Reproduction¶

Part of Inheritance, Variation and Evolution.

Reproduction links cell biology to inheritance. The central question is how organisms make new individuals while passing on genetic information accurately enough for continuity and differently enough for variation.

What You Need to Learn¶

Connect reproduction to the passage of genetic information: how gametes form, how fertilisation restores chromosome number, and how DNA helps determine characteristics. Use inheritance diagrams to predict outcomes and consider screening for inherited conditions. Higher Tier work follows protein production and explains how changes in DNA can affect proteins or gene activity.

Sexual and Asexual Reproduction¶

Asexual reproduction — advantageous when conditions are stable: no need to find a mate, and successful genotypes are copied quickly. Examples: strawberry runners and bulb division in daffodils. Many plants also reproduce sexually by seeds, combining the advantages of both methods. In animals, sperm fuses with an egg; in flowering plants, the male gamete carried by pollen fuses with an egg.

Sexual reproduction — advantageous when conditions change: the genetic variation it produces may include individuals better suited to new challenges.

Some organisms can do both. The malaria parasite (Plasmodium) reproduces asexually in humans and sexually in mosquitoes. Many fungi reproduce both sexually and asexually (via spores).

Meiosis¶

Meiosis is cell division that produces gametes (sex cells). It produces four daughter cells, each with half the number of chromosomes (haploid, 23 in humans). When two gametes fuse at fertilisation, the full chromosome number (46 in humans, arranged as 23 pairs) is restored.

• Gametes are haploid (n); body cells are diploid (2n).
• Meiosis produces genetically unique cells due to:
• Chromosomes shuffling (independent assortment).
• Sections of chromosomes swapping (crossing over).

After DNA is copied once, the cell divides twice to produce four cells with one chromosome set each. Fertilisation combines two chromosome sets in a single cell. That cell divides by mitosis to form an embryo; its cells later differentiate into specialised types.

DNA, Chromosomes and Genes¶

• Chromosomes are condensed structures in the cell nucleus, containing a long DNA molecule associated with proteins.
• DNA (deoxyribonucleic acid) has a double helix structure. It is composed of two strands made from four bases: adenine (A), thymine (T), cytosine (C) and guanine (G). Bases pair specifically: A–T and C–G.
• A gene is a section of DNA whose base sequence specifies the amino-acid sequence of a protein. Proteins help produce an organism’s characteristics.
• The genome is the complete set of genetic material, including non-coding DNA, in an organism.

DNA Structure and the Genome¶

Each DNA strand is a polymer of nucleotides. A nucleotide contains a sugar, a phosphate group and one of the four bases. Alternating sugar and phosphate groups form the backbone, with bases projecting from it. The order of bases carries information: a triplet of bases specifies an amino acid, so changing base order can change the protein produced.

Understanding the human genome helps identify genes associated with disease, investigate and treat inherited disorders, and trace past migrations by comparing inherited DNA variants between populations.

Explore DNA From Building Block to Strand¶

Start with one nucleotide, then compare the repeating backbone with the changing sequence of bases in a strand. The model uses only a short section of DNA and shows its connections flat; a real molecule is much longer and twisted into a double helix. The final view practises Higher Tier complementary base pairing.

Open full interactive

Try and explain: two genes contain the same types of sugar and phosphate but produce different proteins. Which part of their DNA must you compare to explain the difference? At Higher Tier, write the complementary bases for A–C–G and explain the rule you used.

Protein Production and DNA Variants — Higher Tier¶

DNA’s two strands are complementary: A pairs with T and C with G. A template carrying the information from a gene directs protein synthesis at a ribosome. Carrier molecules deliver particular amino acids in the order specified by the template. The completed chain folds into a shape suited to its function, for example an enzyme’s active site or a strong structural protein.

Mutations occur continually, but most have little or no effect on protein function. A mutation in coding DNA can change an amino acid and therefore the protein’s shape. An enzyme may bind its substrate less effectively, or a structural protein may become weaker. Non-coding DNA can regulate whether a gene is active; a variant there may change how much protein is made without changing its amino-acid sequence.

Inheritance: Alleles, Genotype and Phenotype¶

• Allele: a different version of the same gene. Many genes have two common alleles.
• Dominant allele: expressed in the phenotype whether one or two copies are present (denoted by a capital letter, e.g. D).
• Recessive allele: only expressed when two copies are present (denoted by a lowercase letter, e.g. d).
• Codominant alleles: both alleles are expressed simultaneously (e.g. blood group AB).
• Homozygous: both alleles are the same (e.g. DD or dd).
• Heterozygous: two different alleles for a gene (e.g. Dd).
• Genotype: the allele combination of an organism.
• Phenotype: the observable characteristics produced by the genotype (and environment).

Genetic Diagrams (Punnett Squares)¶

Monohybrid inheritance uses a Punnett square to predict the probability of phenotypes from a single gene cross.

Example — a simplified mouse fur-colour cross (D = dark fur, dominant; d = light fur, recessive):

If one mouse is heterozygous dark (Dd) × one mouse is homozygous light (dd):

Result: 2 Dd (dark) : 2 dd (light) → 1:1 ratio; 50% chance of dark or light fur. These probabilities apply to each offspring independently, rather than guaranteeing exactly half a litter has each colour. Most characteristics, including human hair colour, involve several genes rather than this simple model.

Explore Single-Gene Probability¶

Choose parental genotypes, identify their possible gametes and combine alleles in a Punnett square. Then compare predicted probabilities with sampled offspring. Completing and interpreting a square is core knowledge; constructing a cross and using probability to make predictions is a Higher Tier skill.

The activity uses a fictional leaf trait with one dominant and one recessive allele. It assumes equally likely gametes, random fertilisation and equal offspring survival. A predicted ratio describes probabilities across many offspring, not a guaranteed result for a small family. Most real characteristics depend on several genes.

Open full interactive

Try and explain: cross Aa with aa, then sample a small group of offspring. If the group is not exactly half recessive, does that disprove the predicted 1:1 ratio? Explain why another offspring still has a one-half chance of the recessive phenotype.

Codominance: Blood Groups¶

Human blood groups use three alleles: I^A, I^B and I^O.

• I^A and I^B are codominant — genotype I^AI^B gives blood group AB.
• I^O is recessive — genotype I^OI^O gives blood group O.

Family Pedigrees¶

A pedigree diagram shows which members of a family have an inherited condition and which are carriers. They allow you to work out if a disease allele is dominant or recessive.

Example: if an unaffected pair of parents have an affected child, the disease allele must be recessive (both parents are carriers: Ff × Ff → 25% chance of ff child).

Sex Determination¶

Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes:

• XX = female
• XY = male

All egg cells carry an X chromosome. Sperm cells carry either X or Y. Therefore the father's sperm determines the sex of the child. There is a 50% chance of each sex in any pregnancy.

Inherited Disorders¶

Some disorders are caused by faulty alleles and can be passed on through families.

Polydactyly¶

• Extra fingers or toes.
• Caused by a dominant allele, so only one copy is needed for the condition to appear.
• An affected parent has at least one copy of the dominant allele; there is a 50% or 100% chance of passing it on depending on their genotype.

Cystic Fibrosis¶

• Faulty chloride channels cause abnormally thick mucus in the lungs, gut and reproductive organs.
• Caused by a recessive allele (F = normal, f = cystic fibrosis).
• Genotype ff → cystic fibrosis. Genotype Ff → carrier (no symptoms, but can pass it on).
• Both unaffected parents can be carriers (Ff) and have an affected child (ff) — 25% probability per pregnancy.

Sickle Cell Anaemia¶

• A change in the base sequence of the haemoglobin gene produces abnormal haemoglobin. Red blood cells become sickle-shaped, blocking blood vessels. Symptoms include fatigue, pain and fever.
• Caused by a recessive allele (HbA = normal, HbS = sickle).
• Carriers (HbAHbS) have resistance to malaria — an evolutionary advantage in malaria-endemic regions, explaining why the sickle cell allele is common in African populations.

Embryo Screening and Prenatal Testing¶

Testing can identify some inherited conditions, but embryo screening before implantation differs from testing during pregnancy:

• Amniocentesis — a sample of amniotic fluid from around the foetus is tested.
• Chorionic villous sampling — a sample from the placenta is tested.
• During IVF — a cell can be removed from the embryo before implantation and tested.

Screening embryos during IVF can help avoid passing on a serious inherited disorder, but tests do not guarantee a healthy child. Decisions involve cost and access to treatment, uncertainty in results, the value placed on embryos, and concerns about selecting which embryos develop. Prenatal testing raises separate decisions about an existing pregnancy.

Common Confusions¶

• Meiosis vs mitosis: meiosis produces 4 genetically different haploid gametes; mitosis produces 2 genetically identical diploid cells.
• Carrier vs affected: a carrier of a recessive disorder has one copy of the faulty allele (Ff) and shows no symptoms, but can pass the allele on. An affected individual has two copies (ff).
• Dominant disorder inheritance: for a dominant disorder like polydactyly, an affected parent has at least a 50% chance of passing the allele to each child. You do not need both copies for the condition to show.
• Sex chromosomes: the sex of a baby is determined by which sperm fertilises the egg (X-carrying gives girl; Y-carrying gives boy) — not by the mother's egg.

Key Terms¶

• Gamete: a sex cell with half the normal number of chromosomes (haploid).
• Meiosis: cell division that produces four genetically different haploid gametes.
• Allele: a different version of the same gene.
• Genome: the complete set of genetic material in an organism.
• Dominant: describes an allele expressed when only one copy is present.
• Recessive: describes an allele expressed only when two copies are present.
• Codominance: when both alleles are expressed simultaneously in the phenotype.
• Homozygous: having two identical alleles for a gene (e.g. DD or dd).
• Heterozygous: having two different alleles for a gene.
• Genotype: the combination of alleles an organism possesses.
• Phenotype: the observable characteristics of an organism; determined by genotype and environment.
• Carrier: an individual who has one copy of a recessive disease allele but shows no symptoms.
• Polydactyly: an inherited condition causing extra digits; caused by a dominant allele.
• Cystic fibrosis: an inherited condition causing thick mucus; caused by a recessive allele.
• Sickle cell anaemia: an inherited condition causing sickle-shaped red blood cells; caused by a recessive allele.
• Punnett square: a grid diagram used to predict the probability of different genotypes and phenotypes in offspring.

Connected Pages¶

• Variation and Evolution
• Biology Key Terms Glossary