Class Notes (809,497)
Canada (493,753)
Biology (Sci) (2,418)
BIOL 202 (224)

1. Single Gene Inheritance

10 Pages
Unlock Document

McGill University
Biology (Sci)
BIOL 202
Tamara Western

Single gene inheritance: Genetic analysis begins with mutants. By comparing the wild type species, which is found naturally, to a genetic mutant, it is possible to observe phenotypes This is, in fact, the basis of genetics o From this analysis, we can go on to interpret whether these phenotypes arise from singe gene mutations or multiple genes o For example, look at the different phenotypes which arise from different mutations of a certain bacteria Prior to Mendel, genetics was governed by a rule known as blending inheritance o In this theory, the offspring of a parent are a genetic mixture of both parents For example, the child of a small mother and tall father would have a medium height o This theory was later disproved by Mendel Mendel chose to study the inheritance of characters, called phenotypes, whose expression is strong (usually visible) and strongly influenced by gene action He chose pea plants as a model, which luckily showed phenotypes of a strong, simple genetic basis o Round or wrinkled seeds o Yellow or green seeds o Purple or white petals o Inflate or pinched ripe pods o Green or yellow unripe pods o Axial or terminal flowers o Long or short stems There are two types of crosses which Mendel used on his pea plants o Cross pollination involves transferring pollen of one flowers anthers to the stigma of another flower The anthers are cut off the receiving flower to ensure that it doesnt pollenate itself o Selfing is when the pollen of one flower are brushed onto its own stigma, which is essentially crossing the slower with itself A monohybrid cross involves the crossing of two parental plants to form an F1 generation. This F1 generation is then selfed to produce an F2 generation This cross is not limited to a specific trait, but for this example take small plants vs. short plants The parental generation P is crossed using one tall plant and one short plant o This produces an F1 generation of all tall plants This disproves the blending inheritance theory, which would have expected medium height plants The F1 hybrids are then selfed, crossing a tall with a tall o According to Mendels research, this produced: 787 tall and 277 short, roughly a 3:1 ratio As stated before, this phenomenon was not limited to a specific trait. According to the following table, all of the traits Mendel experimented with roughly followed this 3:1 pattern in the F2 generation Mendel created a model to explain his findings from the monohybrid cross. He came up with the first notion of a gene, which at this point was just a unit of inheritance Mendel has no idea of the molecular mechanisms involved in the expression of these traits, but was able to come up with a model using alleles, which are units of inheritance o The monohybrid cross introduced the idea of dominance and recessiveness, where once allele is able to mask the presence of another o It also introduced the idea of equal segregation For example, if an organism contains two different alleles for a specific trait, the chance of passing oneach allele to its progeny is 50% The parental generation involved two homozygous pea plants, one dominant (denoted as Y/Y) and one recessive (y/y) o The F1 generation resulted from this crossing, therefore, will all be Y/y because of the law of equal segregation The F1 generation looks yellow because the dominant Y (yellow) allele masks the green (y) allele The F1 generation is then crossed producing a Y/y X Y/y cross. To help solve all of the possible gamete transfers, a Punnett square is a useful tool o Looking at the resulting genotypes from this gross, the ratio is 1 Y/Y : 2 Y/y : 1 y/y o This corresponds to the resulting phenotype of 3 yellow (both Y/Y and Y/y) : 1 green (y/y only) Crossing instead the heterozygous Y/y with a y/y plant, it is possible to make another Punnet square to view the resulting progeny o In this case, the F2 generation will by spread out over a genotype ratio of 1 Y/y : 1 y/y o The phenotypic ratio is 1 yellow : 1 green This is an example of hypothetical deductive science, which involved the creation of models to explain unknown results o Som
More Less

Related notes for BIOL 202

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.