Genetics and evolution Vocab

Testes (testis)

Produce sperm and the hormone testosterone.

Scrotum

Skin sac that holds the testes outside the body to keep them cooler than body temperature — essential for sperm production.

Sperm

Male gamete. Haploid cell containing 23 chromosomes. Has a tail to swim toward the egg.

Sperm duct (vas deferens)

Tube that carries sperm from the testes to the urethra.

Prostate gland

Adds fluid to sperm to create semen — nourishes and carries sperm.

Urethra

Tube carrying semen or urine out of the penis.

Penis

Organ used to deliver sperm into the vagina.

Ovary

Produces eggs (ova) and hormones like oestrogen.

Egg (ovum)

Female gamete. Haploid cell containing 23 chromosomes. Much larger than sperm.

Oviduct (fallopian tube)

Tube where fertilisation occurs — also transports the egg from the ovary to the uterus.

Uterus (womb)

Where the embryo implants and develops into a foetus.

Cervix

Narrow opening between the uterus and vagina.

Vagina

Birth canal — receives sperm during intercourse.

Gametes

Sex cells (sperm and egg). Always haploid — contain 23 chromosomes in humans.

Fertilisation

Fusion of a sperm and egg to form a zygote. Occurs in the fallopian tube (oviduct).

Zygote

First diploid cell (46 chromosomes) formed immediately after fertilisation. Stage lasts 0–2 weeks. Not yet implanted in uterus.

Embryo

Stage after the zygote implants into the uterus wall. Does not yet resemble a human. Weeks 3–9 of pregnancy.

Foetus

Stage from week 10 onward when the developing baby starts to resemble a human. Continues until birth at week 40.

Haploid cell

Cell with half the usual number of chromosomes. In humans: 23 chromosomes. Only gametes are haploid. Written as n.

Diploid cell

Cell with the full set of chromosomes. In humans: 46 chromosomes (23 pairs). All body cells are diploid. Written as 2n.

Chromosome

Long DNA molecule containing many genes. Humans have 46 chromosomes in every body cell arranged in 23 pairs.

DNA

Molecule carrying genetic instructions. Shaped as a double helix (twisted ladder). Found in the nucleus of every cell.

Base pairs

The four bases that make up DNA: Adenine (A) always pairs with Thymine (T), and Cytosine (C) always pairs with Guanine (G).

Gene

Section of DNA that codes for one specific characteristic (e.g. eye colour). Many genes make up one chromosome.

Genome

The complete set of all DNA in an organism — every gene across all chromosomes.

DNA replication

The copying of DNA before cell division so every new daughter cell receives a complete identical copy of the genetic information.

Mitosis

Cell division producing 2 genetically identical diploid daughter cells. Purpose: growth and repair of body cells.

Meiosis

Cell division producing 4 genetically different haploid cells (gametes). Purpose: sexual reproduction. Halves the chromosome number so fertilisation restores the correct diploid number.

Allele

A different version of the same gene. e.g. the eye colour gene has alleles for brown, blue, and green.

Genotype

The specific alleles an organism has for a characteristic. Written as letter combinations e.g. RR, Rr, rr.

Phenotype

The physical characteristic that is actually shown — what you can observe. e.g. round seeds, brown eyes.

Dominant allele

Allele that is always expressed even when only one copy is present. Written as a capital letter (e.g. R).

Recessive allele

Allele that is only expressed when two copies are present (homozygous recessive). Written as lowercase (e.g. r).

Homozygous

Having two identical alleles for a characteristic. e.g. RR (homozygous dominant) or rr (homozygous recessive).

Heterozygous

Having two different alleles for a characteristic. e.g. Rr. Always shows the dominant trait in Year 10.

Punnett square

Diagram used to predict all possible genetic outcomes of a cross between two parents. Shows genotype ratios of offspring.

Variation

Differences in characteristics between individuals of the same species. Caused by genes and/or the environment.

Discrete variation

Variation that falls into clear separate categories with no values in between. Controlled by one gene. e.g. blood type, tongue rolling, sex.

Continuous variation

Variation that shows a full range of values between two extremes. Controlled by many genes and environment. e.g. height, weight, skin colour.

Mutation

A change in the DNA sequence — an accidental alteration in the genetic code. Can be beneficial, harmful or neutral.

Mutagen

An agent that causes mutations. Examples: UV radiation, X-rays, certain chemicals, errors during cell division.

Beneficial mutation

A mutation that improves an organism's chances of survival and reproduction. e.g. elephants born without tusks avoiding poachers.

Harmful mutation

A mutation that decreases survival or health. e.g. cystic fibrosis, sickle cell anaemia, some cancers.

Neutral mutation

A mutation that has no effect on the organism's survival either way.

Evolution

Change in the inherited characteristics of a species over many generations through natural selection.

Natural selection

The process where individuals with advantageous traits are more likely to survive, reproduce and pass those traits to offspring. Over time the population changes.

Selective breeding

Humans choosing which organisms breed together to produce offspring with desired traits. e.g. breeding dogs for size, cows for milk production.

Fitness (biological)

How well an organism survives and reproduces in its environment — its ability to pass genes to the next generation.

Scientific theory

A well-tested, broad explanation of natural phenomena supported by extensive evidence, observation and repeated testing. Not a guess.

Common ancestor

A species from which two or more different species evolved. All life shares common ancestors.

Speciation

The formation of a new species when populations become reproductively isolated from each other over time.

Evolutionary tree

Diagram showing evolutionary relationships between species — branches show where species diverged from common ancestors. Evolution is a branching tree, not a straight ladder.