Pearson Edexcel International A Level Biology: Stem Cells, Differential Gene Expression and Phenotype (YBI11)

This guide follows the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, and covers specification statements 3.17 to 3.21 in Topic 3, Cell Structure, Reproduction and Development. Topic 3 sits in the International AS (IAS) part of the course, in Unit 2. There are no tiers. Statement 3.17 is split into (i) and (ii), and statement 3.20 into (i), (ii) and (iii); all parts are covered below.

Find the rest of the course on the Edexcel A Level Biology hub and tick off progress on the printable checklist. Companion pages: revision notes and practice questions. To find your weak spots first, try the free 10-minute diagnostics. For wider exam technique see the Edexcel IAL Biology exam preparation guide.

What this unit covers

Statement	What you must be able to do
3.17 (i)	Explain stem cell, pluripotent, totipotent, morula and blastocyst
3.17 (ii)	Discuss how society uses scientific knowledge to decide about stem cells in medical therapies
3.18	Explain how differential gene expression produces specialised cells in animals and plants
3.19	Explain how one gene can give more than one protein through changes to mRNA after transcription
3.20 (i)	Explain phenotype as an interaction between genotype and environment
3.20 (ii)	Describe how DNA methylation and histone modification alter gene activation
3.20 (iii)	Explain how epigenetic modifications can be passed on after cell division
3.21	Explain effects of multiple alleles and polygenic inheritance, and how polygenic inheritance gives continuous variation

3.17 Stem cells and early development

A stem cell is a cell that has not specialised. It keeps dividing by mitosis to produce copies of itself, and some of its daughter cells can differentiate into specialised cell types.

Potency describes how many cell types a stem cell can become:

• Totipotent: can become any cell type in the organism and also the extra-embryonic tissues such as the placenta, so it can give rise to a whole organism. A human zygote and the cells of the first few cleavage divisions are totipotent.
• Pluripotent: can become any of the body’s cell types but cannot form a whole organism on its own, because it does not make the extra-embryonic tissues.

After fertilisation the zygote divides by repeated mitosis (cleavage) without growing. The cells are not yet specialised.

• The morula is a solid ball of cells formed by cleavage, with no cavity.
• The blastocyst is a hollow ball. Its outer layer, the trophoblast, goes on to form part of the placenta. A cluster of cells, the inner cell mass, sits to one side of a fluid-filled cavity and forms the embryo. Inner cell mass cells are pluripotent and are the source of embryonic stem cells.

Adult (tissue) stem cells, such as those in bone marrow, replace cells throughout life but make a narrower range of cell types than embryonic stem cells.

Worked example 1. A zygote divides every cycle with no cell death. How many cells are there after 5 divisions, and after how many divisions does the embryo first exceed 100 cells?

Cells double each division, so the number is 2ⁿ. After 5 divisions: 2⁵ = 32 cells. 2⁶ = 64 and 2⁷ = 128, so it first exceeds 100 cells after 7 divisions.

Stem cells in medicine: how society decides (3.17 ii)

Science can tell you what stem cells can do and what the risks are. It cannot settle whether an action is right. Society decides using scientific evidence together with ethics, law, cost and public opinion.

Source	Possible benefit	Concerns
Embryonic stem cells (inner cell mass)	Pluripotent, so wide range of therapies; divide readily in culture	Embryo is destroyed, and people disagree about its moral status; risk of tumour formation; possible immune rejection
Adult stem cells (for example bone marrow)	No embryo destroyed; patient’s own cells can be used, so little rejection	Narrower range of cell types; harder to obtain and grow
Umbilical cord blood	Collected at birth without harm to the baby	Limited cell types and numbers

A balanced discussion gives points on both sides, names who is affected (patients, donors, embryos), and says what evidence would change the view. Laws and regulators set limits, and these differ between countries.

3.18 Differential gene expression

Almost every body cell of an organism carries the same genes, because all arise from the zygote by mitosis. Cells differ because they express different genes. In a given cell type only some genes are transcribed into active mRNA, meaning mRNA that is translated. The proteins made then control cell processes (as enzymes, carriers, hormones) or determine structure (as cytoskeleton, collagen, haemoglobin).

The sequence is: signal or transcription factor binds to the promoter region of a gene, so the gene is transcribed, then mRNA is translated, then protein is made, then the cell has its specialised function. Genes that are not needed stay switched off.

• Pancreatic beta cells transcribe the insulin gene; muscle cells carry the gene but make no insulin mRNA.
• Developing red blood cells transcribe haemoglobin genes in large amounts.
• In plants, cells that become xylem vessels express genes for enzymes that make lignin, giving thick strengthened walls, while palisade cells express genes for chlorophyll and photosynthesis enzymes.

Worked example 2. Relative mRNA levels (arbitrary units) found in three cell types:

Gene	Liver cell	Beta cell	Neurone
Insulin	0	540	0
Albumin	410	0	0
Actin	95	90	100

Actin mRNA is similar in all three, so actin is expressed in every cell type. Insulin mRNA appears only in the beta cell and albumin mRNA only in the liver cell. The cells differ in which genes are expressed, not in the genes they hold. Within the beta cell, insulin mRNA is 540 ÷ 90 = 6 times the actin mRNA level.

3.19 One gene, more than one protein

In eukaryotes the primary transcript (pre-mRNA) contains exons (sequences that stay in the mRNA) and introns (sequences removed). Introns are cut out and exons are joined, which is splicing. A 5’ cap and a poly-A tail are also added, which help protect the mRNA and its translation.

In alternative splicing, different combinations of exons are kept in the mature mRNA from the same pre-mRNA. Each different mRNA is translated into a different polypeptide. This is why the number of different proteins a human makes is far larger than the roughly 20,000 protein-coding genes.

Examples:

• The same gene gives calcitonin in thyroid cells and a different peptide (CGRP) in some neurones.
• The fruit fly Dscam gene can produce more than 38,000 different mRNAs.

Worked example 3. A gene has five exons with lengths in nucleotides: exon 1 = 90, exon 2 = 60, exon 3 = 105, exon 4 = 45, exon 5 = 72. Exons 1 and 5 are always kept, and the mature mRNA contains exactly one of exons 2, 3 or 4. Exon 5 ends with a stop codon. Find the number of amino acids in each protein.

Coding length = 90 + 72 + exon length. Divide by 3 for codons, then subtract 1 for the stop codon.

Exon kept	mRNA coding length	Codons	Amino acids
2	222	74	73
3	267	89	88
4	207	69	68

One gene gives three polypeptides of 73, 88 and 68 amino acids. Because every exon length is divisible by 3 the reading frame is kept.

3.20 Phenotype, environment and epigenetics

Phenotype is the result of an interaction between genotype and environment (i). The genotype sets the range of possible phenotypes and the environment influences where in that range an individual falls.

• In Himalayan rabbits the pigment enzyme works only at lower temperatures, so dark fur grows on the cooler ears, nose, feet and tail.
• Hydrangea flower colour varies with soil acidity even in genetically identical plants.
• Height depends on many genes and on nutrition in childhood.
• People with phenylketonuria have a faulty enzyme for phenylalanine, but a low-phenylalanine diet prevents the damaging phenotype.

Worked example 4. Five genetically identical cuttings were grown at each of two sites. Heights after 8 weeks (cm): shaded site 18.0, 20.5, 19.0, 21.5, 20.0; open site 27.5, 29.0, 26.0, 30.5, 28.0.

Mean shaded = 99 ÷ 5 = 19.8 cm. Mean open = 141 ÷ 5 = 28.2 cm. Difference = 8.4 cm, which is 8.4 ÷ 19.8 × 100 = 42.4% taller in the open. The genotype is the same, so the difference is due to environment. The spread within each site also arises from environment, since genotype is constant.

Epigenetic modification (ii)

Epigenetic changes alter whether a gene is active without changing the DNA base sequence.

• DNA methylation: methyl groups are added to cytosine bases, often in the promoter region. Methylation reduces binding of transcription factors, so transcription falls and the gene is switched off.
• Histone modification: DNA is wound around histone proteins. Adding acetyl groups to histones loosens the DNA–histone contact, so the DNA is accessible and transcription rises. Removing acetyl groups, or adding methyl groups at some sites, packs the chromatin tightly and lowers transcription.

Environmental factors such as diet and smoking can alter these marks.

Passing marks on after cell division (iii)

During DNA replication the new strand is unmethylated. Enzymes recognise the methylated parent strand and methylate the matching new strand, so both daughter cells keep the same pattern. This lets a specialised cell lineage keep the same genes switched on or off after many mitotic divisions.

3.21 Multiple alleles and polygenic inheritance

Multiple alleles: a gene has more than two alleles in the population, though a diploid individual carries only two. With n alleles there are n(n + 1) ÷ 2 genotypes, so 3 alleles give 6.

The ABO blood group has three alleles: IA and IB are codominant, and IO is recessive to both. Four phenotypes arise: A (IA IA or IA IO), B (IB IB or IB IO), AB, and O.

Rabbit coat colour has four alleles in a dominance series: C (full colour) > cch (chinchilla) > ch (Himalayan) > c (albino).

Worked example 5. A chinchilla rabbit (cch c) is crossed with a Himalayan rabbit (ch c). Gametes: cch, c and ch, c. Offspring: cch ch, cch c, ch c, c c. Phenotypes: 2 chinchilla : 1 Himalayan : 1 albino. Probability of albino = 1/4.

Polygenic inheritance: a trait controlled by many genes, each adding a small effect. Alleles act additively, so offspring show many intermediate phenotypes and the trait shows continuous variation, with a bell-shaped distribution. The environment adds further spread. Height, skin colour and grain yield are examples. Discontinuous variation, such as blood group, comes from one gene with distinct phenotypes.

Worked example 6. Two unlinked genes each have an additive allele (capital) and a non-additive allele. Cross AaBb × AaBb. Number of capital alleles among 16 equally likely gamete combinations: 0 in 1, 1 in 4, 2 in 6, 3 in 4, 4 in 1. That is five phenotype classes in the ratio 1:4:6:4:1. A fraction 6/16 = 3/8 is intermediate. With three genes there are seven classes, closer to a smooth curve.

Common errors

• Saying specialised cells have “different genes” instead of different genes expressed.
• Confusing totipotent with pluripotent. Only totipotent cells make the placenta.
• Saying alternative splicing changes the DNA or removes exons from the gene itself.
• Saying methylation changes the base sequence.
• Writing that polygenic traits are “caused by environment”. Both genes and environment contribute.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology, Specification, Issue 2 (February 2021), Pearson Education Limited: Topic 3, statements 3.17 to 3.21.