UNIT 3.4

Question 1

Mendels Law

Which Laws:
Mendelian Ratios:

Answer 1

Father of genetics → performed experiments on a variety of different pea plants → discovered principles of inheritance

Law of segregation: Genes come in pairs and are inherited as distinct units, one from each parent

Principle of dominance: recessive alleles will be masked by dominant alleles

Non-mendelian ratios:
*Co-dominance of specific alleles eg: pink flowers from red and white ones
*Sex-linked effects eg: color blindness
*Environmental influence on inheritance

Question 2

Haploid Cells

Answer 2

Gametes are haploid and contain one copy of each chromosome, therefore one allele of each gene
Formed by the process of meiosis – males produce sperm and females produce ova
The two alleles of each gene separate into different haploid daughter nuclei during meiosis
As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs

Question 3

Fusion of Gametes

Answer 3

When male and female gametes fuse during fertilization → form a zygote (diploid) containing two alleles for each gene (can be the same or different alleles)
For any given gene, the combination of alleles can be categorized as follows
If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene
If the maternal and paternal alleles are different, the offspring is said to be heterozygous for that gene

Exception: Males have only one allele for each gene located on a sex chromosome, as these chromosomes aren’t paired (XY)

Question 4

Models of Inheritance

Phenotype
Genotype

Answer 4

Proteome (total number of proteins) is larger than genotype

Genotype
*Allele combination: homozygous or heterozygous
Eg: bb or BB or Bb

Phenotype
*Physical expression: determined by both the genotype and environmental influences
Eg: green eyes, brown eyes

Question 5

Dominant Allele

Answer 5

Dominant allele – Allele that is expressed in the heterozygous genotype

Heterozygous state: if present it will mask the recessive allele
Eg: BB or Bb → brown eyes

Question 6

Recessive Allele

Answer 6

Recessive allele – Allele that is not expressed in a heterozygous genotype

Only being present in a homozygous state of two recessive
Eg: bb → blue eyes

Question 7

CO-Dominance (punnet square)

*Check Good Notes for Diagrams

Answer 7

Co-dominant alleles – Pairs of alleles which are both expressed equally in the phenotype of a heterozygous individual (they have a joint effect)

Flower Petals:

Parent Genotypes: CRCW x CRCW
Parent Phenotype: Pink x Pink

Offspring Genotype Ratio: 1:2:1 (CRR:CRw: CWW)
Offspring Phenotype Ratio: 1:2:1 (Red: Pink: White)

Question 8

Inheritance of ABO Blood Groups - Multiple Alleles

A B
O
Blood Transfusions
Glycoproteins

Answer 8

Controlled by a single gene with multiple alleles (A,B,O)

*A and B alleles are codominant and each modifies the structure of the antigen to produce different variants

*O allele is recessive and does not modify the basic antigen structure

As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups

Consequences: agglutination (clumping) –> haemolysis

The glycoproteins (found in the cell membrane are the markers or sensors)

Question 9

Inheritance of ABO Blood Groups - Multiple Alleles (AB)

Answer 9

Can receive: any other type
Because: they already possess both antigenic variants on their cells)

(iA iB)

Question 10

Inheritance of ABO Blood Groups - Multiple Alleles (A)

Answer 10

Cannot receive: B blood of AB blood
Because: the isoantigen produced by the B allele is foreign
(iAiA or iAi)

Question 11

Inheritance of ABO Blood Groups - Multiple Alleles (B)

Answer 11

Cannot receive: A blood or AB blood
Because: as the isoantigen produced by the A apple is foreign
(iBiB or iBi)

Question 12

Comparison of Predicted and Actual Outcomes of Genetic Crosses

Answer 12

The genotypic and phenotypic ratios calculated via Punnett grids are only probabilities and may not always reflect actual trends

When comparing predicted outcomes to actual data, larger data sets are more likely to yield positive correlations

Question 13

Inheritance of ABO Blood GRoups - Multiple Alleles (O)

Answer 13

Can receive: only from O blood
Can donate: Universal
Because: both antigenic (A and B) variants are foreign
(ii)

Question 14

Gene Diseases

Somatic Mutations
Germline Mutations

Answer 14

Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function → development of a disease phenotype

Somatic mutations → occur in non-germline tissues → cannot be inherited → eg: breast tumour

Germline mutations → present in egg or sperm → can be inherited

Question 15

Sickle Cell Anaemia - Codominant

Allele Nature
Combinations
Location of Mutation
Sex Linked
Symptoms

Answer 15

A codominant genetic disease only requires one copy of the faulty allele to occur (eg: sickle cell anaemia)

Heterozygous individuals will have milder symptoms due to the moderating influence of a normal allee

Allele Nature: Co-Dominant

Combinations:
HbA → Normal Haemoglobin
HbS → Allele for sickle Hemoglobin

HbAHbA → Homozygous HbA → Normal
HbAHbS → Heterozygous → Carrier → Malaria protection
HbSHbS→ Homozygous HbS → Sickle Cell Disease → Malaria protection

Location of Mutation:

HBB genes on chromosome 11
GAG mutated into GTG

Sex Linked: NO

Symptoms: Clots in blood vessels (capillaries because of their abnormal shape
Immune to malaria

Question 16

Cystic Fibrosis - Autosomal Recessive

Allele Nature
Combinations
Location of Mutation
Sex Linked
Symptoms

*Check Good Notes for Diagrams

Answer 16

An autosomal recessive genetic disease will only occur if both alleles are faulty (eg: cystic fibrosis)

Carrier: the heterozygous individual will possess one copy of the faulty allele but not develop disease symptoms

Allele Nature: Autosomal Recessive

Combinations:
CC → Normal
Cc → Normal carrier
cc → cystic fibrosis

Location of Mutation: CFTR gene on chromosome 7

Sex Linked: No

Symptoms: Causes secretion of mucus to become very thick. The thick mucus blocked the airway tubes, especially in the lungs.

Question 17

Phenylketonuria (PKU)

Allele Nature
Combinations
Location of Mutation
Sex Linked
Symptoms

Answer 17

Allele Nature: Autosomal Recessive

Combinations:
TT → normal
Tt → carrier
tt → PKU

Location of Mutation: (PAH), found on chromosome 12

Sex Linked: No

Symptoms:
Mutation due to base substitution where there is a change in a single base results in a different amino acid
- tyrosine
- hydroxylase

phenylalanine cannot be converted to tyrosine in the body, making it to build up
- Developmental problems
- Seizures

Question 17

Huntington’s Disease

Allele Nature
Combinations
Location of Mutation
Sex Linked
Symptoms

*Check Good Notes for Diagrams

Answer 17

An autosomal dominant genetic disease requires one copy of a faulty allele to cause the disorder (eg: Huntington’s disease)
Homozygous dominant and heterozygous individuals will both develop a full range of disease symptoms

Allele Nature: Dominant

Combinations:
Hh → Huntington’s disease
hh → normal

Location of Mutation: HTT gene on chromosome 4

Sex Linked: No
Symptoms: Neuron degeneration will lead to brain disorder, affecting the ability to think, talk and move

Question 18

Sex Linked Diseases
*Check Good Notes for Diagrams

Answer 18

Due to mutations in the base substitutions
When a gene controlling a characteristic is located on sex chromosomes (X or Y)
*The Y chromosome is much shorter than the X chromosome and contains only a few genes
*The X chromosome is longer and contains many genes not present on the Y chromosome

Sex-linked conditions are usually Y-linked
*They are more common in males as there is only one X chromosome, so any allele present in the non-homologous part of the X chromosome will be expressed
*Females have 2 X chromosomes → 2 alleles → X-linked dominant traits are more common in females
*Human males have only 1 X chromosome → 1 allele → X-linked recessive traits are more common → they cannot be masked by a second allele → sex-linked disease is more common in men

Only females can be carriers

Question 19

Red Green ColorBlindness

Allele Nature
Combinations
Location of Mutation
Sex Linked
Symptoms

Answer 19

Allele Nature: Recessive

Combinations:
XA → unaffected (normal vision)
Xa → affected (color blindness)

Location of Mutation: Mutation to the red or green retinal photoreceptors
X chromosome

Sex Linked: Yes

Symptoms: Failure to distinguish between red and blue. Loss of certain frequencies of light

Question 20

Hemophilia

Allele Nature
Combinations
Location of Mutation
Sex Linked
Symptoms

Answer 20

Allele Nature: Recessive

Combinations:
XH → unaffected (normal blood clotting)
Xh → hemophilia

Location of Mutation: X chromosome

Sex Linked: Yes

Symptoms:
Clotting response to injury does not work
The patient may bleed to death

Question 21

Pedigree Chart

Sex Linked, Recessive
Autosomal, Recessive
Autosomal, Dominan

*Check Good Notes for Diagrams

Answer 21

Males → squares
Women → circles
Shaded symbols → individual affected by a condition
Unshaded symbols → individual not affected

*Sex Linked, Recessive
- Affects only one gender (males are predominantly affected)
- It is able to skip generations

*Autosomal, Recessive:
- Affects males and females
- Less population with disease than with disease

*Autosomal, dominant
- Affects women and male
- More people with disease than without the disease
- Present in all generations

Question 22

Radiation and Causes of Mutation: Mutagens

Answer 22

Mutagens: Mutagens are agents that cause or increase the frequency of mutations by altering the genetic material of an organism.

They include various types of radiation and chemicals.

Exposure to mutagens can result in the formation of new alleles, some of which may be harmful and contribute to the development of genetic diseases or cancer.

Question 23

Radiation and Causes of Mutation: Chemical Mutagens

Answer 23

Chemical Mutagens: Chemical mutagens are substances that can trigger changes in an organism’s genetic material.

Examples are workplace chemicals, cigarette smoke, and specific additives like nitrites present in cured meats.

These chemicals can increase the mutation rate and contribute to the development of genetic diseases and cancer.

Question 24

Radiation and Causes of Mutation: Radiation

Answer 24

Radiation: Radiation refers to the emission of energy in the form of waves or particles.

Certain forms of radiation, such as ionizing radiation, have sufficient energy to break chemical bonds, including those in DNA.

Sources of radiation that can cause DNA damage include ultraviolet (UV) light, X-rays, and various types of radiation emitted during the decay of radioactive elements.

Question 25

Radiation and Causes of Mutation: Impact of Nuclear Disasters

Long term concequences

Answer 25

Chernobyl and Hiroshima

Long Term Concequences of Exposure to these
1) An increased incidence in cancer development (with a strong correlation between dose of radiation and frequency of cancer)
2) Reduced T cell counts and altered immune functions, leading to higher rates of infection
3) A wide variety of organ-specific health effects (e.g. liver cirrhosis, cataract induction, etc.)

Question 26

Radiation and Causes of Mutation: Chernobyl

Answer 26

The Chernobyl accident occurred in April 1986, when an explosion at the reactor core caused the release of radioactive material

The Chornobyl meltdown involved far more fissionable material and produced different isotopes with much longer half-lives

*Thyroid disease was a common consequence of the Chernobyl accident due to the release of radioactive iodine
*Estimated 250% increase in congenital abnormalities following the Chernobyl meltdown
*Certain Regions of Chornobyl remain unsafe for human habitation
*There is anecdotal evidence to suggest that radiation levels around Chernobyl have caused variations in local flora and fauna
*The presence of residual radiation in the environment can become concentrated in organisms via bioaccumulation

Question 27

Radiation and Causes of Mutation: Hiroshima

Answer 27

The nuclear bombing of Hiroshima (and Nagasaki) occurred in August 1945, during the final stages of World War II

The Hiroshima nuclear bomb was detonated above ground and radiation was dispersed, resulting in less irradiation of the soil

*There was no significant increase in birth defects following the Hiroshima bombing
*Still habitable and well populated, certain regions