Chapter 14: Mendel and the Gene Idea
One possible explanation for heredity is a “blending” hypothesis =>
mixing of traits (blue and yellow = green).
• Will eventually give rise to a uniform population of individuals.
• Breeding experiments do not support it.
An alternative model is “particulate” inheritance => genetic material contributed by each parent retain separate identities in offspring.
Gregor Mendel documented a particulate mechanism of inheritance.
14.1 Gregor Mendel’s Discoveries
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments (around 1857).
Why pea plants?
• Available in many varieties with distinct variant traits.
• Strict control of mating different plants.
• Have both male (stamens) and female sexual organs (carpal).
Pea plants typically self-fertilize, but Mendel also cross- pollinated them tracking only those traits that varied in an “either-or” manner.
His experiments started with true-breeding varieties (P generation).
Sample experiment:
P generation => Tall x Short = All tall in F1 generation (hybrid)
What happened to the short trait?
Hybrids self-pollinate to produce an F2 generation => ¾ tall, ¼ short
Mendel did similar monohybrid crosses with a total of seven pairs of traits (table 14.1). In every case, one trait disappeared in F1 and reappeared in F2. The trait that was observed was dominant. The trait that disappeared was recessive.
Conclusion: When an organism is hybrid for a pair of contrasting traits only the dominant trait can be seen => Principle of Dominance.
Mendel also concluded that each organism has two “factors” that control each trait. These factors (alleles) separate from each other when gametes are formed and recombine randomly at fertlilization => Law of Segregation
Mendel’s Model:
1. Alternative versions of genes account for variations in inherited characters. These alternate versions are called alleles.
Ex. The gene (section of chromosome with a specific nucleotide
sequence) for flower color (character) in pea plants exists in
two versions (alleles) purple and white.
Each gene resides at a specific locus on a specific chromosome.
2. For each character, an organism inherits two alleles. Can be pure (homozygous) or hybrid (heterozygous) for that character.
3. Principle of Dominance
4. Law of Segregation accounts for the 3:1 ratio in the F2 generation.
A Punnett square predicts the results of a genetic cross between individuals of known genotype. Capital letter => dominant allele; lowercase letter => recessive allele.
An organism’s traits are called its phenotype => what is observed
Ex. tall, blue eyes, has disease, purple flowers
Genotype => genetic makeup
Ex. Homozygous dominant or recessive (TT or tt), heterozygous (Tt)
How can we tell the genotype of an individual with a dominant phenotype? => testcross => the mystery individual is bred with a homozygous recessive individual. If any offspring display the recessive, the mystery parent must be heterozygous.
Law of Independent Assortment => each pair of alleles segregates
independently into gametes. Mendel identified this law by
following two characters at the same time (dihybrid cross).
Ex. seed shape (yellow seeds are dominant to green, round seeds
are dominant to wrinkled) => YYRR x yyrr => do certain alleles stay together? Result: all four possible phenotypes (9:3:3:1) were produced in F2 generation.
This law applies only to genes located on different, nonhomologous chromosomes. Genes located near each other on the same chromosome tend to be inherited together.
14.2 The Laws of Probability
The laws of probability govern inheritance. The probability scale ranges from 0 (an event with no chance of occurring) to 1 (an event that is certain to occur).
Ex. heads coin – ½; 3 with die – 1/3
The outcome of one event has no impact on the outcome of the next event.
The multiplication rule is used to determine the chance that two or more independent events will occur together in some specific combination. Multiply the individual probabilities to obtain the overall probability of these events occurring together.
Under the rule of addition, the probability of an event that can occur two or more different ways is the sum of the separate probabilities of those ways.
We can combine the rules of multiplication and addition to solve complex problems in Mendelian genetics.
14.3 Extending Mendelian Genetics
Other patterns of inheritance:
1. codominance => two alleles affect the phenotype together in separate, distinguishable ways
ex. roan coat in cattle, MN blood groups of humans
2. incomplete dominance => heterozygotes show a distinct
intermediate phenotype not seen in homozygotes
ex. four o’clock flower (red x white = pink)
This is not blending inheritance because the traits are separable (particulate), as shown in further crosses.
A dominant allele is not necessarily more common in a population than the recessive allele.
Ex. one baby in 400 is born with polydactyly => extra fingers or
toes due to dominant allele.
3. Multiple Alleles => genes that exist in more than two alternate
forms
Ex. ABO blood groups in humans are determined by three
alleles, IA, IB, and i (IA and IB are codominant;i is recessive)
possible genotypes: possible blood types:
Matching compatible blood groups is critical for blood transfusions because a person produces antibodies against foreign blood factors causing clumping.
4. Pleiotropy => one gene affects more than one phenotypic
character (most genes)
Ex. sickle cell anemia causes wide range of symptoms
5. Epistasis => a gene at one locus alters the phenotypic
expression of a gene at a second locus.
Ex. coat color in mice depends on two genes
Presence (C) is dominant to absence (c) of pigment.
The second gene determines whether the pigment to be
deposited is black (B) or brown (b).
6. Polygenic Inheritance => additive effects of two or more genes
on a single phenotypic character; indicated by quantitative variations
Ex. human skin color controlled by at least three independent
genes AABBCC => very dark, aabbcc => very light
Phenotype depends on environment and genes. Discuss and list examples.
norm of reaction => phenotype range for a genotype
14.4 Human Inheritance
Rather than manipulate mating patterns of people, geneticists analyze the results of matings that have already occurred.
pedigree => genetic family tree (fig 4.14)
A pedigree can help us understand the past and predict the future.
Genetic disorders are not evenly distributed among all groups of humans. This results from the different genetic histories of the world’s people during times when populations were more geographically and genetically isolated.
Many human disorders follow Mendelian patterns of inheritance.
Thousands are inherited as simple recessive traits and occur because the allele codes for a malfunctioning protein or for no protein at all.
1. albinism => lack of pigmentation => susceptibility to skin cancer
and vision problems
2. cystic fibrosis =>
cause: Cl− channels are defective or absent
effect: abnormally high extracellular levels of Cl− leads to
mucus coats of certain cells to become thicker and stickier than normal.
Symtoms: Mucus buildup in the pancreas, lungs, digestive
tract, and elsewhere causes poor absorption of nutrients, chronic bronchitis, and bacterial infections
most common: whites of European descent
prognosis: Without treatment => die before five;
with treatment => live past their late 20s or even 30s.
3. Tay-Sachs =>
Cause: dysfunctional enzyme that fails to break down specific
brain lipids
effect: lipid accumulation in the brain
symptoms: seizures, blindness, and degeneration of motor and
mental performance a few months after birth
most common: people of central European Jewish descent
prognosis: child dies after a few years
4. sickle cell disease =>
cause: substitution of a single amino acid in hemoglobin
effect: sickle-cell hemoglobin aggregate into long rods that
deform red blood cells into a sickle shape that clump and clog capillaries
symptoms: brain damage, pain fatigue, paleness, shortness of
breath
most common: people of African descent
prognosis: regular blood transfusions to treat problems
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