Transcript Section 5-1

Section 5-1
What is Genetics?
I. What have you inherited?
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Heredity is the passing of traits from parents to
offspring
Genes control traits
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Genes are segments of DNA
A. How are traits inherited?
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Genes are found on chromosomes in the
nucleus of a cell
When pairs of genes separate into sex cells
during meiosis, pairs of genes also separate from
one another
Each sex cell winds up with one form of a gene
for each trait
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Examples of traits that can be inherited:
Eye color
 Hair color
 Hair line (straight or widow’s peak)
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One sex cell may receive the trait for straight
hair line and another sex cell may receive the
trait for a widow’s peak
The different forms a gene may have for a trait
are its alleles
The study of how traits are inherited is the
science of genetics
II. The Father of Genetics
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The scientific study of traits pass from one
generation to the next was done by Gregor
Mendel
Mendel’s background in science and math
helped him with his studies of genetics
Mendel studied the inheritance of traits in pea
plants
Mendel is known as the “Father of Genetics”
III. In Mendel’s garden
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Why did use pea plants in his experiment?
They are true-breeding – can self-pollinate and
produce identical offspring
 They can also be cross-bred – one plant pollinates
another to produce a genetic combination in
offspring
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An organism that always produces the same in
its offspring is called a purebred
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Example: A tall plant that always produces a tall
plant
A. Dominant and Recessive Factors
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Mendel’s first experiment: cross pollinated a
purebred of tall plants with a purebred of short
plants
The tall trait is considered a dominant factor
because it dominated or covered up the short
factor
The form of the trait that seemed to disappear is
called the recessive.
Mendel’s second experiment
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Self-pollinated the tall offspring and planted the
seeds
The results of this experiment were in a 3:1
ratio. For every 3 tall plants there was 1 short
plant. The recessive form reappeared
Genotype:
B. Predictions Using Probability
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Probability is a branch of mathematics that helps
you predict the chance that something will
happen
What made Mendel’s predictions accurate?
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He produced a large number of plants
By increasing the numbers, Mendel increased his
chance of seeing a predicted 3-to-1 ratio
IV. Using a Punnett Square
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A handy tool used to predict results in
Mendelian genetics is a Punnett square
A dominant trait is written with a capital letter
and a recessive trait is written with a lower case
letter. The letters show the genetic makeup of
an organism called the genotype
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Phenotype refers to the physical appearance of the
trait
A. Alleles determine traits
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Most cells in your body have two alleles for
every trait
An organism with two alleles for a trait that are
exactly the same is called homozygous (TT, tt)
An organism that has two different alleles for a
trait is called heterozygous (Tt)
The genotype determines an organisms physical
traits called the phenotype
B. Determining genotypes and
phenotypes
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In a Punnett square, the letters representing the
two alleles from one parent are written along the
top of the square and the alleles from the other
parent are written along the side of the square
The letters that you filled in each square
represent the possible genoytypes for the
offspring the parents could produce
C. Mendel’s success
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Mendel succeeded in describing how inherited
traits are transmitted from parent to offspring
3 summary statements:
Traits are controlled by alleles on chromosomes
 An allele’s effect is dominant or recessive
 When a pair of chromosomes separates during
meiosis, the different alleles for a trait move into
separate sex cells
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