Flashcards in Genetics (Chapter 16) Deck (84)
What is a dominant allele?
One whose effect on the phenotype of a heterozygote is identical to its effect on a homozygote
What is a recessive allele?
One that is only expressed (in the phenotype) when no dominant allele is present
What are codominant alleles?
These both have an effect on the phenotype of a heterozygous organism
What is linkage?
The presence of two genes on the same chromosome so that they tend to be inherited together and do not assort independently
What is a test cross?
A genetic cross in which an organism showing a characteristic caused by a dominant allele is crossed with an organism that is homozygous recessive
- the phenotype of the offspring can be a guide to whether the first organism is homozygous or heterozygous
What is the F1 generation?
The offspring resulting from a cross between an organism with a homozygous dominant genotype, and one with a homozygous recessive genotype
What is the F2 generation?
The offspring resulting from a cross between two F1 (heterozygous) organisms
What is the phenotype?
The characteristics of an organism, often resulting from an interaction between its genotype and its environment
What is the genotype?
The alleles possessed by an organism
What does homozygous mean?
Having two identical alleles of a gene
What does heterozygous mean?
Having two different alleles of a gene
What is a mutation?
An unpredictable change in the genetic material of an organism
What is a gene mutation?
A change in the structure of a DNA molecule, producing a different allele of a gene
What is a chromosome mutation?
Mutations that cause changes in the structure or number of whole chromosomes in a cell
How do mutations occur?
Completely randomly (copying errors) or environmental factors e.g. ionising radiation or mutagens
What is a mutagen?
A substance that increases the chances of mutation occurring
What are the 3 different ways in which the sequence of bases in a gene may be altered during a gene mutation?
1) Base substitution
2) Base deletion
3) Base addition (insertion)
What happens during base substitution?
One base takes the place of another
What happens during base deletion?
One or more bases are lost from a sequence
What happens during base addition?
One or more extra bases are added to the sequence
Why do base additions/deletions usually have a very significant effect on the structure and therefore the function of the polypeptide that the allele codes for?
- Because they alter every set of 3 bases that follows them in the DNA molecule - they cause 'frame shifts' in the code
- Often the effects are so large that the protein made is totally useless
- Or, the addition/deletion may introduce a 'stop' triplet part way through a gene, so that a complete protein is never made
What is a silent mutation?
A mutation that has no apparent effect on an organism
Why do base substitutions often have no effect at all?
- They are often silent mutations because many amino acids have more than one triplet code, so even if one base is changed, the amino acid is still coded for
- They also have no effect on any of the triplet codes that follow, only the one that is directly affected
How could base substitutions have a very large effect?
If the substituted base makes the triplet a 'stop' triplet
What is one example of a base substitution that has a significant effect on the phenotype?
Sickle cell anaemia
Describe the substitution in sickle cell anaemia
- In people with the Hbs (sickle cell) allele, the base sequence CTT is replaced by CAT, changing one amino acid in the sequence which changes the beta-globin polypeptide
What is the effect of the small difference in the amino acid sequence in people with sickle cell anaemia?
- It makes little difference to the Hb molecule when combined with oxygen
- But when not combined with oxygen, the unusual beta-globin polypeptides make the Hb molecule much less soluble
- The molecules tend to stick to each other, forming long fibres inside the RBCs
- The RBCs are pulled out of shape, into a half-moon/sickle shape
- The distorted cell cannot carry oxygen and also gets stuck in capillaries, stopping any unaffected cells from getting through
- A person with this unusual beta-globin can suffer severe anaemia and may die
What is anaemia?
A lack of oxygen transported to the cells
What disease is an example of a relationship between a gene, enzyme and human phenotype?