Download Genes, Alleles, and Traits (recovered)

GENES, ALLELES, AND TRAITS
Learning Outcomes
After completing this lesson, you will be able to:
 Draw Punnett squares to illustrate the transfer of single traits
 Use Punnett squares to predict phenotype and genotype probabilities
 Demonstrate an understanding of the terms DNA, chromosomes, genes, genotype,
traits, dominant, recessive, and phenotype by defining them and describing them
Genes, alleles, and traits are closely related terms and are easily confused.

Genes
Genes are a piece of chromosome that contains the actual code for
a certain trait. There must be a gene for hair colour, lefthandedness, and so on. Every characteristic we have must have a
corresponding gene in our chromosomes.

Alleles
An allele is a form of the gene. For example, in the homologous
chromosomes shown in the diagram there is a gene for hair colour.
Alleles for hair colour can be dark (D) or light (d), where dark hair
colour is the dominant allele.
After meiosis has taken place, only one of the homologous
chromosomes will give genetic information from this parent to the
offspring. This means the parent in the example can contribute
either a dark hair dominant allele or light hair recessive allele to
the offspring. Whether the offspring will have light or dark hair can
depend on the alleles contributed by both parents.

Traits
Traits are closely related to phenotype. Traits refer to the single characteristic of the
individual whereas phenotype refers to all the observable characteristics that make up
the individual. For example, a trait in the offspring could be dark hair.
Specific genes determine hereditary traits. A gene specifies a single inherited
characteristic. There are genes for height, weight, eye colour, earlobe attachment, and
so on.
Variations of a gene that relate to the same characteristic are called alleles.
Each person carries two genes for a given characteristic. One gene comes from the
mother’s egg and one from the father’s sperm.
One of the two genes is dominant over the other. The dominant allele will mask the
other recessive allele. For example, if the mother provides a blue eye-colour allele
and the father a brown eye-colour allele, the offspring will have brown eyes since
brown eye colour is dominant.
The observable characteristic in the offspring is called the phenotype and the genetic
structure creating the characteristics is called the genotype.
The British mathematician/biologist R.C. Punnett developed a method for visually
representing the possible combinations of alleles from the parents. It was then
possible to predict the possible outcomes of the offspring.
Before taking a closer look at Punnett squares, we will describe the parents more
carefully. Parents, both male and female, each have two alleles for any trait due to
the 23 pairs of chromosomes. Recall that each pair of chromosomes is homologous,
that is, they have genes responsible for the same characteristic.
If we were tracking the tallness trait, then we know that each parent has two alleles for
tallness, one on each chromosome that contains the tallness allele. There is a
dominant allele for tallness and a recessive allele for shortness. The easiest way to
represent the alleles is to give them a symbol. Usually the symbol for an allele is to
give it the first letter of the dominant characteristic – in this case T for tallness. The
recessive allele is given the lower case letter of the dominant allele – in this case t for
shortness. A Punnett square assigns two alleles for tallness to each parent and
predicts the outcome.
We will assume there are parents who are both tall. The mother is tall with two alleles
for tallness. The father is tall but has one tall and one short allele.
A Punnett square is shown below.
Each parent contributes one allele to
form the trait in the offspring.
Notice how gametes from the father combine with gametes from the mother to form
offspring with two gametes, one from each parent.
The possible offspring with the tallness trait are shown below.
1. Two homozygous tall TT
2. Two heterozygous tall Tt
How many tall and how many short offspring will there be from these parents?
Tall - 100% Short - 0%
You can predict the offspring that come from parents having heterozygous genes.
Both parents in the example below are heterozygous tall. Place the gamete symbols in
their correct form for the two parents (Tt in both).
Place the gametes for the mother on top and
for the father on the side of the Punnett
square. Fill in the Punnett square.
How many of the offspring will be tall and how
many will be short?
Tall – 75% Short – 25%
You can see from this exercise that Punnett
square are helpful in predicting what
characteristics to expect from offspring if you
know the parental gametes.
Punnett squares are probability predictors. In
the practice Punnett square you have just
completed, you are predicting that 75% of the
offspring are going to be tall. This does not
mean that parents will these gametes having four children will produce three tall and
one short offspring. It is possible for them to produce two tall and two short, for
example. However, with a large sample of families with the same gametes, we would
find that very close to 75 percent of the offspring would be tall.
Punnett squares give us three types of information.
o They show us the gametes each parent can produce.
o They show us the genotype combinations that are possible.
o They tell us the probability that a given genotype will occur.
Here is another example of a Punnett square where more terminology is included.
The trait is earlobes. Free earlobes (F) are
dominant and attached earlobes (f) are recessive.
Mother homozygous attached, father
heterozygous free 
Complete the Punnett square for the parents
described above.
Describe the probable phenotypes and genotypes that come from the Punnett square.
Indicate the percentage probability of each.
Phenotypes
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Genotypes
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Summary
Mendel was able to explain the transfer of traits from one generation to the next.
Mendel stated that each parent contributed one gene to the offspring. He described a
single trait as coming from a single gene. He also stated the dominant gene masked
the recessive gene.
The probability of traits showing up in offspring can be represented using Punnett
squares. Punnett squares allow the prediction of the phenotypes and genotypes for
any trait.