Part 2: Inheritance of Genetic Traits The chromosomes you studied in the previou
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Part 2: Inheritance of Genetic Traits
The chromosomes you studied in the previous part of this investigation contain discrete units called genes. These genes contain all of the information attributed to the physical features (e.g. height, eye color) and chemical features (e.g. proteins) of an organism. An individual organism is made up of two sets of genes; one set of genes is contributed via the sperm and a second set of genes is contributed via the egg. These genes may be the same or different. Different versions of the same gene are called alleles.
To demonstrate how genes are inherited, work through the following problem.
The ability to roll your tongue is an inherited genetic trait. Let’s assign the genotype RR to the father who has the ability to roll his tongue and the mother has genotype rr with recessive alleles and she cannot roll her tongue. For the sake of argument this is a dominant trait, which means that every time "R" is present in the genotype the individual will be a tongue roller. The child they produce will have the genotype Rr and retain the ability to roll their tongue (1 allele from each parent).
Now what happens when this individual has a child with someone who is a non-tongue roller? Using a Punnett Square helps illustrate the genotype possibilities (refer to your textbook). Recall from your exercise on meiosis that the gametes only receive one set of chromosomes each.
Parent 1: genotype Rr; gametes "R" & "r"
Parent 2: genotype rr; gametes "r" & "r"
Possible offspring genotypes: Rr; Rr; rr; rr
In this 2nd generation a child has a 50:50 chance of having the ability to roll their tongue.
This is the simplest case of inheritance. Patterns of inheritance are generally more complex than this. Characters may be linked together, i.e. the responsible genes are close together on the chromosome and are usually inherited together; sometimes they are only associated with the sex chromosomes and traits are visible in males or females. You can read about this some more in your chapter on Mendelian Genetics.
This next part of the investigation is a demonstration of how populations evolve through the process of natural selection. To show this we have to apply the principles of Mendalian genetics discussed above.
Objectives:
Understand basic principles of how genes are passed from one generation to the next
Understand how selection pressure can change the frequency of alleles in a population
Understand that if allele frequencies are changing then the population is evolving to increase the likelihood of survival
Materials:
Student provides
Black colored glass rocks (from craft store)
White colored glass rocks (from craft store)
3 paper or cloth sac to hold rocks
Lab Kit
Nothing form lab kit
Natural Selection Exercise:
The black rocks represent the dominant allele B for brown coat color in rabbits; the white rocks represent the recessive allele b. Rabbits which have the genotype bb will have a white coat.
To determine the genotype frequency and allele frequency in a population we turn to the math equation known as the Hardy-Weinberg principle. This equation is written as follows
where p and q represent the frequencies of alleles B and b; p + q = 1.0
Your original "Parental Population" (see below) has the following gene frequencies (recall gametes only receive one set of chromosomes containing alleles)
15 BB individuals produce 30 gametes
30 Bb individuals produce 30 B gametes + 30 b gametes
15 bb individuals produce 30 gametes
Total number of gametes = 120
Allele frequency is calculated by dividing the number of times an allele is present in the population by the total number of alleles in the population
The frequency of alleles B and b in the original parental population is
Absence of selection pressure
Open your bags of black and white rocks
Label your brown paper sac "Parental Population"
Add 15 individuals which are homozygous for the dominant allele (BB); this is the equivalent of 30 black rocks to the "Parental Population"
Add 15 individuals which are homozygous for the recessive allele (bb); this is the equivalent of 30 white rocks
Add 30 heterozygous individuals (Bb) to the Parental Population" ; this is the represented by 30 black rocks and 30 white rocks
Shake the bag to mix the rocks
Assume that the adult organisms mate at random. Simulate this by retrieving alleles from the bag to represent the offspring for this generation
Without looking, pull 2 rocks (representing 2 alleles) from the bag and make a note of the offspring genotype
Repeat until you have the genotypes of 30 offspring
Record how many of each type of offspring genotypes were produced
Post your results on the discussion board
Pool the results of 5 students in the class and note the allele and genotype frequencies in the table below NOTE: A worked example of calculating allele frequency is shown in Announcements from Instructor.
Effect of Selection Pressure
White rabbits living in an area that has brown colored undergrowth are at a distinct disadvantage in that they are easily seen by predators. Now observe what happens to the rabbit population when this negative selection pressure is applied to our population of rabbits.
Establish the "parental population" as you did in the previous exercise
Label a 2nd paper sac "Next Generation"
Label the 3rd paper sac "cannot reproduce/dead"
Assume random mating in the population; without looking pull 2 rocks from the parental population to represent the first offspring
Make a note of the genotype: if the genotype is bb (white coat) those rocks are placed in the paper sac labeled "cannot reproduce", those offspring (and alleles) are effectively removed from the population since they do not contribute genes to future generations; BB and Bb genotypes are placed in the bag labeled "next generation"
Continue producing offspring for the first generation until the "parental population" is depleted.
Place the contents from the "next generation" into the "parental population" bag
Repeat the process of simulating offspring now for the 2nd generation, once again removing individuals with the genotype bb
Repeat through 5 generations each time recording the number of different genotypes
Record your findings in the table below and post them on the discussion board
Pool the data of 5 students and record the numbers below
NOTE: In this exercise we exerted 100% negative selection pressure to exaggerate the concept. In reality, most selection pressure is not this extreme.
Number of individuals according to genotype in each generation:- Individual data
NOTE: You should have a total of 60 individuals in generation 1.
Number of individuals according to genotype in each generation:- Pooled data from 5 students
NOTE: You should have a total of 300 (60 x 5) individuals in generation 1.
Graph your pooled data as a bar graph (you can use excel or other software program). "Generation" will be the variable on the x-axis; "Number of Individuals" will be the variable on the y-axis. Each generation will have 3 bars showing the frequency of the different genotypes (BB; Bb;bb). Try and generate all 5 generations on the same page (this will make the results easier to visualize). Submit your graph for grading.
In the space below discuss what happens to this population (1) in the absence of selection pressure and (2) over successive generations with 100 percent negative selection pressure. Discuss your answer in terms of genotype, phenotype and selection pressure applied. [12 pts]
Rabbit coat color p2 + 2pq + q2 = 1.0 BB Bb bbExplanation / Answer
Based on the given data, you will get
Total number of gametes = 240
The frequency of alleles B and b in the original parental population is:
So,