Genetics

Question 1

What is a gene?

Answer 1

Unit of inheritance, transferred from parent to offspring.

Section of DNA sequence that encodes for a single protein

Question 2

What is the Genome? How many coding genes are in the human genome?

Answer 2

Genome is the entire set of genes in an organism. There are around 20,000 genes that code for proteins.

Question 3

What is a chromosome?

Answer 3

Packaged DNA.

Question 4

How is DNA, the genetic material, stored in the cell?

Answer 4

DNA is stored in the nucleus. The DNA strands wrap around histone proteins, which form into (8 histones) nucelosomes, nucleosomes fold together to form long loops which are then folded together to fibres called chromatin.

Question 5

What is the structure of DNA?

Answer 5

DNA is a polymer made nucleotide monomers. Each nucleotide consists of a nitrogenous base, a deoxyribose sugar and a phosphate group. The sugar and phosphate group build the alternating backbone of the DNA strand which forms a double helical structure with the complementary strand.

Question 6

What are the nitrogenous bases that make up DNA?

Answer 6

Adenosine, Thymine, Guanine and Cytosine

Question 7

What are pyrimidines?

Answer 7

Pyrimidines are the nitrogenous bases of DNA and RNA that consist of a single ringed structure: Thymine, Cytosine and Uracil

Question 8

What is a purine?

Answer 8

Purines are the nitrogenous bases of DNA and RNA that consist of a doubled-ringed structure: Adenosine and Guanine.

Question 9

What is chromatin?

Answer 9

The condensed form of DNA stored in the nucleus while the cell is not dividing.

Question 10

What are the types of chromatin?

Answer 10

Euchromatin and hetero chromatin

Question 11

What is the difference between euchromatin and heterochromatin?

Answer 11

Euchromatin is also known as light chromatin because it absorbs less light. The DNA in euchromatin is not as tightly packaged because these genes need to be available for transcription factors and other molecules to bind to. These are the active genes in the chromosomes so they are more accessible for transcription.

Heterochromatin is also known as dark chromatin. These sequences of DNA are more tightly packaged and therefore absorbs more light because of the tightly packed fibrous structure. The genes in these regions are tightly packed as they do not need to be accessible for transcription. They are not active genes.

Question 12

What is a Karyotype?

Answer 12

The visual representation of an individuals complete set of chromosomes.

Question 13

What dye is used for the staining of karyotypes?

Answer 13

Giesma

Question 14

What cells are chromosomes usually collected from for karyotyping?

Answer 14

White blood cells arrested in metaphase

Question 15

Law of Segregation

Answer 15

during gamete formation (meiosis), the alleles from each gene segregate from each other so that the gamete carries only one allele for each gene. An offspring then receives one allele from each parent organism.

According to this law each gamete has an equal probability of obtaining either allele of the gene.

Question 16

Law of Independent Assortment

Answer 16

genes for different traits can segregate independently during the formation of gametes. Alleles for separate traits are passed independently.

If genes are on the same chromosome they will be linked and therefore will not segregate independently, so will not display classic Mendelian inheritance.

Question 17

Law of Dominance

Answer 17

some alleles are dominant, others are recessive. An organism with at least one dominant allele will display the effect of the dominant allele. Genotype v Phenotype. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it.

Dominant alleles hide the phenotypic traits of the recessive alleles, the phenotypic traits of recessive alleles are only visible in the homozygous form of the gene.

Question 18

Define codominance with respect to Mendelian Genetics?

Answer 18

Different alleles of the same gene can both be expressed which will yield different traits in the individual.

One allele is not dominant over the other.

Question 19

What is a frameshift mutation?

Answer 19

When a nucleotide if inserted or deleted from the DNA sequence that alters the reading frame of the polypeptide chain and changes the codons of all amino acids that are coded for after the insertion or deletion. This will alter the polypeptide produced and affect the protein product.

If 3 nucleotides are added or deleted from the DNA sequence it does not change the sequence just adds or deletes a codon

Question 20

What is a missense mutation?

Answer 20

change in base that codes for a different amino acid - but only affects one amino acid in the polypeptide

Question 21

What is a nonsense mutation?

Answer 21

change of a nucleotide base that alters the codon to code for a stopcodon and results in the translation ending prematurely

Question 22

What is a silent mutation?

Answer 22

a change in nucleotide base in the DNA sequence that does not change the amino acid that is coded for

Question 23

What is a read-through mutation or error?

Answer 23

when a nucleotide is changed that results in the stop codon being changed for a different amino acid and protein synthesis continues

Question 24

What is autosomal dominance? What are some common conditions that follow this inheritance pattern?

Answer 24

When a parent passes down a condition to a child via autosomes. The gene is dominant and therefore will likely be expressed in the offspring if they inherit at least one copy of the gene.

Males and females are equally likely to inherit the gene. Vertically inherited through generations.

Common conditions include: Huntington’s Disease, Marfan’s Syndrome and Polycystic Kidney Disease.

Question 25

What is autosomal recessive inheritance and what are some common conditions associated with this pattern of inheritance?

Answer 25

The passing of a recessive autosomal gene from parent to offspring. Both copies of the abnormal gene must be inherited for the trait to be expressed.

Both parents are carriers of the mutated/recessive gene.

Males and females are equally likely to inherit the gene. Likely to skip generations, unaffected parents and affected children.

Common conditions include: Cystic Fibrosis, Sickle Cell anaemia and Thalassaemia.

Question 26

What are X-linked disorders?

Answer 26

Conditions where the mutant gene is on the X-chromosome but not on the Y chromosome. There are dominant and recessive variants.

Males more likely to be affected because they only have one copy of the X gene whereas females have 2 copies and are more likely to be carriers.

Females will always be carriers of their male parents X linked conditions. We do not see male-male transmission.

Common conditions include: Duchenne’s muscular dystrophy, Fragile X Syndrome and Hemophilia A.

Question 27

What is multifactorial inheritance?

Answer 27

Genetic variation that involves multiple genes and environmental factors. Multifactorial disease can occur in isolation with affected children born to unaffected parents, although familial aggregation is also common.

Environmental factors can increase or decrease the risk of disease. These diseases occur more frequently in a specific ethnic group.

Question 28

How do the nucleotide bases connect, and why are some base pairings stronger than others?

Answer 28

Complementary bases of nucleotides bind together through hydrogen bonds. 2 H bonds form between A-T pairings and 3 H bonds form between G-C pairings therefore G-C base pairings are more stable and less likely to be broken.

Question 29

How do DNA replication occur?

Answer 29

DNA replication occurs in the 5’-3’ direction. DNA strands break apart and the template strand is read by RNAP in the 3’-5’ direction which forms the new strand in the 5’-3’.

Question 30

Is DNA a charged molecule?

Answer 30

Yes, DNA is negatively charged molecule. The phosphate groups in the backbone of DNA give the molecules it’s negative charge.

Question 31

How does the DNA backbone form?

Answer 31

The 2-deoxyribose sugar of the backbone contains 5 carbon molecules. Oxygen atoms in the phosphate groups connect the deoxyribose sugars by binding with the 3rd and the 5th carbons in the molecule.

Question 32

What is a single nucleotide polymorphism (SNP)?

Answer 32

Common genetic variations across populations that can be caused by a single base change or larger changes but do not cause disease.

SNPs can be indicators of resemblance between families or links to genes that cause disease.

Question 33

What are benign variants?

Answer 33

Mutations or changes in genes that do not cause disease

Question 34

List types of pathogenic variations in the DNA sequence that can cause disease?

Answer 34

point mutations, base substitutions, alterations in splice sites, trinucleotide repeats

Question 35

Describe the difference between genetic and somatic mutations?

Answer 35

Mutations can be inherited or occur sporadically.

Genetic mutations are inherited through the germline transmitted to offspring through the gametes. Germline mutations will be present in every cell of the body.

Somatic mutations occur after fertilisation and affect the cell lines after division. Somatic mutations affect one cell and are replicated through cell division. These mutations do not affect the gametes and will not be passed on through generations.

Question 36

Describe 3 common chromosomal abnormalities that consider aneuploidy in autosomes?

Answer 36

Trisomy 21 - Down’s Syndrome
Trisomy 13 - Patau’s Syndrome
Trisomy 18 - Edward’s Syndrome

Question 37

What are the 3 genetics mechanisms that cause Trisomy 21?

Answer 37

Non-disjunction, Robertsonian translocation and mosaicism.

Question 38

Chromosomal rearrangements occur during genetic recombination and can have different affects on the daughter cell. Describe balanced translocation.

Answer 38

exchange of chromosomal material between chromosomes of a similar size so there is no loss of genetic information

Question 39

Chromosomal rearrangements occur during genetic recombination and can have different affects on the daughter cell. Describe unbalanced translocation.

Answer 39

exchange of genetic material during recombination that occurs between chromosomes of varying sizes which can lead to the loss or gain of genetic material - this can result in health problems

Question 40

Chromosomal rearrangements occur during genetic recombination and can have different affects on the daughter cell. Describe Robertsonian translocation.

Answer 40

a chromosomal abnormality that occurs when the long arms, Q arms of two different, acrosomal, chromosomes fuse together.

Only involves chromosomes 13, 14, 15, 21 and 22 because the ends of their short, P, arms, have similar DNA sequences making them more likely to fuse.

Question 41

Chromosomal rearrangements occur during genetic recombination and can have different affects on the daughter cell. Describe reciprocal translocation.

Answer 41

a form of balanced translocation that results in the genes changing locations.

Question 42

Chromosomal rearrangements occur during genetic recombination and can have different affects on the daughter cell. Describe paracentric inversion.

Answer 42

inversion of two regions of the chromosome in the arms of the chromosome – does not involve the centromere. Most of the time a cell can withstand these chromosomal rearrangements because the chromosome can still pair up and loop, it is not stretched across the spindle

Question 43

Chromosomal rearrangements occur during genetic recombination and can have different affects on the daughter cell. Describe pericentric inversion.

Answer 43

inversion of the two regions of the chromosome at either side of the centromere.

Likely to cause chromosome breakage because when the chromosomes try to combine for replication because the complementary regions of DNA are on different side of the centromere. This causes loops to occur in the chromosomes when they try to recombine – chromosome breakage or loss of regions

Question 44

What is the tetralogy of Fallot?

Answer 44

A combination of four heart defects present at birth - PORV

P - pulmonic stenosis
O - overriding aorta
R - right ventricular hypertrophy
V - ventricular septal defect

Can be isolated congential heart malformations or may be part of a wider syndrome

Question 45

What causes DiGeorge’s Syndrome?

Answer 45

Microdeletion of genetic material on chromosome 22

Question 46

What are the symptoms of DiGeorge’s Syndrome?

Answer 46

chromosome 22 -> CATCH 22:

C - cardiac abnormalities
A - abnormal facial features
T - Thymic aplasia (absence of the thymus)
C - cleft palate
H - hypocalcemia

Question 47

What is VATER or VACTERL syndrome?

Answer 47

A group of growth abnormalities that occur in fetal development.

V - vertebral abnormalities
A - anorectal abnormalities
C - Cardiovascular anomalies
TE - Tracheoesophageal fistula
R - Renal abnormalities
L - limb defects

Don’t always see all defects together and doesn’t have high recurrence risk

Question 48

What is CHARGE syndrome?

Answer 48

Sporadic genetic mutation that causes abnormalities in development

C - colomba: missing part of the eye
H - heart defects
A - atresia choanae: narrow nasal passage
R - retardation of growth
G - genital anomalies
E - ear anomalies

Question 49

What gene is associated with CHARGE syndrome?

Answer 49

CHD7 - this gene provides instructions for making proteins that regulate gene expression through chromatin remodeling. Mutations in the gene cause disruption of gene expression.

Question 50

What is a Polygenic Risk Score (PRS)?

Answer 50

Estimates an individuals likelihood of developing a particular condition based on their genome.