Section 5:DNA and Traits - part b

S4.C2.PO(1-4) S2.C1.PO(1-4) C2.PO(1-4) S3.C2.PO(3)

the study of the traits of organisms.

click to find the answer to today's question What is a gene pool?

Sexual reproduction involves the combining of a sperm cell and an egg cell from two individuals. The offspring of this "cross" will have characteristics from both "parents". Organisms that are produced by crossing individuals with different traits are known as hybrids. In most cases, hybrid organisms are more successful than those that are produced from a very limited gene pool. This is sometimes called hybrid vigor. While gene sharing is well understood today, before Gregor Mendel the process was a mystery. Homeboy Mendel

Gregor Mendel link to an Internet Website is known as the father of modern genetics. Mendel's genius was that he noticed differences in peas being grown in his gardenlink to a local picture and applied the scientific method to determine the cause of these differences. He published a paper in 1865 with these three conclusions to his research:

At the time, chromosomes and the process of meiosis were unknown. Mendel's work was considered obscure and unimportant until 1900, when Walter Sutton proposed the Chromosome Theory.

Important genetic terms:

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.

Gene - A discrete unit of heredity information consisting of a specific nucleotide sequence of DNA carried on chromosomes.

Allele - An alternative forma of a gene. (Stem length:  T = tall stem   t = short stem)

Codominant - both alleles are expressed in heterozygous individuals (AB blood type).

Complete dominance - the dominant allele has the same phenotypic effect whether present in one or two copies

Incomplete dominance - a phenotype of a heterozygous individual (Aa) is intermediate between the phenotype of the two types of homozygous individuals (AA, aa). (does not blend)

Sex linked gene - A gene located on either sex chromosome.



To understand genetics, one must understand probabilities.

Random events and probability 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?

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

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.  

Assignment 1.5b - DNA and Traits
Scoring criteria
link to a local webpage

Part 1: History of Genetics

  1. Why is Gregor Mendel considered the father of genetics?
  2. View this video on Mendel and Classical Genetics     in order to answer the following video questions.
    1. Describe two rejected hypothesis about heredity and why they were rejected?
    2. How did Mendel figure out that blending was not responsible for pink flower offspring of red and white flowers?
    3. Draw a diagram describing and defining the term allele.
    4. Explain how alleles are related to the terms homozygous and heterozygous?
    5. What is the relationship between phenotype and genotype?
    6. Explain how probability is part of genetics?
    7. How does the canary breeder get reds into the genetics of canaries?
    8. Why are test crosses important in genetics?
    9. What lead to Mendel's discovery of the principles of inheritance?
  3. Visit this site link to an Internet Website and explore inheritance patterns just as Gregory Mendel did. Complete the first four sections recording all data and questions on paperlink to a local webpage for each of these sections.
    1. Pure breeding   
    2. Do Traits Blend
    3. Patterns of Inheritance


Part 2: 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 (in green boxes, problems 1,2, 3) here. Include the genotype and phenotype percentages for the f1 generation.

Part 3: 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 for 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:

  1. Use this link to visit the University of Arizona's  link to an Internet Website The Biology project Monohybrid Cross problem set. Read the directions at the site carefully and complete questions 1-8. Copy each of the questions along with the work you did to get your answers on a sheet of paper or in this assignment.


Research Links:



Biology Class Nucleic Acid Page










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