Genetics And Genetics In Practise

Question 1

1- describe the structure of DNA

Answer 1

Deoxyribose (sugar)
Nucleic (nucleotides)
Acid (phosphate)

Nucleotides ate made up of 4 nitrogenous bases (A-T, C-G)
Nucleotides are joined together by sugar phosphate backbone
There are two strands of DNA held together by weak hydrogen bonds
-A and T are bonded by two hydrogen bonds
-C and G are bonded using three hydrogen bonds

Question 2

1- describe the function of DNA

Answer 2

To store all of the genetic information that an organism needs to develop

Sequences convert to messages, messages produce proteins, proteins are complex molecules that carry out most of the work in our bodies

Question 3

1- describe the structure of RNA

Answer 3

Ribo (sugar)
Nucleic (nucleotide)
Acid (phosphate)

Single stranded (unlike DNA which is double)
Shorter chain of nucleotides (unlike DNA which is longer)
Nucleic base pairs are different, RNA contains URACIL which pairs with Adenine- RNA hasn’t got any thymine!

Question 4

1- describe the function of RNA

Answer 4

Involved in regulating gene expression
So the DNA contains the genetic material, which is then used to make the RNA and allow it to work with proteins carry out functions in the human body

Question 5

1- describe the structure of genes

Answer 5

Genes are groupings of DNA

Genetic code is based on 3 base groupings called CODONS
E.g. ATT, GGC, GTA
Each codon codes for a PROTEIN

Question 6

1- describe the function of genes

Answer 6

Each codon within a gene codes for a specific protein
This is done through a process of transcription (decoding)
Once decided they go through transcription (become a protein)
Proteins carry out jobs within the body

Full set of codons = the genetic code/genome/25,000 genes

Genes construct characteristics in humans

Question 7

1- describe the structure of chromosomes

Answer 7

Tightly coiled strand of DNA around a histone

Cells in the body contain 46 chromosomes (23 pairs)- DIPLOID cells
Sex cells only contain 1 copy of each chromosome, they aren’t paired- HAPLOID cells

Question 8

1- describe the function of chromosomes

Answer 8

To carry DNA
Transfer genetic information from parents to their offspring
Allow the DNA to be accurately copied
Stop the DNA from getting tangled

Question 9

2- what is an allele?

Answer 9

A variant form of a gene
One of two versions of DNA at one genomic location
An offspring inherits two alleles, one from each parent for each genomic location

Identical alleles= homozygous
Different alleles= heterozygous

Question 10

2- what is a locus?

Answer 10

The physical location of a specific gene or chromosome

“Humans are diploid organisms because they have two alleles at each genetic locus”

Question 11

2- what is genotype?

Answer 11

The genetic makeup of an organism
Determines/contributes to phenotype
Represented in symbols, e.g. BB, Bb, bb

Question 12

2- what is phenotype?

Answer 12

The observable characteristics of an individual, result from expression of genes
E.g. blue eyes, brown hair, Huntington’s disease

Question 13

2- what is homozygous?

Answer 13

When two alleles are the same
E.g. BB, bb

Question 14

2- what is heterozygous?

Answer 14

When two alleles are different
E.g. Bb

Question 15

2- what is dominance?

Answer 15

When one allele of a gene, on one copy of chromosomes, overrides the impact of the other allele

Question 16

2- what is co-dominance?

Answer 16

A type of inheritance where two versions of an allele of the same gene are fully expressed in the phenotype
E.g. flower alleles for colour gene are Rr, the flowers phenotype will be red and white!

Question 17

2- what is recessive?

Answer 17

A trait only expressed when a genotype is homozygous, it is always masked by a dominant gene
E.g. bb

Question 18

2- what are homologous chromosomes?

Answer 18

Pairs of matching chromosomes in an organism, where one comes from each parent
Same gene sequence, loci and length
E.g. the sex chromosomes XX are homologous (XY are not)

Question 19

2- what is a mutation?

Answer 19

An alteration of bases in the genetic pair
The outcome is that the proteins transcribed are malformed

Question 20

2- what are autosomes?

Answer 20

Any chromosome that is not a sex chromosome
The gene in question is located on one of the numbered, non sex chromosomes

Question 21

2- what are sex chromosomes?

Answer 21

One pair (of the 23 pairs) of chromosomes that determine the biological sex of an individual
XX= female
XY= male

Question 22

2- what is karyotype?

Answer 22

The whole collection of an individuals chromosomes (the whole 46 pairs)

It is used as a lab technique, we can screen an individuals karyotype and look for abnormal numbers or structure of chromosomes to diagnose genetic diseases

Question 23

3- draw a punnet square to indicate the genotypic and phenotypic traits to demonstrate the inheritance of autosomal dominant traits in HUNTINGTONS
E.g. mother- Hh, father- hh

Question 24

3- draw a punnet square to indicate the genotypic and phenotypic traits to demonstrate the inheritance of autosomal recessive traits in PKU
E.g. mother- Pp, father- Pp

Question 25

3- draw a punnet square to indicate the genotypic and phenotypic traits to demonstrate the inheritance of sex-linked traits in HAEMOPHILIA A
E.g. mother- XhXH, father- XHY

Question 26

3- what is meant by autosomal dominant (single gene) diseases? Give some examples

Answer 26

The more dominant gene is the abnormal gene
E.g. Huntington’s disease (HH or Hh), achondroplasia (AA or Aa)

Question 27

3- what is meant by autosomal recessive (single gene) diseases? Give some examples

Answer 27

The abnormal gene requires two copies to cause the disorder, as it is the recessive allele that carries it
E.g. cystic fibrosis (cc), sickle cell anaemia (ss)

Question 28

3- what is meant by sex-linked diseases? Give some examples of ‘X linked recessive’, X linked dominant’ and ‘Y linked’

Answer 28

Diseases passed down through families via one of the sex chromosomes, either X or Y chromosome

X linked recessive: haemophilia, colour blindness
X linked dominant: Fragile X syndrome
Y linked: male infertility

Question 29

3- what is meant by ‘co-dominance’ inheritance? (An example of complex inheritance)

Answer 29

Some genes don’t show complete dominance, instead they share the phenotype between two alleles
E.g. Flower- Rr, it will be red and white in phenotype because both alleles are ad dominant as each other

Question 30

3- what is meant by ‘multiple allele’ inheritance? (An example of complex inheritance)

Answer 30

Some genes have more than two alternate forms
E.g. ABO blood groups

Question 31

3- what is meant by ‘polygenic’ inheritance? (An example of complex inheritance)

Answer 31

Several genes responsible for one trait
E.g. eye colour, hair colour

Diseases like cancers and heart disease are an example of this

Question 32

3- what is meant by ‘multifactoral’ inheritance? An example of complex inheritance

Answer 32

A complex interaction of genes with the environment causes some diseases
E.g. skin cancer- sunlight and genes

Question 33

4- discuss co-dominance in relation to the ABO blood group

Answer 33

ABO blood groups are an example of multiple allele (more than two alternate forms) and co-dominance inheritance (phenotype is shared between the alleles)

There are 4 blood types (phenotypes)
They result from 6 possible combinations (IAIA, IAi, IBIB, IBi, IAIB, ii)
Of 3 different alleles (IA, IB, i)
Of a single gene (I)

Question 34

4- for the genotype IAIA or IAi, what would the phenotype be?

Answer 34

Blood type A

Question 35

4- for the genotype IBIB or IBi, what would the phenotype be?

Answer 35

Blood type B

Question 36

4- for the genotype IAIB, what would the phenotype be?

Answer 36

Blood type AB

Question 37

4- for the genotype ii, what would the phenotype be?

Answer 37

Blood type O

Question 38

4- could a mother of blood group A and a father of blood group B have a child with blood group O?

Answer 38

Yes, if they were both heterozygous

Question 39

4- draw a punnet square to demonstrate the potential phenotype inheritance of offspring from a mother with blood group A heterozygous (IAi) and father of blood group B heterozygous (IBi)

Question 40

5- briefly differentiate between chromosome, single gene, polygenic and multifactoral disorders

Answer 40

Chromosomal disorder-
Result from changes in the number or structure of the chromosomes

Single gene disorder-
Caused by mutations in the DNA sequence of one specific gene

Polygenic disorder-
Disorders that arise from the mutations on more than one gene

Multifactorial disorder-
Associated with the effects of multiple genes interacting with the environment

Question 41

6- what is the difference between mitosis and meiosis?

Answer 41

MITOSIS
Occurs in all organisms (except viruses)
Creates all body cells
Involves four cell divisions
Produces two diploid daughter cells (2n)
Daughter cells are genetically identical
Creates a human cell with 46 chromosomes

MEIOSIS
Occurs only in plants, animals and fungi
Creates only sex cells
Involves two successive cell divisions
Produces four haploid daughter cells (n)
Daughter cells are genetically different
Creates a human cell with 23 chromosomes

Question 42

7- what are the symbols called in a family tree? Try and draw/remember as many as you can

Answer 42

Pedigree symbols

Question 43

8- name some antenatal screening tests used to identify genetic conditions

Answer 43

The NHS offers screening tests to all pregnant women in order to detect any of the five most common chromosomal disorders
This is done through a blood test where they will find three copies of a chromosome instead of two

If abnormalities are found in this test, mothers will be offered:
‘CVS’ or ‘amino’ tests which will give a definitive answer
CVS= chorionic villus sampling. Takes a sample of placental tissue to test
Amino= removing and testing a small sample of cells from amniotic fluid

Ultrasound scans are also offered at 12 and 20 weeks gestation
They show images of a baby’s body inside and out. It can’t detect genetic abnormalities but may show physical abnormalities that can indicate towards a genetic abnormality.
Unexpected findings will lead to more invasive investigations like CVS or amino screening

Question 44

9- name some multifactoral genetic disorders and briefly describe them

Answer 44

Type 2 diabetes- Genes, low exercise, poor diet
Heart disease- Genes, smoking, fatty diet, low exercise levels, undernourishment
Depression- Genes, lack of sunlight, bereavement, unemployment, undernourishment
Multiple sclerosis- Genes, smoking, lack of sunlight, poor diet

Question 45

10- what is the role of a genetic counsellor and specialist service?

Answer 45

Genetic counsellors work with patients in order to offer genomic information and support. This allows the patient to make better health decisions

Jobs:
Ordering and recommending genetic tests
Clarify health risks
Support and empower
Work with scientists to diagnose, manage and predict conditions
Helping patients to interpret test results
Work as an MDT (midwives, cancer teams, neurology, ophthalmology etc)