3.4 Inheritance Flashcards
1
Q
Describe how Mendel made his discoveries
A
- Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants
- First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
- Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
- These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)
2
Q
What did Mendel discover as a result of his experiments?
A
- 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 - When Mendel self-fertilized 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
3
Q
What conclusions did Mendel make based on his findings?
A
- Organisms have discrete factors that determine their features (these ‘factors’ are now recognized as genes)
- Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognized as alleles)
- Each gamete contains only one version of each factor (sex cells are now recognized to be haploid)
- Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
- For each factor, one version is dominant over another and will be completely expressed if present
4
Q
While there are caveats to Mendel’s conclusions, certain rules can be established. What are these?
A
- Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
- Law of Independent Assortment: The segregation of alleles for one gene occurs independently of that of any other gene*
- Principle of Dominance: Recessive alleles will be masked by dominant alleles†
*The law of independent assortment does not hold true for genes located on the same chromosome (i.e. linked genes)
† Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance
5
Q
Diagram showing Mendel’s garden pea plant experiment
A
6
Q
What are gametes and how are they produced?
A
- Gametes are haploid sex cells formed by the process of meiosis – males produce sperm and females produce ova
- During meiosis I, homologous chromosomes are separated into different nuclei before cell division
- As homologous chromosomes carry the same genes, segregation of the chromosomes also separates the allele pairs
- Consequently, as gametes contain only one copy of each chromosome they, therefore, carry only one allele of each gene
7
Q
Diagram showing male vs. female gametes
A
8
Q
What is the significance of the fact that gametes are haploid?
A
- Gametes are haploid, meaning they only possess one allele for each gene
- When male and female gametes fuse during fertilization, the resulting zygote will contain two alleles for each gene
- Exception: Males have only one allele for each gene located on a sex chromosome, as these chromosomes aren’t paired (XY)
9
Q
How can the combination of alleles for any given gene be categorized?
A
- 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
- Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene
10
Q
Diagram showing the types of zygosity
A
11
Q
What is the genotype?
A
- The gene composition (i.e. allele combination) for a specific trait
- The genotype of a particular gene will typically be either homozygous or heterozygous
12
Q
What is the phenotype?
A
- The observable characteristics of a specific trait (i.e. the physical expression)
- The phenotype is determined by both the genotype and environmental influences
13
Q
Complete dominance
A
- Most traits follow a classical dominant/recessive pattern of inheritance, whereby one allele is expressed over the other
- The dominant allele will mask the recessive allele when in a heterozygous state
- Homozygous dominant and heterozygous forms will be phenotypically indistinguishable
- The recessive allele will only be expressed in the phenotype when in a homozygous state
- When representing alleles, the convention is to capitalize the dominant allele and use a lowercase letter for the recessive allele
- An example of this mode of inheritance is mouse coat color – black coats (BB or Bb) are dominant to brown coats (bb)
14
Q
Diagram of complete dominance (mouse coat color)
A
15
Q
Co-dominance
A
- Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual
- Heterozygotes, therefore, have an altered phenotype as the alleles have a joint effect
- 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 (C^B) and white (C^W) feathers create a speckled coat (C^BC^W)
16
Q
Diagram of co-dominance (chicken feathering)
A
17
Q
How can human red blood cells be categorized?
A
- They can be categorized into different blood groups based on the structure of a surface glycoprotein (antigen)
- The ABO blood groups are controlled by a single gene with multiple alleles (A, B, O)
- The A, B, and O alleles all produce a basic antigen on the surface of red blood cells
- The A and B alleles are co-dominant and each modifies the structure of the antigen to produce different variants
- The O allele is recessive and does not modify the basic antigenic structure
18
Q
What is the letter I used for when representing blood group alleles?
A
To represent the different antigenic forms (isoantigens)
A allele = I^A ; B allele = I^B ; O allele = i (recessive)
19
Q
Table summarizing the genotypes for the different blood groups
A
20
Q
Explain why blood transfusions are not compatible between certain blood groups
A
- As humans produce antibodies against foreign antigens, blood transfusions are not compatible between certain blood groups
- AB blood groups can receive blood from any other type (as they already possess both antigenic variants on their cells)
- A blood groups cannot receive B blood or AB blood (as the isoantigen produced by the B allele is foreign)
- B blood groups cannot receive A blood or AB blood (as the isoantigen produced by the A allele is foreign)
- O blood groups can only receive transfusions from other O blood donors (both antigenic variants are foreign)
21
Q
Summary table of the ABO blood groups
A
22
Q
Diagram showing the consequence of incompatible blood transfusion
A
23
Q
Steps of drawing monohybrid crosses
A
- A monohybrid cross determines the allele combinations for potential offspring for one gene only
- Monohybrid crosses can be calculated according to the following steps:
- Step 1: Designate letters to represent alleles (dominant = capital letter ; recessive = lower case ; co-dominant = superscript)
- Step 2: Write down the genotype and phenotype of the prospective parents (this is the P generation)
- Step 3: Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)
- Step 4: Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)
- Step 5: Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (this is the F1 generation)
24
Q
Diagram showing an overview of a monohybrid cross
A
25
Q
Comparison of predicted and actual outcomes of genetic crosses using real data
A
- 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
- When comparing predicted outcomes to actual data, larger data sets are more likely to yield positive correlations
- Gregor Mendel performed over 5,000 crosses as part of his pea plant experiment
- However, many statisticians believe Mendel’s results are too close to the exact ratios predicted to be genuine
26
Q
Table showing Mendel’s pea plant experimental data
A
27
Q
Methodology of Mendel’s pea plant experiments
A
- Mendel crossed different varieties of pea plants and recorded the characteristics of the resultant offspring
- Initially, he crossed purebred dominant and purebred recessive plants 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
28
Q
What causes genetic diseases?
A
- Mutations to a gene (or genes) that abrogate normal cellular function, leading to the development of a disease phenotype
- Genetic diseases can be caused by recessive, dominant, or co-dominant alleles
29
Q
When will an autosomal recessive genetic disease only occur?
A
- If both alleles are faulty
- Heterozygous individuals will possess one copy of the faulty allele but not develop disease symptoms (they are carriers)
- An example of an autosomal recessive genetic disease is cystic fibrosis
30
Q
Autosomal dominant genetic diseases
A
- These only require one copy of a faulty allele to cause the disorder
- Homozygous dominant and heterozygous individuals will both develop the full range of disease symptoms
- An example of an autosomal dominant genetic disease is Huntington’s disease
31
Q
Genetic diseases caused by co-dominant alleles
A
- If a genetic disease is caused by co-dominant alleles, it will also only require one copy of the faulty allele to occur
- However, heterozygous individuals will have milder symptoms due to the moderating influence of a normal allele
- An example of a genetic disease that displays co-dominance is sickle cell anemia
32
Q
Diagram showing inheritance patterns for dominant and recessive genetic disorders
A
33
Q
Cystic fibrosis
A
- Cystic fibrosis is an autosomal recessive disorder caused by a mutation to the CFTR gene on chromosome 7
- Individuals with cystic fibrosis produce mucus that is unusually thick and sticky
- This mucus clogs the airways and secretory ducts of the digestive system, leading to respiratory failure and pancreatic cysts
- Heterozygous carriers who possess one normal allele will not develop disease symptoms
34
Q
Diagram of chest X-rays for people without and with cystic fibrosis
A
35
Q
Huntington’s disease
A
- Huntington’s disease is an autosomal dominant disorder caused by a mutation to the Huntingtin (HTT) gene on chromosome 4
- The HTT gene possesses a repeating trinucleotide sequence (CAG) that is usually present in low amounts (10 – 25 repeats)
- More than 28 CAG repeats are unstable and cause the sequence to amplify (produce even more repeats)
- When the number of repeats exceeds ~40, the huntingtin protein will misfold and cause neurodegeneration
- This usually occurs in late adulthood and so symptoms usually develop noticeably in a person’s middle age (~40 years)
- Symptoms of Huntington’s disease include uncontrollable, spasmodic movements (chorea) and dementia
36
Q
Diagram of proteins and brains of people without and with Huntington’s disease
A
37
Q
Identification of genetic diseases in humans
A
- There are over 4,000 identified single gene defects that lead to genetic disease, but most are very rare
- Any allele that adversely affects survival and hence the capacity to reproduce is unlikely to be passed on to offspring
- Recessive conditions tend to be more common, as the faulty allele can be present in carriers without causing disease
- Dominant conditions may often have a late onset, as this does not prevent reproduction and the transfer of the faulty allele
38
Q
What is sex linkage?
A
- When a gene controlling a characteristic is located on a sex chromosome (X or Y)
- The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
- The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp; ~ 2,000 genes)
- Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome
39
Q
Diagram of X and Y chromosomes
A
40
Q
Why do sex-linked inheritance patterns differ from autosomal patterns?
A
- Because chromosomes aren’t paired in males (XY)
- This leads to the expression of sex-linked traits being predominantly associated with a particular gender
41
Q
Why are X-linked dominant traits more common in females?
A
- As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous
- Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)
42
Q
Why are X-linked recessive traits more common in males?
A
- Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits
- X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele
43
Q
What trends hold true for X-linked conditions?
A
- Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers
- Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
- Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)
44
Q
Diagram showing inheritance of an X-linked recessive disease condition
A
45
Q
What are red-green color-blindness and hemophilia examples of?
A
- Sex-linked inheritance/ X-linked recessive inheritance
- Consequently, they are both far more common in males than in females (males cannot mask the trait as a carrier)
46
Q
Conventions for assigning alleles for a sex-linked trait
A
- The convention is to write the allele as a superscript to the sex chromosome (X)
- Haemophilia: X^H = unaffected (normal blood clotting) ; X^h = affected (haemophilia)
- Colour blindness: X^A = unaffected (normal vision) ; X^a = affected (colour blindness)
47
Q
Hemophilia
A
- Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired
- The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
- When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
- Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)
48
Q
Red-green color blindness
A
- Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues
- This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
- Red-green colour blindness can be diagnosed using the Ishihara colour test
49
Q
What is a gene mutation?
A
A change to the base sequence of a gene that can affect the structure and function of the protein it encodes
50
Q
How can mutations arise?
A
They can be spontaneous (caused by copying errors during DNA replication) or induced by exposure to external elements
51
Q
Examples of factors that induce mutations
A
- Radiation – e.g. UV radiation from the sun, gamma radiation from radioisotopes, X-rays from medical equipment
- Chemical – e.g. reactive oxygen species (found in pollutants), alkylating agents (found in cigarettes)
- Biological Agents – e.g. bacteria (such as Helicobacter pylori), viruses (such as human papilloma virus)
52
Q
What are mutagens?
A
- Agents that increase the rate of genetic mutations
- They can lead to the formation of genetic diseases
53
Q
What are carcinogens?
A
Mutagens that lead to the formation of cancer
54
Q
Diagram showing types of mutagens
A
55
Q
Nuclear bombings of Hiroshima and the accident at Chernobyl
A
- These are two examples of a catastrophic release of radioactive material
- The nuclear bombing of Hiroshima (and Nagasaki) occurred in August 1945, during the final stages of World War II
- The Chernobyl accident occurred in April 1986, when an explosion at the reactor core caused the release of radioactive material
- Of the two incidents, more people died from the nuclear bombing, but the meltdown released far more radiation (~400×)
- 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
56
Q
Long-term consequences of radiation exposure following Hiroshima and Chernobyl
A
- An increased incidence of cancer development (with a strong correlation between the dose of radiation and the 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.)
- 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
57
Q
Are Hiroshima and Chernobyl still habitable?
A
- While Hiroshima is still habitable and well populated, certain regions of Chernobyl remain unsafe for human habitation
- 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
58
Q
What is a pedigree?
A
A chart of the genetic history of a family over several generations
59
Q
Describe how pedigree charts are drawn
A
- Males are represented as squares, while females are represented as circles
- Shaded symbols mean an individual is affected by a condition, while an unshaded symbol means they are unaffected
- A horizontal line between man and woman represents mating and resulting children are shown as offshoots to this line
- Generations are labeled with roman numerals and individuals are numbered according to age (oldest on the left)
60
Q
Determining autosomal inheritance
A
Dominant and recessive disease conditions may be identified only if certain patterns occur (otherwise it cannot be confirmed)
61
Q
Determining autosomal inheritance- autosomal dominant
A
- If both parents are affected and an offspring is unaffected, the trait must be dominant (parents are both heterozygous)
- All affected individuals must have at least one affected parent
- If both parents are unaffected, all offspring must be unaffected (homozygous recessive)
62
Q
Determining autosomal inheritance- autosomal recessive
A
- 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)
63
Q
Determining X-Linked inheritance
A
- 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
64
Q
Determining X-Linked inheritance- X-linked dominant
A
- 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)
65
Q
Determining X-Linked inheritance- X-linked recessive
A
- 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)
66
Q
Incomplete dominance
A
- Sometimes, when two alleles are present in an individual, the phenotype is different than in either homozygous state.
- This is incomplete dominance, where neither of the two alleles is dominant, and both impact the phenotype.
- The notation for co-dominant alleles is a capital letter (indicating the trait) with a capital superscript (indicating the allele).
- For example, snapdragons show incomplete dominance for flower color.
- C^R is the allele for red flowers while C^W is the allele for white.
- A plant will have red flowers if it has the genotype C^RC^R and white flowers if the genotype is C^WC^W.
- However, if its genotype is C^RC^W, it will have pink flowers (a blend of red and white).
