3.4 Inheritance

Question 1

Phenotype

Answer 1

physical characteristic/expression of an organism

Question 2

Genotype

Answer 2

the combination of alleles which an organism has

Question 3

Homozygous

Answer 3

two copies of the same allele

Question 4

Heterozygous

Answer 4

two different alleles (sometimes called a carrier if the recessive allele is disease-causing)

Question 5

Dominant

Answer 5

an allele which is always expressed when it is present in both homozygous and heterozygous genotypes (uppercase)

Question 6

Recessive

Answer 6

an allele which is only expressed when two copies are present; only in a homozygous genotype (lowercase)

Question 7

Mendel’s principles of inheritance

Answer 7

Mendel worked with the garden pea, Pisum sativum:

Its seeds were readily available;
Pollination is relatively straight forward and controllable;
Several physical traits of the garden pea are easy to see - pea colour/shape, pod colour/shape, height, flower colour
‘Pure breeds’ created (i.e. offspring always purple-flowered)
Fast reproduction to create large sample s

Question 8

Mendel Noticed…

Answer 8

The “short” phenotype disappeared in the F1 generation and reappeared in the F2 generation

The “short” gene must have been “hiding” → today we use the term recessive

Concluded that traits depend on two “factors” → today we call these alleles

Question 9

independent assortment

Answer 9

means that traits are inherited independently of one another, so the inheritance of one trait doesn’t depend on the inheritance of another

I.e. certain traits are not linked or inherited together (e.g. hair colour is not connected to eye colour; if you have brown hair it doesn’t mean you will have brown eyes)

Due to the random orientation during Metaphase I & II.

Question 10

complete dominance (Mendelian):

Answer 10

Heterozygous individuals will express a DOMINANT allele

Question 11

incomplete dominance (non-Mendelian):

Answer 11

Heterozygous individuals will express a BLEND of alleles

Question 12

Co-dominance (non-Mendelian):

Answer 12

heterozygous individuals will express BOTH alleles equally

Question 13

Blood type O is known as…

Answer 13

the universal donor because they can donate to anyone.

Question 14

Blood type AB is known as…

Answer 14

the universal acceptor because they can accept from anyone.

Question 15

The ABO blood type is controlled by a single gene:

Answer 15

the ABO gene

Question 16

alleles of ABO gene

Answer 16

i O allele (no antigen is produced)
IA A allele (type “A” antigen is produced)
IB B allele (type “B” antigen is produced)

Question 17

dominance and codominance in blood type alleles

Answer 17

Dominance: IA and IB are dominant to i.
Co-dominance: IA and IB are both expressed if heterozygous

Question 18

ii genotype

Answer 18

• no antigen production on RBC
• A and B antibody in plasma
• Blood type O

Question 19

IAIA and IAi genotype

Answer 19

• A antigen production on RBC
• B antibody in plasma
• Blood type A

Question 20

IBIB and IBi genotype

Answer 20

• B antigen production on RBC
• A antibody in plasma
• Blood type B

Question 21

IAIB genotype

Answer 21

• A and B antigen production on RBC
• no antibody in plasma
• Blood type AB

Question 22

X-chromosome in the non-homologous region:

Answer 22

Alleles in this regions are expressed whether they are dominant or recessive, as there is no alternate allele carried on the Y chromosome.

Therefore sex-linked genetic disorders are more common in males.

Question 23

Examples of sex-linked genetic disorders

Answer 23

hemophilia
colour blindness

Question 24

How is colour-blindness inherited?

Answer 24

The red-green gene is carried at locus Xq28.

This locus is in the non-homologous region, so there is no corresponding gene (or allele) on the Y chromosome.
Normal vision is dominant over colour-blindness.

Question 25

Pedigrees

Answer 25

A pedigree chart displays a family tree, and shows the members of the family who are affected by a genetic trait.

Question 26

Autosomal Recessive Inheritance Pattern

Answer 26

In order to be “affected” an individual must have 2 recessive alleles → parents must at least carry the gene.

Affected individuals often skip a generation.

Males and females equally affected

Carriers may be half-shaded. Not all pedigrees will show heterozygous individuals (carriers) as those individuals are not always easily identified

Question 27

Autosomal Dominant Inheritance Pattern

Answer 27

In order to be affected the individual will be affected when they have one or two alleles of the trait → can get the allele from either parent

Affected individuals often in every generation.

Males and females equally affected

Secondly, there will never be unseen carriers (all carriers will exhibit the trait!).

Question 28

X-linked Recessive Inheritance Pattern

Answer 28

Affected males receive defective allele from mother

Affected females receive defective allele from father

Fathers cannot pass down defective allele to son

May skip generations

Males more frequently affected

Question 29

X-linked Dominant Inheritance Pattern

Answer 29

Affected father always has affected daughter

Fathers cannot pass down defective allele to son

Affected individuals often in every generation.

Males and females equally affected

Question 30

The number of genes present in the human genome along with the fact that most conditions are autosomal recessive:

Answer 30

it is unlikely that one parent will have a mutation on a disease related gene, but the probability that both parents have a mutation on the same gene is very small.

Question 31

Cystic Fibrosis (CF)

Answer 31

autosomal recessive genetic disease caused by a mutation on the CFTR gene on chromosome 7.

This mutation causes secretions (e.g. mucus, sweat and digestive juices) to become thicker than normal.

Instead of acting as a lubricant, the secretions block tubes, ducts and passageways, especially in the lungs and pancreas.

Despite therapeutic care lung problems in most CF sufferers leads to long-term digestive and respiratory issues and early death (life expectancy is between 35 and 50 years).

Question 32

Huntington’s Disease (HD)

Answer 32

brain disorder caused by an autosomal dominant allele caused by a mutation of the HTT gene on chromosome 4.

This causes progressive neurodegeneration beginning between the ages of 30 to 50.

Life expectancy is ~20 years after the onset of symptoms.

Due to the late-onset, many parents have already had children before they become symptomatic.

This gene is extremely rare, most affected individuals are heterozygous

Question 33

Sickle Cell anemia

Answer 33

Another example of codominance.

The mixed phenotype gives protection against malaria, but does not exhibit full-blown sickle cell anemia.

Question 34

Hemophilia

Answer 34

Results from a lack of clotting factors. These are globular proteins, which act as enzymes in the clotting pathway.

A recessive X-linked mutation in hemophiliacs results in one of these clotting factors not being produced.

Therefore, the clotting response to injury does not work and the patient can bleed to death.

Question 35

A mutation

Answer 35

change in an organisms genetic code.

A Gene mutation is a change in the nucleotide sequence of a section of DNA coding for a particular feature

Question 36

Mutagens

Answer 36

agents that cause gene mutations such as:

chemicals that cause mutations (like some found in tobacco smoke) are referred to as carcinogens

high energy radiation such as X-rays

ultraviolet light

Some viruses*

Question 37

Mutations can be classed as being…

Answer 37

beneficial, neutral (due to the degenerate nature of DNA) or harmful.

Most mutations are neutral or harmful.

Question 38

Mutations that occur in body (somatic cells) remain within the organism while…

Answer 38

Mutations that occur in gametes can be inherited by offspring: this is how genetic diseases arise.

Question 39

accident at Chernobyl nuclear power station

Answer 39

Radioactive isotopes released into the environment exposing humans and other organisms to potentially dangerous levels of radiation.

A large area of pine forest downwind of the reactor turned brown and died.

Horses and cattle near the plant died from radiation damage to their thyroid glands.

Bioaccumulation of radioactive caesium in fish (Scandinavia and Germany) and lamb (Wales) - contaminated meat was banned from sale for years afterward.

Drinking water (and milk) contaminated with radioactive iodine - at least 6,000 thyroid cancer attributed to radioactive iodine.

No clear evidence to support an increase in the rate of leukemia other cancers – in part due to the widely dispersed variable radiation and measures taken in European populations.

Question 40

nuclear bombing of Hiroshima

Answer 40

Elevated rate of Leukemia (with the greatest impact in children and young adults)

Elevated rates of other cancers

No evidence of stillbirth or mutations in the children of those exposed to radiation