2. SLE (Lupus). (Click
here for an article about lupus and genetics.)
3. Weight. (Click
here for an article about obesity and genetics.)
4. Eye Color. (Click
here for an article about eye color.)
5. Intelligence.
6. Skin Color.
7. Many forms of behavior.
Click
here to see Genetic Heterogeneity, Twinnings and Sibblings described by MITOPENCOURSEWARE.
Lecture 32. probability and pedigrees
In Mendel’s time he used statistics to account for his observations on his experiments on peas, and, thanks to the results he abtained, he could formulate his two famous laws of genetics-- the Law of segregation and the Law of independent assortment, which were based on statistical segregation ratio 3:1, 9:3:3:1, 1:1:1:1 etc…
Nowadays in genetic research and especially in medical genetic counseling, statistics is needed for calculating the risks of genetic diseases in human pedigrees. The risks in these cases are expressed in terms of so-called probability.
The probability of an event is the chance that it will happen. The probability of tossing a coin to land heads up is roughly ½.
The probability of an impossible event is 0.
the probability of a certain event is 1.
If the probability of event x is p then the probability of 'not x' is 1-p.
The probability of two independent events ocurring at the same time is the product of their two indivdual probabilities.
So, for example, in the cross above, in the F2 the
probability of a wrinkled seed is ¼; the probability of a green seed is also ¼, and the probability of being both green and wrinkled is therefore ¼ x ¼ = 1/16.
The probability of being not wrinkled (i.e. smooth) is 1-¼ = ¾. The probability of being both smooth and green is therefore ¾ x ¼ = 3/16 and so on.
In the
example below about the coefficient of inbreeding of children from first cousin marriages, we considered a number of probabilities of ½ which we multiplied together to reach a final probability of 1/16 that any gene was homozygous by descent.
Autosomal recessive
A recessive trait will only manifest itself when homozygous. If it is a severe condition, it will be unlikely that homozygotes will live to reproduce, and thus most occurrences of the condition will be in matings between two heterozygotes (or carriers). An autosomal recessive condition may be transmitted through a long line of carriers before, by ill chance, two carriers mate. Then there will be a ¼ chance that any child will be affected. The pedigree will therefore often only have one 'sibship' with affected members.
a) A 'typical' autosomal recessive pedigree and b) An autosomal pedigree with inbreeding If the parents are related to each other, perhaps by being cousins, there is an increased risk that any gene present in a child may have two alleles identical by descent. The degree of risk that both alleles of a pair in a person are descended from the same recent common ancestor is the degree of inbreeding of the person. Let us examine b) in the figure above.
Considering any child of a first cousin mating, we can trace through the pedigree the chance that the other allele is the same by common descent. Let us consider any child of generation IV, any gene which came from the father, III3 had a half chance of having come from grandmother II2, a further half chance of being also present in her sister, grandmother II4 a further half a chance of having been passed to mother III4 and finally a half chance of being transmitted into the same child we started from. A total risk of ½ x ½ x ½ x ½ = 1/16.
