3.4 Inheritance Flashcards
Phenotype
physical characteristic/expression of an organism
Genotype
the combination of alleles which an organism has
Homozygous
two copies of the same allele
Heterozygous
two different alleles (sometimes called a carrier if the recessive allele is disease-causing)
Dominant
an allele which is always expressed when it is present in both homozygous and heterozygous genotypes (uppercase)
Recessive
an allele which is only expressed when two copies are present; only in a homozygous genotype (lowercase)
Mendel’s principles of inheritance
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
Mendel Noticed…
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
independent assortment
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.
complete dominance (Mendelian):
Heterozygous individuals will express a DOMINANT allele
incomplete dominance (non-Mendelian):
Heterozygous individuals will express a BLEND of alleles
Co-dominance (non-Mendelian):
heterozygous individuals will express BOTH alleles equally
Blood type O is known as…
the universal donor because they can donate to anyone.
Blood type AB is known as…
the universal acceptor because they can accept from anyone.
The ABO blood type is controlled by a single gene:
the ABO gene
alleles of ABO gene
i O allele (no antigen is produced)
IA A allele (type “A” antigen is produced)
IB B allele (type “B” antigen is produced)
dominance and codominance in blood type alleles
Dominance: IA and IB are dominant to i.
Co-dominance: IA and IB are both expressed if heterozygous
ii genotype
- no antigen production on RBC
- A and B antibody in plasma
- Blood type O
IAIA and IAi genotype
- A antigen production on RBC
- B antibody in plasma
- Blood type A
IBIB and IBi genotype
- B antigen production on RBC
- A antibody in plasma
- Blood type B
IAIB genotype
- A and B antigen production on RBC
- no antibody in plasma
- Blood type AB
X-chromosome in the non-homologous region:
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.
Examples of sex-linked genetic disorders
hemophilia
colour blindness
How is colour-blindness inherited?
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.
Pedigrees
A pedigree chart displays a family tree, and shows the members of the family who are affected by a genetic trait.
Autosomal Recessive Inheritance Pattern
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
Autosomal Dominant Inheritance Pattern
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!).
X-linked Recessive Inheritance Pattern
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
X-linked Dominant Inheritance Pattern
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
The number of genes present in the human genome along with the fact that most conditions are autosomal recessive:
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.
Cystic Fibrosis (CF)
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).
Huntington’s Disease (HD)
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
Sickle Cell anemia
Another example of codominance.
The mixed phenotype gives protection against malaria, but does not exhibit full-blown sickle cell anemia.
Hemophilia
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.
A mutation
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
Mutagens
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*
Mutations can be classed as being…
beneficial, neutral (due to the degenerate nature of DNA) or harmful.
Most mutations are neutral or harmful.
Mutations that occur in body (somatic cells) remain within the organism while…
Mutations that occur in gametes can be inherited by offspring: this is how genetic diseases arise.
accident at Chernobyl nuclear power station
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.
nuclear bombing of Hiroshima
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
