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Chapter 14: Mendel and the Gene Idea (Mendel's Model (First Concept…
Chapter 14: Mendel and the Gene Idea
Laws of Inheritance
Mendel: basic principles of heredity
by breeding garden peas in carefully planned experiments
Quantitative Approach
deduce principles that had remained elusive to others
character: heritable feature that varies among individuals (such as flower color)
trait: each variant for a character, such as purple or white color for flowers
Peas used by Mendel
Short generation time
Large numbers of offspring
Mating could be controlled; plants could be allowed to self-pollinate or could be cross-pollinated
Peas were available to be used in many different varieties
Two Distinct Alternative Forms
true-breeding:
plants that produce offspring of the same variety when they self-pollinate
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
The Law of Segregation
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 purple flowers to one white flower in the F2 generation
Results of Segregation
only the purple flower factor was affecting flower color in the F1 hybrids
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
“heritable factor” is what we now call a gene
the same pattern of inheritance in six other pea plant characters,
each represented by two traits
Mendel's Model
hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
Four related concepts make up this model
what we now know about genes and chromosomes
First Concept
First: alternative versions of genes account for variations in inherited characters
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 Concept
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
Third Concept
if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance
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
Fourth Concept
Fourth (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
The model accounts for the 3:1 ratio observed in the F2 generation of Mendel’s crosses
Possible combinations of sperm and egg can be shown using a Punnett square
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele
Genetic Vocabulary
An organism with two identical alleles for a character is called a
homozygote
It is said to be
homozygous
for the gene controlling that character
An organism with two different alleles for a gene is a heterozygote
Unlike homozygotes, heterozygotes are not true-breeding
heterozygous for the gene controlling that character
An organism’s traits do not always reveal its genetic composition
phenotype, or physical appearance
genotype, or genetic makeup
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
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, heterozygous for one character
A cross between such heterozygotes is called a monohybrid cross
Dihybrids
Crossing two true-breeding parents differing in two characters
in the F1 generation, heterozygous for both characters
dihybrid cross: a cross between F1 dihybrids
two characters are transmitted to offspring as a package or independently
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
Probability Laws
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
the alleles of one gene segregate into gametes independently of another gene’s alleles
govern Mendelian inheritance
(X) and (+) RULES
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
Addition
probability that any one of two or more mutually exclusive events will occur
calculated by adding together their individual probabilities
figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
Solving Complex Genetic Problems
We can apply the rules of probability to predict the outcome of crosses involving multiple characters
A multicharacter 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
Inheritance Patterns
The relationship between genotype and phenotype is rarely as simple as in the pea plant
Many heritable characters are not determined by only one gene with two alleles
the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
often more complex than predicted by simple Mendelian genetics
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
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
is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain
Alleles
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 forms
The enzyme encoded by the IA allele adds the A carbohydrate,
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
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
pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases
cystic fibrosis and sickle-cell disease
Two or More Genes
Some traits may be determined by two or more genes
In polygenic inheritance, multiple genes independently affect a single trait
Epistasis
expression of a gene at one locus alters the phenotypic expression of a gene at a second locus
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
one gene affects the phenotype of another due to interaction of their gene products
Polygenic Inheritance
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
Skin color in humans is also controlled by many separately inherited genes
Nature and Nurture
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
Patterns of Inheritance
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
Pedigree Analysis
family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees
be used to make predictions about future offspring
use the multiplication and addition rules to predict the probability of specific phenotypes
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
Many genetic disorders are inherited in a recessive manner
These range from relatively mild to life-threatening
Cystic Fibrosis
most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent
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
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)
Symptoms include physical weakness, pain, organ damage, and even paralysis