Two ways to make new life

Living things reproduce in two very different ways:

• Sexual reproduction — two parents, each giving a gamete (sex cell). Gametes fuse at fertilisation, mixing genes, so the offspring show variation and are not identical to the parents.
• Asexual reproduction — one parent, no gametes and no fusion. Offspring are produced by mitosis, so they are genetically identical clones of the parent.

Examples: sexual — humans, flowering plants; asexual — bacteria, and plants that spread by runners (e.g. strawberry) or bulbs.

Which reproduction?

Inside an insect-pollinated flower

A flower is a plant's reproductive organ. The male parts (stamen) make pollen; the female parts (carpel) receive it.

Flower part ↔ job

Tap a part on the left, then tap its job on the right.

Insect vs wind pollination

Pollination is the transfer of pollen from an anther to a stigma. Flowers are built for their carrier:

Reading the flower

From pollination to fruit

After pollination, a pollen tube grows down the style so the male nucleus can reach an ovule:

• Pollination: pollen lands on the stigma.
• Fertilisation: the pollen nucleus fuses with the nucleus of an ovule in the ovary.
• Each fertilised ovule → seed; the whole ovary → fruit around the seeds.

Germination conditions

A seed stays dormant until conditions are right. To germinate (start growing) a seed needs three things:

Light is not needed to germinate — seeds germinate underground. Light matters only later, once the shoot reaches the surface and starts photosynthesis.

Your turn — germination

The reproductive systems

Humans reproduce sexually. Each system makes gametes and delivers or protects them:

• Male: testes make sperm; the scrotum holds them; sperm travel via the sperm duct, pick up fluid from glands, and leave through the urethra in the penis.
• Female: ovaries release egg cells (ova); the oviduct (fallopian tube) carries the egg — this is where fertilisation happens; the uterus holds the developing baby; the cervix and vagina lead to the outside.

The menstrual cycle & its hormones

The ~28-day cycle prepares the uterus for a possible pregnancy, controlled by four hormones:

Which hormone?

Fertilisation & pregnancy

Fertilisation happens in the oviduct: a sperm nucleus fuses with the egg nucleus to form a zygote. This divides and implants in the uterus lining, where two structures protect and feed it:

• Placenta — exchanges materials between mother and fetus. Oxygen, glucose and amino acids pass in; CO₂ and urea pass out. Blood does not mix.
• Amnion — a membrane that holds the amniotic fluid, cushioning the fetus from bumps.

Your turn — pregnancy

DNA, genes and chromosomes

Your instructions are written in DNA — a molecule shaped as a double helix (two strands twisted together):

• A gene is a short length of DNA that codes for one protein (one characteristic).
• A chromosome is a long, coiled thread of DNA carrying many genes. Humans have 23 pairs (46 total).

Mitosis vs meiosis

Cells divide in two different ways for two different jobs:

• Mitosis — makes 2 cells, genetically identical to the parent, with the full chromosome number. Used for growth, repair and asexual reproduction.
• Meiosis — makes 4 gametes, each genetically different, with half the chromosome number. Used to make sex cells for sexual reproduction.

Naming the division

The language of genetics

Genes come in versions called alleles. You have two alleles for each gene (one from each parent):

• Dominant allele (capital letter) — shows in the phenotype if even one copy is present.
• Recessive allele (small letter) — only shows if both alleles are recessive.
• Homozygous = two the same (BB or bb). Heterozygous = two different (Bb).
• Genotype = the alleles you have (Bb). Phenotype = the characteristic you see (e.g. brown eyes).

Monohybrid cross & Punnett squares

To predict offspring, cross two heterozygous parents (Bb × Bb) where B = brown eyes (dominant), b = blue eyes:

Ratios are probabilities, not guarantees. A 3 : 1 ratio means each child has a 3-in-4 chance of brown eyes — not that exactly 3 of every 4 children will be brown.

Your turn — read the cross

A different cross: Bb × bb

Crossing a heterozygous parent with a homozygous recessive one (Bb × bb) gives a different ratio:

The same two alleles give a 3 : 1 ratio from Bb × Bb but a 1 : 1 ratio from Bb × bb — always draw the square, never guess.

Codominance

Usually one allele dominates. In codominance, both alleles in a heterozygote are fully expressed together — neither is hidden.

• Classic example: ABO blood groups. Alleles I^A and I^B are codominant, so genotype I^AI^B gives blood group AB — both antigens appear.
• Another: a red-flower allele and a white-flower allele that together give red-and-white speckled flowers (not pink).

Sex determination: XX & XY

One of your 23 pairs is the sex chromosomes. Females are XX, males are XY. Eggs always carry X; sperm carry X or Y:

Sex-linked inheritance

A sex-linked characteristic is controlled by a gene on a sex chromosome — usually the X. The Y is much shorter and carries few genes.

• Because males (XY) have only one X, a single recessive allele on it shows up — so conditions like red-green colour blindness and haemophilia are far more common in males.
• Females (XX) usually need the recessive allele on both X chromosomes to be affected, so they are more often unaffected carriers.

Match cross ↔ ratio

Tap a cross, then tap the box for the phenotype ratio it gives.

Variation: genes and environment

Individuals differ. That variation comes from two sources:

• Genetic variation — different alleles inherited from parents (e.g. blood group, eye colour). Passed on to offspring.
• Environmental variation — caused by surroundings and lifestyle (e.g. a scar, a suntan, a language you speak). Not inherited.
• Many features (like height) are influenced by both genes and environment.

Which kind of variation?

Natural selection & evolution

Variation + a struggle to survive drives evolution by natural selection (Darwin):

• Individuals in a species vary (from mutation and sexual reproduction).
• There is competition — more offspring are produced than can survive.
• Individuals with advantageous features are more likely to survive and reproduce ("survival of the fittest").
• They pass those alleles on, so over many generations the population changes — this is evolution. Antibiotic-resistant bacteria are a fast, real example.

Explaining resistance

The big ideas to know

You've covered the whole of Edexcel 4BI1 Section 3 — Reproduction & Inheritance. Press Finish to see your score.