A document comprising genetic related definitions and examples, listed in a table form. It also includes a self-written answer for 2019 past exam questions.
Monosomy The absence of one copy of a pair of chromosomes, due to non-
disjunction of sister chromatids during first or second meiotic
division. (only occurs on sex chromosomes)
Fusing normal and empty gametes.
Turner syndrome:
Karyotype: 45, XO
Phenotype: female
Clinical presentation:
- Short
- Webbed neck
- Small jaw
- Ovarian failure
- Normal intelligence; potential learning disabilities
Partial Monosomy Cri du chat syndrome:
(chromosome mutation) Karyotype: 46, -5p (deletion on chromosome 5)
Clinical presentation:
- small head size
- narrow eye
- shout of a cat
- heart defects
- developmental delays
Pleiotropy An expression of a single gene has multiple phenotypic effects.
Marfan syndrome:
- encodes a mutant connective tissue protein fibrillin.
- Affects the lens of the eye, aorta, bones
e.g. in mouse
molecular level: The ASP gene is ubiquitously expressed (expressed
everywhere not just in the skin) and bines with MC4R, which is
involved in energy homeostasis.
Pericentric inversion An inversion type that includes the centromere, which changes the
relative lengths of the two arms of a chromosome.
Evolution of Human chromosomes:
- Human chromosome 4 vs chimpanzee chromosome 4
Haploid The presence of a single set of chromosomes in an organism’s cells.
In sex cells:
- E.g. sperm, egg, pollen
- Reason they undergo meiosis
Threshold trait Quantitative characteristics which is simply present of absent but
requires some threshold to express the phenotype.
Type II diabetes
Homozygous A pair of chromosomes with identical genetic information in every
site.
Cystic fibrosis:
- A homozygous dominant condition which causes the build
up of thick mucus, leading to frequent lung infections.
Incomplete dominance Lack of dominance in 2 different alleles in heterozygotes, leading to
an intermediate phenotype.
Wavy hair in human:
- Mix of curly and straight hair
Genetic linkage The closeness of genes to another on the same chromosome,
making them more likely to be inherited together without or with
little extent of crossing over.
Complete linkage:
, - No crossing over
- Producing 2 types of gametes (forms 4 gametes)
Incomplete linkage:
- crossing over between chromosome of non-sister
chromatids, according to the interlocus distance.
- Producing 2 crossover gamete and 2 non-crossover gametes
Trisomy A condition with 3 copies of a chromosome, due to non-junction of
sister chromatids, fusing diploid and normal gametes to form
trisomy. (2n +1 chromosomes)
Klinefelter syndrome:
Karyotype: 47, XXY
Phenotype: male
Clinical Symptoms:
- Tall with long arms and legs
- Underdeveloped testes and prostate gland
- No facial hair
- Infertile
- Normal intelligence; may be slow learner/ social
development issues.
Trisomy X:
Karyotype: 47, XXX
Phenotype: female
Clinical symptoms:
- Mostly normal (normal in a sense it’s not lethal)
- Some may have underdeveloped secondary sex
characteristics.
- Some may have menta retardation
Patau Syndrome
Karyotype: trisomy 13
Clinical symptoms:
- Complex developmental disorder
- 80% of live births die in first year of life
Edwards syndrome
Karyotype: trisomy 18
Clinical symptoms:
- Median lifespan of 5-15 days
Down syndrome: Mosaicism in human
Karyotype: Mosaic trisomy 21
- The error occurs after fertilization at some point during
early cell division.
- Individuals have two cell lines: one with normal number of
chromosomes and one with an extra chromosome 21
Epistasis Epistasis refers to heritable changes in gene expression (active
versus inactive genes) that does not involve changes to the
underlying DNA sequence; a change in phenotype without a change
in genotype
Pigmentation in dogs:
Labrador retriever coat colour genes only come in black or
chocolate.
The inheritance of dog pigment colour is dihybrid.
BB: dark pigment enzyme
bb: loss of function enzyme
EE: functional MC1R
ee: loss of function MC1R
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