Chapter 14 Notes Flashcards
Mendel discovered the
basic principles of heredity by breeding garden peas in carefully planned experiments
Advantages of pea plants for genetic study
- There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
- Mating can be controlled (via cross pollination)
- Each flower has sperm-producing organs (stamens) and an egg-producing organ (carpel)
- Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another
Mendel chose to track only those characters that occurred in
two distinct alternative forms.
He also used varieties that were true-breeding
(plants that produce offspring of the same variety when they self-pollinate)
In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called
hybridization
The true-breeding parents are the
P generation
The hybrid offspring of the P generation are called the
F1 generation
When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the
F2 generation is produced
When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants,
all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids,
many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about
three to one, purple to white flowers, in the F2 generation
Mendel reasoned that
only the purple flower factor was affecting flower color in the F1 hybrids
Mendel called the purple flower color
a dominant trait and the white flower color a recessive trait
The factor for white flowers was not diluted or destroyed because
it reappeared in the F2 generation
Mendel observed the same pattern of inheritance in
six other pea plant characters, each represented by two traits
What Mendel called a “heritable factor” is what we now call
a gene
Alternative versions of genes account for variations in inherited characters
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
These alternative versions of a gene are now called
alleles
Each gene resides at a
specific locus on a specific chromosome
For each character, an organism inherits two alleles, one from each parent.
Mendel made this deduction without knowing about the role of chromosomes
The two alleles at a particular locus may be
identical, as in the true-breeding plants of Mendel’s P generation
Alternatively, the two alleles at a locus may
differ, as in the F1 hybrids
If the two alleles at a locus differ, then one (the dominant allele) determines the
organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
-In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant
(now known as the law of segregation):
the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the organism.
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis.
Mendel’s segregation model accounts for the 3:1 ratio he observed in
the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square,
a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a
lowercase letter represents a recessive allele
-Potential gametes are on the outside of the square
Punnett Square Steps
Step 1- Write the genotypes of the parents
Step 2- Determine the gametes each parent can produce
Step 3- Place the gametes on the outside of the punnett square
Step 4- Fill in the boxes
An organism with two identical alleles for a character is said to be
homozygous for the gene controlling that character
An organism that has two different alleles for a gene is said to be
heterozygous for the gene controlling that character
Unlike homozygotes, heterozygotes are not
true-breeding
Because of the different effects of dominant and recessive alleles,
an organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its
genotype, or genetic makeup
In the example of flower color in pea plants,
PP and Pp plants have the same phenotype (purple) but different genotypes
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross:
breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype,
the mystery parent must be heterozygous
all kids dominant =
AA (homozygous)
1/2 kids dominant
1/2 kids recessive =
Aa (heterozygous)
Phenotype
physical appearance (like the color)
Genotype
genetic makeup (like the Bb, BB, bb letter stuff)
BB genotype (two capital letters) =
homozygous dominant
bb genotype (two lower case letters) =
homozygous recessive
Bb genotype (one capital and one lower case letter) =
heterozygous
Mendel derived the law of segregation by
following a single character
The F1 offspring produced in this cross were monohybrids,
individuals that are heterozygous for one character
A cross between such heterozygotes is called a
monohybrid cross
Mendel identified his second law of inheritance by
following two characters at the same time
Crossing two true-breeding parents differing in two characters produces
dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine
whether two characters are transmitted to offspring as a package or independently
Using a dihybrid cross, Mendel developed the
law of independent assortment
The law of independent assortment states that
each pair of alleles segregates independently of each other pair of alleles during gamete formation
Strictly speaking, this law (law of independent assortment) applies only to genes on different,
nonhomologous chromosomes or those far apart on the same chromosome
Genes located near each other on the same chromosome tend to be
inherited together
Mendel’s laws of segregation and independent assortment reflect the
rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
The multiplication rule states that
the probability that two or more independent events will occur together is the product of their individual probabilities –AND-
The addition rule states that
the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities –OR-
We can apply the multiplication and addition rules to predict
the outcome of crosses involving multiple characters
In calculating the chances for various genotypes,
each character is considered separately, and then the individual probabilities are multiplied
What’s the probability of getting an individual with the genotype AABbCc? From a cross of AaBbCc x AaBbCc
-1st- Break each trait apart AA – probability ¼ Bb – (Bb =1/4 OR bB = ¼) probability ½ Cc – (Cc = ¼ OR cC = ¼) probability ½ -Now decide to add or multiply? AND = multiply ¼ x ½ x ½ = 1/16 ANSWER
The relationship between genotype and phenotype is rarely as simple as
in the pea plant characters Mendel studied
Many heritable characters are not determined by only one gene with two alleles
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
- When alleles are not completely dominant or recessive
- When a gene has more than two alleles
- When a gene produces multiple phenotypes
Complete dominance occurs when
phenotypes of the heterozygote and dominant homozygote are identical
In incomplete dominance,
the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
((cross two traits and there will be a mixture of something. like cross red and white and get pink))
In codominance,
two dominant alleles affect the phenotype in separate, distinguishable ways
Ex: AB blood type
((like there being two color stripes on something))
A dominant allele does not subdue a recessive allele; alleles don’t interact that way
Alleles are simply variations in a gene’s nucleotide sequence
For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain
- At the organismal level, the allele is recessive
- At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
- At the molecular level, the alleles are codominant
Dominant alleles are not necessarily more common in populations than recessive alleles
- For example, one baby out of 400 in the United States is born with extra fingers or toes
- The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
- In this example, the recessive allele is far more prevalent than the population’s dominant allele
Most genes exist in populations in more than two allelic forms
(multiple alleles)
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A carbohydrate,
whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
multiple alleles
when you have more than 2 alleles of choices
Most genes have multiple phenotypic effects, a property called
pleiotropy
For example, pleiotropic alleles are responsible for the
multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
pleiotropy
one gene affects many traits
Some traits may be determined by two or more genes
Epistasis
Polygenic inheritance
In epistasis,
a gene at one locus alters the phenotypic expression of a gene at a second locus
(epistasis)
For example, in Labrador retrievers and many other mammals, coat color depends on two genes
- One gene determines the pigment color (with alleles B for black and b for brown)
- The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
Quantitative characters are those
that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance
an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of
polygenic inheritance
(eye color is also an example of this)
polygenic inheritance
many genes influence one trait
Another departure from Mendelian genetics arises when the
phenotype for a character depends on environment as well as genotype
The norm of reaction is the
phenotypic range of a genotype influenced by the environment
-For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Norms of reaction are generally broadest for polygenic characters
Such characters are called multifactorial because
genetic and environmental factors collectively influence phenotype
An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior.
An organism’s phenotype reflects its overall genotype and unique environmental history.
Humans are not good subjects for genetic research
Generation time is too long
Parents produce relatively few offspring
Breeding experiments are unacceptable
-However, basic Mendelian genetics endures as the foundation of human genetics
A pedigree is a
family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using
pedigrees
Pedigrees can also be used to make predictions about future offspring.
We can use the multiplication and addition rules to predict the probability of specific phenotypes.
Many genetic disorders are inherited in
a recessive manner
These range from relatively mild to life-threatening
Recessive inherited disorders
- Albinism
- Cystic Fibrosis
- Sickle-cell syndrom/disease
Recessively inherited disorders show up only in individuals homozygous for the allele
AA and Aa don’t have the disorder
aa does have the disorder
Carriers are
heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents (Aa)
Most people who have recessive disorders are born to
two carrier parents
Albinism is a recessive condition characterized by
a lack of pigmentation in skin and hair
If a recessive allele that causes a disease is rare, then
the chance of two carriers meeting and mating is low
Consanguineous matings (i.e., matings between close relatives) increase
the chance of mating between two carriers of the same rare allele
-Most societies and cultures have laws or taboos against marriages between close relatives
Cystic fibrosis is the
most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in
defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell
Cystic fibrosis Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Chronic bronchitis, foul stools, recurrent bacterial infections
If children with cystic fibrosis remain untreated, the children die before their 5th birthday
Treatment options include:
- Physical chest clearing
- Daily antibiotic regimens
- Life expectancy is about 30 years
Sickle-cell disease affects
one out of 400 African-Americans
Sickle-cell disease is caused by
the substitution of a single amino acid in the hemoglobin protein in red blood cells
In homozygous individuals,
all hemoglobin is abnormal (sickle-cell)
Sickle-cell disease symptoms include
physical weakness, pain, organ damage, and even paralysis
Sickle-cell disease and cystic fibrosis are examples of
pleiotropy
In sickle-cell disease
hh- full blown disease
Hh – sickle-cell trait
HH - normal
Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous
Some human disorders are caused by
dominant alleles
Dominant alleles that cause a lethal disease are
rare and arise by mutation
Achondroplasia is a form of
dwarfism caused by a rare dominant allele
DD – lethal
Dd- dwarfism
dd – normal height
This is an example where the recessive allele is more common than the dominant allele
The timing of onset of a disease significantly affects its
inheritance
Huntington’s disease is
a degenerative disease of the nervous system
Huntington’s disease has no obvious phenotypic effects until the individual is
about 35 to 40 years of age
Hh - affected
hh-normal
Once the deterioration of the nervous system begins the condition is irreversible and fatal
Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both
genetic and environmental components
Little is understood about the
genetic contribution to most multifactorial diseases
Genetic counselors can provide information to prospective parents concerned about
a family history for a specific disease
Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders.
Probabilities are predicted on the most accurate information at the time; predicted probabilities may change as new information is available.
For a growing number of diseases, tests are available that
identify carriers and help define the odds more accurately
In amniocentesis,
the liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS),
a sample of the placenta is removed and tested
Other techniques, such as
ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
Some genetic disorders can be detected at birth by
simple tests that are now routinely performed in most hospitals in the United States
- PKU
- -Can’t metabolize phenylalanine – diet
Huntington’s disease and Achondroplasia are
dominantly inherited disorders
gene-
a heritable unit that determines a character; can exist in different forms
allele-
an alternative version of a gene
character-
a heritable feature that varies among individuals
trait-
a variant for a character
dominant allele-
determines phenotype in a heterozygote
recessive allele-
has no effect on phenotype in a heterozygote
genotype-
the genetic makeup of an individual
phenotype-
an organism’s appearance or observable traits
homozygous-
having two identical alleles for a gene
heterozygous-
having two different alleles for a gene
testcross-
a cross between an individual with an unknown genotype and a homozygous recessive individual
monohybrid cross-
a cross between individuals heterozygous for a single character
