GENE 303 L4

what is the ratio of the phenotypes in the F2 progeny when segregating 2 or more genes

9:3:3:1

• this is what the seed color phenotype v seed shape phenotype produced

what is the principle of independent assortment

in the formation of gametes, segregation of the members of any pair of alleles is independent of the segregation of the other pairs

penetrance

the proportion of orgaisms having a particular genotype that actually express the corresponding phenotype

think of it as a yes/o question that measures the entire population

• if a gene has 80% penetrance, that means that 80 out of 100 people show the trait in some form regardless of the severity

expressivity

the degree of the phenotypic expression

more of an indivdiual scale

• mild expressivity vs severe expressivity

what are the characteristics of rare dominant alleles with complete penetrance

1. 1. female and male are equally likely to be affected → non sex chromosome link

2. 2. affected offspring have one affected parent → equal likelihood of either parent

3. 3. half the individuals in offspring from an affected parent are affected

example of a rare dominant allele with complete penetrance

huntington disease

• homozygous dominant or heterozygote = affected by the disease

characteristics of recessive alleles with complete penetrance

1. 1. male and female equally likely to be affected

2. 2. affected individuals usually have unaffected progeny

3. 3. affected individuals usually have unaffected parents

4. 4. the parents of affected individuals are often relatives

5. 5. among siblings of affected offspring, the proportion of affected individuals is about 25%

example of recessive allele with 100% penetrance

albinism

what is probability in transmission genetics

the probability of a particular outcome of a cross is equivalent to the proportion of times this outcome is expected to be realized in numerous repeated trials

what is the addition rule and when is it used

P{A or B} = P{A} + P{B}

used for mutually exclusive events

ex in a cross of Aa x Aa, what is the probability the offspring is either AA or Aa?

AA = 1/4 , Aa = ½
P{AA or Aa} = ½ + ¼ = ¾

what is the multiplication rule and when is it used

P{A and B} = P{A} x P{B}

for independent events happening at the same time

ex) a cross of AaBb x AaBb (two independently assorting genes) what is the probability of a aabb offspring?
P(aa) = ¼ , P(bb) = ¼
P(aabb) = ¼ x ¼ = 1/16