4: Mendelian inheritance in humans Flashcards
What are pedigree charts used for?
Used to infer mode of inheritance
Genetic counselling e.g risk of offspring inheriting condition
What are the pedigree symbols for males (unaffected and affected)?
Unaffected = square
Affected = filled in square
What are the pedigree symbols for females (unaffected and affected)?
Unaffected = circle
Affected = filled in circle
What is the pedigree symbol for unknown sex?
Diamond
What is the pedigree symbol for a carrier?
Square/circle with dot in the middle
What is the pedigree symbol for dead?
Square/circle with dash through
What is the pedigree symbol for marriage?
line connecting shapes
Describe autosomal dominant
- Affected person usually has at least 1 affected parent
- Affects either sex & transmitted by either sex = autosomal
- Child of unaffected x (hetero) affected mating has 50% chance of disease
- 8/14 with affected parent have disease = ~50%
Give some examples of autosomal dominant phenotypes in humans and mendel’s peas
Humans:
Achondroplasia → form of dwarfism
Polydactyly → extra fingers/ toes
Window’s peak
Mendel’s peas:
Yellow dom to green
Purple flowers dom to white
Smooth seeds dom to wrinkled
Describe an autosomal recessive
Typically, an affected person will not have affected parents
Parents usually carriers
Affects either sex
Often seen in pedigree where inbreeding is common
Carriers/non-carriers are indistinguishable
If 2 carriers mate offspring have ¼ chance of being affected and ½ chance of being a carrier
Give some examples of autosomal recessive phenotypes
Albinism
Sickle cell anaemia
Cystic fibrosis
Attached/ detached earlobes in humans
describe cystic fibrosis
→ in european pop.s it is the most common autosomal recessive disease
Affects lungs, increased mucus secretion
Median survival ~50 years (20 yrs ago was 31 years on av)
Heterozygotes may have (or had) a selective advantage through resistance to cholera, typhoid of other diseases
Genetics:
Affecteds have 2 inactive copies of the CFTR gene → involved in transport of chloride ions
Very large gene, so many diff mutations can cause CF = severity of disease depends on where mutation occurs
Most common mutation: Δ508
What disorder do X-linked recessive disorders often get mistaken for?
Autosomal recessive
Why do X-linked recessive disorders only commonly effect males?
XX females have another allele to compensate for the other, whereas XY males only have one → so if they have the disordered allele they will be affected
In X-linked recessive disorders, what % chance do male offspring of female carriers have of being affected?
50% chance
In X-linked recessive disorders, what is the only situation where females can be affected?
If father is affected and mother is a carrier
Give some examples of X-linked recessive disorders
Duchenne Muscular dystrophy
Red-green colorblindness
Haemophilia
What makes X-linked recessive disorders much more common within a family?
In breeding
e.g European Royal Families
describe X-linked dominant disorder
Affects either sex
Affected male will always have affected daughters
Affected female’s offspring will have 50% chance of being affected
No male children of affected males are affected
Give an example of x-dominant disorder
Quite rare so v few examples:
Hypophosphatemia → form of vitamin D-resistant rickets
Describe X-chromosome inactivation
In mammals female are XX and males XY
This inequality compensated for by X-inactivation (lyonization)
In each cell of females one X is randomly inactivated
Occurs early in development and daughter cells have the same X inactivated
Give an example of X-chromosome inactivation
Tortoiseshell cats → always female
Have 2 diff versions of a gene in their X chromosome
Dominant = orange
Recessive = black
Because some parts of the chromosome are inactive/ active the coat develops diff colours
= Because its X linked males will either be orange OR black (they only have 1 X)
describe Y-linked disorder
Affects only males
All sons of an affected man are affected
Give an example of Y-linked disorder, explain why there are very few examples
Very few examples:
Hairy ear rims possibly
Y chromosome eroding → has v few functioning genes
Beyond determining sex (SRY gene) it does very little
