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Chapter 11
Introduction to Genetics
INTRODUCTION TO GENETICS
• GREGOR MENDEL=Monk who became interested in plants-why do some
grow tall, short, green and yellow seeds, smooth or wrinkled. The pea plants
he worked with were able to produce both types of gametes this allowed for
cross fertilization. He could control the mating and breeding of Pea Plants.
He was able to obtain pure lines.
• GENETICS= Study of heredity-passing of traits from one generation to
another-inherited characteristic
• TRUE BREEDING – meaning that if they were allowed to self-pollinate,
they would produce offspring identical to themselves
• TRAITS – is a specific characteristic, such as seed color or plant height, that
varies from one individual to another.
• HYBRID – The offspring of crosses between parents with different traits
• GENES= Are the factors that control traits – 1 gene has 2 alleles
• ALLELES= Different forms of a gene, some are dominant some recessive –
TT or tt or Tt
• SEGREGATION – separation of alleles during gamete formation
• GAMETES=A male or female reproductive cell that carries genetic
information.Male=Sperm,Female=Egg
b
PROBABILITY = Chance P=#of chances for an event/ # of possible events
•
• PUNNETT SQUARE – diagram showing the gene combinations that might
result from a genetic cross
• HOMOZYGOUS= The pair of alleles or genotype are identical – TT or tt
• HETEROZYGOUS= The pair of alleles or genotype are different – Tt
• PHENOTYPE=The way an organisms traits are expressed, physical
appearance, an expression of genotype
• GENOTYPE= Genes pairing found on chromosomes
• INDEPENDENT ASSORTMENT –independent segregation of genes during
the formation of gametes
• INCOMPLETE DOMINANCE – Cases in which one allele is not completely
dominant over another (pink)
• CODOMINANCE – both alleles contribute to the phenotype of the organism
(roan)
• MULTIPLE ALLELES – Many genes have more than two alleles
• POLYGENIC TRAITS - Traits controlled by two or more genes
• DOMINANT= A gene whe present covers up a recessive gene
• RECESSIVE= Masked or covred up whenever the dominant allele is
present, the trait will show through if there is no dominant trait, two recessive
traits.
Cross-Pollination
• To cross-pollinate pea
plants, Mendel cut off
the male parts of one
flower and then
dusted it with pollen
from another flower.
• Question? How did
this procedure
prevent selfpollination?
Mendel’s Seven F1 Crosses on Pea Plants
From these experiments, Mendel concluded some
alleles are dominant and others are recessive
Segregation
Mendel allowed the F1 x F1 = F2, by self
pollination
When Mendel allowed the F1 plants to reproduce by selfpollination, the traits controlled by recessive alleles reappeared
in about one fourth of the F2 plants in each cross.
QUESTIONS
• DID MENDEL CROSS POLLINATE F1 PLANTS
TO GET F2 PLANTS?
– No, he allowed them to self pollinate
• WAS THE RECESSIVE TRAIT, SHORTNESS
LOST IN THE F1 GENERATION?
– No, it was masked by the dominant trait tallness
• ARE THE F1 PLANTS TRUE BREEDING?
– No, they did not produce off spring identical to
themselves
Segregation – the alleles were segregated from each
other during the formation of the reproductive cells,
gametes
• During gamete
formation, alleles
are segregated
from each other so
that each gamete
carries only a
single copy of each
gamete. Each F1
plant produces two
types of gametes –
those with the
allele for tallness
and those with the
allele for
shortness.
QUESTIONS
• WHAT ARE THE DOMINANT AND RECESSIVE ALLELS?
– Dominant allele: form of a trait that always shows up in an organism;
recessive allele: form of a trait that shows up only when the dominant
allele is not present
• WHAT IS SEGREGATION? WHAT HAPPENS TO ALLELES DURING
SEGREGATION?
– Separation of alleles; the alleles are separated so that each gamete
carries only a single copy of each gene.
• WHAT DID MENDEL CONCLUDE DETERMINES BIOLOGICAL
INHERITANCE?
– Factors that are passed from one generation to the next.
• DESCRIBE HOW MENDEL CROSS POLLINATED PEAS.
– Mendel cut away the male parts of one flower, then dusted it with pollen
from another flower.
• WHY WERE TRUE BREEDING PEAS IMPORTANT FOR MENDEL’S
EXPERIMENTS?
– True-breeding peas have two identical alleles for a gene, so in a genetic
cross each parent can contribute only one form of an allele.
• WHAT IS A PUNNETT SQUARE?
– A diagram that shows the gene combinations that might result from a
genetic cross
11-2
Probability and Punnett Squares
• Probability – the likelihood that a particular
event will occur.
• Example: Coin flip
• If you flip a coin three times in a row what
is the probability that it will land heads up?
• ½ X ½ X ½ = 1/8
• The principle of probability can be used to
predict the outcomes of genetic crosses.
Punnett Squares
• - the gene
combinations
that might
result from a
genetic cross
can be
determined by
drawing a
diagram.
• Organisms that have two identical alleles
for a particular trait—TT or tt in this
example—are said to be homozygous
• Organisms that have two different alleles
for the same trait are heterozygous
• phenotype, or physical characteristics
• genotype, or genetic makeup.
• Phenotypes and
Genotypes
Although these
plants have
different genotypes
(TT and Tt), they
have the same
phenotype (tall).
INDEPENDENT ASSORTMENT
–
–
Genes that segregate independently, such
as genes for seed shape and seed color in
pea plants, do not influence each other’s
inheritance
Genes for different traits can segregate
independently during the formation of
gametes
The principle of independent assortment states
that genes for different traits can segregate
independently during the formation of gametes.
DOMINANT AND RECESSIVE
–
Incomplete
dominance –
which 1 allele is
not completely
dominant over
another
•
Example – red
flower x white
flower = pink
flower
DOMINANT AND RECESSIVE
– Codominance – both alleles contribute to the
phenotype of the organism
• Example – in cattle the allele for red hair is
codominant with the allele for white hair – both
alleles are roan, coats are mixed, red and white.
• Example – chicken heterozygous – speckled both
black and white feathers
MULTIPLE ALLELES
–
Genes with more than 2 alleles- more than
2 possible alles
•
Example – blood typing, A and B Codominant,
O is recessive
•
POLYGENIC TRAITS – controlled by 2
or more genes
•
Example – skin color, hair color
QUESTIONS
– EXPLAIN WHAT INDEPENDENT ASSORTMENT
MEANS.
» Genes for different traits do not influence each
other’s inheritance
– DESCRIBE TWO INHERITANCE PATTERNS
BESIDES SIMPLE DOMINANCE.
» Incomplete dominance, codominance, multiple
alleles, or polygenic traits
– WHY ARE FRUIT FLIES AN IDEAL ORGANISM FOR
GENETIC RESEARCH?
» They are small, easy to keeping the laboratory,
and produce large numbers of offspring in a short
period of time.
LINKAGE AND GENE MAPS
–
–
–
–
Thomas Hunt Morgan – researched on fruit
flies led him to the principle of linkage.
It is the chromosomes, however, that
assort independently, not individual genes.
The farther apart two genes were, the more
likely they were to be separated by a
crossover in meiosis.
Gene map – relative locations of each
known gene on the chromosomes.
QUESTIONS
– HOW DOES THE PRINCIPLE OF INDEPENDENT
ASSORTMENT APPLY TO CHROMOSOMES?
» It is the chromosomes that assort independently, not
individual genes
– WHAT ARE GENE MAPS, AND HOW ARE THEY PRODUCED?
» A gene map shows the relative locations of genes on a
chromosome. The frequency of crossing-over between
genes is used to produce a map of distances between genes.
– HOW DOES CROSSING-OVER MAKE GENE MAPPING
POSSIBLE?
» The farther apart two genes are, the more likely they are to
be separated during a crossover in meiosis. Therefore, the
frequency of crossing over is equal to the distance between
two genes.
– IF GENES ARE ON THE SAME CHROMOSOME BUT
USUALLY ASSORT INDEPENDENTLY, WHAT DOES THAT
TELL YOU ABOUT HOW CLOSE TOGETHER THEY ARE?
» The two genes are located very far apart from each other.