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Genes Ch. 14 (Mendelian model applies to Inheritance of Human Variations,…
Genes
Ch. 14
Mendelian model applies to Inheritance of Human Variations
Human traits follow Mendelian patterns of inheritance
However, basic Mendelian genetics endures as the foundation of human genetics
Humans are not good subjects for genetic research
Generation time is too long
Parents produce relatively few offspring
Breeding experiments are unacceptable
Pedigree Analysis
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
Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
These range from relatively mild to life-threatening
The Behavior of Recessive Alleles
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
Most individuals with recessive disorders are born to 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, it is unlikely that two carriers will meet and mate
Consanguineous matings (i.e., between close relatives) increase the chance that both parents of a child carry the same rare allele
Most societies and cultures have laws or taboos against marriages between close relatives
Cystic Fibrosis
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
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Sickle-Cell Disease
About one out of ten African Americans has sickle-cell trait, an unusually high frequency
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common
Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
Dominantly Inherited Disorders
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
Multifactorial Disorders
Many diseases, such as heart disease, cancer, alcoholism, and mental illnesses, have both genetic and environmental components
No matter what our genotype, our lifestyle has a tremendous effect on phenotype
The timing of onset of a disease significantly affects its inheritance
Huntington’s disease
is a degenerative disease of the nervous system
The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
Once the deterioration of the nervous system begins, the condition is irreversible and fatal
Genetic Testing & Counseling
Counseling based on Mendelian Genetics
Suppose a couple both have a brother who died from the same recessively inherited disease
f both members of the couple had a sibling with the recessively inherited illness, both of their parents were carriers
Thus each has a ⅔ chance of being a carrier
If both are carriers, there is a ¼ chance of each child having the recessive illness
The overall probability of them having a child with the illness is ⅔ × ⅔ × ¼ = 1/9
A genetic counselor can help determine the risk that a couple will have a child with a particular disease
It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings
Testing Carriers for Identifying Carriers
For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately
The tests enable people to make more informed decisions about having children
However, they raise other issues, such as whether affected individuals fully understand their genetic test results, and how the test results are used
Fetal Testing
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, allow fetal health to be assessed visually in utero
Probability laws govern Mendelian inheritance
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 & Addition Rules Applied to Monohybrid Crosses
The
multiplication rule
states that the probability that two or more independent events will occur together is the product of their individual probabilities
Probability in an F1 monohybrid cross can be determined using the multiplication rule
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
The
addition rule
states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities
The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
Using the rule of addition, then, we can calculate the probability of an F2 heterozygote as 1/4 + 1/4 = 1/2
Solving Complex Genetics Problems
The rules pf probability can be used to solve complex genetics problems
A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances of the various offspring genotypes from such crosses, each character is first considered separately and then the individual probabilities are multiplied
Patterns of Inheritance are more Complex
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 as observed in Mendel's experiment
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
Mendelian Genetics for a Single Gene
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
Degrees of Dominance
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
In
codominance,
two dominant alleles affect the phenotype in separate, distinguishable ways
Dominance & Phenotype
In the case of pea shape, the dominant allele codes for an enzyme that converts an unbranched form of starch in the seed to a branched form
The recessive allele codes for a defective form of the enzyme, which leads to an accumulation of unbranched starch
This causes water to enter the seed, which then wrinkles as it dries
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
Frequency of Dominant Alleles
Dominant alleles are not necessarily more common in populations than recessive alleles
One baby out of 400 in the United States is born with extra fingers or toes
This condition, polydactyly, is caused by a dominant allele, found much less frequently in the population than the recessive allele
Multiple Alleles
Most genes exist in populations in more than two allelic form
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme 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
Pleiotrophy
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
Mendelian genetics for 2 or More Genes
Some traits may be determined by two or more genes
In epistasis, one gene affects the phenotype of another due to interaction of their gene products
In polygenic inheritance, multiple genes independently affect a single trait
Epistasis
The phenotypic expression of one gene affects the expression of another gene
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 E for color and e for
no color) determines whether the pigment will be deposited in the hair
Polygenic Inheritance
A single phenotypic character is affected by 2 or more genes
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
Height is a good example of polygenic inheritance: Over 180 genes affect height
Mendel's Theory of Inheritance
Mendel's Pea Experiment
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
Mendel chose to work with peas b/c of the many varieties that there are. For example one variety has purple flowers while another variety has white flowers
A heritable feature that varies among individuals (such as flower color) is called a
character
Each variant for a character, such as purple or white color for flowers, is called a
trait
Other advantages of using peas
Short generation time
Large numbers of offspring
Mating could be controlled; plants could be allowed to self-pollinate or could be cross-pollinated
For the breeding experiment Mendel cross-pollinated some pea plants
Mendel chose to track only those
characters
that occurred in two distinct alternative forms, such as purple or white flower color
He also started with varieties that were
true-breeding
(plants that produce offspring of the same variety when they self-pollinate)
Mendel cross-pollinated 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
Mendel's analysis of the F2 plants form thousands of genetic crosses like these allowed him to deduce 2 fundamental principles of heredity (The 2 laws he proposed)
Law of segregation
During the experiment Mendel observed 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 purple flowers to one white flower 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
Mendel's model
explains the 3:1 inheritance pattern that he observed among the F2 offspring in his pea experiment
4 related concepts make up Mendel's model
First:
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 called
alleles
Each gene resides at a specific locus on a specific chromosome
Second:
for each character, an organism inherits two alleles, one from each parent
Mendel made this deduction without knowing about chromosomes
The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
Or the two alleles at a locus may differ, as in the F1 hybrids
A genetic locus is actually represented twice in a diploid cell, once on each homolog of a specific pair of chromosome
Third:
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 organism's appearance
Accordingly Mendel's F1 plants had purple flowers because the allele for that trait is dominant
And the allele for white flowers is recessive
Fourth the
law of segregation
:
states that the two alleles for a heritable character segregate (separate from each other) 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
These concepts can be related to what we now know about genes and chromosomes
A
Punnet square
, a handy diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic material
A capital letter represents a dominant allele
A lowercase letter represents a recessive allele
Useful Genetic Vocabulary
An organism that has a pair of identical alleles for a gene encoding a character is called a
homozygote
It is said to be
homozygous
for the gene controlling that character
In the parental generation in Figure 14.5, the purple-flowered pea plant is homozygous for the dominant allele (PP), while the white plant is homozygous for the recessive allele (pp)
Figure 14.5
Homozygous plants "breed true" because all of their gametes contain the same allele-either P or p in this example
An organism with two different alleles for a gene is a
heterozygote
and is said to be
heterozygous
for the gene controlling that character
Unlike homozygotes, heterozygotes are not true-breeding
For example, P- and p-containing gametes are both produced by our F1 hybrids. Self-pollination of the F1 hybrids thus produces both purple-flowered and white-flowered offspring
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
The
Testcross
An individual with the dominant phenotype could be either homozygous dominant or heterozygous
To determine the genotype we can 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
Law of Independent Assortment
Mendel derived the law of segregation by following a
single
character, such as flower color
The F1 offspring produced in this cross were
monohybrids
, meaning they were heterozygous for one particular character being followed in the cross
We refer to a cross between such heterozygotes is called
a
monohybrid cross
Mendel worked the 2nd law of inheritance by following
two
characters at the same time, such as seed color and seed shape
Seeds (peas) may be either yellow or green. They may also be either round (smooth) or wrinkled
Mendel applied info derived from the law of segregation about single character crosses and knew the allele for yellow seeds is dominant (Y), and the allele for green seeds is recessive (
y
)
For the seed-shape character, the allele for round is dominant (R), and the allele for wrinkled is recessive (
r
)
Imagine crossing 2 true-breeding pea varieties that differ in
both
of these characters- a cross between a plant with yellow-round seeds (YYRR) and a plant with green-wrinkled seeds (
yyrr
)
A
dihybrid cross
, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
The F1 plants will be
dihybrids
, individuals heterozygous for the 2 characters being followed in the cross (YyRr)
Using a dihybrid cross, Mendel developed the
law of independent assortment
It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation
This law 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
