Week 2 Flashcards
Mendelian genetics, probabilities, binomial equation, and dihybrid crosses
Why was it important that Mendel’s peas were highly in-bred?
Because it ensured homozygousity
What is the difference between a purebred and hybrid offspring?
- Purebred: homozygous for each chosen trait
- Hybrid: heterozygous genotype
How were Mendel’s experiments designed?
In a way that allowed him to study one trait at a time
What is a gene?
An inherited factor (encoded by DNA) that helps determine a characteristic
What is an allele
One of two or more alternative forms of a gene
What is a characteristic/character?
An attribute or feature possessed by an organism (determined by allele)
What is a locus?
The specific place on a chromosome occupied by an allele
What is a genotype?
The set of alleles possessed by an individual organism
What is a heterozygote?
An individual organism possessing two different alleles at a locus
What is a homozygote?
An individual organism possessing two of the same alleles at a locus
What is a phenotype/trait?
The appearance/manifestation of an allele
What do dominant alleles determine?
The expressed factor
How are dominant alleles denoted?
With an uppercase letter
What do recessive alleles determine?
The latent factor (not expressed unless homozygous)
What is the P (parental) generation?
The first generation of a genetic cross
What is the F1 (first filial) generation?
- The second generation of a genetic cross
- The offspring of the parents in the P generation
What is the F2 (second filial) generation?
- The third generation of a genetic cross
- The offspring of the F1 generation
In Mendel’s experiments, how did the F1 generation reproduce?
Via self-fertilization
What is Mendel’s principle of segregation?
Each individual organism possess two alleles at one locus (one from each parent)
What three observations led Mendel to conclude the principle of segregation?
- Each individual organism possesses two alleles encoding a trait
- Alleles separate when gametes are formed
- Alleles separate in equal proportions
What is Mendel’s principle of independent assortment?
Alleles at different loci separate independently (ex. allele at locus for wrinkled vs round is at a different locus than the allele for plant height
Assuming no crossing over occurs, when do segregation and independent assortment take place?
Anaphase I
If crossing over takes place, where might segregation and independent assortment take place?
Anaphase II of meiosis
What is Mendel’s principle of dominance?
In a heterozygote, one allele may conceal the presence of another
What is the multiplicative rule?
If events A and B are independent, the probability that they occur together is the product of their individual probabilities
What is the key word indicating the multiplicative rule should be used?
“and”
What is the additive rule?
If events A and B are independent and do not overlap, the probability that at least one of them occurs is the sum of their individual probabilities
What is the key word indicating the additive rule should be used?
“or”
What does binomial probability help determine?
- Any combination of events
- the probability that for n progeny, exactly x will fall into one class and y into another
What is the equation for binomial probability?
P=[n!/(x!y!)] (p^x) (q^y)
What does each variable in the binomial probability equation represent?
- Equation: P=n!/(x!y!)(q^y)
- P= overall probability of event X and Y
- n= total number of potential outcomes (x+y)
- x= the number of times we want X to occur
- y= the number of times we want Y to occur
- p= probability of X occurring
- q= probability of Y occurring
What two conditions need to be met when using the binomial equation?
- There are only two possible outcomes (ex. affected vs unaffected)
- Each event is independent of the other
What is a simple cross?
A single locus is under consideration and one of the alleles is dominant to the other
What is the phenotypic ratio of offspring when both parents are heterozygous?
3:1 (A_:aa)
What is the phenotypic ratio of offspring when one parent is heterozygous and the other is homozygous recessive?
1:1 (Aa:aa)
When does uniform progeny occur?
When all offspring have the same phenotype
What types of crosses produce uniform progeny?
- Any cross between two homozygotes (AA x AA, aa x aa, AA x aa)
- Homozygous dominant and heterozygous (AA x Aa) (dominant trait will be expressed in all progeny)
What is the genotypic ratio of offspring when both parents are heterozygous?
1:2:1 (AA:Aa:aa)
What is the genotypic ratio of offspring when one parent is heterozygous and the other is homozygous (dominant or recessive)
1:1 (Aa:AA or Aa:aa)
What is the genotypic ratio of offspring when both parents are homozygous (dominant or recessive)?
- All progeny are genotypically uniform
- AA x AA = all AA
- aa x aa = all aa
- Aa x Aa = all Aa
What is the difference between monohybrid crosses and dihybrid crosses?
- Monohybrid: cross between two individuals that differ in a single characteristic
- Dihybrid: cross between two individuals that differ in two characteristics
In a dihybrid cross, what is the genotypic ratio of offspring when one parent is homozygous dominant for both traits and the other is homozygous recessive?
All progeny are heterozygous at both loci (AABB x aabb = all AaBb)
In a dihybrid cross, what is the phenotypic ratio of offspring when both parents are heterozygous for both traits (the F2 generation in Mendel’s dihybrid cross)?
- 9:3:3:1
- dom both traits : dom trait 1 and rec trait 2 : dom trait 2 and rec trait 1 : rec both traits
- Mendel’s cross - round, yellow : round, green : wrinkled, yellow : wrinkled, green
How many different gametes can be produced in a dihybrid cross of a parent who is heterozygous for both traits (AaBb)?
Four (AB, ab, Ab, aB)
Which of Mendel’s laws allows multigene crosses to be broken down into multiple monohybrid crosses?
The law of independent assortment
What is a test cross?
An individual with an unknown genotype is crossed with an individual who is homozygous recessive to determine the genotype of the unknown individual
