Summary 6.1.2 Patterns of Inheritance Revision Notes (OCR A)
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6.1.2 PATTERNS OF INHERITANCE
a. i) the contribution of both environmental and genetic factors to phenotypic variation
Both environmental and genetic factors influence phenotypic variation.
Genetic factors – mutations to DNA or to the gross structure of the chromosome.
Environmental factors – e.g. diet (animals), etiolation and chlorosis (plants).
Genetic factors:
Physical, chemical and biological mutagens can contribute can increase the rate of mutation.
o Physical agents – e.g. X-rays, gamma rays, UV light.
o Chemical agents – e.g. nitrous acid, reactive oxygen species, benzopyrene.
o Biological agents – e.g. some viruses, transposons, mycotoxins from fungi.
Chromosome mutations may also occur during meiosis.
o Deletion – part of a chromosome is lost.
o Inversion – a section of a chromosome may break off, turn through 180 o then join again.
o Translocation – a piece of one chromosome breaks off and then becomes attached to
another chromosome (e.g. Philadelphia translocation).
o Duplication – a piece of a chromosome may be duplicated.
o Non-disjunction – one pair of chromosomes or chromatids fails to separate, leaving one
gamete with an extra chromosome (e.g. Down syndrome).
o Aneuploidy – the chromosome number is not an exact multiple of the haploid number for
that organism (e.g. trisomy, Down syndrome).
o Polyploidy – having more than two sets of chromosomes (e.g. if a diploid gamete is
fertilised by a haploid gamete).
Mutations must occur in the germ line in order to be passed on to offspring.
Environmental factors:
In animals:
o The loss of a limb following injury.
o Diet affects an animals’ body mass, height and build.
In plants:
o Etiolation – if a plant is kept in the dark, its stem will grow long and thin in an attempt to
try and access more light.
o Chlorosis – if a plant does not get enough magnesium, it can no longer produce
chlorophyll. The plant’s leaves grow yellow and it can no longer photosynthesise.
ii) how sexual reproduction can lead to genetic variation within a species
Genetic variation in sexual reproduction:
Sexual reproduction increases genetic variation within a species.
Meiosis produces four genetically different gametes.
o This can result from crossing over, independent assortment or other mutations.
o Crossing over – the exchange of alleles between non-sister chromatids during prophase 1.
o Independent assortment – the orientation of chromosomes during metaphase 1 and 2.
o Other mutations – e.g. non-disjunction, translocation.
, Random fertilisation – the random fusion of gametes at fertilisation.
See the mitosis topic for more detail.
b. i) genetic diagrams to show patterns of inheritance
Monogenic inheritance – examining the inheritance of a characteristic determined by a single gene.
Punnett squares can be used to show all the genotypes and phenotypes resulting from the
possible combinations of gametes.
o P1 (parent) generation – the original population.
o F1 (first filial) generation – the offspring of the P1 generation.
o F2 (second filial) generation – the offspring of the F1 generation.
E.g. height of stem of pea plants.
o Crossing a heterozygous mother with a homozygous recessive father.
o Half the offspring are heterozygous and half are homozygous recessive.
T t
t Tt tt
t Tt tt
Dihybrid inheritance – examining the simultaneous inheritance of two characteristics.
E.g. seed shape and seed colour in pear plants.
o Crossing two heterozygous parents, where yellow and round are dominant.
o We see a 9:3:3:1 ratio of phenotypes.
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
Autosomal linkage – gene loci present on the same autosome that are often inherited together.
When the two alleles are on different chromosomes, they are assorted independently of each
other. Inheriting one allele has no effect on the likelihood of inheriting another.
However, when two gene loci are on the same autosomal (non-sex) chromosome, they are said to
be autosomally linked.
If we assumed no crossing over, then two genes on the same autosomal chromosome will always
be inherited together, as there is no independent assortment.
However, crossing over can result in the formation of recombinant gametes.
The further apart the two gene loci are on a chromosome, the greater is the chance of
recombinant gametes forming.
o There is a larger chance that the recombination of alleles (due to crossing over) will occur
in the length of DNA between the two gene loci.
Multiple alleles – a characteristic for which there are three or more known alleles at a specific gene
locus in the population’s gene pool.
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