Fundamentals of genetics Flashcards
(120 cards)
1
Q
What is a gene?
A
A segment of DNA that encodes a protein or RNA; determines traits.
2
Q
What is an allele?
A
A version of a gene (e.g., dominant or recessive).
3
Q
What is a trait?
A
Observable characteristic (e.g., eye color).
4
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What is mutation?
A
A change in a DNA sequence, creating genetic diversity.
5
Q
What is a chromosome?
A
DNA molecules with part/all of the genetic material. Humans have 46 (23 pairs).
6
Q
What is a genotype?
A
The actual genetic makeup (e.g., AA, Aa, aa).
7
Q
What is a phenotype?
A
The observable traits resulting from genotype.
8
Q
What is a dominant allele?
A
Expressed if present (e.g., A).
9
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What is a recessive allele?
A
Only expressed if both alleles are recessive (e.g., aa).
10
Q
What is mitosis?
A
Cell division that creates identical diploid cells.
11
Q
What is meiosis?
A
Cell division that produces haploid gametes (sperm, egg, pollen).
12
Q
What is a gamete?
A
A sex cell (haploid), which combines during fertilisation.
13
Q
What are menders 3 laws?
A
Law of dominance
Law of segregation
Law of independent assortment
14
Q
What is mendels law of dominance?
A
Law of Dominance
In a heterozygote (e.g., Aa), one allele (dominant) masks the effect of the other (recessive).
15
Q
What is mendels law of segregation?
A
Law of Segregation
During gamete formation, the two alleles for a gene separate (segregate), so each gamete gets only one allele.
16
Q
What is mendels law of independent assortment?
A
Law of Independent Assortment
Genes for different traits segregate independently during gamete formation (applies when genes are on different chromosomes).
17
Q
Why did mendels experiments use peas?
A
Easy to grow, with clear traits.
Naturally self-fertilising (true-breeding).
Mendel used artificial cross-pollination with forceps and a brush to control inheritance.
18
Q
What is the monohybrid cross?
A
Cross between two organisms focusing on one trait (e.g., round vs wrinkled seeds).
F1 generation: All dominant phenotype.
F2 generation: 3:1 phenotype ratio (dominant:recessive).
Used to test Mendel’s laws.
19
Q
What are punnet squares?
A
Letters represent alleles (e.g., R = round, r = wrinkled).
Use capital letters for dominant traits, lowercase for recessive.
Helps predict offspring genotype and phenotype ratios.
20
Q
What are inheritance problems?
A
Translate descriptions into symbols (e.g., Tall = T, short = t).
Apply rules of inheritance to solve crosses.
Chi-square (χ²) analysis can test whether observed ratios match expected ones, checking the validity of genetic hypotheses.
21
Q
What is theory?
A
Explains why something happens (can be revised).
22
Q
What is law?
A
Describes what happens consistently (e.g., Mendel’s Laws).
23
Q
What are the 4 different types of genetics?
A
Transmission genetics: Inheritance of traits from parents to offspring.
Molecular genetics: Structure and function of genes at molecular level.
Population genetics: Gene frequency in populations.
Evolutionary genetics: How genetic variation drives evolution.
24
Q
What were mendels observation from monohybrid crosses?
A
Reciprocal crosses yielded the same results → trait not sex-linked.
F1 generation: all showed the dominant trait.
F2 generation: 3:1 ratio (dominant:recessive) always appeared.
Traits inherited as autosomal (not sex-linked).
Traits followed particulate inheritance (not blending).
Each trait is controlled by two alleles (one from each parent).
25
What are Mendel’s Three Laws of Inheritance?
Law of Dominance
One allele (dominant) masks the other (recessive) in heterozygotes (e.g., Aa shows dominant trait).
Law of Segregation
During meiosis, the two alleles for a gene separate so each gamete receives only one allele.
Law of Independent Assortment
Genes for different traits assort independently during gamete formation — if on different chromosomes.
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What is Chromosome Theory of Inheritance?
Mitosis: Cell division that produces genetically identical diploid cells.
Meiosis: Cell division that forms haploid gametes, each with one allele per gene.
Fertilisation restores the diploid state, combining alleles in new ways.
Mendel’s laws are physically explained by chromosome behavior in meiosis.
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What is recombination (crossing over)?
Occurs during Prophase I of meiosis.
Homologous chromosomes pair up and exchange genetic segments.
This reshuffles alleles and increases genetic variability.
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Punnett Squares vs Branch Diagrams
Punnett Squares: Grid system for predicting offspring genotypes/phenotypes.
Branch Diagrams: Use probabilities and the product rule; better for complex crosses (e.g., dihybrids).
📌 Product Rule: Multiply probabilities of independent events.
Example: P(round) = P(R_) × P(Y_) in a dihybrid cross.
29
What are test crosses?
Used to determine unknown genotype showing a dominant phenotype (e.g., is it AA or Aa?).
Cross with a true-breeding recessive:
If all offspring show the dominant trait → likely homozygous dominant.
If any offspring show the recessive trait → must be heterozygous.
30
What is the Chi-Square (χ²) Test?
A statistical tool to determine if observed data fits expected Mendelian ratios.
Used to accept or reject a genetic hypothesis (null hypothesis = no difference between observed and expected).
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What is disjunction?
Normal separation of chromosomes during meiosis/mitosis.
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What is non-disjuction?
Abnormal separation → leads to missing or extra chromosomes.
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What is crossover?
Exchange of genetic material between homologous chromosomes in meiosis.
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What is meiosis?
Cell division that produces haploid gametes with genetic diversity.
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What is mitosis?
Cell division producing identical diploid cells for growth/repair.
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What is recombination?
DNA mixing during crossing over, creating genetic variation.
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What is gametogenesis?
Formation of gametes (sperm/egg) via meiosis.
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What is a phenotype?
Observable traits.
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What is a genotype?
Genetic makeup (e.g., Aa, BB).
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What is an allele?
version of a gene (e.g., A or a).
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What is a gene?
DNA sequence that codes for a protein.
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What is meant by dominant/recessive?
Dominant alleles mask recessive ones in heterozygotes.
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What is a testcross?
Cross with a homozygous recessive to reveal genotype of a dominant-appearing organism.
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What is true breeding?
Organisms that consistently pass on the same traits.
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What is diploid/haploid?
Diploid = two sets of chromosomes; Haploid = one set.
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What is autosomal?
Not on sex chromosomes.
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What is germline?
Cells that lead to gametes.
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What is somatic?
All other (body) cells.
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What is recombination in meiosis?
Happens during Prophase I of meiosis.
Homologous chromosomes pair and exchange segments at chiasmata.
This process increases genetic diversity and ensures proper segregation (disjunction).
Crossovers help keep chromosomes paired until separation.
50
What are chromosomal structural mutations?
These are large-scale changes in chromosome structure, often due to breaks and faulty repair
51
What is a deletion mutation?
Loss of a DNA segment.
Can delete one or more genes → loss of gene function.
Caused by:
Transposon activity (DNA "jumping" elements).
Faulty repair of double-strand breaks (especially via NHEJ).
🧠 Example: Turner syndrome (XO) – missing a whole X chromosome (monosomy).
52
What is duplication mutation?
Doubling of a chromosome region.
Can cause disease or lead to evolutionary innovation.
CNVs (copy number variations) make up 5–10% of the human genome.
Extra gene copies can:
Gain new functions.
Share functions of original gene.
53
What is inversion mutation?
A chromosome segment is reversed.
Paracentric (no centromere) vs Pericentric (includes centromere).
May affect phenotype or cause meiosis problems due to mispairing.
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What is translocation?
A segment is moved to another chromosome.
Balanced (conservative) if no genetic material is lost.
Can:
Create fusion genes → altered protein function.
Cause disease.
🧠 Example: Philadelphia chromosome → fusion of ABL and BCR genes → leukemia (CML).
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What is the chromosome naming system?
Each chromosome has a p (short) arm and a q (long) arm.
Divided into:
Regions → Bands → Sub-bands (numbered away from centromere).
Helps pinpoint mutation locations.
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What are small-scale point mutations?
These affect one or a few base pairs:
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What is a silent mutation?
Alters a codon but doesn’t change the amino acid (due to genetic code redundancy).
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What is a missense mutation?
Changes amino acid → may alter protein function.
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What is a nonsense mutation?
Converts a codon into a STOP codon → truncated, non-functional protein.
Note: Most of the genome is non-coding.
Only ~1% of human DNA codes for proteins.
Mutations in non-coding DNA are usually silent but can affect regulation.
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What is a transposon?
"Jumping genes" that move around the genome.
Make up ~50% of the human genome.
Can disrupt genes, cause rearrangements, or increase genome size.
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Why was the Model Organism: Drosophila melanogaster (Fruit fly) used in experiments?
Short life cycle
Easy to culture
High reproductive rate (50 eggs/day/female)
Cheap and small
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What was Morgans breakthrough in sex linkage?
Thomas Hunt Morgan discovered a white-eyed male fruit fly (most had red eyes).
When he crossed this male with red-eyed females, the white-eyed trait only appeared in males of later generations.
This led him to conclude: the gene for eye colour must be located on a sex chromosome—specifically the X chromosome.
63
How was sex determination linked to chromosomes?
Nettie Stevens (1905) discovered that male mealworms had two types of sperm:
One with a large chromosome (X)
One with a small chromosome (Y)
The chromosome that fertilised the egg determined the offspring's sex:
X + X = Female
X + Y = Male
This laid the foundation for understanding X and Y chromosomes.
64
What is linkage?
Linked genes are genes located close together on the same chromosome.
They tend to be inherited together because they are less likely to be separated by recombination during meiosis.
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What is recombination?
During meiosis, homologous chromosomes can exchange genetic material—this is crossing over.
If two genes are far apart, crossing over between them is more likely, producing recombinant offspring.
If genes are close together, they’re more likely to be inherited together (less recombination = stronger linkage).
66
How is recombination frequency measured?
Recombination frequency =
Number of recombinant offspringTotal offspring×100Total offspringNumber of recombinant offspring ×100
This value is expressed in centiMorgans (cM) or map units:
1 cM = 1% recombination = genes are relatively close together.
More cM between genes = further apart = more likely to recombine.
67
What was Alfred Sturtevants insight into recombination?
Greater recombination = greater physical distance between genes.
He used this idea to build the first genetic map of the X chromosome in fruit flies (1913).
68
What are three factor crosses?
Involve three genes.
Double recombinants (very rare) help determine the gene order.
Confirm that genes are arranged in a linear, not branched, fashion.
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Why Do Genetic Mapping Today if We Have Genome Sequences?
Even with full DNA sequences, we often don’t know which gene causes a particular trait or disease.
Genetic mapping:
Narrows down the DNA region likely to contain the gene of interest.
Identifies candidate genes based on proximity to known genetic markers
70
How to tell if a gene is sex linked?
How to tell if a gene is sex-linked:
Sex-linked traits, usually on the X chromosome, show specific inheritance patterns:
More common in males (only one X chromosome).
Affected males often inherit the trait from their carrier mothers.
Autosomal genes show no difference in inheritance between sexes.
71
What is the basic concept of genes as units of inheritance?
Genes are units of inheritance located at specific positions (loci) on chromosomes.
In sexually reproducing organisms, each individual has two copies of each gene—one from each parent.
These two alleles segregate during gamete formation, so each gamete gets only one.
The combination of alleles (genotype) determines the observable traits (phenotype)
72
What are Mendelian traits?
Traits governed by a single gene with two alleles: one dominant (R) and one recessive (r).
Classic 3:1 phenotypic ratio seen in F2 generation after a monohybrid cross.
Example:
MC1R and red hair: Red hair results when a person inherits two copies of the red-hair allele of the MC1R gene. Other features associated include freckles, fair skin, and high Vitamin D synthesis.
Other Mendelian traits:
Dominant: Cleft chin, dimples, freckles.
Recessive: Albinism.
73
What causes albinism? What are the 2 types? Symptoms? Genotypes?
Albinism is caused by mutations that disrupt melanin synthesis.
Oculocutaneous albinism (OCA) and ocular albinism (OA) are inherited in different ways:
OCA: Autosomal recessive.
OA: Usually X-linked recessive.
Symptoms: Lack of pigmentation, nystagmus (involuntary eye movement), vision issues.
Genetics:
Autosomal recessive: Both parents are carriers → 25% chance affected child.
X-linked recessive: Sons more likely to be affected if mother is a carrier; fathers cannot pass to sons.
74
What are polygenic traits?
Traits controlled by many genes, each contributing a small effect.
Examples: Height, skin color, intelligence.
Still follows Mendelian segregation, but the overall pattern is complex.
75
What is incomplete dominance?
Neither allele is completely dominant.
Heterozygote shows an intermediate phenotype.
E.g., red × white snapdragons = pink offspring
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What is codominance?
Both alleles are expressed equally in the phenotype.
Example: Human ABO blood group:
IA and IB are codominant; both expressed in type AB blood.
i is recessive (null allele).
77
how is Labrador coat colour an example of 2 gene interaction?
Two unlinked Mendelian genes:
E: Required for pigment production.
B: Controls pigment deposition.
E is epistatic to B: ee genotype blocks pigment formation → yellow coat regardless of B allele.
Phenotypic ratio: 9 black : 3 chocolate : 4 yellow (due to gene interaction, not classic 9:3:3:1).
78
What makes up gene structure?
Coding region = sequences that code for protein.
Regulatory region = sequences that control gene expression (e.g., promoters, enhancers).
79
What are the 5 types of mutations and what do they do?
Null (Amorphic): No gene function (e.g., gene deletion).
Hypomorphic: Reduced gene function.
Loss-of-function: Includes both null and hypomorphic.
Gain-of-function: New or increased function.
Dominant-negative: Mutant protein interferes with normal protein from the other allele.
80
What is allelic series?
A collection of mutations at a single gene locus showing different effects.
Often created experimentally using transposons (mobile DNA elements that disrupt gene function).
81
What is a pedigree analysis? What are the main symbols?
Pedigrees help track inheritance patterns across generations.
Symbols:
Square = male, circle = female.
Filled = affected, empty = unaffected.
Horizontal line = mating, vertical line = offspring.
82
What is autosomal dominant?
Autosomal dominant: Appears every generation, both sexes equally affected.
E.g., Huntington's disease.
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What is autosomal recessive?
Autosomal recessive: Can skip generations, carriers possible.
E.g., cystic fibrosis.
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What is x-linked recessive?
Mostly affects males, skips generations.
E.g., hemophilia, red-green color blindness.
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What is x-linked dominant?
X-linked dominant: Affects females more often, no male-to-male transmission.
E.g., familial Vitamin D–resistant rickets.
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What is non-nuclear inheritance?
Mitochondria and chloroplasts have their own DNA.
Maternally inherited: Only mothers pass mitochondrial genes to offspring.
These do not follow Mendelian patterns
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What is Loss-of-function mutations? What are the 2 types?
The mutation reduces or eliminates the activity of the gene product.
Types:
Amorph (null): Complete loss of function
Hypomorph: Partial loss of function
Most LOF mutations are recessive: One functional copy is usually enough for normal function.
+/m (heterozygote) = no phenotype
m/m (homozygote) = phenotype
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What is gain-of-function (GOF) mutations?
The mutation increases activity or gives the gene a new function.
Often causes the gene to be active at the wrong time/place or level.
Most GOF mutations are dominant: One mutated copy is enough to cause an effect.
GOF/+ (heterozygote) = phenotype
🧬
89
What is RAS? What are RAS mutations?
RAS = small GTPase, switches between active (GTP-bound) and inactive (GDP-bound) states, controls cell growth.
GOF mutations (like G12D) make RAS stay active, leading to cancer:
More GTP binding
Less GTP hydrolysis
Higher interaction with RAF → signals for cell proliferation
LOF RAS mutations in Drosophila disrupt cell differentiation and viability, especially in the eye.
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What is an allelic series?
There are many ways to mutate a gene, each giving a different mutant allele.
Together with the wild-type (+) version, these form an allelic series or multiple alleles of a gene.
Different mutant alleles can lead to different levels of gene activity → different phenotypes.
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Give apterus as an example for an allelic series.
🧪 Example: Apterous (wing development gene)
Amorphic (null) allele → no wings
Hypomorphic allele → small or deformed wings
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What are the 2 types of allelic interactions?
+/m → wild-type or mutant phenotype (depends on dominance)
m1/m2 → two different mutant alleles; may show intermediate phenotype
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What is dominance?
Dominance describes how two alleles of a gene interact in a heterozygote.
It's not an inherent property of an allele but a relationship between alleles
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What is complete dominance?
One allele completely masks the other (example, Mendels peas)
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What is incomplete dominance?
heterozygote is intermediate between homozygotes (eg pink snapdragons)
96
What is co-dominance?
Both alleles are expressed in heterozygote (ABO blood types)
97
How can overdominance be a heterozygote advantage (example?)
Heterozygote has higher fitness than either homozygote.
Classic example: Sickle Cell Trait (AS):
AA: normal, susceptible to malaria
AS: mild or no symptoms + malaria resistance ✅
SS: sickle cell disease ❌
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What is pleiotropy?
A single gene mutation affects multiple traits or processes.
Why? Because many gene products are used in multiple cell types or pathways.
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What is an example of pleiotropy?
🧬 Example: Sickle Cell Allele
Affects:
Hemoglobin shape
Red blood cell (RBC) stability
Oxygen transport
Resistance to malaria
RBC count at high altitude
100
How is protein phophatase 1 an example of pleiotropy?
Interacts with 200+ different regulatory subunits
Mutation in the catalytic subunit affects:
Cell cycle
Cell movement
Metabolism
101
What is a complementation test? How does it work? Example?
Used to determine whether two mutations are in the same gene or in different genes.
🧪 How It Works:
Cross two homozygous mutants:
If the offspring has a normal phenotype, the mutations complement each other → in different genes.
If the offspring is mutant, the mutations fail to complement → in the same gene (i.e., alleles of that gene).
🧬 Example:
Mutant 1/Mutant 2 = no wings → failed complementation → both affect Apterous gene
Mutant 1/Mutant 2 = normal wings → complementation → mutations in different genes
102
What are the 4 different point mutations?
Silent, missense, nonsense and frame shift
103
What is a silent mutation?
No change in amino acid
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What is a missense mutation?
Changes one amino acid
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What is a nonsense mutation?
Introduces a premature stop codon
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What is a frame shift mutation?
Insertion or deletion shifts the reading frame
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What is LOF
Loss of function
- often recessive
- one working allele (+/m) can provide normal function
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What is GOF
Gain of function
- often dominant
- one mutant allele (GOF/+) can cause phenotype
109
What is meant by ‘dominance is relative’
Depends on the trait observed and allele interactions
110
What are transitions vs transversions?
Purines = A, G | Pyrimidines = C, T
Transitions = Purine ↔ Purine or Pyrimidine ↔ Pyrimidine
E.g., A ↔ G or C ↔ T
Transversions = Purine ↔ Pyrimidine
E.g., A ↔ T or G ↔ C
🧬 These changes can cause missense, nonsense, or silent mutations depending on the codon affected
111
Do genes work alone?
Genes don’t act alone - they operate in pathways and complexes
Mutation in one gene can influence how a mutation in another gene behaves
112
What are the 4 ways of finding genes that interact?
Candidate approach - based on known pathways
Screens - look for unexpected genetic interactions
Biochemical - find physical binding partners
Genetic - using modifier or epistasis screens
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What is epistasis?
When one gene’s effect masks or modifies another genes effect (example: mutation A may only cause a phenotype if mutation B is also present)
114
What is a modifier screen?
Look for genes that enhance or suppress a known phenotype
115
What is penetrance and what are the 2 types?
Penetrance = how often a genotype shows the expected phenotype
Complete penetrance: all individuals with genotype show phenotype
Incomplete penetrance: some individual with genotype don’t show phenotype
116
What is meant by ‘expressivity’?
The degree to which a phenotype is expressed
- variable expressivity: all have the same genotype but show different severity of phenotype
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What are the causes of penetrance and expressivity?
Modifier genes, environmental factors, epigenetics (eg methylation in twins)
118
What is pleiotropy?
One gene affects multiple traits (eg sickle cell disease affects RBC shape, oxygen transport, malaria resistance etc)
119
What are multigenic (polygenic) traits?
Traits like height or hair colour are influenced by many genes, leading to continuous variation (not just discrete categories like blood type)
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