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Chapter 14 Genetics (Complex Inheritance Problems ((The relationship…
Chapter 14 Genetics
Mendel & Laws of Inheritance
Mendel chose to track only characters that occurred in two distinct alternative forms.
He also started with true-breeding varieties (those that produced the same variety when self-pollinated)
Def
. Hybridization: Mating two contrasting, true-breeding, varieties.
F1 generation: Hybrid offspring of P generation.
F2 generation: Offspring of F1 generation by self-pollination or cross-pollination.
P generation: True-breeding parents.
Mendel discovered the basic principle of heredity through carefully planned garden pea breeding experiments
Approach
Each character variant, was called a trait.
Mendel had access to multiple varieties of pea plants.
Heritable features, that varied among individuals, was called a character.
Advantages of using Pea Plants
Large number of offspring
Mating could be controlled; Self-pollination or cross-pollination.
Short generation times
Law of Segregation
When true-breeding white and purple- flowered pea plants where crossed, all of F1 hybrids were purple.
When the F1 hybrids where crossed, many of the F2 plants had purple flowers while some had white.
The ratio was about 3 purple flowers for every white flower.
Mendel reasoned that only the purple flower factor was affecting the flower color of the F1 hybrids.
Mendel called the purple color a dominant trait and the white color a recessive trait
The factor for white flowers wasn't diluted or destroyed because it reappeared in the F2 generation.
Mendel observed the same patterns of inheritance in six other pea plant characters, each represented by two traits.
Mendel's "heritable factors" are what we now call genes.
The law of Independent Assortment
Mendel derived the law of segregation by following a single character
The F1 offspring produced in this cross were monohybrids, heterozygotes for one character.
A cross between such heterozygotes is called a monohybrid cross.
Mendel identifies the second law of inheritance by following two characteristics at the same time.
Crossing two true-breeding parents differing in two characters produce 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.
The law of independent assortment states that each pair of alleles segregates independently of any other pair of alleles during gamete formation.
This law applies to genes on different nonhomolgous 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 Model
Mendel developed a hypothesis to explain the 3:1 inheritance pattern
Alternative versions of genes accounts for variations inherited characters.
These alternative versions of a gene are called allele.
Each gene resides at a specific locus on a specific chromosome.
For each character, an organism inherits two alleles, one from each parent.
Or the two alleles at one locus may differ, as in the F1 hybrids.
If the two alleles at one locus differ, the dominant allele determines the organisms appearance.
While the recessive allele has no noticeable effect on appearance.
The two alleles at a particular locus may be identical, as in the true-breeding plants of P generation.
The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes.
Thus, an egg or sperm gets only one of the two alleles that are present in an organism.
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis.
Genetic Vocabulary
Def
. Homozygote: An organism with two identical alleles for a character (homozygous)
Def
. Heterozygote: An organism with two different alleles for a gene (heterozygous)
Not true breeding
An organism's traits do not always reveal it's genetic composition.
Therefore, we distinguish between an organism's physical appearance and genetic make up.
Def
. Phenotype: An organism's physical expressions of it's genes.
Def
. Genotype: An organism's genetic make up.
Test Cross
An individual with the dominant phenotype, could be either homozygous dominant or heterozygous.
To determine the genotype, a test cross can be carries out.
Def
. Test cross: Breeding the mystery individual with a homozygous recessive individual.
If any offspring display the recessive phenotype, the mystery parent must be heterozygous.
Probability Laws Governing Inheritance
Rules Applied to Monohybrid Crosses
The multiplication rule states the probability that two or more independent events will occur together is the product of their individual probabilities.
Probability in a F1 monohybrid cross can be determined using the multiplication rule.
Segregation in a heterozygous plant: each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance to carry 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.
Used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous.
Complex Genetics Problems
The rules of probability can be used to predict the outcome of crosses involving multiple characters.
A multi-character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously.
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied.
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.
Similarly, with all alleles of one gene segregate into gametes independently of another gene's alleles.
Complex Inheritance Problems
The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied.
Many heritable characters are not determined be only one gene with two alleles.
The basic principles of segregation and independent assortment still apply.
Extending Mendelian Genetics
Inheritance of characters, by a single gene, may deviate from simple Mendelian patterns.
Genes may have more than two alleles.
Genes can produce multiple phenotypes.
Alleles are not always completely dominant or recessive.
Degrees of Dominance
Def
. Incomplete Dominance: the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
Def
. Codominance: two dominant alleles affect the phenotype in separate, distinguishable ways
Def
. Complete Dominance: occurs when phenotypes of the heterozygote and dominant homozygote are identical
Dominance and Phenotype Relationship
In the case of pea shape, the dominant allele codes for an enzyme that converts an unbranched starch in the seed into the branched form.
The recessive allele codes for a defective 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
Biochemical level: the phenotype (enzyme activity level) is incompletely dominant.
Organismal level: the allele is recessive.
Molecular level: the alleles are codominant.
A dysfunctional enzyme causes an accumulation of lipids in the brain.
Frequency of 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 forms.
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
Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy.
Pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.
Extending Mendelian Genetics for Two 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
In Labrador retrievers and many
other mammals, coat color depends on two genes.
One gene determines the pigment color, The other gene determines whether the pigment will be deposited in the hair.
Expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.
Polygenic Inheritance
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype.
Quantitative characters are those that vary in the population along a continuum.
Height is a good example of polygenic inheritance: Over 180 genes affect height
Skin color in humans is also controlled by many separately inherited genes.
Mendelian View of Heredity and Variation
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.
Nature and Nurture
Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype.
The phenotypic range is broadest for polygenic characters.
Traits that depend on multiple genes combined with environmental influences are called multifactorial.
Human Traits follow Mendelian Patterns of Inheritance
Humans are not good subjects for genetic research.
Parents produce relatively few offspring
Breeding experiments are unacceptable
Generation time is too long
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
Multiplication and addition rules can be used to predict the probability of specific phenotypes.
Behavior of Recessive Alleles
Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner,
These range from relatively mild to life-threatening.
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 (between close relatives) increase the chance that both parents of a child carry the same rare allele.
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
Sickle-cell disease affects one out of 400 African-Americans.
About one out of ten African Americans has sickle-cell trait, an unusually high frequency.
It 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)
Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous in regions where malaria is common
Symptoms include physical weakness, pain, organ damage, and even paralysis.
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.
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.
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.
Genetic Testing and Counseling
Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease.
Suppose a couple both have a brother who died from the same recessively inherited disease.
Each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings.
If 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
Tests for Identifying Carriers
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
Def
. Amniocentesis: the liquid that bathes the fetus is removed and tested.
Def
. 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.
Newborn Screening
Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States.
One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000–15,000 births in the United States.
