Download Chapter 10 Basic Genetics

Chapter 10
Basic Genetics
ALABAMA 7TH GRADE SCIENCE STANDARDS COVERED IN THIS CHAPTER INCLUDE:
8
Describe the function of chromosomes.
9
Identify the process of chromosome reduction in the production of sperm and
egg cells during meiosis.
HEREDITY AND GENETICS
Parents pass on physical characteristics to their children. A few that you might think
of are height, eye color and hair color. Parents may also pass on characteristics like
temperament and agility to their offspring. The process of passing characteristics
from parent to offspring is called heredity. The study of the chemical basis of
heredity is called genetics.
GENES AND CHROMOSOMES
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Hereditary characteristics are determined by DNA. (We will examine the structure of
DNA more closely in Chapter 1 1 For now, just know that DNA carries all the
information necessary to duplicate the organism.) Chromosomes are organized
structures of DNA and protein. They control cell processes and determine the
characteristics of an organism.
.
Chromosomes contain genes. Genes are specific portions of
that are known to carry genetic information. If you
;NKS DNA
could unravel all the DNA in a chromosome, it might be
easier to see. Imagine DNA as a long twisted phone cord.
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Specific areas on the cord contain packets of chemical
information. This information is the basis of the hereditary
characteristics, called traits, that you have. When you and your spouse someday
have children, you will pass those genes on to them.
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Basic Genetics
But you and your spouse won’t be passing the exact
same genes, will you? Of course not. You are two
different people and you have different DNA. But
you will still have genes that are chemically
designed to affect height, hair color, eye color,
temperament and so on. The gene itself is not the
same from person to person. But theJunction of the
gene is the same. These alternate forms of a gene are
called alleles.
Allelea
Alleles are expressed using letters from the alphabet.
Each parent passes on one allele for each trait to the
offspring. The actual presence of these alleles in
your chromosomes is called your genotype. Your
genotype is a chemical profile. It is not visible.
The physical expression of the genes you have is
visible. This is called your phenotype. It is how you
look, act, move, etc.
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Allele A
Allele b
Allele B
AlleleC
AlleleD
Allele c
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“Unravelled” DNA
fromMom
Allele e
“Unravelled” DNA
from Dad
figure 10.1 Alleles in a Chromosome
So, a person’s genotype is the actual genes they carry for a particular trait. Their
phenotype is the physical characteristic that results from those genes.
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Figure 10.2 Alleles for Height
For example, let’s say that your mom passed on to you
an allele for being tall (T) and your dad passed along an
allele for being short (t). Your genotype for height
would be Tt. Now let’s say you actually are tall. Tall is
your phenotype for height.
The combination of chromosomes from your parents
leads to an entirely unique you. You have genetic
information from your parents, mixed up in your own
way. Meiosis ensures that what you pass on to your
offspring will be equally unique.
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MElosis
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Reproductive cells found in the ovaries of women are called egg cells. Reproductive
cells in the testes of men are called sperm cells. All your reproductive cells were not
donated from your parents. Your body had to make them, which means that cell division
was required. This was done by meiosis. Melosis is the way reproductive cells divide.
Meiosis is essential to sexual reproduction and occurs in all (well, almost all)
eukaiyotes.
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Chapter 10
Cell division by melosis produces four new reproductive cells called gametes. Each one
contains half the numbefof chromosomes as the parent cell. The chromosomes in the
gametes are not identical to those in the original cell. This means that each reproductive
cell in your body is unique. They have the same genetic origins, but are mixed up in
different ways. Every egg and sperm cell forms a different person. That is why two
sisters born of the same parents are not the same person. They would be, if every one of
the parent’s egg and sperm cells were the same.
The result of meiosis is gametes containing half the number of chromosomes of the
parent cell. The parent cell is said to be diploid. Once divided into a gamete, it is
haploid. So, the diploid number of chromosomes is twice the number of haploid
chromosomes. Said another way, haploid is halfthe diploid number of chromosomes.
Each possible gamete has a unique
arrangement of chromosomes The
more chromosomes an organism has,
the greater the number of gamete
.
Possible
gametes
figure 10.3 Four Different
Gametes Formed
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arrangements that are possible
Figure 10 3 and Figure 10 4 each
show a reproductive cell of an
organism. Figure 10.3 shows an
organism with two pairs of
chromosomes. Its diploid number is
four. The haploid number of the
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10.4 Eight Different
Gametes Formed
gamete is two. The four gametes all
have different arrangements. The
organism in Figure 1 0.4 has a diploid number of six chromosomes. It forms gametes
with a haploid number of three chromosomes each. But look how many more different
arrangements are possible.
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You may know that humans have 46 chromosomes. That makes 8,388,608 unique
gametes possible. Now, do you see why we are all so uniquely different?
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Number ofChromosomes Number ofChromosomes
in Reproductive Cells
in Body Cells
Human
46
Barley/Wheat
14
Earthwo
36
Domestic Sheep
54
Gypsy Moth
62
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Basic Genetics
CHAPTER
1 0 REVIEW
1. The combination of inherited alleles is called the
A
B
heterozygote.
phenotype.
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genotype.
genetic translocation.
2. What is the expression of traits called?
A
B
C
B
phenotype
genotype
mutation
allele
3. What are different forms of a gene called?
A
B
C
B
alleles
DNA
mutations
phenotypes
4. How is asexual reproduction different from sexual reproduction?
A
B
C
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Asexual reproduction requires two parents, while sexual reproduction
only requires one parent.
Sexual reproduction requires two parents, while asexual reproduction
only requires one parent.
Sexual and asexual reproduction are not different in any way.
Sexual reproduction produces alleles, while asexual reproduction
producesDNA.
5. What are genes?
A
B
C
B
the passing along of characteristics to offspring
a double stranded structure of molecules found in chromosomes
specific portions ofDNA
different traits of offspring
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