Download Ch. 08 Mendel and Heredity

Mendel and Heredity
Chapter 8
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The Origins of Genetics
• Heredity is the passing of traits from parents to
offspring.
• Gregor Mendel did experiments with pea plants
on the passing of seven traits.
• He discovered patterns in the passing of traits
and these patterns are called genetics.
• Mendel’s parents were peasant farmers and he
was a priest and teacher.
• Mendel used an experiment of T.A. Knight but
kept records.
Useful Features in Peas
• Characteristics occur in two distinct forms.
– Flower color is a character, purple & white are traits of
the flower color character.
• Pea plants self-fertilize or make clones.
– To cross-pollinate remove the stamen of one plant
and transfer pollen form another stamen
• Peas are small, grows & matures quickly, and
produces many offspring.
– This gives quick results that are easy to count.
• See Table 1 on page 163.
Monohybrid Cross
• Monohybrid cross involves crossing two
plants with one pair of contrasting traits.
• To ensure true-breeding plants selfpollinated for several generations.
• The parents are the P Generation
• The offspring of the P generation is the
First Filial or F1 Generation
• A cross of two F1 plants gives the F2 or
second Filial generation.
Mendel’s Results
• The P generation was ½ purple & ½ white.
• In the F1 generation one trait disappeared
in all of the plants. (All purple, no white)
• In the F2 generation the white trait
reappeared in ¼ of the plants.
• The ratio of purple to white flowers was
3:1.
• This pattern was the same for all 7
characteristics.
Mendel’s Theory
• Before Mendel people believed in blending
• Mendel saw “heritable factors” were
passed to offspring from parents.
• The father passed one of his two “factors”
as did the mother to the baby.
• When gametes fuse to make a zygote the
“factors” are combined.
Mendel’s Theory
• Each individual has two copies of a character,
one from each parent.
• A character is now called a gene.
• There are alternate versions of the genes are
called Alleles. purple, white are alleles
• One Allele is dominant & makes protein.
• One allele is recessive & doesn’t make protein.
• Genes making protein are called expressed.
Gametes and Alleles
• Mendel said when gametes form in a plant
the alleles from that plants parents
segregate independently of each other.
• Each gamete has one allele in it.
• When the gametes unite the baby will
have two alleles for each trait. One from
the mother and one from the father.
Genetic Vocabulary
• The physical appearance of an organism
is its phenotype. (purple flowers)
• The combination of alleles is the
genotype. (PP, Pp, pp)
• If an organism has the same alleles in its
genotype it is called Homozygous. (TT)
• If an organism has different alleles for a
trait it is called Heterozygous. (Tt)
Laws of Heredity
• Law of Segregation: states that when
gametes form (meiosis) the two alleles a
person has separate into different
gametes.
• Law of Independent Assortment: states
that the segregation of one pair of alleles
has no affect on the segregation of a
different set of alleles.
Studying Heredity
• Scientists use Punnett Squares to predict
the possible outcomes of breeding parents
• Punnett Squares have four boxes with the
alleles of one parent on top and the alleles
of the other parent on the side. The
alleles passed to the offspring are inside
the boxes.
• Phenotypic and genotypic ratios are
determined from the Punnett Squares.
Test Crosses
• A test-cross crosses an unknown
genotype with a homozygous recessive (tt)
• If no recessive shows then the unknown
was homozygous dominant, if the
recessive shows the unknown was
heterozygous.
• This cross will determine the genotype of
an unknown organism.
Probability of a Cross
• Probability is the chance a specific event
will occur. (Coin Flip heads to tails)
• Probability= # of 1 kind of outcome / total #
of possible outcomes
• Probability of an allele in a gamete is ½
• Probability of a cross is determined by
multiplying the individual events.
• Flip two different coins, four possible
outcomes. Same for alleles in parents.
Pedigrees & Sex-linked Genes
• A pedigree is a diagram that shows history
of a trait through many generations.
• Genes on the Sex chromosomes (XY) are
sex linked. Genes on the autosomes (122) are not linked to a sex.
• Sex-linked disorders are more common in
males than females. Females have 2 X’s
Complex Patterns of Heredity
• Incomplete Dominance- occurs when a single
dominant does not make enough protein to
mask the recessive.
• Codominance- occurs when two alleles both
make their protein.
• Multiple Alleles- occurs when there are more
than two alleles. (Human Blood type)
• Polygenetic Traits- occurs when a trait has more
than one location
• Environmental conditions can alter proteins
made by the genes.
Figure 11-11 Incomplete Dominance in
Four O’Clock Flowers
Genetic Disorders
• Genes maybe copied incorrectly or damaged
causing proteins not to function.
• Sickle Cell Anemia has hemoglobin made
incorrectly and red blood cells change shape
when lacking oxygen.
• Cystic Fibrosis has a defective gene to move
mucus from cells.
• Hemophilia is on the X chromosome and does
not allow blood to clot. (AHF protein)
• Huntington’s Disease is dominant but does not
take affect until later in life after it is passed to
the kids.
Gene Therapy
• Genetic Counseling will determine the
risks an individual with genetic disorders in
the family to pass the gene to their kids.
• Gene therapy uses viruses to inject
working genes into cells with nonworking
copies.