Download Inheritance of Traits

Since Mendel’s time, our knowledge of the mechanisms of genetic inheritance has grown
immensely. For instance, it is now understood than inheriting one allele can, at times,
increase the chance of inheriting another or can a affect how and when a trait is expressed
in an individual's phenotype. Likewise, there are degrees of dominance and
recessiveness. With some traits, the simple rules of Mendel’s inheritance do not always
apply. Let's look at the inheritance of traits.
To predict inheritance is to determine the chances of inheriting a given trait. Scientists
use Punnett squares and symbols to represent the genes. We use upper case letters to
represent dominant genes. We use the lowercase letters to represent recessive genes.
For example: T = represents the gene for TALL in pea plants.
t = represents the gene for short in pea plants. So: TT and Tt both result in a TALL
plant, because T is dominant over t. t is recessive. tt will result in a short plant.
Remember there are two genes for every trait!
Mendel's principle of dominance
Some genes (alleles) are dominant and others are recessive. The phenotype (trait) of a
dominant gene will be seen when it is paired with a recessive gene (Tt).
Let’s cross a totally dominant tall plant (TT) with a short plant (tt). Totally dominant
means homozygous. Homozygous means alike; the two alleles are the same.
Each plant will give only one of its two genes to the offspring or F1 generation.
TT x tt
Mendel's law of segregation
Each gene (allele) separates from the other so that the offspring get only one gene from
each parent for a given trait.
Punnett squares
You draw a box that looks like a window. Label the top of the box with one parent's
genes. Label the side of the box with the other parent's genes. I like to use father's genes
on the top and mother’s genes on the side.
Once you have the box labeled, you will pull down one gene from the father and pull
over the other gene from the mother so that each Square has one paternal gene and one
maternal gene inside. Each box is filled with two genes Tt. This represents the first
generation.
Interpreting the results
The genotype for all the offspring is Tt. The genotype ratio is four out of four or 100%
(Tt).
The phenotype for all the offspring is tall. The phenotype ratio is four out of four or
100% tall.
Now it is your turn to practice, draw the Punnett square on your sheet of paper. Use the
same genotypes that are provided. Fill in the squares.
Check your answer!
Now record the genotype and phenotype ratios of the offspring for the second generation.
Check your answers! Remember to include the percentages. For example, when you
cross the offspring of the second generation with each other, one out of four or 25% of
the offspring should be homozygous tall (TT).
This cross was a monohybrid cross. We worked with only one trait, the height of the
plant. Mono means one.
Later we will work with a dihybrid cross, using the height of the plants and either seed
color or seed shape. Di means two.
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