Chapter 14 - Genetics Flashcards
What is a mono hybrid cross?
A cross in which only one trait is being studied
State the four basic tenets of Mendel’s first law (the law of segregation)
- Genes exist in alternative forms (alleles)
- An organism has two alleles for each gene, one inherited from each parent.
- The two alleles segregate during meiosis, resulting in gametes that Carey only one allele for any inherited trait.
- If two alleles in an individual are different, only one will be fully expressed (the dominant allele), and the other will be silent (the recessive allele).
What are the generational designations used in Mendelian genetics?
P generation - the parent generation. These are the individuals being crossed.
F generation - the offspring, the filial generation. Subscripts designate generational distance from the parent generation.
What is a Test Cross?
Test crosses are used to determine unknown genotypes. Note that only homozygous recessive individuals are used in test crosses as such crosses will always yield the ability to determine the unknown genotype based on the phenotype distribution of the offspring (F generation).
Test crosses are often called back crosses sine we are attempting to determine the genotype of the parent by those of its offspring.
In a test cross, what can we glean about the unknown genotype given 100% of offspring are of the dominant phenotype? 50% dominant?
If all of the offspring are of the dominant phenotype then all of the offspring are heterozygous dominant (Pp) the unknown is homozygous dominant (PP).
If only half of the offspring express the dominant phenotype then half must be heterozygous dominant (Pp) and half homozygous recessive (pp). This can only occur if the unknown is a heterozygote (Pp).
What is Mendel’s second law called, and what does it state?
The Law of Independent Assortment. This law is based on dihybrid crosses and states that each gene’s inheritance (assortment) is independent of (unrelated to) the inheritance of other genes. This is only true of genes that are unlinked.
What are the predicted frequencies (genotype and phenotype) associated with a dihybrid cross of homozygous dominant (TTPP) and a heterozygous (TtPp) parents.
The genotype frequencies are 25% TTPP, 25% TtPP, 25% TTPp, and 25% TtPp. The phenotype frequencies are 100% for the dominant trait.
What are the predicted frequencies (genotype and phenotype) associated with a dihybrid cross of homozygous dominant (TTPP) and homozygous recessive (ttpp) parents.
The genotype frequencies will be 100% TtPp, and the phenotype frequencies will be 100% for the dominant phenotype.
What are the predicted distributions (genotype and phenotype) associated with a dihybrid cross of two heterozygous (TtPp) parents.
The genotypic frequencies will be 1/16 each for TTPP ttPP, TTpp, and ttpp; 1/8 each for TtPP, TTPp, ttPp, and Ttpp; and 1/4 for TtPp.
The phenotypes will have a 9:3:3:1 relationship. Note that each trait associates individually in a 3:1 ratio, as in a monohybrid cross. There are 9 DP and 3 Dp, for a total of 12 D; similarly, there are 3 dP and 1 dp, for a total of 4 d. Hence the D:d ratio is 12:4 = 3:1. The P:p ratio follows the same pattern, yielding 3:1.
Statistics - need to clarify the question and work on answer. This was in one if the questions at the back of the book…
Sum the probilities of each pathway when multiple independent pathways to the same outcome.
Multiply the probability of each event when multiple independent events must occur along one path to yield a specific outcome.
Codominance vs incomplete dominance
Codominance occurs when two dominant alleles are fully expressed by a heterozygous dominant individual, e.g. with the ABO blood type, types A and B are both dominant to type O, and Type AB individuals completely express the phenotypes of both A and B. Note that this is not an intermediate.
Incomplete dominance occurs when the a heterozygous individual expresses an intermediate phenotype, e.g. if dominant is red and recessive is white, a heterozygote will be pink.
