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Heredity
Chapter 5
Heredity and Genetics
section 5-1 H.W. pg 132 ques. 1-5
 Heredity is the passing down of traits
from parent to offspring.
 You inherit traits or body features from
your parents, and them from their
parents, etc…
 All of the traits that you express and carry
are inside your DNA on what we call
alleles.
 An allele is basically one half, of one
piece of DNA that is responsible for gene.
Heredity and Genetics
 Sex cells contain one half of all of the
alleles in your DNA (or half of your DNA).
 So when a sperm and egg fertilize each
other, their alleles combine to produce
genes. (now full DNA,46 Chromosomes)
 And the study of how traits are inherited
through the interactions of alleles is what
we call genetics.
Mendel
 An Austrian monk named Gregor Mendel
is known as the father of genetics.
 Through science and mathematics, he
studied how traits get passed on from
generations to generation, using pea
plants.
 He used pea plants because they grew
quickly and they had different colored
pea flowers. This made it easy to see
where the traits were passed on to.
Mendel
 He looked at characteristics like the color of the
peas, color of the pea pod, shape of the pea
pod, height of the pea plant, and color of the
flowers.
 The way he studied the plants is by cross
mating two pea plants that expressed totally
different traits.
 He found that the offspring from these 2
different plants produced an offspring which
always looked like on of the parent plants.
 He called these offspring plants Hybrids,
because they received different alleles for a
trait from each parent.
Mendel
 He found that it was easy to breed pea plants
that had pure traits. That means that an
organism that always produces the same traits
generation after generation, and is called a
pure breed.
 An example is that tall plants will always
produce tall offspring.
 To see how genetics worked in nature he took
pollen from short plants and fertilized a tall
plant seed, just like a bee might carry pollen
randomly from one plant to another.
 When you mate two pure breeds of different
traits (short and tall for example) it is called
cross breeding.
Mendel
 The result of this cross breeding, was that all the
offspring were tall. Whatever it was that made the
short plants short appeared to be gone.
 He than referred to the tall trait as a dominant trait
because it overcame the short trait or covered it
up.
 He referred to the short trait as recessive because
it seemed to disappear.
 However, he found that these recessive traits didn’t
disappear because as he kept cross breeding his
pea plants he found that they reappeared.
 So, the recessive traits did not disappear, they
were just not expressed because a dominant trait
was present.
Alleles and Punnett Squares
 Almost every cell in your body has two alleles for
every trait. One from mommy and one from daddy.
 These alleles are on your chromosomes inside the
nucleus of your cells.
 An organism that has two alleles that are the same
for one trait is said to be called homozygous. TT
 An organism that has two different alleles for a
single trait is called heterozygous. Tt
 We use these terms to express an organisms
phenotype, or a trait that can physically can be
seen without knowing what alleles it has.
 Ex: I have black hair, so my phenotype for hair
color is black.
Alleles and Punnett Squares
 We also use these terms to express what is
called, an organisms genotype. This is
genetic make up of the alleles an organism
has.
 Ex: I might have black hair, but I might not
have 2 alleles for black hair, but b/c one allele
is black, and black is dominant over any other
color allele, I have black hair. Bb
 We can not assume what an organisms
genotype is just by looking at its phenotype.
Alleles and Punnett Squares
 To test the probability of what traits will get
passed down from the cross breeding of
parent genotypes, to offspring, we use
Punnett squares.
 In a Punnett square we use 2 letter
abbreviations for traits.
 Capital letters stand for dominant traits and
lower case letters represent recessive traits.
Punnett Square
Alleles and Punnett Squares
 Even though Mendel didn’t know about
DNA and Chromosomes he did teach us
3 principles of heredity.
 1) Traits are controlled by alleles on
chromosomes.
 2) An allele can be dominant or
recessive.
 3) When a pair of chromosomes separate
during meiosis, the different alleles for a
trait move into separate sex cells.
Genetics Since Mendel
section 5-2 H.W. pg 140 ques. 1-4
 Since we have found what is called
incomplete dominance. This is when the
offspring of 2 homozygous parents produce
an intermediate or middle offspring.
 Ex: A pure white flower and a pure red flower
could give a pink flower.
 This occurs with hair color of horses and
other animals and feather color of birds.
Multiple alleles
 We also learned that many traits are controlled
by more than just two traits.
 A trait that is controlled by more than two
alleles is said to be controlled by multiple
alleles.
 An example is blood type. There are 3 alleles
for blood type: A B O
 A could be AA or AO
 B could be BB or BO
 AB is only AB
 O is only OO
Polygenic Inheritance
 Some traits are produced because many
genes combine to form it. When a group of
genes act together to produce a trait its
called polygenic inheritance.
 An example is skin color. It is estimated that
3-6 genes control what skin color an
offspring will have.
 Eye color is also a polygenic trait, among
others.
Mutations
 Sometimes during cell division DNA is not
copied correctly. And these incorrect copies of
DNA are called mutations and can be harmful
or advantages to an organism. (diseases or a 4
leaf clover)
 There can also be chromosome disorders.
These disorders occur when an organism
contains to few or to many chromosomes.
 When this happens the fetus or embryo usually
dies before birth.
 If it does live it could result in mental or
physical disorders like down syndrome (3
chromosome 21’s)
Recessive Genetic
disorders
 A lot human genetic disorders, like cystic
fibrosis, are caused by mutated recessive
genes.
 To get this disease one would have to
inherit 2 recessive alleles. aa
 This could mean that the parents of this
offspring could have only been carriers of
the disease and had no shown symptoms
of it.
Sex Determination
 The sex of an organism is determined by
special sex chromosomes.
 The female sex chromosome is the X
chromosome and the male is the Y
chromosome.
 In order for a female to be produced it
needs to inherit 2 X chromosomes: XX
 If a Y is inherited the result is a male: XY
 YY is not possible
Sex linked disorders
 Some genetic disorder occur because of
mutations of genes on sex chromosomes
or sex linked genes.
 These disorders can only be expressed
in certain sexes.
 Ex: Color blindness is only expressed on
the X chromosome. So men only need
one allele for colorblindness to become
color blind where women will need two.
Pedigree Chart
 We use a pedigree chart to find out how
a trait was passed on from generation to
generation.
Genetic Engineering
 Genetic engineering is used to correct genetic
disorder. We want to fix a piece of DNA that
is damaged or missing something.
 We do it in a couple of ways: one way is with
recombinant DNA. We do this with diabetics a
lot.
 Here we place DNA for insulin inside of a
bacteria’s DNA.
 This will cause the bacteria to produce insulin
that can be used for people with diabetes.
Genetic Engineering
H.W. pg 149 & 150 ques 1-18
 Another way is called gene therapy. Here
we try to fix a bad gene inside of a cell.
 We take a good gene and place it into a
virus genome. Then we infect a person
that doesn’t have the good gene with this
virus and hopefully it corrects the
problem.
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