Genetics Flashcards
Autosomal recessive inheritance - risk of 2 parent carriers
1:4 chance 2 normal genes (unaffected, non-carrier)
1:4 chance affected carrier
2:4 chance unaffected carrier
Features of Turner syndrome
- Short stature
- Lymphoedema of hands and feet in newborns
- Webbed neck, low-set ears, broad chest, widely spaced nipples
- Delayed puberty, primary amenorrhoea (gonadal dysgenesis - streak ovaries and infertility)
- Heart defects - bicuspid aortic vavle, CoA
- Increased risk of autoimmune disorders (hypothyroidism, coeliac)
Condition?
Turner syndrome - X,O (absence of all or part of one X chromosome)
Turner syndrome inheritance pattern
- Not typically inherited - occurs due to random meiotic error.
- 45,X karyotype, although some may have mosaicism with a 46,XX or 46,XY cell line.
- Not generally passed on to offspring; affected individuals are usually infertile.
- About half of the population with Turner syndrome have monosomy X (45,XO). The other 50% of the population has a mosaic chromosomal component (45,X with mosaicism).
- Rarely, Turner syndrome can result from a partial deletion of the X chromosome, and this can pass from one generation to the next.
Describe segregation and linkage analysis
Segregation analysis is used to determine the pattern of inheritance for a trait.
Linkage analysis is used to determine the genetic location of a disease gene (ie identify a piece of DNA of known location only inherited by affected family members). Once the DNA is found, one knows the disease is close by.
What is a phenotype?
An individual’s observable traits, such as height, eye colour, and blood type. The phenotype results from the interaction between the genotype and the environment. ‘Pheno’ means ‘observe’ and comes from the same root as the word ‘phenomenon’.
What is a genotype?
A genotype is an individual’s collection of genes.
What is a haplotype?
A haplotype is a set of DNA variations, or polymorphisms, that tend to be inherited together.
What is a karyotype?
A karyotype is an individual’s collection of chromosomes.
What is penetrance?
The proportion of a population of individuals who carry a disease-causing allele who express the related disease phenotype aka how likely it is that a condition will develop. Eg the penetrance for retinoblastoma is 90%, as 10% with the AD retinoblastoma causing mutation don’t develop it.
What is expressivity and variable expressivity?
Expressivity describes the extent to which a genotype shows its phenotypic expression in an individual aka the severity of the phenotype. Variable expressivity means the same genotype can have varying degrees of the phenotype.
Name 2 conditions with incomplete penetrance
Retinoblastoma, Huntington’s
Name 2 conditions with 100% penetrance
Achondroplasia
Conditions with AD pattern inheritance (7)
- Neurofibromatosis type 1&2 2. Tuberous sclerosis
- Marfan’s, achondroplasia
- Myotonic dystrophy
- Noonan’s syndrome
- Retinoblastoma
Conditions with autosomal recessive pattern (13)
- Phenylketonuria
- Homocystinuria
- Hurler’s syndrome
- Galactosaemia
- Tay-Sach’s disease
- Friedrich’s ataxia
- Wilson’s disease
- CF
- Sickle cell
- Thalassaemias
- MCAD deficiency
- Spinal Muscular Atrophy
- Alpha-1 Antitrypsin Deficiency
Conditions with x-linked dominant inheritance pattern (4)
- Vit D resistant rickets
- Rett syndrome
- Aicardi syndrome
- Alport syndrome
Conditions with x-linked recessive inheritance pattern (7)
- Duchenne muscular dystrophy
- Becker muscular dystrophy
- Haemophilia A&B
- G6PD deficiency
- Red-green colour blindness
- Hunter’s (MPS II)
- Lesch-Nyhan
Explain autosomal dominant inheritance pattern
- Conditions are expressed in individuals who have just one copy of the mutant allele.
- Affected M&F have an equal probability of passing on the trait to offspring, and M&F equally affected.
- Affected individual’s have 1 normal copy of the gene and 1 mutant copy of the gene, thus each offspring has a 50% chance on inheriting the mutant allele.
- Transmitted from one generation to the next (vertical transmission)
- Parents usually one affected heterozygote and one unaffected homozygote.
- If only one parent affected there is a 50% chance that a child will inherit the mutated gene.
Pattern of inheritance?
Autosomal dominant
Explain autosomal recessive inheritance pattern
- Conditions are clinically manifest only when an individual has 2 copies of the mutant allele. When just one copy of the mutant allele is present, an individual is a carrier of the mutation, but does not develop the condition.
- F&M equally affected.
- When two carriers mate, each child has a 25% chance of being homozygous wild-type (unaffected); a 25% chance of being homozygous mutant (affected); or a 50% chance of being heterozygous (unaffected carrier).
- Both parents’ usually unaffected heterozygotes.
Pattern of inheritance?
Autosomal recessive
Explain x-linked recessive pattern of inheritance
- Not clinically manifest when normal copy of the gene.
- Traits fully evident in males b/c they have only one x chromosome, but women are rarely affected (but are when have 2 copies of the mutant allele).
- No father to son transmission, but there is father to dtr and mother to dtr/son transmission.
- If male affected with an x-linked recessive condition, all dtrs will hv one copy of the mutant gene
- A female carrier has a 50% chance of having affected sons and a 50% chance of having carrier dtrs
Pattern of inheritance?
X-linked recessive
Pattern of inheritance?
X-linked dominant
