Sesh 7: Genotype, Phenoype And Inheritance Flashcards
What is a phenotype?
The observable/ physical characteristics of an individual caused by the proteins they express, affected by genotype and interactions with its environment.
Genetics is the study of ___________ and ___________.
- Inheritance
2. Variation
What is an allele?
A version of a gene. Each individual has 2 alleles of each gene. Different mutations give rise to different alleles. There are many alleles of a gene within a population.
What do you know about that alleles of an individual if they are a:
- Homozygote
- Heterozygote
- Hemizygote
- The 2 alleles of a gene they have are the same.
- 2 alleles of that gene are different.
- Only have 1 allele of a gene on the X chromosome (Males only- hemizygous for all genes on the X chromosome).
Define dominant and recessive alleles.
Dominant= the dominant allele in a heterozygote determines the phenotype. Recessive= the non dominant allele in a heterozygote.
Give an example of co-dominant alleles.
Human ABO blood types.
The human isoglutamin gene codes for glycoproteins on the surface of rbcs.
Alleles A and B are dominant over O, but are not dominant over each other i.e. Are co-dominant….when A and B are co-expressed, it results in a different phenotype(blood gp) to A, B or O.
What is recombination frequency?
How often crossing over between genes is occurring.
Describe features of autosomal recessive inheritance.
- Have to be homozygous for the allele to be affected
- M+F equally affected
- 2 heterozygotes have 1/4 chance having affected offspring
- 2 affected individuals will only have affected offspring
- Disease seems to ‘come out of nowhere’ and can skip generations
E.g. Cystic Fibrosis
Describe features of autosomal dominant inheritance.
- Heterozygotes affected (only need 1 allele for it to be observed in phenotype)
- M+F equally affected
- Rarely found in homozygous state (often fatal in development)
- Affected individuals have 50% chance of having affected offspring
- Every generation affected- cannot skip
E.g. Huntington’s disease
How would you identify X-linked recessive inheritance?
- Hemizygous and homozygous females affected
- More common in males (females require both copies of allele to be mutated)
- Female carrier has 1/2 chance of having affected sons
- Affected males cannot pass on to sons
- Every affected male will have at least a carrier mother
E.g. Haemophilia A
Describe features of X-linked dominant inheritance.
- Heterozygous males and females affected
- Affected males cannot pass on to sons, but will pass on to all daughters
E.g. X-linked hypophosphataemia
Why is Y-linked inheritance so rare?
As the Y chromosome is so small and carries few genes.
Can only be passed from father to son.
Through what line is mitochondrial inheritance passed?
Maternal.
What is polygenic inheritance?
Where more than one gene is involved in producing the phenotype.
Often involves linked genes.
What are ‘linked’ and ‘non-linked’ genes?
- Linked= genes on the same chromosome. Don’t show random assortment at meiosis, but can get recombination
- Non-linked= genes on different chromosome
What does recombination frequency between 2 linked genes depend on?
The distance between the genes.
- genes close together= tightly linked, so less opportunity for chiasmata. Likely to travel together.
- genes far apart almost behave as unlinked genes, so cross over more
When taking a family history, how many generations should you take?
Aim for at least a 3 generation history.
Define the penetrance for autosomal dominant inheritance.
The proportion of heterozygotes for a dominant gene who express a trait, even if mildly.
What is expressivity?
Variation in the severity of the phenotypic features of a particular gene.
Whether someone expresses a genetic mitochondrial disease depends on…..?
The proportion of abnormal mitochondrial DNA to normal DNA.
What does recombination frequency of 2 genes on the same chromosome give you an indication of?
Their closeness.
If closely linked, then lower frequency as less chance of chiasmata forming than if further apart.
