ANA 209 Human Genetics Flashcards
definition of heredity
passing of a trait from parent to offspring
What is genetics
the study of heredity
what is the law of segregation?
The law of segregation states that during the production of gametes, alleles segregate from each other during meiosis, so that offspring acquire one factor from each parent
Also known as the first law of inheritance
What is the law of dominance?
states that hybrid offspring will only exhibit the dominant trait in the phenotype
What is the law of independent assortment?
states that alleles will assort independently of one another during gamete formation
2nd law of inheritance
Gene
Basic unit of heredity
Allele
variation of a gene
Homozygous
an individual with 2 identical alleles
Heterozygous
an individual with 2 different alleles
Genotype
the composition of alleles in an individual
Phenotype
The physical appearance of an individual
Gene locus
a specific position on a chromosome where a gene is located
cytogenetics
the study of chromosomes
Chromosome
a thread-like structure consisting of DNA
Chromatid
A chromatid is one of the two identical halves of a chromosome that has been replicated in preparation for cell division.
Parts of a chromosome
Chromatid
centromere
telomere
Telomere
A telomere is a region of repetitive DNA sequences at the end of a chromosome
Genome
Genetic material of an organism
Types of chromosomal abnormalities
- Numerical abnormalities
- Structural abnormalities
What is continuous variation?
Continuous variation is the type of genetic variation showing an unbroken range of phenotypes in a population
What is discontinuous variation?
discontinuous variation is where the different types of variations are placed into discrete categories.
It cannot be measured or quantified
It is caused by a small number of genes. The environment has little or no effect e.g. blood group and eye color
Incomplete dominance
Both alleles of a gene at a locus are partially expressed, often resulting in an intermediate phenotype of e.g skin and hair
Based on the position of centromere and length of chromosomal arms, the chromosomes are classified into 4 groups:
(1). Telocentric chromosomes
(2). Acrocentric chromosomes
(3). Sub-metacentric chromosomes
(4). Metacentric chromosomes
Describe a Telocentric chromosome
- Centromere is located at the proximal end (tip/telomere) of the chromosome
- Long rod-like
- Appear as an ‘i’ shaped strcutre in metaphase stage
- Only had one chromosome arm (q arm)
5.very rare, reported in very few speciea
Describe Acrocentric chromosomes
- Very short p arm and vert long q arm
- Appears as a ‘J’ shaped structure in metaphase
- Acrididae group (grasshoppers) show this type of chromosomes
- Aka sat chromosomes
- In humans Chromosomes 13,15,21, and 22 are sat-acrocentric
Describe Sub-metacentric chromosomes
- Centromere is located near the center (not the exact center) of the chromosome
- Unequal arms (small p and large q)
- Appears ‘L’ shaped during metaphase
- Majority of human chromosomes are sub-metacentric
Describe metacentric chromosomes
- Centromere at the exact center
- 2 equal size arms
- Primitive type
- Seen in amphibians
- Seen in humans in chromosome 1 and 3
- Appear ‘V’ shaped during metaphase
Types of chromosomal abnormalities (based on type of change)
- Structural changes: Any change which alters the basic chromosome structure is known as a structural change.
- Numerical changes
Types of Structural changes in chromosomal abnormalities
Deletions
Duplications
Translocations
Inversions
What is deletion?
A portion of the chromosome is missing or deleted. The chromosome becomes shorter due to the loss of one or more genes.
Large deletions will most probably be lethal
Smaller deletions may allow survival
E. coli: deletions of up to 1% have been observed in living cells
D. melanogaster : deletions of up to 0.1% observed
What is duplication?
A portion of the chromosome is duplicated, resulting in extra genetic material.
Give examples of deletions in humans
- Cri-du-chat syndrome
Microdeletion of chromosome 5 - Di-George syndrome
Micro deletion of chromosome 22 - Schizophrenia & Obsessive Compulsive Disorder
Micro deletion of chromosome 22 associated - Angelman syndrome
Micro deletion of chromosome 15 - Prader-Willi syndrome
Micro deletion of chromosome 15
What is Cri-du-chat syndrome?
1st autosomal deletion on chromosome 5
Characteristic cat-like cry, which disappears with age
Microcephaly (small head)
Severe mental retardation
Congenital heart disease
Hypertelorism (widely separated eyes)
Low birth weight and poor growth
Severe cognitive, speech, and motor delay
Behavioral problems
Excessive drooling
What is Prader-Willi Syndrome
Lack of muscle tone in newborn
Poor swallowing reflex
As adult - gross obesity
Mean I.Q. ~ 50
Microdeletion of 15
What is Cri-du-chat syndrome?
1st autosomal deletion described
Characteristic cat-like cry, which disappears with age
Microcephaly (small head)
Severe mental retardation
Congenital heart disease
Hypertelorism (widely separated eyes)
Low birth weight and poor growth
Severe cognitive, speech, and motor delay
Behavioral problems
Excessive drooling
Deletion in chromosome 5
What is Angelman Syndrome
Developmentally delayed
Jerky movements
Stiff, fixed smile
Uncontrolled laughter
Abnormal E.E.G., epilepsy
Microdeletion of 15
What is translocation?
A portion of one chromosome is transferred to another chromosome. There are three main types of translocation.
Simple translocation- In simple translocation, segment from one chromosome will break and attached to another chromosome.
Reciprocal translocation- In a reciprocal translocation, segments from two different chromosomes have been exchanged.
What is Robertsonian translocation?
Robertsonian translocation- In a Robertsonian translocation, an entire chromosome arm has attached to another at the centromere.
Loss of chromosomal segment.
What is an isochromosome?
Centromere of the chromosome divides transversely instead of longitudinally
One arm is missing and the other arm duplicated
What is a ring chromosome?
Occurs due to loss of both the ends of a chromosome
The broken ends rejoin to form a ring-like chromosome
Rare anomaly
What are the two types of robertsonian changes in chromosomes?
Fusion: 2 chromosomes join to form one
Fission: One chromosome split to form 2
What is inversion?
A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted.
An inversion is produced when there are two breaks in a chromosome and the intercalary segment reunites in reverse order i.e., the segment rotate by 180°.
Types of inversion
Pericentric inversion- If the inverted segment includes the centromere, the inversion is called pericentric inversion
Paracentric inversion- If it does not include centromere the inversion is called paracentric inversion.
Codominace
Both alleles are expressed in the offspring e.g. ABO blood grouping
Causes of variation
Crossing over
Recombination
Mutation
Environmental effects
What are heteroploids?
Individuals possessing the variant chromosome numbers
Types of heteroploidy
- Aneuploidy
- Euploidy
Aneuploidy
•It involves addition or deletion of one or few chromosomes to the usual diploid set of chromosomes.
•Aneuploid changes in chromosome number do not involve the whole genome
Types of Aneuploidy
Hypoploidy: Monosomy (2n-1) Nullisomy (2n-2)
Hyperploidy Trisomy (2n+1) Tetrasomy (2n+2)
Hypoploidy
They arise by the loss of one chromosome from the diploid set i.e., 2n-l.
Describe monosomies of chromosomes
•Presence of only one member of a chromosome pair in a karyotype
•More detrimental than equivalent trisomy
•Can involve autosomes or sex chromosomes
•Usually abort spontaneously
•Monosomy of X chromosome results in XO condition called Turner syndrome
What is Turner syndrome?
•Chromosome complement: 45,XO
•Phenotype: Female
•Incidence: 1:5000-8000
Features of Turner syndrome
•Short statured female
•Sexual infantilism with primary amenorrhoea and sterility
•Short, webbed neck
•Prominent ears with defective hearing
•Small mandible
•Defective vision
•Epicanthal folds
•Low posterior hair line
•Cubitus valgus
•Broad chest with widely spaced nipples
•Cardiovascular anomalies
•Hyperconvex finger nails
•Pigmented nevi
•Sex chromatin negative
What is amenorrhea going
Absence of menstruation due to underlying disease or condition
What is cubitus valgus?
Cubitus valgus is a deformity in which the forearm is angled out away from the body when the arm is fully extended.
Nullisomy
These arise by the loss of a particular pair of chromosomes i.e., 2n-2.
Trisomy
These arise by addition of an extra chromosome to the normal diploid set with the genetic formula, 2n + 1.
Examples of trisomy if sex chromosomes
•Klinefelter syndrome
•Triple X syndrome
•Double Y syndrome
What is Klinefelter syndrome?
*Chromosome complement: 47,XXY
*Phenotype: Male
*Incidence: 1:1000
Features of Klinefelter Syndrome
Tall stature; thin build; long lower limbs
•Testicular atrophy
•Female pattern of pubic hair
•High pitched voice
•Infertility (aspermatogenesis)
•Gynaecomastia
•Low level of intelligence
•Serum testosterone levels low to normal
•FSH and LH levels very high
•Sex chromatin positive
What is Testicular atrophy?
Testicular atrophy refers to the shrinking of testicles
Triple X syndrome (superfemale)
•Chromosome complement: 47,XXX
•Phenotype: Female
•Incidence: 1:1000
Features of triple X syndrome
•Normal in appearance
•Difficulty in speech, learning and emotional responses
•Mild mental retardation in 15-25% cases
•Two sex chromatin Barr bodies
•Infertility
•Wide-set eyes
•Amenorrhoea
•Expressionless face
•Enamel hypoplasia
•Deficient language skills
•Delayed development of motor skills
What is enamel hypoplasia?
Dental enamel hypoplasia is an enamel defect characterized by thin or absent enamel
What is a Barr body?
the inactive X-chromosome in the somatic cells of mammalian females
Double Y syndrome
•Chromosome complement: 47,XYY
•Phenotype: Male
•Incidence: 1:1000
Features of double y syndrome
•Normal in appearance
•Tall stature
•Aggressive behaviour
•Problems in motor and language development
Tetrasomy
•These arise by the addition of an extra pair of chromosome to the diploid set with a chromosomal formula 2n + 2.
•By this a particular chromosome is represented in four doses instead of normal two.
Euploidy
addition or loss of complete one set (n) or more than one set of chromosomes
Types of Euploidy
Monoploidy: Presence of a single copy of a single genome is known as monoploidy, denoted by x.
Polyploidy: Organisms having more than two normal sets of chromosomes (2n) are called polyploids.
Organisms with three sets of chromosomes (2n + n) = 3n, are triploids.
•Those with four sets of chromosomes (2n + 2n) = 4n, are tetraploids and those with five sets (2n + 3n) = 5n, are pentaploids and so on.
Mechanism of polyploidy
(a)Failure of pulling apart of 2 chromatids to opposite ends after metaphase stage of mitosis.
(b)Reduplication of chromosomes without dissolving of nuclear membrane.
(c)Failure of cytoplasmic division.
Types of polyploids
*Autopolyploids and allopolyploids
Autopolyploidy
Autopolyploids are those polyploids which have same basic set of chromosomes multiplied.
For instance, if a diploid species has two similar sets of chromosomes (AA), an autotriploid will have three similar sets (AAA) and an autotetraploid will have four such sets (AAAA).
Allopolyploidy
*Allopolyploids are those polyploids which contains two or more distinct (different) genomes.
* If we double the chromosome number in a F1 hybrid (AB) which is derived from two distinctly different species, the resulting polyploidy will be allotetraploid (AABB).
*common wheat is an allohexaploid having genome configuration AABBDD (three distinct species).
Mechanism of Aneuploidy
Non-dysjunction: failure of separation of chromosomes during cell division.
•Formation of 2 types of gametes (both abnormal)
•Fusion of either of these abnormal gametes with a normal gamete can result in trisomy or monosomy
•May involve autosomes or sex chromosomes
Examples of trisomy’s of autosomes
*Trisomy 13 or D-trisomy (Patau syndrome)
*Trisomy 18 or E-trisomy (Edward syndrome)
*Trisomy 21 or G-trisomy (Down syndrome)
Trisomy 13 (Patau Syndrome)
•1st described by Bartholin (1657) & redefined by
Patau (1960).
•Chromosomal complement: 47,XX,+13 (female) or
47,XY,+13 (male)
•Phenotype: Male or female
•Incidence: 1:12,000 (increases with the age of
mother)
Features of Patau syndrome
Mental deficiency
•Low birth weight
•Abnormal development
of frontal lobe
•Absence of corpus callosum
•Hypoplasia of cerebellum
•Sloping forehead
•Scalp defects
•Malformed ears
•Congenital heart defects
•Renal tract anomalies
•Microphthalmia
•Bilateral cleft lip/palate
•Polydactyly with
rudimentary digits
•Rocker-bottom heel
Trisomy 18 (Edward Syndrome)
•Chromosomal complement: 47,XX,+18 (female) or
47,XY,+18 (male)
•Phenotype: Male or female
•Incidence: 1:8000
Features of Edward Syndrome
•Mental deficiency
•Growth retardation
•Prominent occiput with elongated head
•Webbing of the neck
Short sternum
Micrognathia
•Low-set malformed ears
•Ventricular septal defects
•Renal anomalies
•Clenched fists with overlapping of fingers
•Hypoplastic nails
Micrognathia
Micrognathia is a condition in which the mandible is undersized (madibular hypoplasia)
What is hypoplasia?
Hypoplasia is the incomplete development or underdevelopment of an organ or tissue.
Trisomy 21 (Down Syndrome)
•Chromosomal complement: 47,XX,+21 (female) or 47,XY,+21 (male)
•Phenotype: Male or female
•Incidence: 1:800 (increases with the age of mother)
Features of Down Syndrome
•Short height
•Severe mental deficiency with decline in the IQ with age
•Brachycephaly with flat face and occiput
•Flat and low nasal bridge
•Upward slant to palpebral fissures
•Malformed large ears
•Epicanthal folds of the eyes
•Brushfield spots in iris
•Renal anomalies
•Prominent and protruding tongue (scrotal tongue)
•Simian crease
•Clinodactyly of 5th digit
What is Brachycephaly?
is an infant skull deformity characterized by a shortened anteroposterior skull length and a widened biparietal diameter (a flattened area at the back of the skull)
What is simian crease/ single palmar crease
A single palmar crease (simian crease) is a single line that runs across the palm of the hand. People most often have 3 creases in their palms.
What are Brushfield spots?
Brushfield spots are small, white or greyish/brown spots on the periphery of the iris in the human eye due to aggregation of connective tissue, a normal constituent of the iris stroma
What is clinodactyl?
Fingers curving to one side
What are the palpebral fissures?
The palpebral fissure is the area between the open eyelids.
What is the occiput?
Back of the head
What is microphthalmia?
Microphthalmia is when one or both of a baby’s eyes are small
What is a ventricular septal defect (VSD)?
A ventricular septal defect (VSD) is a hole in the heart. It’s a common heart problem present at birth (congenital heart defect)
What are hypoplastic nails?
the nails in young children, especially the toenails, may be unusually small and underdeveloped
