Science Homework
Due: Friday, October 14
Name: ______________________________
th
, 2011
Teacher: ____________
Period: ____
DNA, Genes and Chromosomes
A gene is a section of DNA on a chromosome. A
model of a chromosome is shown to the left,
separated into gene sections. Each gene
section on a chromosome contains a code of
DNA bases that tell the cell what kind of
proteins to build. The type of proteins that are
built in the cell determine the outward traits,
also called the phenotype, of the organism.
During reproduction, chromosomes are passed
from parent to offspring. During sexual
reproduction, an offspring gets one
chromosome from the mother and one from
the father.
1.
Another way to say “the outward appearance an organism has” is its _____________________.
Along the chromosome, there are small
dark bands that mark off the chromosome
into sections called genes. A gene is a part
of the chromosome that controls a trait,
such as hair color or eye color. For each
trait, an offspring gets one gene from the
mother and one from the father during
sexual reproduction.
2.
In ________ the offspring gets one gene from the mother and one gene from the father.
a. Crossing over
b. reproduction c. sexual reproduction
d. asexual reproduction
In our fly activity we looked at just a few of the traits that are passed from parents to offspring, and saw many
differences or variations in a small set of visible characteristics. These differences happen because each
offspring has two sets of genes, one from the father and one from the mother. This means that offspring have
two forms of the same gene for each trait. The two different forms of genes are called alleles, and can either
be the dominant or recessive form of the gene. The two alleles that an organism has for a trait is called the
genotype, for example Bb.
3.
The two ______________ that an organism has for a trait is called the __________________.
In the genotype, dominant genes are represented with a capital letter. The capital letter stands for the
dominant form of the gene, and is written first in the gene pair. If a dominant gene is present in a gene pair,
its trait will be expressed or seen.
Recessive traits are represented with a lower case letter. The lower case letter stands for the recessive allele
of the gene that codes for the trait. Notice that the letter of the trait is determined by the dominant allele,
and the recessive allele is simply given the lower case letter.
In dragonflies, the dominant allele for black body color is shown by writing B. The recessive allele for body
color, brown, is shown by b. An organism that has two recessive alleles, bb, is called homozygous recessive,
and will have a brown body. A dragonfly with two dominant alleles is called homozygous dominant, and is
written BB. Drangonflies with a genotype of BB will have black bodies. The third type of genotype is
heterozygous, written Bb. Dragonflies that are heterozygous for body color have a black phenotype, because
the dominant allele “covers up” the instructions from the recessive allele.
Directions: Using the data in Table 1, write the genotype that corresponds to the phenotype
descriptions
Table 1: Traits for Dragonflies:
Allele
E
Red eyes
e
White eyes
B
Black body color
b
Brown body color
L
Long legs
l
Short legs
In words
4. Homozygous recessive for
eye color
5. Homozygous dominant for
body color
6.
Genotype
Phenotype
White eyes
BB
7. Heterozygous for leg
length
8.
9. Homozygous recessive for
leg length
10.
Short legs
Ll
ll
12.To the left you see the alleles for
a dragonfly.
13.To the left you see the alleles for
a dragonfly.
What is its genotype?____________
What is its genotype?____________
What is its phenotype?___________
What is its phenotype?___________