Biology Week 5:DNA and Traits - day 3

Genotype - the genetic makeup of an organism.
Phenotype - the external appearance of an organism.
Homozygous trait - the genes for that trait are the same.
Heterozygous trait - the genes for that trait are not the same.
Monohybrid cross - a cross between individuals with one pair of contrasting genes.
Dihybrid cross - a cross between individuals with two pairs of contrasting genes.
Parents - the two organisms whose genes produce offspring.
F₁ generation - the offspring from parents.
F₂ generation - the offspring produced by crossing two F₁ individuals.  

The traits that are passed from parents to offspring are determined by the combination of genes that are in the gametes. This is a random event in nature. The probability of a particular event occurring is the fraction of outcomes in which that event occurs.

What is the probability of the number 5 coming up when a dice is rolled?

• A single roll of a dice has the possible outcomes 1, 2, 3, 4, 5, 6.
• There are a total of 6 possible outcomes, one of those being the number 5.
• The probability of the number 5 coming up is 1/6.

What is the probability of rolling an even number with a dice?

• The even numbers are 2, 4, 6.
• Each of these has a 1/6 chance of occuring.
• This gives us three chances in six of rolling an even number. 3/6 = 1/2

What is the probability of cutting an ace from a deck of shuffled cards?

Introduction to Punnett Squares:

A Punnett Square is a chart drawn to determine the probable results of a genetic cross. To be able to draw a Punnett Square, you must know the genotype of both parents.

Sample Problem 1.   In roses, red flowers are dominant over white flowers. What are the possible offspring when a homozygous red rose is crossed with a homozygous white rose.

Nine steps are needed to solve problem 1: Write genotype for each parent. Use a capital case letter of the dominant trait to represent a dominant gene. R - for the dominant trait red. Use a small case letter of the dominant trait to represent a recessive gene. r - for the recessive white. If no dominant trait is present, use a small letter of each trait. Both parents are homozygous in the sample problem. The genotypes for the cross are - RR X rr Write the possible gametes of one parent across the top. Write the possible gametes of the other parent down the side. Draw the lines of your Punnett Square. This is the first time you know how many squares are inside the Punnett Square. Combine the gametes from each parent in the squares. Always write capital letters first. Count the number of each possible combination present in your Punnett Square. These are the offspring genotypes. 4   R r The phenotypes are the way the offspring appear. 4   red Since the genes are not the same, this trait is heterozygous. Divide the number of each combination type by the total combinations. You now have the probability ratio for the offspring of the parents. 4 / 4 Four out of Four, or all (100%), of the offspring will be heterozygous red, R r

Sample Problem 2.   Draw a Punnett Square showing the possibilities when two of the Rr offspring above are crossed.   What genotypes are predicted by this square?   What phenotypes are predicted by this square?

Sample Problem 3.   In carnations, neither red nor white is dominant. What are the possible results when two pink carnations are crossed? Since neither trait is dominant, no capital letters are used in the Punnett Square. Use the first letter of each trait to indicate that gene in the square.

Day 3 Assignment - DNA and Traits
Scoring criteria

History of Genetics

1. Who is considered the father of genetics?
2. Summarize in your words the scientific conclusions he published in his paper in 1865 about the differences he noticed in pea plants.
3. Mendel's paper explaining his experiment has been translated into English. Read the first three sections to answer the following questions:
   1. How long did Mendel run the experiment?
   2. Why did Mendel think members of the family Leguminosae would be best for the experiment?
   3. What is the scientific name (genus and species) of the plant Mendel used?
   4. What seven characteristics in Mendel's plants were chosen for comparison during his experiment?

Probability and The Punnett Square

1. Explain how probability can be used to predict the outcome of a random event?
2. What is needed to construct a Punnett square?
3. Explain why a Punnett square would be used?
4. What does a capital letter represent in a Punnett Square?
5. When you see a Punnett Square with no capital letters in it, what do you know about the traits represented in the square?
6. Explain the difference between an organisms genotype and phenotype.
7. Is the offspring used in sample question 2 above heterozygous or homozygous for the trait?
8. Complete the above sample Punnett square problems here (problems1, 2 & 3). Include the genotype and phenotype for the f₁ generation.

Practice monohybrid crosses (show all Punnett square work)

1. Let's say that in seals, the gene for the length of the whiskers has two alleles.  The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers.

   a)  What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?
   b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers?

    

2. In chimpanzees, straight fingers are dominant to bent fingers. Complete a Punnett square to show the genotypes and phenotypes expected for the following cross: heterozygous straight x homozygous bent.

 

Honors biology: complete 1,2 & 3 above and the following section

1. Use this link to visit the University of Arizona's  The Biology project Monohybrid Cross problem set. Read the directions at the site carefully and complete each of the questions. Copy each of the questions and your correct answers on a sheet of paper.

 

Research Links:

• Genetics - Arthro limited, Education on the internet
• Help with Punnett squares - Baby Steps through Punnett Squares
• Genetic transfer and the Punnett square - Liverpool central school district
• Java Genetics - BioCrypt

A gene pool refers to all the genes that can be shared by reproduction within a population.