Case 13- Genetics Flashcards
X and Y linked disease
Genes can be X-linked or Y-linked. If its Y-linked only men can get it. Even if a condition is autosomal dominant females may have a milder form of the disease as they have an extra X-chromosome.
Hardy-Weinberg principle
Dominant and recessive genes are in equilibrium within a population. The percentage of different alleles remains stable.
Exceptions to the Hardy-Weinberg principle
- Non-random mating
- Mutation- can mean you are less likely to reproduce i.e. a mutation which affects fertility. These alleles will drop in the population.
- Selection pressure- more favourable allele will increase. I.e. carriers for sickle cell anaemia are less likely to die of malaria so the allele increases in the population.
- Small population size- due to random chance. Has a greater effect in smaller populations.
- Gene flow (migration)- families which contain certain alleles may move out of a population.
Where is cystic fibrosis common
In Caucasians in Northern Europe
Where is sickle cell disease common?
Africa
Genetic susceptibility
An increased likelihood of getting a particular disease due to genes
Types of gene inheritance
- Single gene inheritance
- Complex interaction of genes- multiple risk genes, doesn’t guarantee that the person will get the disease.
- Modified by environment- genes plus environmental risk factors can cause the disease.
Gene associated with autism, learning disabilities and epilepsy
Deletion of 1q21.1
Complex interaction of genes
Common complex diseases often have multiple susceptibility genes i.e. type 1 diabetes, type 2 diabetes, hypertension, cancer and Alzheimer’s disease. Low recurrence rate as genes dont guarantee you get the disease. Heavily influences by environmental factors
Unifactorial disease
Unifactorial diseases (simple genetics) are due to the inheritance of one gene, the environment does not play a part. There is a high recurrence risk, of passing the gene to further generations i.e. Downs surgery.
Environmental disease
Some diseases are completely environmental and are not due to genetics i.e. the cold or scurvy
Hardy-Weinberg equations
p+q =1
p^2 + 2pq + q^2 = 1
Explanation for Hardy-Weinberg equations
You use these equations to calculate genotype frequency. For example, if you know aa is 0.01 you can square root it to find out the frequency of P, take it away from 1 then square to find AA. You then take AA and aa away from 1 to find Aa.
When can cells exit the cell cycle
At G0. The cell cycle enables the cell to divide and replicate. Some cells in the adult leave the cell cycle i.e. neurones which are post mitotic.
Stages of mitosis
1) Prophase
2) (Prometaphase)
3) Metaphase
4) Anaphase
5) Telophase
Mitosis- Prophase
Nuclear membrane breaks down. DNA compacts into tight chromosomes. Mitotic spindle starts to form, the centriole extends the microtubules.
Mitosis- Prometaphase
Mitotic spindle start to attach to centromere at the protein complex. Distinct chromosomes visible. The microtubules attached to the chromosomes are called kinetochore. Polar microtubules are not attached to the chromosome.
Mitosis- Metaphase
Mitotic spindle fully attached to the centromeres. Chromosomes line up along the middle
Mitosis- Anaphase
Microtubules shorten pulling the chromosomes towards the centrioles. The mitotic spindle separates the chromatids and pulls them to opposite poles of the cell.
Mitosis- Telophase
Chromosomes have reached the pole of the cell. The nuclear membrane starts to reform, the chromosomes become more diffuse. 2 cells form with a diploid number of chromosomes.
Mitosis- Cytokines
Occurs after mitosis, the DNA is separated into 2 separate cells. The other stages of mitosis are part of the nuclear division stage
Chromosome
One half of a duplicates chromosome
Characteristics of Mitosis
Stages= Prophase, Metaphase, Anophase and Telophase
Number of cell divisions= 1
Number of daughter cells= 2
Role= growth, repair, replace
Amount of genetic material= Diploid (2n)
Genetic composition of daughter cells= Identical (clones)
Crossing over= none
Characteristics of Meiosis
Stages= Prophase, Metaphase, Anophase and Telophase
Number of cell divisions= 2
Number of daughter cells= 4
Role= Gametes
Amount of genetic material= Haploid (n)
Genetic composition of daughter cells= Variable
Crossing over= At Prophase 1
Meiosis
Meiosis is used for the production of gametes i.e. sperm and egg cells. The chromosome number needs to be halved to go from diploid (46) to haploid (23). There are two meiotic divisions, so from 1 cell you form 4 gametes. In egg cells only one of these cells will go on to form a functioning egg, the other 3 form polar bodies which are non-functioning.
Meiosis Prophase 1
1) Nuclear membrane breaks down.
2) DNA compacts into tight chromosomes.
3) Mitotic spindle starts to form, the centriole extends the microtubules.
4) Homologous chromosomes pair up, this is synapsis. Each pair is a bivalent.
5) At the synapsis there may be crossing over of DNA as chromatids break and reconnect to each other, the chiasma is where the crossing over occurs.
6) There is exchange of alleles between maternal and paternal chromosomes, increasing genetic diversity. Called recombination of genes.
Meiosis Prometaphase 1
1) Mitotic spindle start to attach to centromere at the protein complex.
2) Distinct chromosomes visible.
3) The microtubules attached to the chromosomes are called kinetochore.
4) Polar microtubules are not attached to the chromosome.
Meiosis Metaphase 1
1) Mitotic spindle fully attached to the centromeres.
2) Chromosomes lined up along the middle in their homologous pairs.
3) The chromosomes line up in a random assortment (independent segregation), this increases diversity as there is random assortment of maternal and paternal chromosomes into the gamete.
Meiosis 1 Anaphase
Microtubules shorten pulling the chromosomes towards the centrioles. The mitotic spindle pulls the chromosomes to opposite poles of the cell.
Meiosis 1 Telophase
1) Chromosomes have reached the pole of the cell.
2) The nuclear membrane starts to reform, the chromosomes become more diffuse.
3) 2 cells form with half the number of chromosomes but still two copies of each chromosome (chromatids).
How is variation produced in offspring
Independent assortment of homologous chromosomes and crossing over of chromatids produces genetic variation in meiosis and variation in offspring.
Interphase meiosis
Between the two cell divisions of meiosis you have a brief interphase, the cells each have a haploid number of chromosomes
Meiosis 2
- Prophase 2- Centrosomes and centrioles replicate and move to opposite poles of the cells. The nuclear envelope breaks down
- Metaphase 2- Chromosomes line up separately across the equator of the spindle
- Anaphase 2- Centromeres divide and spindle microtubules pull the chromatids to opposite poles.
- Telophase 2- Four haploid daughter cells are formed after cytokenesis. Nuclear envelope re-forms. Chromatids have reached the poles of the spindle.
Products of meiosis
4 daughter cells which are all genetically different
How do errors arise in meiosis
1) Failure in Homologous chromosomes to separate
2) Failure of chromatids to separate
Failure in homologous chromosomes to separate
May be an error in the mitotic spindle. In the first division one cell will have both copies of the chromosome and the other cell will not contain that chromosome. So, your egg cells will contain either 22 or 24 chromosomes.
Failure of chromatids to separate
In the second division the chromatids do not separate. One cell contains 2 chromosome and the other cell has nothing. So, two daughter cells will have the right number of chromosomes, one will have 24, the other will have 22.
Formation of triosmic chromosomes
If an abnormal sperm cell containing 24 chromosomes fertilises a normal egg cell then you will end up with 47 chromosomes. The embryo will be trisomic for a particular homologous pair of chromosomes, 2 paternal chromosomes and one maternal chromosome for a homologous pair.
Formation of Monosomic chromosomes
If an abnormal sperm cell only has 22 chromosomes. The offspring will have 45 chromosomes and be Monosomic for a particular homologous pair of chromosomes.
Outcome of errors in meiosis
1) In both mechanisms, aneuploidy can be produced in both autosomes and sex chromosomes
2) Most embryos that form errors in meiosis spontaneously abort, typically very early before the women realises she is pregnant
Gene
A discrete section of DNA that codes for a specific protein or series of similar proteins
Allele
Different versions of the same gene
Locus
The position of a gene on a chromosome
