Case 13- Genetics

Question 1

X and Y linked disease

Answer 1

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.

Question 2

Hardy-Weinberg principle

Answer 2

Dominant and recessive genes are in equilibrium within a population. The percentage of different alleles remains stable.

Question 3

Exceptions to the Hardy-Weinberg principle

Answer 3

• 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.

Question 4

Where is cystic fibrosis common

Answer 4

In Caucasians in Northern Europe

Question 5

Where is sickle cell disease common?

Answer 5

Africa

Question 6

Genetic susceptibility

Answer 6

An increased likelihood of getting a particular disease due to genes

Question 7

Types of gene inheritance

Answer 7

• 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.

Question 8

Gene associated with autism, learning disabilities and epilepsy

Answer 8

Deletion of 1q21.1

Question 9

Complex interaction of genes

Answer 9

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

Question 10

Unifactorial disease

Answer 10

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.

Question 11

Environmental disease

Answer 11

Some diseases are completely environmental and are not due to genetics i.e. the cold or scurvy

Question 12

Hardy-Weinberg equations

Answer 12

p+q =1

p^2 + 2pq + q^2 = 1

Question 13

Explanation for Hardy-Weinberg equations

Answer 13

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.

Question 14

When can cells exit the cell cycle

Answer 14

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.

Question 15

Stages of mitosis

Answer 15

1) Prophase
2) (Prometaphase)
3) Metaphase
4) Anaphase
5) Telophase

Question 16

Mitosis- Prophase

Answer 16

Nuclear membrane breaks down. DNA compacts into tight chromosomes. Mitotic spindle starts to form, the centriole extends the microtubules.

Question 17

Mitosis- Prometaphase

Answer 17

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.

Question 18

Mitosis- Metaphase

Answer 18

Mitotic spindle fully attached to the centromeres. Chromosomes line up along the middle

Question 19

Mitosis- Anaphase

Answer 19

Microtubules shorten pulling the chromosomes towards the centrioles. The mitotic spindle separates the chromatids and pulls them to opposite poles of the cell.

Question 20

Mitosis- Telophase

Answer 20

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.

Question 21

Mitosis- Cytokines

Answer 21

Occurs after mitosis, the DNA is separated into 2 separate cells. The other stages of mitosis are part of the nuclear division stage

Question 22

Chromosome

Answer 22

One half of a duplicates chromosome

Question 23

Characteristics of Mitosis

Answer 23

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

Question 24

Characteristics of Meiosis

Answer 24

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

Question 25

Meiosis

Answer 25

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.

Question 26

Meiosis Prophase 1

Answer 26

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.

Question 27

Meiosis Prometaphase 1

Answer 27

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.

Question 28

Meiosis Metaphase 1

Answer 28

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.

Question 29

Meiosis 1 Anaphase

Answer 29

Microtubules shorten pulling the chromosomes towards the centrioles. The mitotic spindle pulls the chromosomes to opposite poles of the cell.

Question 30

Meiosis 1 Telophase

Answer 30

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).

Question 31

How is variation produced in offspring

Answer 31

Independent assortment of homologous chromosomes and crossing over of chromatids produces genetic variation in meiosis and variation in offspring.

Question 32

Interphase meiosis

Answer 32

Between the two cell divisions of meiosis you have a brief interphase, the cells each have a haploid number of chromosomes

Question 33

Meiosis 2

Answer 33

• 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.

Question 34

Products of meiosis

Answer 34

4 daughter cells which are all genetically different

Question 35

How do errors arise in meiosis

Answer 35

1) Failure in Homologous chromosomes to separate

2) Failure of chromatids to separate

Question 36

Failure in homologous chromosomes to separate

Answer 36

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.

Question 37

Failure of chromatids to separate

Answer 37

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.

Question 38

Formation of triosmic chromosomes

Answer 38

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.

Question 39

Formation of Monosomic chromosomes

Answer 39

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.

Question 40

Outcome of errors in meiosis

Answer 40

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

Question 41

Gene

Answer 41

A discrete section of DNA that codes for a specific protein or series of similar proteins

Question 42

Allele

Answer 42

Different versions of the same gene

Question 43

Locus

Answer 43

The position of a gene on a chromosome