Mendel and Punnet Squares

Mendel and Punnet Squares

Mendel

Father of geneticsParents were farmers whoinvested in Mendel’s future by sending him to a good school.•became an Augustinian priest•Work was largely ignored untilEarly 20th century, when it wasre-discovered.

Mendel’s Experiments

Chose to work with peas, because:

1. They were easy to cross (breed)

2. showed a variety of contrasting traits, like purple versus white flowers, tall versus short stems, round versus wrinkled seeds.



Mendel bred a pure-bred (means if two pure-bred individuals are bred together, they would only produce offspring with the same characteristics) with a plant of the opposite trait. For example, a pea plant with purple flowers to a plant with white flowers.

These two individuals were called the parental (P) generation.


The offspring of the parental generation were called the first filial (F1) generation.- Mendel counted and recorded the numbers and characteristics of these offspring.

He then bred 2 individuals from the F1 together.The offspring of the F1 crossed with F1 were

then called the second filial (F2) generation.- Mendel counted and recorded the numbers and characteristics of these offspring.


Mendel’s Results and Conclusions

The F1 generation always showed only one trait (he later called this the dominant trait), the other was ‘hidden.’

The F1 generation must be carrying the ‘hidden’ trait from the original parents - the white trait

Some individuals in the F2 generation showed the hidden trait, 1/4 of the F2 generation had it (he later called this hidden trait the recessive trait).

Each individual has two "factors" that determine what external appearance the offspring will have. (We now call these factors genes or alleles)

Mendel’s Laws

1. The Principle of Dominance and Recessiveness - one trait is masked or covered up by another trait

2. Principle of Segregation - the two factors (alleles) for a trait separate during gamete formation

3. Principle of Independent Assortment - factors of a trait separate independently of one another during gamete formation; another way to look at this is, whether a flower is purple has nothing to do with the length of the plants stems - each trait is independently inherited.

Modern Genetics

Mendel's factors are now called ALLELES. For every trait a person have, two alleles determine how that trait is expressed.

We use letters to denote alleles, since every gene has two alleles, all genes can be represented by a pair of letters.

PP = purple, Pp = purple, pp = white Homozogyous: when the alleles are the same, the

individual is said to be homozygous, or pure breeding. Letters designating a homozgyous individual could be capital or lowercase, as long as they are the same. Ex. AA, bb, EE, dd

Heterozygous: when the alleles are different, in this case the DOMINANT allele is expressed. Ex. Pp, Aa

Modern Genetics

Monohybrid cross = a cross involving one pair of contrasting traits. Ex. Pp x Pp

Punnet Square: used to determine the PROBABILITY of having a certain type of offspring given the alleles of the parents

Genotype: letters used to denote alleles (BB, Pp..etc)

Phenotype: what an organism looks like (brown, purple..)

Punnet Squares

Used to predict the outcome of a cross.

Sample Problem -

*In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. Using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous (purebred) tall pea plant and a homozygous (purebred) short pea plant.

How to solve, Step 1

Designate letters which will represent the genes/traits. Capital letters represent dominant traits, and lowercase letters represent recessive traits.

T = tall t = short


Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.

TT X tt (tall) (short) - both homozygous (same) or

purebred


Draw a Punnett square - 4 small squares in the shape of a window. Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.


Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

Tt Tt

Tt Tt

T T

t

t


List the possible genotypes and phenotypes of the offspring for this cross.

The letters inside the boxes indicate probable genotypes (genetic makeup) of offspring resulting from the cross of these particular parents. There are 4 boxes, and the genotypic results can be written either as fractions or percents. In this case, all 4 boxes out of the 4 are showing the Tt genotype. Therefore, each of the offspring has a 4/4 or 100% chance of showing the Tt genotype.

Punnet Squares

What are the possible genotype(s) of a tall plant?

What are the possible genotype(s) of a short plant?

What would be the phenotype of TT? What would be the phenotype of tt? Why is the phenotype of Tt tall and not

medium/average?

Documents

Mendel and Punnet Squares