Genes, Chromosomes, Alleles, Traits
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Big Question:
A gene is a segment of the chromosome that has the information for a certain trait. Each gene is in a
specific location on the chromosome. For example, if a gene on position 9 on one chromosome in
humans codes for type of ear lobe (free or attached), then the gene on position 9 of another chromosome
in another human also codes for type of ear lobe. But one gene may code for attached lobe while the
other gene may code for free lobe. We always get ½ of our chromosomes from our mother from her egg
and ½ of our chromosomes from our father from is sperm. The different sets of chromosomes in different
eggs are different. The different sets of chromosomes in different sperms are different.
I) Genes, Chromosomes, Alleles, Traits
A) Gene: Genes are chemical factors within the cell that give
information about traits. Genes are segments of the DNA
molecule.
B) Chromosome: When the DNA molecule coils it is called a
chromosome. So, genes are segments on the chromosome.
Each gene occupies a specific place on the chromosome.
C) Alleles: Alleles are different versions of the SAME gene. For
each gene, there are TWO alleles. ONE allele is passed from
the father in the sperm. ONE allele is passed from the mother
in the egg.
D) Trait: A trait is a characteristic of a living organism, for
example green eyes, or red hair. Traits are NOT passed down
from parent to offspring. INFORMATION about traits is
passed down when the DNA of the parent is transferred to the
offspring.
II) Dominant and Recessive Alleles
Sometimes, one allele seems to be “stronger” than another.
The stronger allele is called DOMINANT. The information it
carries gets translated into a trait. The weaker allele is called
RECESSIVE. The information it carries does NOT get
translated into a trait.
III) Using Letters to Show Alleles
A) We use a CAPITAL letter (for example, T) to show a
dominant allele. We use a lowercase letter (for example, t) to
show a recessive allele. We always write the capital letter
first.
B) Each gene is given its own letter. Only the case changes to
show dominant (CAPITAL) and recessive (lower case). For
example, “B” for black and “b” for white.
IV) Examples of Writing Alleles
A) Hybrid or Heterozygous: Both dominant and recessive alleles
are present. For example, Gg.
B) Purebred or Homozygous Dominant: Only dominant alleles
are present. For example, TT.
C) Purebred or Homozygous Recessive: Only recessive alleles
are present. For example, bb.
Rev: 2/9/2016
Genes, Chromosomes, Alleles, Traits
1.
Factors that give information about traits are
called ________________.
2.
Different forms of a gene are called
__________________.
3.
True or False? Recessive alleles are never
present when dominant alleles are present.
4.
True or False? Recessive alleles hide dominant
alleles.
5.
True or False? A trait controlled by a dominant
allele always shows up in the organisms when the
allele is present.
6.
True or False? Only pea plants that have two
recessive alleles for short stems will be short.
7.
A dominant allele is represented by a(n)
_________________ letter.
8.
How might a geneticist write the alleles to show
that a tall pea plant has one allele for tall stems
and one allele for short stems?
________________________
9.
True or False? “Ww” means that ONLY
recessive alleles are present.
10. True or False? Hybrid means that ONLY
recessive alleles are present.
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Mini-Essay (10 pts) Read, Understand, and Answer: Green (G) is the dominant color for pods in pea
plants. Yellow (g) is recessive. Is it possible to have a heterozygous yellow pod? Explain.
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Self:
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Honest and fair self-evaluation helps you improve.
Other:
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You are evaluating the work of the person
NOT the person.
If your handwriting is hard to read, maximum score is 5. You can type your essay and paste in the space.
Rev: 2/9/2016