Download Chapter 11 – Mendelian Genetics

Chapter 11 – Mendelian Genetics
11.1 - Mendel’s Principles of Heredity
The Study of Heredity
When two gametes fuse, a zygote is formed. The haploid number of each gamete
combine to form a diploid number zygote. The offspring may have some similarities
with one or both of the parents and due to crossing-over, the zygote will also have its own
unique characteristics. Genetics is the study of hereditary information as it is passed on
from parents to offspring. Gregor Mendel was the first to study genetics in the 1800’s.
Mendel’s Experiments
Gregor Mendel was a monk who used pea plants to study genetics. The advantages to
using pea plants as his model organism were; ease of growing them, matured relatively
quickly, and traits or characteristics were easily visible with the naked eye. He kept
records of the following characteristics, which came in two forms;
1. seed shape – round or wrinkled
2. seed color – yellow or green
3. seed coat color – grayish brown or white
4. pod color – green or yellow
5. pod shape – inflated or wrinkled
6. stem length – long or short
7. flower position – lateral or terminal
He used mathematics to explain his results and he published his work in a journal and
some libraries. His ideas were not widely accepted at the time and he died without
recognizing his fame.
The Law of Dominance
Mendel discovered that he could create purebred plants. For example, when he allowed
short plants to self-pollinate, eventually 100% of he offspring would be short. The same
was true for the tall plants. Then he wondered what size offspring would result from
crossing a short plant with a tall plant. The short and tall plants are called the parent
generation or the P generation. The offspring are called the first filial generation or the
F1 generation. When Mendel crossed a purebred tall and a purebred short P generation,
the F1 generation was 100% tall. From this cross Mendel concluded that the tall trait was
the dominant one. More accurately, he concluded that when an organism is hybrid for a
pair of contrasting traits, only the dominant trait can be seen in the hybrid. This is the
law of dominance. Then, to satisfy his curiosity, Mendel crossed two of the tall offspring
(no longer purebred) from the F1 generation and found something interesting. The F2
generation of the second filial generation was 75% tall and 25% short.
Law of Segregation
Taking the F1 and F2 data into account, Mendel had to explain why the short or recessive
trait was not seen in the F1 generation and was seen in the F2 generation. Rather than
using the word gene, which did not exist to Mendel, he used the word factor to explain
his results. He said there were two factors for a trait and each factor could be one of two
forms. In this case the factors were tall or short and each plant could have a tall or short
factor, two tall factors, or two short factors.
Mendel theorized that in the hybrid F1 cross each plant had one tall and one short factor.
He went on to theorize that the factors separate from each other during the formation of
gametes or meiosis and then recombine together during fertilization. This is Mendel’s
Law of Segregation.
The Gene-Chromosome Theory
Long after Mendel’s death, a graduate student, W.S. Sutton, observed homologous
chromosomes in diploid cells separate during spermatogenesis. Sutton realized that the
separated chromosomes would be recombining in the fertilization process. After looking
at Mendel’s work, Sutton concluded that Mendel’s factors are carried on the
chromosomes, since the chromosomes were doing the same thing Mendel’s factors were
theorized to do, that is separate and recombine. In Sutton’s day, the word gene was used
instead of factors and the Gene-Chromosome Theory was developed.
Fundamentals of Genetics and Genotypes and Phenotypes
Vocabulary Review
1. Alleles – different forms for one gene (for example, regarding the gene that
determines plant height; the alleles are short (t) or tall (T))
2. Homozygous – the alleles are the same (for example, in a purebred tall plant, both
alleles are tall (TT))
3. Heterozygous – the alleles are different (for example, in a hybrid tall plant, one allele
is tall the other is short (Tt))
4. Genotype – the genetic makeup of an organism (for example, (Tt))
5. Phenotype – the physical trait of an organism (for example, tall)
11.2- Probability in Genetics
The law of probability, or chance, says if there are several possible events that might
happen, and no one of them is more likely to happen than any other, then they will all
happen in equal numbers over a large number of trials. Your lab better illustrates this
concept.
11.2 - The Punnett Square
The Punnett square diagram is an easy way to show the results of any cross. Refer to the
bottom of page 504 for directions on how to use this helpful tool.
The Punnett square results for a purebred cross is always 100% hybrid genotype showing
the dominant phenotype. The Punnett square results for a hybrid cross is 25% pure
dominant and 25% pure recessive genotypes, 50% hybrid genotypes, 75% tall
phenotypes, and 25% short phenotypes. Refer to the top of page 506.
The Test Cross
When you are trying to find out the genotype of an organism, you simply cross it with a
known pure recessive mate. If 100% the F1 generation show the dominant phenotype, the
unknown organism was purebred for the dominant trait. If 50% of the F1 generation was
dominant and 50% was recessive, then the unknown organism was a hybrid. Refer to the
Punnett squared on page 507 for an illustration of this concept.
11.3
Other Concepts in Genetics
The Law of Independent Assortment – (refer to the bottom of page 508) states that during
meiosis, gene for different traits are separated and distributed to gametes independently
of one another (note: there are exceptions to this rule).
11.4 - Meiosis
Meiosis, Another Kind of Cell Division
Organisms use meiosis to produce gametes, which are sex cells such as sperm and egg
cells. This is important because it’s the fusion of male and female gametes that produces
offspring. Without meiosis, there would be no gametes and no fertilization, which is the
process of combining the nuclei of the male gamete with the female gamete.
Diploid and Haploid Chromosome Numbers
The cells of your body are called somatic cells and you already know that the cells
involved in creating new life are called gametes and there are male and female gametes.
One difference between somatic cells and gametes is that somatic cells contain the total
number of chromosomes (we call this the diploid number), while gametes only contain
half the number of chromosomes (we call this the haploid number). Your somatic cells
contain 46 chromosomes, which make 23 pairs. Your gametes contain 23 chromosomes,
which are not paired off until fertilization.
Stages of Meiosis
General overview:
1. in the beginning, the cell has the diploid number of chromosomes
2. in the process of meiosis, the cell divides twice, so each original cell produces four
daughter cells
3. the chromosomes only replicate once, so each daughter cell contains the haploid
number of chromosomes
4. the process of meiosis is really mitosis happening twice without the replication of
DNA in the second prophase!!!!
1. Prophase I –
 In prophase I, the chromosome has replicated, meaning there are twice as many
chromosomes as normal. Using the diagram on page 275, we see that 6
chromosomes or 3 pairs of chromosomes is normal, or the diploid number.
Doubling causes there to be 6 pairs of chromosomes or 12 chromosomes.
 The chromosomes have combined at the centromere to their homologous pairs in a
process called synapsis. The pair of homologous chromosomes is called a tetrad. Do
you see the 6 pairs of chromosomes (12 chromosomes)?
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Sometimes tetrads twist with one another and segments of genes exchange places.
This is called crossing over.
The nuclear membrane disappears, spindle fibers form, and the tetrads move towards
the equator of the cell.
2. Metaphase I –
 The tetrads line up at the equator with the spindles attached to the centromeres
3. Anaphase I –
 Tetrads begin to separate and move to the poles, this is called disjunction
 At this point there is the diploid number of chromosomes at each pole (6
chromosomes or 3 pairs)
4.
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Telophase I –
The cytoplasm divides
The cell pinches
Two daughter cells are formed
The next mitotic division takes place immediately, but the DNA does not replicate or
double
5. Prophase II –
 Each daughter cells starts out with the diploid number of chromosomes (3 pairs or 6
chromosomes).
 The chromosomes move toward the equator
6. Metaphase II –
 Spindle microtubules fasten to centromeres as they line up at the equator
7. Anaphase II –
 Centromeres divide and the divided chromosomes move to the poles
8.
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Telophase II –
Each daughter cell divides
The nuclear membrane forms
Each of the four gamete cells contains the haploid number of chromosomes (1.5 pairs
or 3 chromosomes compared to the diploid number of 3 pairs or 6 chromosomes)
Refer to page 278 for a comparison between mitosis and meiosis.
Gametogenesis: Meiosis in Females and Males
The creation of gametes is called gametogenesis. In females, creating the egg is called
oogenesis and in males, creating the sperm is called spermatogenesis.
Oogenesis:
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In human females, the oogonia (immature egg cells) are all developed before birth
and stored in the ovaries, these cells are at the first mitotic division (diploid)
When the time is right, an oogonia will undergo the second mitotic division and
create a haploid egg
Refer to page 278 for details on this process
Spermatogenesis:
 In the testis, sperm are developed from spermatogonia (immature sperm cells).
 Spermatogonia have the diploid number of chromosomes, they must undergo meiosis
to make the haploid sperm cells
 Refer to page 278 for details on this process
Fertilization and Zygote Formation
When the haploid sperm and haploid egg fuse together, a diploid zygote is produced. So, the normal
number of chromosomes is present.