who crossed pea plants in the mid-19th century
He transferred pollen by hand from one plant to another, so the parentage of every seed was certain.
He studied plant height, flower colour and seed coat texture, among others.
He used large numbers and repeated crosses, so his ratios were reliable.

Results

The P generation was pure-breeding (homozygous) for different phenotypes.
Every plant in F1 resembled one parent only.
F2 plants showed both parental forms in a ratio of about 3:1.

Units of inheritance

Mendel's term for what we now call genes, some dominant and some recessive

Flower types

Flower types both sexes in one flower, separate male and female flowers on one plant, or separate male and female plants
A plant with both sexes can self-pollinate, which keeps useful traits in a line.
Growers can cross-pollinate chosen plants artificially to combine desirable traits.

Section 3 of 13Monohybrid crosses

Monohybrid cross

Monohybrid cross a cross involving one gene with two alleles, starting from pure-breeding parents

Punnett grid

Punnett grid a table that predicts the probability of offspring genotypes and phenotypes

Choose allele symbols and write the parental phenotypes and genotypes.

List every type of haploid gamete each parent can produce.

Put one parent's gametes along each axis of a 2 × 2 grid.

Fill each box by combining the gametes to give the offspring genotypes.

Convert the genotypes into phenotypes.

State the genotype and phenotype ratios.

Example

Example green pods G are dominant to yellow g
GG × gg gives all Gg, all green, in F1.
Gg × Gg gives 1 GG : 2 Gg : 1 gg in F2, a phenotype ratio of 3 green : 1 yellow.

Probability

Probability predicted ratios are probabilities, and small samples often differ from them

Letters

Letters choose symbols whose capital and lower-case forms look different

Section 4 of 13Dominance and phenotype

Dominance and phenotype

Dominant allele

Dominant allele needs to be inherited from only one parent for the trait to show

Recessive allele

Recessive allele must be inherited from both parents, and it is hidden in a heterozygote

Feature	Complete dominance	Codominance	Incomplete dominance
Heterozygote phenotype	Same as dominant homozygote	Both traits visible	A blend or intermediate
Phenotypes (two alleles)	Two	Three	Three
Heterozygote × heterozygote	3:1	1:2:1	1:2:1
Example	Pea stem height	Speckled chickens C^WC^B, AB blood	Pink flowers C^WC^R in the four o'clock (marvel of Peru)
Symbols	T and t	Superscripts	Superscripts

Dominance and phenotype

Factors deciding phenotype

Genotype only, as in ABO blood group.
Environment only, as in scars and accent.
Both together, as in height and skin colour.

Phenotypic plasticity

Phenotypic plasticity the genotype is fixed but its expression varies with the environment
UV radiation activates enzymes that raise melanin production and darken the skin.
In the Himalayan rabbit the fur pigment gene is inactive above 35 °C and active at 15-25 °C, so cool ears, feet and nose turn black.

Section 5 of 13Phenylketonuria

PKU

PKU an autosomal recessive condition
A mutation in the PAH gene makes phenylalanine hydroxylase non-functional.
Phenylalanine builds up, which can cause seizures and learning difficulties.
About 1 in 10 000 births in the UK are affected.
It is controlled by a low-phenylalanine diet.

Carriers

Carriers two unaffected heterozygous parents have a 25% chance per child of PKU and a 75% chance of not having it
Cystic fibrosis follows the same pattern.

Screening

Screening a heel-prick blood sample a few days after birth tests for PKU and other conditions

Section 6 of 13Multiple alleles and blood groups

Multiple alleles and blood groups

SNP

SNP a single base difference between alleles
Alleles occupy the same locus.
A small change can have a large effect on function.

Multiple alleles

Multiple alleles the ABO gene has three alleles, I^A, I^B and i
I^A and I^B are codominant and both are dominant over i.
An individual carries only two alleles.

Group	Possible genotypes	Antigen on red cells	Antibodies in plasma
A	I^AI^A or I^Ai	A	Anti-B
B	I^BI^B or I^Bi	B	Anti-A
AB	I^AI^B	A and B	None
O	ii	Neither	Anti-A and anti-B

Multiple alleles and blood groups

Cross

Cross a heterozygous group A parent and a heterozygous group B parent can have children of all four groups

Transfusion

Transfusion the wrong blood group triggers antibodies, so red cells clump and clots form

Section 7 of 13Sex determination

Sex chromosomes

Sex chromosomes pair 23 is XX in females and XY in mammalian males
Pairs 1-22 are autosomes with no effect on sex.
The rule does not apply to all species.

Role of the father

Role of the father half the sperm carry X and half carry Y
Every egg carries an X.
Daughters get the father's X and sons get his Y, giving a 1:1 sex ratio.

X and Y chromosomes

X and Y chromosomes the X is larger with its centromere nearer the middle and carries about 16 times more genes than the Y
Genes for blood clotting factors are on the X and not the Y.
The Y carries the SRY gene, which triggers testis development and testosterone production.
Without SRY, ovaries develop.

Section 8 of 13Sex-linked inheritance: haemophilia

Sex linkage

Sex linkage a gene on the X chromosome is inherited differently in males and females
Males (XY) have one copy of the gene and are hemizygous.
Females (XX) have two copies, so they can be normal, carriers or affected.

Haemophilia

Haemophilia a recessive X-linked allele fails to make clotting factor VIII, so blood clots slowly
F is the normal allele and f the recessive allele.

Genotype	Sex	Phenotype
X^FX^F	Female	Unaffected
X^FX^f	Female	Carrier with normal clotting
X^fX^f	Female	Affected, very rare
X^FY	Male	Unaffected
X^fY	Male	Affected

A carrier mother makes eggs with X^F or X^f.

An unaffected father makes sperm with X^F or Y.

The gametes fuse at random.

Daughters receive the father's X^F, so none is affected and half are carriers.

Sons receive the father's Y and one maternal X.

Half the sons receive X^f and are affected.

Pattern

Pattern affected sons inherit the allele from their mother and never from their father

Notation

Notation upper-case X and Y with superscripts for the alleles

Section 9 of 13Pedigree charts

Pedigree chart

Pedigree chart traces the inheritance of a characteristic through the generations of a family
Squares are males and circles are females.
Shaded or coloured symbols are affected individuals.
A horizontal line joins parents and their children hang below.
Roman numerals label the generations, with the eldest child on the left.

Recessive allele

Recessive allele two unaffected parents with an affected child shows the allele is recessive
Both parents are carriers.
The trait can skip a generation.

Dominant allele

Dominant allele an affected child always having an affected parent suggests a dominant allele

Sex linkage

Sex linkage an excess of affected males suggests an X-linked trait

Close relatives

Close relatives they are more likely to share harmful recessive alleles, which raises the risk of affected children

Reasoning

Inductive reasoning draws a general rule from specific observations.
Deductive reasoning draws a specific conclusion from known rules.

Section 10 of 13Continuous variation and box plots

Feature	Discrete	Continuous
Categories	Clear separate classes	A range between two extremes
Genes involved	Usually one or few	Many (polygenes)
Environmental effect	Little or none	Often large
Graph type	Bar chart or pie chart	Histogram, box plot or normal curve
Example	Blood group with Rhesus factor	Height, birth mass and skin colour

Polygenic trait

Polygenic trait many genes each add a small effect, so phenotype = genotype + environment
Skin colour depends on genes for melanin production and on UV exposure.
Birth mass shows a normal distribution and is affected by the mother's diet, twins and smoking.

Box plot

Box plot shows the lowest value, first quartile, median, third quartile and highest value
The box shows the interquartile range, the middle 50% of the data.
The whiskers show the lowest and highest 25%.

Outlier

Outlier a point more than 1.5 × the interquartile range beyond a quartile, plotted separately

Section 11 of 13More from the specification

Section 12 of 13Key terms

Term	Meaning
Allele	One of the different forms of the same gene.
Genotype	The combination of alleles that an individual has for a gene.
Phenotype	The observable characteristics of an organism.
Homozygous	Having two identical alleles of a gene.
Heterozygous	Having two different alleles of a gene.
Codominance	Both alleles are fully expressed in the heterozygote.
Sex linkage	Inheritance of a gene carried on a sex chromosome.
Polygenic trait	A trait controlled by many genes, showing continuous variation.

Section 13 of 13Exam technique

Using gene and allele as the same word — a gene has different alleles

Saying a recessive trait always shows in the child of two carriers — only 25% of children are affected

Saying haemophilia passes from father to son — the affected son receives the allele from his mother

Saying predicted ratios are exact — they are probabilities

Last updated: 18 October 2026