Day 2: Mendel’s First Law--The Law of Dominance                Name_________________ Period___

Grading--

For an A, take the quiz and complete all of Day 2 lessons

For a B, take the quiz and complete the section “On Your Own”, including the summary and the questions

For a C, Do all of the “On Your Own”  including the summary.

On Your Own:

The Law of Dominance 

Stated "simply" it goes like so:

     In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting.  When he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall.  Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants.  The same was true for other pea traits:

 

Parent Pea Plants

F1 Pea Plants

Generation 1

tall stem x short stem

all tall stems

yellow seeds x green seeds

all yellow seeds

green pea pods x yellow pea pods

all green pea pods

round seeds x wrinkled seeds

all round seeds

axial flowers x terminal flowers

Flowers found on the tips of the shoots are dubbed 'terminal'. If the flowers are borne on the shoot, where the branches begin, then they are called "axial".

all axial flowers

     

     So, what he noticed was that when the parent plants had contrasting forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the offspring resembled only one of the parent plants with respect to that trait.  So, he said to himself,

     "Greg, there is a factor that makes pea plants tall, and another factor that makes pea plants short. Furthermore Greg ol' boy, when the factors are mixed, the tall factor seems to DOMINATE the short factor".

     Now, in our modern wisdom, we use "allele" or "gene" instead of what Mendel called "factors".   There is a gene in the DNA of pea plants that controls plant height (makes them either tall or short).  One form of the gene (allele) codes for tall, and the other allele for plant height codes for short.  For abbreviations, we use the capital "T" for the dominant tall allele, and the lowercase "t" for the recessive short allele.

Let's revisit the three possible genotypes for pea plant height & add some MORE VOCABULARY.

 

Genotype Symbol

Genotype Vocab

Phenotype

TT

homozygous DOMINANT

or

pure tall

tall

Tt

heterozygous

or

hybrid

tall

tt

homozygous RECESSIVE  

or

pure short

short

Note: the only way the recessive trait shows-up in the phenotype is if the genotype has 2 lowercase letters (i.e. is homozygous recessive).

Also note: hybrids always show the dominant trait in their phenotype (that, by the way,  is Mendel's Law of Dominance in a nutshell).


The PUNNETT SQUARE (P-Square for short)

     OK, now is as good of time as any to introduce you to a new friend, the Punnett Square.  This little thing helps us illustrate the crosses Mendel did, and will assist you in figuring out a multitude of genetics problems.

     We will start by using a P-Square to illustrate Mendel's Law of Dominance.  Recall that he "discovered" this law by crossing a pure tall pea plant & a pure short pea plant.  In symbols, that cross looks like this:

Parents (P):  TT x tt

The P-Square for such a cross looks like this:

     Inside the 4 boxes are the possible genotypes (with respect to plant height) of the offspring from these parent pea plants.  In this case, the only possible genotype is Tt (heterozygous).  In hybrids, the dominant trait (whatever the capital letter stands for) is the one that appears in the phenotype, so all the offspring from this cross will have tall stems.

A summary of this cross would be:

 

Parent Pea Plants  

(P Generation)

Offspring  

(F1 Generation)

Genotypes:

TT x tt

Phenotypes:

tall x short

Genotypes:

100% Tt

Phenotypes:

100% tall

Now, a helpful thing to recognize is this:

ANY TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR A TRAIT, AND ALL THEIR OFFSPRING RESEMBLE ONLY ONE OF THE PARENTS, YOU ARE DEALING WITH MENDEL'S LAW OF DOMINANCE.

All the offspring are heterozygous for the trait, one parent is homozygous dominant, and the other is homozygous recessive.

Share the summary with your table partner. Write and discuss questions. Then, share with the class.

Discuss with a lab table partner.

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