Predicting Traits
Name: ___________________________
Period: _____
Physical traits are characteristics such as having free earlobes, a widow’s peak, or
having light eyes. The instructions for making each of your physical traits come
from a pair of genes located on your chromosomes. Chromosomes are made up of
twisted strands of DNA (deoxyribonucleic acid) that are located in the nucleus of
each of your cells. For each trait, you inherit two genes—one gene from your
mother, and one gene from your father.
Nucleus
Cell
Chromosome
Example of
a gene pair
for a trait
Strands of DNA
In describing the inheritance of genes, some additional terms are useful. An
organism’s phenotype is the physical appearance of a trait. For example, in the
“Class Traits” activity, there were two possible phenotypes for eye color—dark
eyes or light eyes. For Mendel’s peas, the phenotypes for seed shape were round
or wrinkled.
Genotype is the organism’s allele combinations, or gene type. There are three
possible genotypes for eye color—DD, Dd, or dd. You’ll learn more about figuring
out genotypes later in this lesson.
Three additional terms can be used to describe a genotype:
homozygous dominant = both genes are dominant (ex: DD)
homozygous recessive = both genes are recessive (ex: dd)
heterozygous = one dominant and one recessive gene (ex: Dd)
Called purebreds
by Mendel
Called hybrids
by Mendel
Study Skill Hint: By now, you’ve probably begun to recognize that many “hard” science words
are made up of prefixes, roots, and suffixes, and if the word is broken down, the meaning is
easier to remember. For example, in the words above, the prefix “hetero” means different; and
the prefix “homo” means same. So, heterozygous means there are two different alleles;
homozygous means there are two of the same alleles (either two dominant or two recessive).
Dominant traits usually appear more often in a population (except widows peak and
cleft chin). For example, having free earlobes is the dominant form of the trait; so
it will show up more often in a population. When there is at least one dominant
gene in the pair, then the dominant allele masks, or covers up, the recessive allele.
The only time the recessive form of the gene shows up is if an individual inherits
two recessive alleles. This happens less frequently and is why the recessive form
of trait is not as common in a population. This is why you see attached ear lobes
less often.
To help all of this make sense, we will use one of the examples from the Class
Traits activity to look at the probability of having a particular trait. Let’s look at
the probability of eye color. Dark eyes is a dominant trait, so we’ll use the capital
letter D to represent the dominant allele. Light eyes is a recessive trait, so we’ll
use the lower case letter d to represent the recessive allele.
Activity: Let’s assume both parents have dark eyes and are heterozygous for the
trait. This means the genotype for each parent is Dd, and each parent’s possible
alleles can be represented by heads (D) & tails (d) of a coin. You’ll need two coins—
one for the mother and one for the father. Flip both coins together a total of 50
times and tally the resulting genotypes in the Offspring data table below. Make
sure to calculate totals & percents, and describe the phenotype.
OFFSPRING DATA
Homozygous Dominant
(DD)
Total #:
Heterozygous
(Dd)
Homozygous Recessive
(dd)
______
______
______
Percentage: ______
______
______
Phenotype?
Dark Eyes
Dark Eyes
(Hint: to find the percentage, multiply the total # by 2.)
Light Eyes
Questions:
Think about the coins you used…
1. What genotype did both parents have? Dd (heterozygous – D=heads/d=tails)
2. What phenotype did both parents have? Dd has a dominant so the phenotype is
dark eyes.
3. What was the probability of the offspring receiving a recessive allele from one
parent? 50 %
How about receiving a dominant allele from one parent? 50 %
Look at your offspring data…
4. What percentage of the time was the offspring heterozygous? _____%
5. What percentage of the time was the offspring homozygous dominant? ____%
6. What percentage of the time was the offspring homozygous recessive? ____%
During the class discussion, you will answer the following questions…
7.
D
D
d
d
DD
Dd
Dd
dd
Punnett Square – a
tool used to predict
the probability of
offspring with
certain genotypes.
8. From the data collected, and the answers to the questions above, fill in the
following charts:
Genotype
DD
Dd
dd
Phenotype
Dark eyes
Light eyes
Predicted
Actual
Probability (%) Results (%)
¼ or 25%
½ or 50%
¼ or 25%
Predicted
Actual
Probability (%) Results (%)
¾ or 75%
¼ or 25%
9. Based on the results of this activity, are traits in offspring somewhat
predictable? Explain your answer. (Write in complete sentences.)
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