Lecture 19 (Extensions of Mendelian genetics) Flashcards
Why is the phenotype for SS and Ss the same?
This is because S is dominant over s
What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross?
The probability is 1/16
What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross? - How can we achieve ss?
P of parent 1 supplying s = 1/2
P of parent 2 supplying s = 1/2
1/2 x 1/2 = 1/4
What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross? - How can we achieve yy?
P of parent 1 supplying y = 1/2
P of parent 2 supplying y = 1/2
1/2 x 1/2 = 1/4
What is the possibility of getting a ssyy offspring from a SsYy x SsYy dihybrid cross? - what is the probability of ss and yy occurring?
What is the probability of both of these events occurring …
1/4 x 1/4 = 1/16
Product rule
Use the product rule for independent events e.g. chance of s allele from parent one and chance of s allele from parent 2 are totally independent events and therefore can be multiplied together
AaBBCc x AaBbcc = AaBbcc
How can we achieve Aa?
P of parent 1 supplying A= 1/2
P of parent 2 supplying a = 1/2
1/2 x 1/2 = 1/4
P of parent 1 supplying a= 1/2
P of parent 2 supplying A = 1/2
1/2 x 1/2 = 1/4
1/4+1/4 = 1/2 (sum rule)
AaBBCc x AaBbcc = AaBbcc
How can we achieve Bb?
P of parent 1 supplying B= 1
P of parent 2 supplying b = 1/2
1x1/2 = 1/2
AaBBCc x AaBbcc = AaBbcc
How can we achieve cc?
P of parent 1 supplying c= 1/2
P of parent 2 supplying c = 1
1x1/2 = 1/2
AaBBCc x AaBbcc = AaBbcc
What is the probability of the offspring AaBbcc?
(1/4+1/4) x 1/2 x 1/2 = 1/8
Sum rule
The probability that one or the other of two mutually exclusive events will occur is the sum of their individual probabilities. The rule that states that the probability of the occurrence of mutually exclusive events is the sum of the probabilities of the individual events. (for examples where it is a or b)
Multiple alleles
Three or more alternative forms of a gene (alleles) that can occupy the same locus. However, only two of the alleles can be present in a single organism.
Polymorphic
A gene is said to be polymorphic if more than one allele occupies that gene’s locus within a population.
An individual can only have two alleles, one on each homologous chromosome however in a population there can be a bunch of different alleles
Incomplete dominance
Occurs when one allele is not completely dominant over the other. The heterozygote produced shows a blend of the two alleles. (you have an intermediate)
e.g. a red and a white flower show incomplete dominance when they produce a pink flower (shown in a heterozygote)
Incomplete dominance …When f1 has an intermediate phenotype …
When f1 has an intermediate phenotype, it suggests blending inheritance.
However, in the F2 generation (when two organisms with intermediate phenotypes mate), it rejects the hypothesis of blending inheritance and instead supports particulate inheritance
Co-dominance
This is where both phenotypes exist side by side in an organism (both alleles are simultaneously expressed in the heterozygote) (its like superimposing two different phenotypes on each other)
Both parental phenotypes are present in F1
Co-dominance in blood
Blood type is also an example of multiple alleles - the ABO system of blood groups is controlled by three alleles, only two of which are present in an individual.
AB blood type is an example of co-dominance as the A is inherited from one parent and the B is inherited from the other parent and are expressed simultaneously
Polygenic traits
A polygenic trait is one whose phenotype is influenced by more than one gene. Traits that display a continuous distribution, such as height or skin colour, are polygenic.
Phenotype is controlled by many genes that have an additive effect. Characters appear continuous or quantitive i.e. non-discrete steps in range of phenotypes
Controlled by groups of genes. Polygenic traits do not exhibit complete dominance as do Mendelian traits, but exhibit incomplete dominance.
Additive effect of genes
Means that genes are almost working together to create a trait
Additive genetic effects occur when two or more genes source a single contribution to the final phenotype, or when alleles of a single gene (in heterozygotes) combine so that their combined effects equal the sum of their individual effects.
Polygenic trait in a population
Normal’ distribution typical of polygenic trait in a population
The more genes involved, the higher the number of phenotypic classes
The environment on phenotype
Environment (nature and nurture) also affects phenotype e.g. nutrition and height/weight; life expectancy and disease
The environment smooths difference among phenotypes
