3.4 Inheritance Flashcards
Who was Gregor Mendel?
Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants
What was his first experiment?
First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
What did he do with the F1 generation?
Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
How many times did mendel perform his experiments?
These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)
What did Mendel find out about breeding purebred varieties?
When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants
What did Mendel find out regarding self-fertilisation?
When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1
What did Mendel find out about genes?
Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)
What did Mendel find out about alleles?
Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)
What did Mendel find out about gametes?
Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)
What did Mendel find out about parental contribution?
Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
What did Mendel find out about dominant/recessive?
For each factor, one version is dominant over another and will be completely expressed if present
What are Mendel’s 3 rules?
- Law of Segregation
- Law of Independent Assortment
- Principle of Dominance
What is the law of segregation?
Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
What is the law of independent assortment?
The segregation of alleles for one gene occurs independently to that of any other gene*
The law of principle dominance?
Recessive alleles will be masked by dominant alleles†
What is the exception to the law of independent assortment?
The law of independent assortment does not hold true for genes located on the same chromosome (i.e. linked genes)
What is the exception to the principle of dominance?
Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance
What are gametes?
Gametes are haploid sex cells formed by the process of meiosis – males produce sperm and females produce ova
What happens to homologous chromosomes in meiosis I?
During meiosis I, homologous chromosomes are separated into different nuclei prior to cell division
What does the separation of homologous chromosomes also segregate?
As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs
What do gametes carry?
Consequently, as gametes contain only one copy of each chromosome they therefore carry only one allele of each gene
What are gametes categorised as?
Gametes are haploid, meaning they only possess one allele for each gene
What will occur when gametes fuse, due to them being haploid?
When male and female gametes fuse during fertilisation, the resulting zygote will contain two alleles for each gene
What is the exception for gametes fusing and producing a zygote with two alleles for EACH GENE?
Males have only one allele for each gene located on a sex chromosome, as these chromosomes aren’t paired (XY)
What does it mean if offspring is homozygous for a gene?
If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene
What does it mean if offspring is heterozygous for a gene?
If the maternal and paternal alleles are different, the offspring is said to be heterozygous for that gene
What does it mean if offspring is hemizygous for a gene?
Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene
What is the genotype?
The gene composition (i.e. allele combination) for a specific trait is referred to as the genotype
What can the genotype be categorised as?
The genotype of a particular gene will typically be either homozygous or heterozygous
What is the phenotype?
The observable characteristics of a specific trait (i.e. the physical expression) is referred to as the phenotype
What determines the phenotype?
The phenotype is determined by both the genotype and environmental influences
What is complete dominance?
Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other
What allele is expressed in a heterozygous individual?
The dominant allele will mask the recessive allele when in a heterozygous state
Can homozygous dominant and heterozygous be distinguished in terms of phenotype?
Homozygous dominant and heterozygous forms will be phenotypically indistinguishable
When will the recessive allele be expressed?
The recessive allele will only be expressed in the phenotype when in a homozygous state
What allele is capitalised?
When representing alleles, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele
An example of this mode of inheritance is mouse coat colour – black coats (BB or Bb) are dominant to brown coats (bb)
What is codominance?
Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual
What genotype is different, due to co-dominance?
Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect
How are codominant alleles written?
When representing alleles, the convention is to use superscripts for the different co-dominant alleles (recessive still lower case)
An example of co-dominance is feathering in chickens – black (CB) and white (CW) feathers create a speckled coat (CBCW)
How can human red blood cells be categorised?
Human red blood cells can be categorised into different blood groups based on the structure of a surface glycoprotein (antigen)
What are the ABO blood groups controlled by?
The ABO blood groups are controlled by a single gene with multiple alleles (A, B, O)
What do all the ABO alleles produce?
The A, B and O alleles all produce a basic antigen on the surface of red blood cells
Which blood group alleles are co-dominant?
The A and B alleles are co-dominant and each modify the structure of the antigen to produce different variants
Which blood group allele is recessive?
The O allele is recessive and does not modify the basic antigenic structure
What letters are used to represent the blood group alleles?
When representing blood group alleles, the letter I is used to represent the different antigenic forms (isoantigens)
A allele = IA ; B allele = IB ; O allele = i (recessive)
What do different blood groups, therefore, imply about transfusions?
As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups
From what groups can an individual with blood group AB receive?
AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)
From what groups can an individual with blood group A receive?
A blood groups cannot receive B blood or AB blood (as the isoantigen produced by the B allele is foreign)
From what groups can an individual with blood group B receive?
B blood groups cannot receive A blood or AB blood (as the isoantigen produced by the A allele is foreign)
From what groups can an individual with blood group O receive?
O blood groups can only receive transfusions from other O blood donor (both antigenic variants are foreign)
What does a monohybrid cross determine?
A monohybrid cross determines the allele combinations for potential offspring for one gene only
What is the first step of carrying out a monohybrid cross?
Step 1: Designate letters to represent alleles (dominant = capital letter ; recessive = lower case ; co-dominant = superscript)
What is the second step of calculating a monohybrid cross, after designating letters?
Step 2: Write down the genotype and phenotype of the prospective parents (this is the P generation)
What is the third step of calculating a monohybrid cross, after genotype and phenotype?
Step 3: Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)
What is the fourth step of calculating a monohybrid cross, after gametes?
Step 4: Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)
What is the fifth step of calculating a monohybrid cross, after drawing the Punnett grid?
Step 5: Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (this is the F1 generation)
What does the punnett grid provide us with?
The genotypic and phenotypic ratios calculated via Punnett grids are only probabilities and may not always reflect actual trends
E.g. When flipping a coin there is a 50% chance of landing on heads – this doesn’t mean you will land on heads 50% of the time
What evidence supports Mendel’s findings (state experiment)?
Mendel crossed different varieties of pea plants and recorded the characteristics of resultant offspring
Initially, he crossed purebred dominant and purebred recessive plants in order to produce heterozygotes (F1 generation)
He then self-pollinated the heterozygotes to produce an F2 generation and counted the dominant and recessive phenotypes
The expected ratio of dominant : recessive phenotypes was 3 : 1 – this ratio was supported by the experimental data
- more data, more likely to get closer to this ratio
When are genetic diseases caused?
Genetic diseases are caused when mutations to a gene (or genes) abrogate normal cellular function, leading to the development of a disease phenotype
What alleles can cause genetic diseases?
Genetic diseases can be caused by recessive, dominant or co-dominant alleles
When will an autosomal recessive genetic disease occur?
An autosomal recessive genetic disease will only occur if both alleles are faulty
What can an individual be termed as if they only have one “faulty” recessive allele?
Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers)
What is an example of a autosomal recessive genetic disease?
An example of an autosomal recessive genetic disease is cystic fibrosis
What does an autosomal dominant genetic disease require to develop?
An autosomal dominant genetic disease only requires one copy of a faulty allele to cause the disorder
What will a hetero- and homozygous individual develop, in the case of an autosomal dominant genetic disease?
Homozygous dominant and heterozygous individuals will both develop the full range of disease symptoms
What is an example of an autosomal dominant genetic disease?
An example of an autosomal dominant genetic disease is Huntington’s disease
What does a genetic disease caused by codominant alleles require?
If a genetic disease is caused by co-dominant alleles it will also only require one copy of the faulty allele to occur
What will heterozygous individuals with a codominant genetic disease develop?
However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele
What is an example of a co-dominant genetic disease?
An example of a genetic disease that displays co-dominance is sickle cell anaemia
What is cystic fibrosis?
Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
What do individuals with CF produce?
Individuals with cystic fibrosis produce mucus which is unusually thick and sticky
What does the excess mucus do in individuals with CF?
This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
Will heterozygous individuals develop CF?
NO
What is Huntington’s Disease?
Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
What does the HTT gene possess?
The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
When does the sequence generated by the HTT gene become unstable?
More than 28 CAG repeats is unstable and causes the sequence to amplify (produce even more repeats)
When does Huntington’s lead to neurodegeneration?
When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
When do symptoms of Huntington’s develop?
This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)
What are the symptoms of Huntington’s?
Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia
How many single gene defects have been identified?
There are over 4,000 identified single gene defects that lead to genetic disease, but most are very rare
Why are single-gene defects rare?
Any allele that adversely affects survival and hence the capacity to reproduce is unlikely to be passed on to offspring
What type of genetic conditions are more common?
Recessive conditions tend to be more common, as the faulty allele can be present in carriers without causing disease
Therefore why are some dominant genetic conditions still present?
Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele
What is sex-linkage?
Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)
Which sex chromosome is shorter?
The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
Which sex chromosome is longer?
The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes)
Therefore what chromosome are sex-linked diseases linked to?
Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome
Why do sex-linked inheritance patterns differ from autosomal patterns?
Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)
What is the expression of sex-linked traits usually associated with?
This leads to the expression of sex-linked traits being predominantly associated with a particularly gender
How do sex-linked diseases differ for women?
As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous
Therefore what sex-linked traits are more common in females?
Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)
What can males be categorised in terms of x-linked traits?
Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits
Therefore what sex-linked traits are more common in males?
X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele
Can males be carriers of x-linked conditions?
Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers
From what parent will males inherit x-linked trait and how often?
Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
Can females inherit a recessive condition from their unaffected father?
Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)
What are examples of x-linked recessive conditions?
Red-green colour blindness and haemophilia are both examples of X-linked recessive conditions
In what gender are colour blindness and haemophilia more common?
Consequently, they are both far more common in males than in females (males cannot mask the trait as a carrier)
How are sex-linked alleles written?
When assigning alleles for a sex-linked trait, the convention is to write the allele as a superscript to the sex chromosome (X)
Haemophilia: XH = unaffected (normal blood clotting) ; Xh = affected (haemophilia)
Colour blindness: XA = unaffected (normal vision) ; Xa = affected (colour blindness)
What is haemophilia?
Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired
Haemophilia - where are genes for coagulation factors located?
The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
What does a lack of coagulation factors lead to in haemophiliacs?
When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
Is there only one type of haemophilia?
Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)
What is red-green colourblindness?
Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues
What is red-green colourblindness caused by?
This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
What is a gene mutation?
A gene mutation is a change to the base sequence of a gene that can affect the structure and function of the protein it encodes
What two ways can mutations be caused?
Mutations can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements
What are 3 factors that can induce mutations?
radiation
chemical
biological agents
What are examples of radiation?
e.g. UV radiation from the sun, gamma radiation from radioisotopes, X-rays from medical equipment
What are examples of chemical factors?
Chemical – e.g. reactive oxygen species (found in pollutants), alkylating agents (found in cigarettes)
What are examples of biological agents?
Biological Agents – e.g. bacteria (such as Helicobacter pylori), viruses (such as human papilloma virus)
What are mutagens?
Agents which increase the rate of genetic mutations are called mutagens, and can lead to the formation of genetic diseases
What are carcinogens?
Mutagens which lead to the formation of cancer are more specifically referred to as carcinogens
What are two examples of catastrophic releases of radioactive material?
The nuclear bombing of Hiroshima and accident at Chernobyl are two examples of a catastrophic release of radioactive material
When did the nuclear bombing of Hiroshima occur?
The nuclear bombing of Hiroshima (and Nagasaki) occurred in August 1945, during the final stages of World War II
When did the Chernobyl accident occur?
The Chernobyl accident occurred in April 1986, when an explosion at the reactor core caused the release of radioactive material
What was the difference between the radioactive material released?
The Chernobyl meltdown involved far more fissionable material and produced different isotopes with much longer half-lives
The Hiroshima nuclear bomb was detonated above ground and radiation was dispersed, resulting in less irradiation of the soil
What are 3 long-term consequences of these disasters?
An increased incidence in cancer development (with a strong correlation between dose of radiation and frequency of cancer)
Reduced T cell counts and altered immune functions, leading to higher rates of infection
A wide variety of organ-specific health effects (e.g. liver cirrhosis, cataract induction, etc.)
What are specific consequences of the two disasters?
Some of the consequences of radiation exposure are specific to the incident due to the types and amounts of radiation released
Thyroid disease was a common consequence of the Chernobyl accident due to the release of radioactive iodine
There was no significant increase in birth defects following the Hiroshima bombing, but an estimated 250% increase in congenital abnormalities following the Chernobyl meltdown
What is the key difference between hiroshima and Chernobyl?
There is anecdotal evidence to suggest that radiation levels around Chernobyl have caused variation to local flora and fauna
The presence of residual radiation in the environment can become concentrated in organisms via bioaccumulation
What is the key difference between hiroshima and Chernobyl?
There is anecdotal evidence to suggest that radiation levels around Chernobyl have caused variation to local flora and fauna
The presence of residual radiation in the environment can become concentrated in organisms via bioaccumulation
What is a pedigree chart?
A pedigree is a chart of the genetic history of a family over several generations
How are males and females represented on pedigree chart?
Males are represented as squares, while females are represented as circles
What does a shaded shape mean on a pedigree chart?
Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected
What do lines between individuals show on the pedigree charts?
A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line
How are generations and individuals labelled in pedigree charts?
Generations are labelled with roman numerals and individuals are numbered according to age (oldest on the left)
How can we determine that a trait is autosomal dominant from a pedigree chart, considering the parents are heterozygous?
If both parents are affected and an offspring is unaffected, the trait must be dominant (parents are both heterozygous)
How can we determine that a trait is autosomal dominant from a pedigree chart, considering the parents are homozygous recessive?
If both parents are unaffected, all offspring must be unaffected (homozygous recessive)
How can we determine that a trait is autosomal dominant from a pedigree chart, considering an affected individual?
All affected individuals must have at least one affected parent
How can we determine a trait is autosomal recessive?
If both parents are unaffected and an offspring is affected, the trait must be recessive (parents are heterozygous carriers)
If both parents show a trait, all offspring must also exhibit the trait (homozygous recessive)
Is it possible to determine x-linked inheritance from a pedigree chart?
It is not possible to confirm sex linkage from pedigree charts, as autosomal traits could potentially generate the same results
However certain trends can be used to confirm that a trait is not X-linked dominant or recessive
How is it possible to determine an x-linked dominant disease from a pedigree chart?
If a male shows a trait, so too must all daughters as well as his mother
An unaffected mother cannot have affected sons (or an affected father)
X-linked dominant traits tend to be more common in females (this is not sufficient evidence though)
How is it possible to determine an x-linked recessive disease from a pedigree chart?
If a female shows a trait, so too must all sons as well as her father
An unaffected mother can have affected sons if she is a carrier (heterozygous)
X-linked recessive traits tend to be more common in males (this is not sufficient evidence though)
