Due: Friday, October 23th, 2015 / Name: ______
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. Dragonflies 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.
Table 1: Traits for Dragonflies:
AlleleE / Red eyes
e / White eyes
B / Black body color
b / Brown body color
L / Long legs
l / Short legs
In words / Genotype / Phenotype
4. Homozygous recessive for eye color / White eyes
5. Homozygous dominant for body color / BB
6. / Short legs
7. Heterozygous for leg length
8. / Ll
9. Homozygous recessive for leg length / ll
10.