| Allele |
One form
of
a gene. Many genes have two or more forms (alleles) that
each give a different appearance or quality to the organism
they
are in. Different alleles of the same gene are usually symbolised with
one
letter. The capital letter is used for the dominant
allele, while
the lower
case letter is used for the recessive
allele. |
| Asexual |
This
means
"without sex". It is usually used when referring to
reproduction in which one parent produces offspring without producing gametes.
The offspring
are exact replicas of the parent. E.g. Yeast
budding, plants grown from cuttings. The reproduction of cells for
growth is also asexual (see mitosis). |
| Bases |
In a
molecule of DNA,
these are
the "rungs" on the "ladder". There are four of them, Adenine (A),
Thymine (T), Guanine (G) and Cytosine (C). The bases are able to store
genetic information. Each group of three bases on DNA codes for one
amino acid in the protein
it codes for. Bases are found in
pairs
in a DNA molecule. A pairs with T. G
pairs with C. This gives DNA the
ability to make copies of itself during replication.
Not to be confused with the places you run around in
baseball. |
| Centromere |
The
constricted point
where the two sides (chromatids) of a replicated chromosome join. |
| Chromatid |
A
chromosome having only
one DNA strand - not having two sides. Results from meiosis
or mitosis
in which the two sides (chromatids) of a replicated chromosome
are pulled apart. |
| Chromosome |
A
chromosome consists of a core of DNA
which is associated with a number
of protein
molecules called histones. Most of the time, chromosomes are
largely uncoiled and interwoven with other chromosomes in the nucleus
of a cell. As the time for cell division approaches, chromosomes coil
up and appear as an X shape. Each side is an exact copy of the
other side. |
| Clone |
A cell or
a
whole organism that is an exact replica of another cell or organism. |
Deoxyribonucleic
acid |
Also
called
deoxyribose nucleic acid, this is the full name of DNA. DNA is the
molecule in chromosomes that carries the genetic code. Its double helix
structure was worked out in 1953 by Watson, Crick and Wilkins (a New
Zealander), for which they received the Nobel Prize. Rosalind Franklin,
whose work with X-ray crystallography helped them work out the
structure, died before the prizes were awarded so she missed out. |
| Diploid |
Having
the
"double"
number of chromosomes (2n), consisting of two sets of homologous
chromosomes. In humans 2n = 46
Most plants and animals are diploid. In comparison, all
gametes and some simple organisms (including some male insects such as
bees) are haploid
or monoploid. |
| Dominant |
An allele
of a gene is said to be dominant if it is always expressed (has an
effect) when present. A dominant allele will mask the presence of a recessive
allele. |
| Egg |
The
common
name for an ovum. It is a gamete
produced by meiosis
cell division in
the ovary of a female. Eggs are monoploid
or haploid
(n), having only
one set of chromosomes. Eggs are usually much bigger than male gametes
(sperm) and in most species eggs remain inside the female before fertilisation. |
| Embryo |
An early
stage of development of an organism.
It has grown beyond a ball of
similar cells, but has not yet developed easily recognisable features. |
| F1 |
First
Filial generation. This term is used to describe the offspring
of
cross breeding two different, pure breeding
(homozygous)
varieties
of a
species.
For example, when pure breeding furry cats (HH)
are crossed with pure breeding sphinx (hairless) cats (hh), the kittens
produced are the F1.
All of the F1 kittens will
be heterozygous
(Hh) and show the dominant phenotype
(furry). |
| F2 |
Second
Filial generation. The resulting offspring
of interbreeding heterozygous
F1
individuals are called the F2 generation. The offspring would show a 3
to 1 ratio. Three quarters
would show the dominant phenotype,
the other quarter would show the recessive
phenotype.
For example, if the heterozygous (Hh) furry cats mentioned above were
mated with other Hh cats, the resulting kittens would be the F2
generation. You would expect about a quarter of the kittens to be
hairless. |
| Fertilisation |
The
joining
of male and female gametes.
That is, when a sperm cell fuses with an
egg cell. Fertilisation restores the diploid
chromosome number.
It's nothing to do with that smelly stuff that people put on gardens. |
| Gamete |
A sex
cell,
either sperm or egg, produced by meiosis.
Gametes are haploid,
containing only one set of chromosomes. Male gametes are usually
smaller and mobile while female gametes are larger and often remain in
the female. |
| Gene |
A unit of
inheritance.
The unit of heredity passed on from parent to offspring
A part of a chromosome
which controls a certain trait
of an organism.
A segment of DNA
which codes for a certain protein. |
| Genetics |
The study
of how inherited traits
in organisms are
controlled within the
organism and passed on to its offspring. |
| Genome |
All of
the
genes in a cell of an organism.
Or all of the genes/DNA
code in an organism.
Or all of the genes/DNA code in a species
The Human Genome
Project involves working out all of the base pairs in a representative
sample of human DNA and mapping all the functional areas (genes)
contained in the human DNA. |
| Genotype |
The alleles
present in an organism for a certain gene.
A genotype can be written simply as symbols, e.g. Aa.
OR, it can be a few words or a sentence.
E.g. Benny has two alleles for attached earlobes. Or, Rebecca is heterozygous
for the freckle gene. |
| Haploid |
Having
half
the normal number of chromosomes
- one member of each homologous
pair.
Or having one set of chromosomes (n). Also called the monoploid
number.
In humans, n = 23
All gametes
and some simple organisms (including some male insects such as
bees) are haploid or monoploid. In comparison, most plants and animals
are diploid. |
| Helix |
The shape
that results when a wire (for example) is wound around a cylinder with
parallel sides.
Not to be confused with a spiral which would result if the wire were to
be wound around a cone so that one end was wider than the other.
DNA structure is that of a double helix. Like a parallel-sided ladder
that has been twisted around to form two helices - (don't try this at
home with dad's ladder). |
| Heterozygous |
Having
two
different alleles
of a gene.
E.g. having the genotype
Aa or Bb. |
| Homologous |
Refers to chromosomes
having the same form - shape, length, genes in the same
order, centromere
in the same position.
Diploid
cells have two sets of homologous chromosomes.
During meiosis,
homologous pairs of chromosomes temporarily join and
swap pieces. |
| Homozygous |
Having
two alleles
the same for a gene.
This could be two dominant
alleles or two recessive
alleles.
Examples:
Homozygous
dominant, e.g. DD
Homozygous
recessive, e.g. dd.
|
| Locus |
The
point on a chromosome
where a gene
under discussion is found. Often
used in a sentence such as " the b allele is found at the same locus as
the B allele". |
| Meiosis |
The
special
nuclear division that occurs only in gonads or other reproductive
structures
(female ovaries, male testes or anthers).
It produces gametes
(ova in females, sperm cells in males) which are haploid
and used for
sexual reproduction. Meiosis along with fertilisation
contributes
towards variation
in a species as new gene combinations can occur. (See
an animation)) |
| Mitosis |
Normal
cell
division that is usually used for growth or replacement of
cells.
It can occur in all tissues throughout the organism
and the daughter
cells produced have the same chromosome number as the original cell. It
is used for asexual
reproduction in some species. Mitosis does not
contribute to variation
in a species as daughter cells have the same
genes as the parent. (See an animation) |
| Monoploid |
Having
one
set of chromosomes (n). Or Having half
the normal number of chromosomes - one member of each homologous
pair.
Also called the haploid
number.
All
gametes and some simple organisms (including some male insects such as
bees) are haploid or monoploid. In comparison, most plants and animals
are diploid. |
| Mutation |
Any
change
in the DNA, genes,
or chromosomes
in an individual or species.
Mutations are caused by ionising radiation, some chemicals, mistakes
during cell division and other agents. They are often harmful, but can
be beneficial.
Mutations are the ultimate source of variation
within a species and
provide the raw material for evolution. |
| Nucleic
acid |
A complex
chemical which is a polymer (long chain of repeated sub-units) of
nucleotides. Each nucleotide consists of a 5-carbon sugar. a phosphate
group and a nitrogenous base.
There are two types; DNA,
used in chromosomes to carry the genetic code
and RNA
of which there are three types used during the process of
protein
synthesis in cells. |
| Nucleus |
A
membrane-bound structure found in cells. It contains the chromosomes,
usually uncoiled and intertwined giving it a grainy appearance. |
| Offspring |
A word
biologists use when talking about the young individuals produced by the
parents in a genetic cross. In other words, children, kittens, puppies,
foals, calves,
seedlings and so on. |
| Organism |
A living
thing, e.g. cat, dog, cabbage, bacterium, aardvark, pine tree. |
| Ovum |
An egg.
Plural - ova. Haploid
cell produced by meiosis
in female gonads. It is
not usual however to talk about having a couple of fried ova for
breakfast. |
| Pedigree |
Family
tree, often in the form of a diagram which shows two or more
generations. Pedigree diagrams are used to trace the inheritance of a
certain trait
through the family. Females are symbolised by circles,
males by squares. The symbols of individuals with the phenotype
under
study are shown shaded.
Animal breeders, for example dog enthusiasts, cat nuts and such-like,
talk about pedigree animals, meaning they know their family tree and
they have bits of paper to prove they are pure breeding for that
variety, and they can charge you a lot of money for them. |
| Phenotype |
The
outward appearance or the characteristics of an organism, usually in
relation to a genetic trait
under discussion. For example: a blue-eyed
person, a tall pea plant, a corn plant that is sweet due to the
production of sucrose. Phenotypes must be written in words, unlike genotypes
which can be written in the symbols of the alleles present. |
| Protein |
A
molecule which is a polymer of sub-units called amino acids. Genes have
their effects by carrying a code which determines the order in which
the amino acids are joined together in a protein. This order is very
important in determining the job of the protein.
There are 20 amino acids found in living things and since proteins may
have from twenty-odd, to thousands of amino acids of any
sorts, the possible combinations are enormous.
Proteins have functions including structural components such as hair or
connective tissue, to transportation helpers such as haemoglobin in red
blood cells, to hormones - chemical messenger molecules such as
insulin, to enzymes which help chemical reactions occur in cells.
Proteins are also found helping form the structure of chromosomes as
well as being important in the controlling of what genes are turned off
or on in any one cell at any time. |
| Punnett
square |
A diagram
used to work out the possible genotypes
of the offspring
produced in a
cross between parents of know genotypes.
Nothing to do with the little boxes strawberries come in.
The method was invented by
British geneticist,
Professor Reginald Crundall Punnett, (1875 - 1967) |
| Pure breeding |
Homozygous.
An organism that is pure breeding for a trait
can be relied on to
produce gametes
with only one type of allele
for that trait.
For example, all of the gametes produced by a pure breeding
folded-eared cat will contain alleles for folded ears. All of
the gametes from a pure breeding tall pea plant will contain alleles
for tallness. |
| Recessive |
Refers to alleles
that will show up or have their effect only when two of them
are present. Recessive alleles can be hidden or masked if a dominant
allele is present. Recessive alleles are usually symbolised by lower
case letters. |
| Replication |
The
process by which a DNA
molecule makes two exact copies of itself by
splitting in half between its base
pairs then each side acts as a
pattern to which new bases join (A to T, T to A, C to G and G to C).
The two copies are seen as the two sides or chromatids
of a chromosome
in a cell which is ready to divide. |
| Ribonucleic
acid |
RNA. This
is similar to DNA
but is only single stranded, has ribose sugar rather
than deoxribose and has the base, uracil (U) in place of thymine (T).
RNA molecules are also much shorter than DNA.
There are three types of RNA;
mRNA = messenger RNA which is made in the nucleus using the pattern of
the DNA in a gene and carries the code out into the cytoplasm of the
cell.
rRNA = ribosomal RNA which are found in ribosomes, the tiny organelles
which provide an "active site" where the code on the mRNA is used to
make a protein.
tRNA = transfer RNA molecules are short, cloverleaf shaped bits of RNA
that pick up loose amino acids in the cell and bring them to the
ribosome where they can be joined to make a protein. |
| Sexual |
To do
with reproduction that involves the use of haploid gametes
produced by meiosis
in two parents. As compared with asexual
which refers to reproduction that involves only one parent that
produces offspring
identical to itself by the process of mitosis.
Sexual reproduction results in reshuffling of gene combinations in the
production of gametes and the production of new gene combinations
during fertilisation of the ovum by the sperm. This contributes to variation
in the offspring. |
| Sperm |
The male gamete
produced by the process of meiosis
in male gonads (testes) or
anthers of flowering plants. Sperm cells are haploid,
having one
set of chromosomes. |
| Trait |
A nice
short word to use in place of characteristic. The proper way
to pronounce it is "tray" (as it is French in origin). |
| Variation |
Having
variety, a range of different phenotypes
for one or more
characteristics.
Variety is considered to be very important for the
continuation of life. If a species has variety, it is likely that at
least some individuals will survive a change of environmental
conditions or the arrival of a new disease which has the potential to
kill some or many individuals.
Mutation of genes is the original source of variety within a species.
Sexual reproduction results in reshuffling of genes to produce new
combinations which also contributes to variety.
Evolution is the gradual change in allele frequencies which results in
the formation of sub-species, species, families and so on over very
long periods of time. This would not be possible without the original
variety. |
| Zygote |
The first
cell formed when a sperm cell fuses
with an egg cell. It is derived
from a Greek word for yoke (used for harnessing animals to wagons) and
has a meaning of pairing or joining together.
Nothing to do with goats really. |
|
|
A site for
learning
about basic genetics