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Unit 6

Spring 2016: Biology - Classical Genetics (Chapters 6, 7, & 9)
by

Jordan Rowlen

on 19 February 2016

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Transcript of Unit 6

pedigree
: a family tree that records and traces the occurrence of a trait in a family

squares = males
circles = females
colored shapes = individuals that show the trait

Parents are connected by horizontal lines, with their children beneath them in the order of birth.









example: Earlobes are either free or attached. This pedigree tracks the
occurrence of attached earlobes in three generations of a family.
example: red-green colorblindness
















The half-filled circles represent carriers.

(This pedigree shows the appearance of colorblindness in four generations of a family.)

It is rare—but not impossible—for females to exhibit sex-linked (X-linked) traits.
To study the inheritance of a human trait:

collect information about a family's history for the trait

organize the information in a family tree

analyze this information by applying Mendel's concepts of dominant
and recessive alleles and his principle of segregation
Classical Genetics
A pea has gene loci for shape and color on different chromosomes. After this dihybrid pea matures into a plant, its chromosomes shuffle during meiosis, producing four possible genotypes in its gametes.
Unit 6
Chapter 6
Meiosis and Mendel
Chapters 6, 7, & 9
Concept 6.3
Concept 6.5
Concept 6.6
Chapter 7
Extending Mendelian Genetics
Concept 7.2
Concept 7.3
Concept 7.4
Concept 9.6
Mendel and Heredity
Traits and Probability
Meiosis and Genetic Variation
Complex Patterns of Inheritance
Gene Linkage and Mapping
Human Genetics and Pedigrees
Genetic Screening and Gene Therapy
Concept 7.1
Chromosomes and Phenotype
Chapter 9
Frontiers of Biotechnology
trait
: a distinguishing characteristic that is inherited

(example: a plant's trait of having red flowers, or yellow flowers, etc.)
genetics
: the study of biological inheritance patterns and variation
Gregor Mendel
: an Austrian monk who found the basic rules of inheritance through a series of experiments
(He showed that traits are inherited as discrete units!)
blending hypothesis
: hypothesis in 1800s to explain how offspring inherit traits from both parents

(example:
red
and
yellow
flowered plants could produce an
orange
flowered plant)
BUT this was FALSE!
parental plants (P generation)
the gene for white flowers did not disappear in the F1 plants
notice: NOT a blend of purple and white
hybrid offspring (F1 generation)
Traits are inherited as discrete units.
Organisms inherit two copies of each
gene, one from each parent.
The two copies segregate during
gamete formation.
cross-fertilization
: the process Mendel used to cross two true-breeding plants with two contrasting traits
cross seen above:
purple flower
x

white flower
(Mendel experimented with these seven pea plant characters.)
Mendel fertilized

true-breeding

pea plants - meaning when they self-fertilized, the offspring are identical to the parent pea plant.
(example:
purple-flowered
pea plant makes
purple-flowered
pea plant offspring)
Mendel bred pea plants and recorded inheritance patterns in the offspring for 7 years.
(Your book refers to these as
purebred
.)
Concept 6.4
Traits, Genes, and Alleles
hybrid
: the offspring of two different true-breeding varieties
Prior To Mendel
Mendel's Work
True-Breeding Plants
Cross Fertilization
Mendel's
Experiment
Mendel crossed the P generation to produce the F1 generation.

Notice the results:
Conclusion
F1 generation: all plants had
purple
flowers
F2 generation: some plants had
purple
flowers and some had
white
Mendel's
Mendel concluded that the trait for the white flower had not disappeared - it had been hidden.
Mendel performed many crosses to help him observe inheritance patterns.
Cross Results
Hybrids
Mendel's Three Conclusions
( and make up Mendel's Law of Segregation.)
gene
: a piece of DNA that directs a cell to make a certain protein
Genes
Each gene has a
locus
, a specific location on a pair of homologous chromosomes.
(Recall this from Unit 5?!)
alleles
: alternative forms of genes

(example: allele for blue eyes (
b
) vs. allele for brown eyes (
B
))

Each parent donates one allele for every gene.
Alleles
dominant allele
: an allele in a heterozygous individual that appears to affect the trait

(example: allele for brown eyes (
B
))

recessive allele
: an allele in a heterozygous individual that does not appear to affect the trait

(example: allele for blue eyes (
b
))
homozygous
: two alleles that are the same at a specific locus
(example:
BB
or
bb
)
heterozygous
: two alleles that are different at a specific locus
(example:
B
b
)
Genotype and Phenotype
genotype
: genetic makeup or combination of alleles (what the DNA says)
example:
PP

phenotype
: an observable trait (what is physically seen)
example:
purple flowers
Allele Dominance
Punnett square
: a grid system for predicting all possible genotypes resulting from a cross - it gives the ratio of possible genotypes and phenotypes of offspring
Punnett Square
monohybrid cross
: examine the inheritance of only one specific trait
Monohybrid Crosses
dihybrid cross
: examines the inheritance of two traits
Dihybrid Cross
Mendel's dihybrid crosses led to his second law, the
Law of Independent Assortment
. This law states that allele pairs separate
independently
of each other during meiosis.
Independent Assortment
testcross
: a cross between an organism with an unknown genotype and an organism with the recessive phenotype
TestCross
probability
: the likelihood that something will happen
Probability
Additional Videos on Heredity
Monohybrid Cross
Example above:
homozygous dominant
X
homozygous recessive
=
all heterozygous, all dominant
More Examples of
heterozygous
X
heterozygous
=
1 homozygous dominant
:
2 heterozygous
:

1 homozygous recessive

or
3 dominant
:

1 recessive
heterozygous
X
homozygous recessive
=
1 heterozygous
:

1 homozygous recessive
or
1 dominant
:

1 recessive
wrinkled
green
seed
RR
YY
round
yellow
seed
rr
yy
All peas had the dominant phenotype: round and
yellow
.
This produced four phenotypes of peas.

Punnett square prediction:
a phenotypic ratio of 9 : 3 : 3 : 1
example: R can end up with either
Y
or
y
, and r can end up with either
Y
or
y
)
if
PP
, then:
if
P
p
, then:
all offspring would be
P
p
both
purple
(
P
p
) and
white
(
pp
) offspring
genotypic ratio
: the ratio of the genotypes
(
1 PP
:
2 P
p
:
1

pp
)

phenotypic ratio
: the ratio of the phenotypes
(
3 purple
:
1

white
)
Sexual reproduction creates unique combination of genes.


independent assortment of chromosomes in meiosis
random fertilization of gametes




Unique phenotypes may give a reproductive advantage to some organisms.
Genetic Variation
crossing over
: the exchange of chromosome segments between homologous chromosomes; occurs during the first part of meiosis I; results in new combinations of genes
Crossing OVer
Chromosomes contain many genes. The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.

genetic linkage
: when genes located close together on a chromosome tend to be inherited together
Genetic Linkage
Crossing over during Meiosis increases genetic diversity!
How?
AKA - you don't know which gamete will be used to make the zygote!
(Think back to Mendel's law...)
HONORS ONLY
HONORS ONLY
Recall...
Mendel's Law of Independent Assortment
Crossing over can recombine gene loci on homologous chromosomes. This is unlikely when the genes are very close together (Scenario 1). A crossover is more likely to recombine the alleles when the genes are far apart (Scenario 2).
A
and
B
are not linked to
C
and
D
because they are so far apart. Crossing over is likely to occur in the space between genes
B

and
C
, thereby separating
A
and
B

from
C
and
D
.
A
and
B
are referred to as linked because they would likely be inherited together.
C
and
D
are referred to as linked because they would likely be inherited together.
autosomal gene
: a gene located on any of the autosomal chromosomes (chromosomes 1-22 in humans)

Two copies of each autosomal gene affect phenotype.
Autosomal Genes
Mendel’s rules of inheritance apply to autosomal genetic disorders.

carrier
: a heterozygote for a recessive disorder

Disorders caused by dominant alleles are uncommon.
Autosomal Disorders
sex-linked genes
: genes on sex chromosomes
(chromosome 23 in humans)

Y chromosome genes in mammals are responsible for male characteristics.
X chromosome genes in mammals affect many traits.
Sex-Linked Genes
male mammals: XY genotype
All of a male’s sex-linked genes are expressed.
Males have no second copies of sex-linked genes.


female mammals: XX genotype
Expression of sex-linked genes is similar to autosomal genes in females.
X chromosome inactivation randomly “turns off” one X chromosome.
Male and Female Genotypes
example: fruit flies

allele for red eyes = dominant
allele for white eyes = recessive

It is
extremely
rare to find a female with white eyes.
Sex-Linked Cross
Sex-linked disorders are disorders that are inherited as
sex-linked (X-linked) recessive traits.
(the same way as the white-eye trait in fruit flies)

examples: red-green color blindness and hemophilia
(a disease in which blood fails to clot normally)

These are more common in men.
(If a human male inherits the sex-linked recessive allele from his mother, the allele will be expressed - whereas females must inherit two alleles to exhibit the trait!)
Sex-Linked Disorders
This inheritance pattern is located only on the X chromosome. There is no corresponding eye color locus on the Y.

females (XX) carry two copies for eye color
males (XY) carry only one for eye color

(SO - a female will have white eyes only if she has the white-eye allele on both her X chromosomes, but males will only need one allele on their one X chromosome.)
Why might this be?
For some characters of organisms,
neither
allele is dominant.

incomplete domiance
: pattern of inheritance where the heterozygous phenotype is
intermediate
between the two homozygous phenotypes
Incomplete Dominance
Codominant alleles will both be completely expressed.

Codominant alleles are neither dominant nor recessive.
The ABO blood types result from codominant alleles.
CoDominance
polygenic inheritance
: when two or more genes affect a single character - the variation in phenotypes can become even greater

example: height, eye color, and skin color (in humans)
Polygenic Inheritance
An individual's phenotype depends on environment as well as on genes.

(example: a tree's genotype does not change, yet the tree's leaves vary in size, shape, and greenness from year to year (depending on exposure to wind and sunlight))

For humans, many phenotypes vary due to environmental factors.
Environmental Affect
Many characters of organisms have more complicated inheritance patterns than those studied by Mendel.
This inheritance pattern does
not
support the blending hypothesis.
(This is because the parent phenotypes can reappear in the F2 generation.)
example: red and white parents produce F1 hybrid offspring that are pink - neither the red nor white allele is dominant
three alleles for blood type in the human population: A, B, O
(But note that any one person has only two alleles for blood type.)


result in 6 genotypes & 4 phenotypes

Alleles IA and IB are codominant.
Allele i is recessive.

The individual's phenotype is not incomplete, but rather shows the
separate traits of both alleles.
On the other hand - some phenotype with little or no influence from the environment.

example: human blood type

In summary, the product of a genotype is generally not a single, rigidly defined phenotype, but a range of possibilities influenced by the environment.
Want more info on blood types?
Click the link: http://www.redcrossblood.org/learn-about-blood/blood-types
How to Study
Inheritance Patterns
Pedigrees
Most human genetic disorders are recessive.

carrier
: an individual who has one copy of the allele for a recessive disorder and does not exhibit symptoms

example: a particular form of deafness is inherited as a recessive trait.
Types of Disorders
A smaller number of human disorders are inherited as dominant traits.

example: achondroplasia (a form of dwarfism - "little people")
(About 1 out of 25,000 people has achondroplasia - all individuals with this disorder are heterozygous. More than 99.99% of the population is homozygous for the normal, recessive allele so it clear that dominant alleles are not necessarily more plentiful than recessive alleles in a population.)
Disorders Inherited as Dominant Traits
Pedigrees for
Sex-Linked Disorders
Females can carry sex-linked genetic disorders.

Males (XY) express all of their sex linked genes.

Most sex-linked alleles are located on the X chromosome. A male only receives such sex-linked alleles from his mother. (The homologous Y chromosome is always inherited from the father.)

Therefore, if the phenotype is more common in males,
the gene is likely sex-linked.
Disorders Inherited as Recessive Traits
disorders inherited as recessive traits
disorders inherited as dominant traits
sex-linked disorders (both dominant and recessive)
dad & mom
1 daughter
3 sons
How do you figure out genotypes of each?

determine the pattern in which the trait occurs

(example: Notice that the first-born daughter in the third generation has attached lobes, although both of her parents have free earlobes. - so the attached-earlobe trait must be recessive. If the trait for attached earlobes were dominant, then at least one of her parents would have attached earlobes.)

The genotypes of most family members can be determined.
Sex-Linked Disorders
Concept 7.3 covers much of the same information covered in Concept 6.6.

Students are responsible for the material in Concept 6.6 but may refer to 7.3 if desired for a more in-depth look at crossing over and genetic linkage.
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
HONORS ONLY
gene therapy
: can detect genetic disorders; involves the testing of DNA

determines risk of havingor passing on a geneticdisorder
used to detect specificgenes or proteins
can detect some genesrelated to an increasedrisk of cancer
can detect some genesknown to cause geneticdisorders
Genetic Screening
HONORS ONLY
gene therapy
: the replacement of faulty genes

Gene therapy replaces defective or missing genes, or adds new genes, to treat a disease.
Gene Therapy
HONORS ONLY
genetically engineered viruses used to “infect” a patient’s cells
insert gene to stimulate immune system to attack cancer cells
insert “suicide” genes into cancer cells that activate a drug
Gene Therapy Techniques
HONORS ONLY
inserting gene into correct cells
controlling gene expression
determining effect on other genes
Gene Therapy Challenges
HONORS ONLY
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