Topic 15 - Inheritance and Evolution Flashcards
Variation
Differences between individuals of the same species. Variation can be genetic, environmental, or a combination of both.
How genetic variation is produced
Random fusion of gametes at fertilisation means each zygote will have a unique combination of alleles. This is because an egg cell can be fertilised by a single sperm out of millions of sperm cells.
Female sex chromosomes
XX
Male sex chromosomes
XY
As only a father can pass on a Y chromosome, he is responsible for determining the sex of the child
Allele
A different version of the same gene
Phenotype
Physical, observable characteristics of an organism (e.g. eye colour, hair colour)
Genotype
The genetic makeup of an organism - the combination of alleles that control each characteristic (e.g. RR, Rr, rr)
Homozygous
Having only one type of allele present (i.e. BB or bb)
Heterozygous
Having two different types of allele present (i.e. Bb or bB)
Dominant allele
A version of a gene that, if present, is always expressed
Recessive allele
A version of a gene that is only expressed if two copies are present
Monohybrid inheritance
Inheritance of a single characteristic/trait. These characteristics are controlled by a single gene.
Pedigree rules
- Dominant disorders can’t jump generations but recessive ones can. With recessive disorders, two non-sufferers can have a child who has the disease
- It is impossible for two people with a recessive disorder to have children without the disorder as their genotypes will be ff x ff
- It is possible for two people with a dominant disorder to have children without the disorder. This is because their genotypes can be Hh and Hh
Polygenic
Characteristics that are controlled by more than one gene (e.g. eye colour is controlled by several genes, so the wide range of combinations make it difficult to show on a genetic diagram)
Codominance
When both alleles within a genotype are expressed in the phenotype of an individual (e.g. alleles A and B are codominant, so the blood type will be AB)
Mutation
A rare, random change in the sequence of bases in DNA
How mutations can affect phenotype
- when mutations change the base sequence of DNA, the base sequence of the complementary mRNA transcript also changes
- modified mRNA transcript will be translated to code for different amino acids, changing the structure of the chain
- the function of protein is altered
Effects of mutations
- most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
- a small number of mutations code for a significantly altered protein with a different shape, which may affect the protein’s function (e.g. collagen may lose its strength if shape changes)
Causes of mutations
Frequency of mutations can be increased by:
- exposure to gamma rays, x-rays and UV rays (ionising radiation)
- exposure to chemicals (e.g. tar in tobacco)
Evolution
the change in the characteristics of a species over time (millions of years), occurring through the process of natural selection
Darwin’s Theory of Evolution
- individuals show a wide range of variation caused by differences in genes and mutations
- individuals with characteristics that are most suited to the environment are more able to survive and reproduce
- these characteristics are passed on at a higher rate than less suited characteristics, becoming more common in the population
