Download Genetic variation

GENETIC VARIATION
Patterns of Inheritance
LEARNING OUTCOMES
(i) the contribution of both environmental and genetic factors to
phenotypic variation
 To include examples of both genetic and environmental contributions –
environmental examples could include diet in animals and etiolation or
chlorosis in plants.
(ii) how sexual reproduction can lead to genetic variation within a
species
 Meiosis and the random fusion of gametes at fertilisation.
KEY DEFINITIONS
Define the terms genotype and phenotype
Genotype – the genetic make up of an organism
Phenotype – the visible characteristics of an organism
From GCSE, what influences the phenotype?
Both genotype (genetic factors) and the environment
(i) the contribution of both environmental and genetic
factors to phenotypic variation
GENETIC FACTORS
3 things influence the genetic factors:
Gene mutations
Chromosome mutations
Sexual reproduction
GENE MUTATIONS
(i) the contribution of both environmental and genetic
factors to phenotypic variation
Mutagens can influence the rate of mutation. These can include:
Physical agents – X-rays, gamma rays and UV light
Chemical agents – benzopyrene (in tobacco smoke), mustard gas,
free radicals
Biological agents – some viruses, transposons (jumping genes),
food contaminants (mycotoxins from fungi)
Mutations can be harmful, advantageous or neutral.
Mutations that occur during gamete formation are persistent
(transmitted through many generations) and random (not directed
by need)
CHROMOSOME MUTATIONS
These occur during meiosis.
Create a list of the different types of chromosome mutations, how
they occur and the influence they may have.
CHROMOSOME MUTATIONS
(i) the contribution of both environmental and genetic
factors to phenotypic variation
Deletion – part of the chromosome, containing genes and regulatory
sequences, is lost
Inversion – sections of chromosome mat break off, turn through 180o
and join again. All the genes are still present but may be too far
away from their regulatory nucleotide sequence to be expressed.
Translocation – a piece of chromosome breaks off and becomes
attached to another chromosome. This may affect both chromosomes.
Duplication – a piece of chromosome may be duplicated.
Overexpression can be harmful because too many of certain proteins
may disrupt metabolism.
CHROMOSOME MUTATIONS
(i) the contribution of both environmental and genetic
factors to phenotypic variation
Non-disjunction – one pair of chromosomes or chromatids fails to
separate leaving one gamete with an extra chromosome. Eg Downs
syndrome (trisomy 21) and klinefelter syndrome (an extra X
chromosome XXY)
 Aneuploidy – the chromosome number is not an exact multiple of the
haploid number e.g. trisomy
 Polyploidy – if a diploid gamete is fertilized by a haploid gamete, the
resulting zygote will be triploid. The fusion of 2 diploid gametes leads to a
tetraploid zygote. Many cultivated plants are polyploidy
(ii) how sexual reproduction can lead to genetic variation
within a species
SEXUAL REPRODUCTION
How can sexual reproduction lead to genetic variation?
During meiosis:
Allele shuffling – swapping of alleles between sister chromatids during
crossing over in prophase 1.
Independent assortment – of chromosome during metaphase/anaphase
1 and 2
Random fertilisation – any male gamete can combine with any female
gamete.
ENVIRONMENTAL FACTORS
(i) the contribution of both environmental and genetic
factors to phenotypic variation
Some phenotypic variation is caused only by the environment
and not passed on in genes:
Speaking with a regional dialect
Loss of limbs or scars
Some phenotypic variation is caused by the environment
interacting with genes:
Diet in humans
Chlorosis in plants (plants grown in dim light with a lack of
magnesium are yellow and cannot photosynthesis). They have the
genotype for making chlorophyll but environmental factors prevent
them expressing the gene.
KEY DEFINITIONS
Write definitions for the following words we will be using over the
next few lessons
Allele,
heterozygous,
homozygous,
monogenic,
dihybrid,
codominance,
multiple allele,
sex-linked,
autosomal linkage,
epistasis
KEY DEFINITIONS
Allele – a version of a gene
Heterozygous – not true breeding; having different alleles at a
particular gene locus on a pair of homologous chromosomes
Homozygous - true breeding; having identical alleles at a
particular gene locus on a pair of homologous chromosomes
Monogenic – determined by a single gene
Dihybrid – involving 2 gene loci
KEY DEFINITIONS
Codominance – where both alleles present in the genotype of a
heterozygous individual contribute to the individuals phenotype
Multiple alleles – characteristic for which there are 3 or more
alleles in the populations gene pool
Sex-linked – genes present on one of the sex chromosomes
Autosomal linkage – gene loci present on the same autosome (non
sex chromosome) that are often inherited together
Epistasis – interaction of non-linked gene loci where one masks the
expression of the other
LEARNING OUTCOMES
(i) the contribution of both environmental and genetic factors to
phenotypic variation
 To include examples of both genetic and environmental contributions –
environmental examples could include diet in animals and etiolation or
chlorosis in plants.
(ii) how sexual reproduction can lead to genetic variation within a
species
 Meiosis and the random fusion of gametes at fertilisation.