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definitions for Sci 1.9


A molecule containing phosphate-sugar-base, found in DNA.


A sequence of three nucleotide bases found on a DNA strand.

Double Helix

A term used to describe the structure of DNA; a spiral ladder-shaped helix molecule.

Base Pair

Two bases making up each rung of DNA; either C-G or A-T.


A base found in DNA which bonds with Thymine. These make a base pair that form "rung" in the DNA structure.


A base found in DNA which bonds with Guanine. These make a base pair that form "rung" in the DNA structure.


A base found in DNA which bonds with Cytosine. These make a base pair that form "rung" in the DNA structure.


A base found in DNA which bonds with Adenine. These make a base pair that form "rung" in the DNA structure.


A molecule found in the nucleus of cells that codes for an individual's genetic makeup.


A molecule made up of many repeating units.


A distinctive inherited feature of an organism.


Another word meaning characteristic.

Genetic Code

The genetic information that is held in the DNA


An individual that is able to grow, reproduce, respire etc. (MRS GREN)


The structural and functional unit that makes up all living things.

Cell Nucleus

The centre of the cell where the genetic material is found.


A length of DNA, which holds the genes; thread like structures that are visible before a cell divides. They are found in the nucleus of the cell. Humans have 23 homologous pairs.

Cell Division

The formation of two or more daughter cells formed from one original cell.



Daughter Cells

The new cells that form once a cell has divided.


A length of DNA found on a chromosome that carries the code for one feature.


When something is passed on from one generation to another.


Something which is able to provide a pattern or sequence that can be copied.

DNA Replication

The process whereby DNA makes a complete copy of itself.


Cell division used for growth and repair - produces two cells identical to the original.


The type of cell division which produces sex cells/gametes (sperm and eggs; pollen and ova). In animals it happens in the ovaries and testes. Each gamete contains half the genetic information of normal cells (i.e. 23 chromosomes in humans).


A different form of a gene. Each characteristic has 2 alleles.


The characteristic that is expressed due to a particular geneotype i.e. what you see.


The combination of two alleles that an organism has for a particular gene. This is the code for the alleles.

Homologous Pairs

Two copies of a chromosome that have the same shape and structure and carry the same genes.


When both alleles are the same eg RR or rr


When the two alleles are different eg Rr


The allele which is expressed even if just one is present.


The allele which is only expressed if the dominant allele is absent.

Sexual Reproduction

Form of reproduction involving the fusion of two gametes from two parents. Produces variation in offspring.


The sex cells. A cell that contains half the genetic information that has to join with another gamete to form a zygote before further development can occur.

Sex Cells

Gametes formed in the testes and ovaries in animals.


The gamete/sex cell produced by male animals.


The male organ in animals that produces male gametes - sperm.


The gamete/sex cell produced by female animals.


The female organ that produces the female gametes - eggs.


The fusion (joining) of a male gamete and a female gamete to form a zygote.


A fertilised egg. A single cell containing chromosomes from both the male gamete and the female gamete.


The differences between individuals, which help a species survive.

Crossing Over

The exchange of genetic material between homologous chromosomes which occurs during meiosis and contributes to genetic variation.

Independent Assortment

Name for the process where chromosomes line up in random order at the cell equator during meiosis which results in a random combination of alleles in a particular gamete.

Inheritable Variations

Variations that can be passed on from parent to offspring.

Non- Inheritable Variations

Variations that are due to environmental factors.


Photograph of individual chromosomes of a cell arranged in pairs and showing their size number and shape.

Punnet Square

A grid-like diagram used to make predictions about inheritance.

Pedigree Chart

A chart that shows the genetic relationships between members of the same family.


A number expressed as part of a whole number e.g. ⅔ of offspring are blue eyed.


The ratio/relationship of one part to another e.g. 2 out of every 3 offspring are blue eyed.


The relationship of one number to another. E.g. 2 blue eyed offspring: 1 non-blue eyed offspring.

Pure Bred

An organism that is homozygous for a trait.

Test Cross

A breeding test that is carried out to determine the genetic makeup of an organism

Natural Selection

Occurs when the environment "selects" the best phenotypes. Often referred to as "survival of the fittest", as a result the species changes

Selective Breeding

A process that is used to breed for specific required traits e.g. cattle with more muscle, flowers with better smell.


A change in the base sequence of DNA.


Group of plants or animals differing only slightly from others and capable of interbreeding.


A population is all the organisms that both belong to the same species and live in the same geographical area.

Environmental Conditions

The conditions in the environment that can change an organisms appearance e.g. diet, food supply, climate changes, diseases, predators.

Asexual Reproduction

Reproduction involving only one parent that creates genetically identical offspring


Making a genetically identical copy of an organism.

Continuous Variation

Type of variation in which there is a complete range of measurements from one extreme to the other e.g. human height.

Discontinuous Variation

Type of variation where the characteristic fall into a number of distinct classes or categories e.g. human eye colour or tongue rolling.

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