1. Patterns of Inheritance
Genetic variation
Patterns of Inheritance

2. 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 (long splindly plants lacking colour) 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.

3. 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

4. Genetic factors 3 things influence the genetic factors: Gene mutations
(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

5. 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)

6. 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.

7. (i) the contribution of both environmental and genetic factors to phenotypic variation
Chromosome mutations
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.

8. 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

9. (ii) how sexual reproduction can lead to genetic variation within a species
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.

10. 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.

11. 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

12. 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

13. 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

14. 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.