Year 8 Biology - Variation Key Points
Chromosomes, genes and heredity
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A cell nucleus contains chromosomes. Chromosomes are made up of genes
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Most cells in the parent organism contain two sets of chromosomes: they are called diploid (di
means “two”). In human beings most cells contain 46 chromosomes, which exist as 23 chromosome
pairs.
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Gametes or sex cells (sperm cells and egg cells in human beings) contain only one set of
chromosomes: they are called haploid (from a word meaning half). In human beings, egg cells and
sperm cells both contain 23 chromosomes.
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A gene is made of DNA. Each gene gives instructions about or “codes for” an inherited
characteristic such as eye colour.
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Different versions of the same gene are called alleles. For example the red eye gene and the white
eye gene are both alleles of the eye colour gene in flies.
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Genes are passed on from parents to children: this is called heredity.
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An individual will inherit one of each pair of chromosomes from one parent and the other
chromosome from the other parent.
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We can symbolise two possible alleles as R and r. The alleles can be different (Rr) or they could
both be the same (RR or rr).
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Alleles can be dominant or recessive. Suppose in flies R is the (dominant) allele for red eyes and r
is the (recessive) allele for white eyes.
Allele present in gamete from
1 parent
R
r
R
r
Allele present in gamete from
other parent
R
r
r
R
Genotype
Phenotype
RR
rr
Rr
Rr
red eyes
white eyes
red eyes
red eyes
Characteristics and variation
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Variation within a species is called intraspecific variation.
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Offspring produced by sexual reproduction are genetically different from their parents, but may
share some similarities (such as looks, personalities, etc.) with them.
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Individuals within a population show variation of characteristics e.g. eye colour. Closely related
species have similar characteristics
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Some characteristics of an organism are inherited; some are environmental; some are a mixture of
environmental and inherited characteristics.
Inherited (genetic)
characteristic
Eye colour
Male or female
Blood group O, A, etc
Freckled or not
Able to roll tongue or not
Environmental (acquired)
Characteristic
Having teeth filled
Having tattooed skin
Having long hair
Speaking fluent French
Wearing a hat
A mixture of genetic and
environmental characteristics
Being a good athlete
Being good at learning languages
Being good at music
Being good at maths
Being good at making speeches in public
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Some characteristics show a continuous variation across the population: characteristics such as
height, weight, length of fingers and so on can have any value within the range.
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Other characteristics show discontinuous variation as they can only take certain values such as
male, female, blood group etc.
Inheritance of sex
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Sex chromosomes carry the genes that determine the sex of the child. There are two sexdetermining chromosomes, X and Y.
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Male body cells have the genotype XY, female body cells have the genotype XX.
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Egg cells all have the X chromosome; sperm cells have the X or the Y chromosome. When the
sperm fertilises the egg, the fertilised egg or zygote will have the genotype XX or XY:
Egg cell
X
X
Sperm cell
Y
X
Zygote genotype
XY
XX
Sex of child
Male
Female
Selective breeding
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A species is a group of closely related organisms which are capable of interbreeding to produce
fertile offspring.
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A breed is a domesticated variety of an animal.
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Cultivated plants are called varieties or cultivars.
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Selective breeding involves deliberately selecting those individuals that have desired characteristics
and breeding from them. It can take many generations to achieve the desired results.
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Examples of characteristics that might be bred for in racehorses are: speed, calm temperament,
ability to cope with stress, and so on.
Natural Selection, Evolution and the Evidence for Evolution
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Fossils are the preserved remains of animals or plants which existed millions of years ago
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Life on Earth has been changing ever since it formed
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Evolution is the slow change of organisms over time
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Evolution may result in the formation of new species
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The survival of well adapted organisms is called Natural selection
Generation
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Like chromosomes, genes also come in pairs.
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Each of your parents has two copies of each of their genes, and each parent passes along just
one copy to make up the genes you have. Genes that are passed on to you determine many of
your traits, such as your hair color and skin color.
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A Punnett square is a chart that allows you to easily determine the expected percentage of
different genotypes in the offspring of two parents.