Download Lecture 15 Notes CH.14

BIOLOGY 101

CHAPTER 14: Mendel and the Gene Idea:
Drawing from the Deck of Genes
Mendel and the Gene Idea:
Drawing from the Deck of Genes
CONCEPTS:
•
14.1 Mendel used the scientific approach to identify two laws of inheritance
•
14.2 Probability laws govern Mendelian inheritance
•
14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics
•
14.4 Many human traits follow Mendelian patterns of inheritance
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel took an experimental and quantitative approach
•
Mendel grew up on a small farm in what is today the Czech Republic.
•
In 1843, Mendel entered an Augustinian monastery.
•
Mendel studied at the University of Vienna from 1851 to 1853, where he was influenced by a physicist
who encouraged experimentation and the application of mathematics to science and by a botanist who
stimulated Mendel’s interest in the causes of variation in plants.
o
These influences came together in Mendel’s experiments
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel took an experimental and quantitative approach
•
•
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned
experiments
Mendel’s approach allowed him to deduce principles that had remained elusive to others
• A heritable feature that varies among individuals (such as flower color) is called an allele
• Each variant for a character, such as purple or white color for flowers, is called a trait
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel took an experimental and quantitative approach
•
Around 1857, Mendel began breeding garden peas to study inheritance.
•
Pea plants have several advantages for genetic study
o
Pea plants are available in many varieties that have distinct heritable
features, or characters, with different variant traits.
o
Peas have a short generation time, and each mating produces many
offspring
o
Large numbers of offspring
✓
Mating could be controlled; plants could be allowed to self-pollinate or
could be cross pollinated
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel took an experimental and quantitative approach
•
Mendel was able to strictly control the mating between his pea plants.
o
Each pea plant has male (stamens) and female (carpel) sexual organs.
✓
In nature, pea plants typically self-fertilize, fertilizing ova with the
sperm nuclei from their own pollen.
✓
Mendel could also use pollen from another plant for cross-pollination.
•
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)
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel took an experimental and quantitative approach
•
True Breeding plants are plants that only express a single characteristic for
a given trait
•
Recall that diploid organisms have two of each chromosome, therefore,
two alleles for each gene
•
True breeding plants are such that both alleles are the same –
This means they can only pass along a single characteristic
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel took an experimental and quantitative approach
•
In a typical experiment, Mendel mated two contrasting, true-breeding
varieties, a process called hybridization
o
The true-breeding parents are the P generation
o
The hybrid offspring of the P generation are called the F1 generation
o
When F1 individuals self-pollinate or cross- pollinate with other F1
hybrids, the F2 generation is produced
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 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 allowed the F1 hybrids to self-pollenate, 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 and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 The Law of Segregation
•
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
o
The factor for white flowers was not diluted or destroyed because it
reappeared in the F2 generation
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 The Law of Segregation
•
Mendel observed the same pattern of inheritance in six other pea plant
characters, each represented by two traits
o
What Mendel called a “heritable factor” is what we now call a gene
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
Mendel’s Model:
•
Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
•
Four related concepts make up this model
•
These concepts can be related to what we now know about genes and chromosomes
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
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
o
These alternative versions of a gene
are called alleles
o
Each gene resides at a specific locus
on a specific chromosome
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
Mendel’s Model:
•
Second: for each character, an organism inherits two alleles, one from each parent
•
Mendel made this deduction without
knowing about chromosomes
o
The two alleles at a particular locus may
be identical, as in the true-breeding
plants of Mendel’s P generation
o
Alternatively, the two alleles at a locus
may differ, as in the F1 hybrids
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
Mendel’s Model:
•
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 appearance
o
In the flower-color example, the F1
plants had purple flowers because
the allele for that trait
is dominant
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
Mendel’s Model:
•
Fourth (the law of segregation): the two alleles for a heritable character separate (segregate) during
gamete formation and end up in different gametes
o
Thus, an egg or a sperm gets only one of
the two alleles that are present in the
organism
o
This segregation of alleles corresponds
to the distribution of homologous
chromosomes to different gametes in
meiosis
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
•
The model accounts for the 3:1 ratio observed in the F2 generation of
Mendel’s crosses
o
Possible combinations of sperm and egg are shown using a Punnett square
o
A capital letter represents a dominant allele, and a lowercase letter
represents a recessive allele
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
•
The model accounts for the 3:1 ratio observed in the F2 generation of
Mendel’s crosses
o
Possible combinations of sperm and egg are shown using a Punnett square
o
A capital letter represents a dominant allele, and a lowercase letter
represents a recessive allele
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
Useful Vocabulary:
•
An organism with two identical alleles for a character is homozygous for the gene controlling that character
•
An organism that has two different alleles for a gene is
heterozygous for the gene controlling that character
•
Unlike homozygotes, heterozygotes are not true-breeding
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 Mendel developed a hypothesis to explain these results that consisted of four related ideas
Useful Vocabulary:
•
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
o
In the example of flower color in pea plants, PP and Pp plants
have the same phenotype (purple) but different genotypes
Mendel and the Gene Idea:
Mendel used the scientific approach to identify two laws of inheritance
14.1 A testcross can be used to determine the genotype of an individual with the dominant
phenotype
•
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
o
If any offspring display the recessive phenotype, the
mystery parent must be heterozygous
Mendel and the Gene Idea:
Drawing from the Deck of Genes
14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics
•
•
•
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
Mendel and the Gene Idea:
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics
14.3 There are degrees of dominance
•
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following
situations:
o
When alleles are not completely dominant or recessive
o
When a gene has more than two alleles
o
When a gene produces more than two phenotypes
Mendel and the Gene Idea:
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics
14.3 There are 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
Mendel and the Gene Idea:
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics
14.3 There are 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 (all alleles are expressed)
Mendel and the Gene Idea:
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics
14.3 Many genes exist in populations in more than two allelic forms
•
Most genes exist in populations in more than two allelic forms
•
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: A, B,
and i.
•
The enzyme encoded by the A allele adds the A carbohydrate,
whereas the enzyme encoded by the B allele adds the B
carbohydrate; the enzyme encoded by the i allele adds
neither
Mendel and the Gene Idea:
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics
14.3 Many genes exist in populations in more than two allelic forms
•
•
The ABO blood groups in humans are determined by three alleles: IA, IB, and i.
o
Both the IA and IB alleles are dominant to the i allele.
o
The IA and IB alleles are codominant to each other.
Because each individual carries two alleles, there are six possible genotypes and four possible blood
types.
o
Individuals who are IAIA or IAi are type A and have type A carbohydrates on the surface of their red
blood cells.
o
Individuals who are IBIB or IBi are type B and have type B carbohydrates on the surface of their red
blood cells.
o
Individuals who are IAIB are type AB and have both type A and type B carbohydrates on the surface
of their red blood cells.
o
Individuals who are ii are type O and have neither carbohydrate on the surface of their red blood
cells.
Mendel and the Gene Idea:
Inheritance patterns are often more complex than predicted by simple Mendelian
genetics
14.3 Genes and environment determine phenotype
•
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
o
•
Traits that depend on multiple genes combined with environmental influences are called multifactorial
o
A single tree may have leaves that vary in
size, shape, and greenness, depending on
exposure to wind and sun
o
A person’s red and white blood cell count
varies with factors such as altitude,
customary amount of exercise, and
presence of infection
Mendel and the Gene Idea:
Many human traits follow Mendelian patterns of inheritance
14.4 Some human disorders are recessively inherited
•
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
o
Only individuals with two recessive albinism
alleles will express the trait
o
Carriers are phenotypically normal, but can
pass the trait to their offspring
Mendel and the Gene Idea:
Many human traits follow Mendelian patterns of inheritance
14.4 Some human disorders are recessively inherited
Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications
•
•
•
•
Sickle-cell disease affects one out of 400
African-Americans
The 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)
Symptoms include physical weakness, pain,
organ damage, and even paralysis
Mendel and the Gene Idea:
Many human traits follow Mendelian patterns of inheritance
14.4 Some human disorders are recessively inherited
Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications
•
•
•
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
Heterozygotes are less susceptible to the
malaria parasite, so there is an advantage to
being heterozygous in regions where malaria is
common