Download CHAPTER 11 INTRODUCTION TO GENETICS

11-1 The Work of Gregor Mendel
genetics – the scientific study of heredity
A. Gregor Mendel’s Peas
- Austrian monk who worked with pure strains of pea
plants called true-breeding plants
- True-breeding plants would self-pollinate and
produce offspring identical to themselves (one parent)
Ex. tall plants would produce only tall plants
- pea plants can also cross-pollinate meaning male sex
cells in pollen from one plant can fertilize the eggs of a
flower from another plant (two parents)
- Mendel selected certain characteristics and performed
experiments by crossing the seeds of those selected plants
B.Genes and Dominance
- Mendel studied 7 specific pea plant traits
- A trait is a specific characteristic such as seed color or
plant height
- Each of the 7 traits studied, had different forms such as
green seed color and yellow seed color, tall or short height, etc.
These different forms are called alleles
- Mendel crossed each trait and studied the offspring
P = parent generation
F1 = first offspring generation
Hybrids – the offspring of crosses between parents
with different traits
- Mendel’s F1 hybrids showed characteristics of only one
parent and it appeared that the second parental trait disappeared
Traits Mendel Tested
Two conclusions were drawn from Mendel’s
experiments:
1. Biological inheritance is determined by factors that
are passed from one generation to another. These factors are
called genes.
2. Principle of dominance
- some alleles are dominant and some are recessive
- the dominant allele or form of the trait always shows
- the recessive allele is hidden and only is visible when
the dominant form is absent
C. Segregation
- to determine if the recessive alleles disappeared or
were simply hidden, Mendel crossed all F1 hybrid
generations by self-pollination to produce an F2 generation
F1 x F1 = F2 (second filial generation)
1. The F1 cross
- traits controlled by the recessive allele reappeared
- ¼ (25%) of F2 plants showed the trait was controlled
by the recessive allele
2. Explaining the F1 cross
- reappearance of the trait controlled by the recessive
allele indicated that the allele had at some point been
separated from the dominant allele (segregation)
- Mendel suggested that segregation of different alleles
occurs during the formation of gametes (sex cells)
- Each gamete carries only one copy of each gene
- Each F1 plant produces two types of gametes; those
with the dominant and those with the recessive allele
- The alleles get paired up again during fertilization
when the gametes fuse in the F2 generation leaving the F2
generation with new combinations of alleles
11-2 Probability and Punnett
Squares
A. Genetics and Probability
Probability
- the likelihood that a particular event will occur
- past outcomes do not affect future ones
- the way in which alleles segregate is completely random,
therefore, we can use the principles of probability to predict the
outcomes of genetic crosses
B. Punnett Square
- a diagram used to determine the gene combinations that might
result from a genetic cross (See Fig 11-7)
- organisms with two identical alleles for a particular trait (truebreeding) are homozygous (TT)
- organisms that have two different alleles for a particular trait
(hybrid) are heterogygous (Tt)
- the phenotype represents the physical characteristics (what you
see) (tall or short)
- having the same phenotype does not mean they are genetically
the same (genotype)
Ex. TT (homozygous) tall
Tt (heterozygous) tall
- therefore, plants with the same phenotype may have a different
genotype
C. Probability and Segregation
See. Fig 11-7
¼ of F2 plants are TT (tall)
2/4 (1/2) of F2 plants are Tt (tall)
¼ of F2 plants are tt (short)
3 : 1 ratio
3 tall : 1 short
- Mendel’s prediction of 3 tall to 1 short proved to be
correct for each of the 7 crosses and segregation occurred
according to his model
D. Probabilities Predict Averages
- probabilities predict the outcomes of a large number
of events but not of an individual event
- this is also true of genetics; the larger the number of
individuals, the closer the resulting offspring number will
get to the expected values
11-3
Exploring Mendelian
Genetics
Does the segregation of one pair of alleles affect the
segregation of another?
A. Independent Assortment
- Mendel performed an experiment called the twofactor cross
1.The two-factor cross: F1
See Fig 11-9 pg. 270
RRYY (yellow,round) x rryy(wrinkled, green)
- F1 generation was RrYy (round,yellow) showing the
alleles for yellow and round are dominant over the alleles
for wrinkled and green
See. Fig 11-10 pg. 271
- The F1 cross did not show whether or not the genes
assorted or segregated independently
2. The two-factor cross: F2
- Mendel crossed the F1 generation which were all
heterozygous for both seed shape and seed color: RrYy x
RrYy = F2
- the F2 generation offspring showed combinations of alleles
and phenotypes not seen in either parent meaning that the alleles
for seed color segregated independently from those of seed shape
- this is a principle known as independent assortment and the
genes for seed shape and seed color do not influence each other’s
inheritance.
***** the principle of independent assortment states that genes
for different traits can segregate independently during the
formation of gametes
B.Summary of Mendel’s Principles
1. The inheritance of biological characteristics is determined
by individual traits known as genes. Organisms that reproduce
sexually pass genes from parent to offspring.
2. Where two or more forms of a gene exist, some forms of
genes may be dominant and some may be recessive
3. In sexually reproducing organisms, each adult has two
forms of a gene (one from each parent) and those genes are
segregated from each other when the gametes are formed
4. Alleles for different genes usually segregate
independently from one another
C. Beyond Dominant and Recessive Alleles
- there are exceptions to the principles discovered by
Mendel
- some alleles are neither dominant nor recessive, and
many traits are controlled by multiple alleles or multiple
genes
1. Incomplete dominance
- cases in which one allele is not completely dominant
over another Fig 11-11 pg. 272
- the heterozygous phenotype is in between the two
homozygous phenotypes (blending)
Incomplete Dominance
2. Codominance
- both alleles contribute to the phenotype of the
organism
- seen in some varieties of chicken and cattle
3. Multiple Alleles
- some genes have more than two alleles
- seen in rabbit coats and blood types
- the combination of alleles display a pattern of
dominance that can result in different phenotypes
4. Polygenic Traits
- traits controlled by two or more genes exhibiting a
wide range of phenotypes
Ex. skin color: controlled by as many as 4 different
genes
D. Analyzing Mendel’s Principles
- the fruit fly, Drosophila melanogaster, is used
frequently to study genetics because it can breed a new
generation of offspring every 14 days with as many as 100
offspring.
- Mendel’s principles also apply to humans and have
been used to study the inheritance of human traits and
calculate probabilities of their appearance in the next
generation
Ex. Albinism (no melanin)
11-4 Meiosis
The process by which gametes (sex cells) are formed and
end up with only one set of genes
All other cells in the body have two sets of genes, one from
each parent
A. Chromosome number
- cells that contain two complete sets of homologous
chromosomes and two complete sets of genes are said to be
diploid
- this agrees with Mendel’s principle that cells of adult
organisms contain two copies of each gene
- gametes of sexually reproducing organisms contain
only one set of chromosomes and one set of genes
- gametes are said to be haploid
B. Phases of Meiosis
- Meiosis is a process of reduction division by which
the number of chromosomes per cell is cut in half through
the separation of homologous chromosomes in a diploid cell
- Meiosis involves two phases, Meiosis I and Meiosis
II
- At the end of Meiosis II, the diploid cell that entered
Meiosis has become 4 haploid cells
1. Meiosis I
- replication has already taken place
- cells begin to divide in a way similar to mitosis
Prophase I
- each chromosome pairs with its corresponding
homologous chromosome to form a tetrad (contains 4
chromosomes)
- as homologous chromosomes pair up and form a tetrad,
they may exchange portions of their chromatids in a process
called crossing-over
- crossing-over results in the exchange of alleles between
homologous chromosomes and produces new combinations of
alleles
Crossing Over
Metaphase I
- spindle fibers attach to the chromosomes
Anaphase I
- Spindle fibers pull the homologous chromosomes to
the opposite ends of the cell
Telophase I and Cytokinesis
- the nuclear membrane forms, the cell separates into
two new cells each with 46 chromosomes
Meiosis II:
- The second meiotic division without replication
Prophase II
- the chromatids pair up
Metaphase II
- The chromosomes line up in the center of each cell attached
to spindle fibers
Anaphase II
- the paired chromatids separate and move to opposite ends of
the cell
Telophase II and Cytokinesis
- each cell divides into two daughter cells (4 cells) each with
one half the number (haploid) of chromosomes
C. Gamete Formation (sex cells)
Sperm – haploid gametes produced by the male
Egg – haploid female gamete
- divisions at the end of Meiosis I and Meiosis II are
uneven with most of the cytoplasm going to the one egg
cell. The other three are called polar bodies and usually do
not participate in reproduction.
D. Comparing Mitosis and Meiosis
Mitosis
- results in 2 genetically identical diploid cells
Meiosis
- results in 4 genetically different haploid cells
11-5
Linkage and Gene Maps
- A gene map shows the relative location of genes on a
chromosome
- The frequency of crossing over between genes is used
to produce a map of distances between genes
- The farther apart the genes, the more likely they are
to be separated during crossing-over in Meiosis, therefore,
the frequency of crossing-over is equal to the distance
between two genes