Module 6 Lesson 2 - 2

Read pages 589 - 590

Recall that Mendel worked with seven contrasting characteristics. Each characteristic was controlled by a single gene, and each gene had two alternate forms known as alleles. These alleles related to each other as either dominant or recessive.

Dominant alleles are always expressed; they appear in the phenotype of either homozygous dominant or heterozygous individuals. Recessive phenotypes can be displayed when there are only copies of recessive alleles present; thus, they appear only in homozygous recessive individuals.

In working with genetic problems, understanding how alleles move and how they are expressed is important. Mendel's first law, the Law of Segregation, states that each allele pair in the parents is separated in the creation of gametes. This separation is random. Further, these gametes unite with other gametes in equally random ways. This ensures that all possible combinations of gametes appear in the offspring. To demonstrate this, geneticists use a Punnett square.

Read in your text, pages 589 and 590, about how to represent alleles properly and how to construct and use Punnett squares.

Monohybrid Punnett Squares (One Trait)

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A Punnett square is a chart used to predict the outcome of a cross. It shows all the possible combinations of genotypes in offspring. It can be used to determine an allele in one of the parents based on the results of the offspring. Punnett squares are used for either a monohybrid cross (a single trait such as Aa) or a dihybrid cross (two traits such as AaBb). This lesson focuses on the monohybrid cross.

How to Solve Monohybrid Punnett Squares

1. From the information about the trait, determine what is dominant and what is recessive.
   
   
   • Use a capital letter for dominant traits (colour, shape, etc.).
     
   • Use the lower case letter of the dominant trait for recessive traits.
     
   • Write the dominant allele first for a trait. aA is the same as Aa but the capital letter is always written first.
   • Make the P1 = ______ x  _____ generation.
     
   
   

   Keyword	Definition	Genotype
   homozygous	the same alleles, the same letters	TT or tt
   heterozygous	different alleles, different letters	Tt
   pure, or true breeding	homozygous parents	TT or tt
   x	mating, fertilization, pollination, crossed with

   
   

   Example:
   

   A cross between two heterozygous tall plants:
   

   P1 Generation = ___Tt___ x __Tt___





2. Divide the parental genotype into its gametes (separate the alleles). 
   

   • Chromosomes occur in pairs. Each chromosome has an allele for the trait. To form sperm (male) or eggs (female), a pair of alleles for a given trait are separated into separate sperm or egg cells.
     
   
   
   • Tt has two gamete forms: T and t.

     

   
   
   • The pair TT becomes T and T. The pair tt becomes t and t.
   
   
   • These letters (gametes) are used on the top row or left side column of a Punnett square.
     
   
   

   Example: 

   	T	t
   T
   t

   
   
   
3. Complete a Punnett square for the question.
   
   
   • This is a checkerboard method used to find expected results of a cross between parents.
   • Can you spot the error in this Punnett square?
     
     
   • Answer
     

     The dominant allele should be written first in the genotype.  The video indicates "tT" but should be written "Tt".
     

   Punnett Square
   




4. Find the genotype and phenotype.

   • Write the genotype of the first square and record the number of squares that are exactly like this genotype. Proceed until all genotypes are recorded.
     
     
   • The phenotype is the expression of the alleles of the genes: the colour, shape, size, the effect of the genes, etc.
     
     
     • Remember that, in dominance, Tt has the same phenotype as TT.
     • For the first square, record the phenotype and count those that are identical. Locate the next different phenotype; record and determine how many are similar. Repeat until all different phenotypes are accounted for.

   Example: 

   Genotype = 1 TT : 2 Tt : 1 tt
   

   Phenotype = 3 Tall : 1 Short 




5. Determine the ratio or probability, if required.

   • 1:0 = 1 or 100% of the offspring are the same colour.
     
   • 1:1 = 50 / 50 ratio - ½ (50%) are one colour and ½ (50%) are another colour.
   • 3:1 ratio = ¾ (75%) are one colour and ¼ (25%) are another colour. 
   
   

   Example:

   75% (0.75) are tall and 25% (0.25) are short.