Heredity

Every human on earth is a combination of two sets of genes: your mother’s and your father’s. These genes created a blueprint for you, and they make you unique. Genes lead to different traits, or characteristics, such as brown eyes or blue eyes. Parents passing on their genes to their offspring is called heredity. In genetic terms, you are a hybrid of both of your parents: a combination of their many different genes.

Download Project

What’s the probability of receiving a particular gene? The Punnett Square is a tool that allows you to see the different gene combinations that are possible when two parents of any species create offspring.

When looking at the model of inheritance which the Punnett Square illustrates (referred to as Mendelian inheritance), you are observing combinations of dominant alleles and recessive alleles. An allele is a version of a gene (the eye color gene can consist of blue, brown, green, gray, and hazel alleles). Dominant genes mask recessive genes. For example, brown eyes are the dominant gene for eye color, and blue eyes are recessive, so when the genes for brown and blue eyes are combined in offspring, there is a 75% chance that the offspring will have brown eyes. This is why the majority of people in the world have brown eyes.

PUNNETT SQUARE

The basic p-square looks like a window pane :

.

When given enough info about two parent organisms, we can use this window pane to
predict the genotypes & phenotypes of their offspring. Exciting, ain’t it?

---

• • genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype. A capital letter represents the dominant form of a gene (allele), and a lowercase letter is the abbreviation for the recessive form of the gene (allele).
  • phenotype = the physical appearance of a trait in an organism

For example, let’s say that for the red-throated bird (I am making this up), red throat is the dominant trait and white throat is recessive.  Since the “red-throat code” and the” white-throat code” are alleles (two forms of the same gene), we abbreviate them with two forms of the same letter.  So we use “R” for the dominant allele/trait (red throat) and “r” for the recessive allele/trait (white throat).  Our possible genotypes & phenotypes would be like so:

Symbol	Genotype Name	Phenotype
RR	homozygous (pure) dominant	red thoat
Rr	heterozygous (hybrid)	red throat
rr	homozygous (pure) recessive	white throat

Note: Remember, we don’t use “R” for red & “W” for white because that would make it two different genes which would code for two different traits, and throat color is one trait.  What the genotype contains are two codes for the same trait, so we use two forms of the same letter (capital & lowercase).

One more note: A very very helpful thing to memorize is that the ONLY way for a recessive trait to show up in an organism is if that organism’s genotype is homo-zygous recessive (two little letters, like “rr”).

Filling in the top-left box:

Filling in the bottom-left box:

Filling in the top-right box:

Filling in the bottom-right box:

One from the left, one from the top… one from the left, one from the top…one from the left, one from the top…one from the left, one from the top.

 Summarize the results (genotypes & phenotypes of offspring).

• • Simply report what you came up with.  You should always have two letters in each of the four boxes.
  • In this example, where our parent pea plants were Tt (tall) x tt (short), we get 2 of our 4 boxes with “Tt”, and 2 of our 4 with “tt”.  The offspring that are “Tt” would end up with tall stems (the dominant trait) and the “tt” pea plants would have short stems (the recessive trait).
  • So our summary would be something like this:

Parent Pea Plants  (“P” Generation)		Offspring  (“F1” Generation)
Genotypes: Tt x tt	Phenotypes: tall x short	Genotypes: 50% (2/4)  Tt 50% (2/4) tt	Phenotypes: 50% tall 50% short