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Level 1 Genetics, basic meiosis, and mitosis, definitions, etc.

Genetic Code

structures made of DNA molecules that contain the genes.
sections of chromo­som­es/DNA that code for our charac­ter­istics
altern­atives forms of a gene
a geneti­cally determined charac­ter­istic or condition
Homologous Pair
a pair of 2 chromo­somes: 1 from the mother and 1 from the father
an individual animal, plant, or single­-celled life form.
the combin­ation of alleles that an organism has
the charac­ter­istic produced by the genotype
Dominant Allele
always expressed (shows the trait) if present (eg. B)
Recessive Allele
only expressed when the dominant allele is absent (eg. b)
having two identical alleles (eg. bb)
having two different alleles (eg. Bb)
DNA carries the genetic inform­ation in a cell. A gene and an allele are both made of DNA. A gene is a length of DNA that codes for a particular charac­ter­istic, whereas an allele is the altern­ative form of a gene containing a different base sequence. A gene for a charac­ter­istic consists of two alleles.

DNA Structure

genetic material stored in the nucleus of cells
contains the chromo­somes
DNA Structure
double helix which consists of nucleotide subunits
See diagram of DNA structure here:
Comple­mentary Base Pairing
there are four bases, two pairs:
Adenine (A)
Thymine (T)
Guanine (G)
Cytosine (C)
Remember adenine pairs with thymine as 'Apples grow on Trees'.
Remember guanine pairs with cytosine as 'Carrots grow in the Ground'.


Why is variation important?
-Variation increases the chance of survival of a species if there is a change in the enviro­nmental pressure.
-Without variation in a species any sudden enviro­nmental change can mean that no individual has a phenotype that allows it to survive, causing the species to become extinct.

Pure Breeding

An organism that always passes down certain phenotypic traits to its offspring of many genera­tions. The organi­sms's parents and ancestors all belong to the same breed.
Pure bred genotypes are always homozy­gous.
genotype of parent 1: PP, genotype of parent 2: PP
genotypic ratio of the offspring: 4PP, 0pp, 0Pp
phenotypic ratio of the offspring: 4purple, 0white
In pigs, curly tails are dominant. To work out if a curly-­tailed pig is homozygous dominant or hetero­zygous for this trait, mate the curly-­tailed pig with a straig­ht-­tailed pig. We know the straig­ht-­tailed pig's genotype is tt. If any offspring have straight tails we know the curly-­tailed pig is hetero­zygous because they must be carrying the recessive allele for a recessive offspring to be produced. If no offspring have straight tails it is likely that the curly-­tailed pig is TT.


a cell that contains a single set of chromo­somes.
a type of cell division that produces 4 gametes (sex cells- sperm and egg) each with only half the number of chromo­somes (23).
There are two main ways meiosis produces variation:
1. Crossing Over
2. Indepe­ndent Assortment
These steps are important because they mix up allele combin­ations, causing variation.


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