Mrs. Sevgi’s Calamity Day
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Snow Day 1:
Science Daily
Meiosis
In biology, meiosis is the process by which one diploid eukaryotic cell divides
to generate four haploid cells often called gametes.
Meiosis is essential for sexual reproduction and therefore occurs in all
eukaryotes (including single-celled organisms) that reproduce sexually.
A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry
out meiosis and have acquired the ability to reproduce by parthenogenesis.
Meiosis does not occur in archaea or bacteria, which reproduce via asexual
processes such as mitosis or binary fission.
During meiosis, the genome of a diploid germ cell, which is composed of
long segments of DNA packaged into chromosomes, undergoes DNA
replication followed by two rounds of division, resulting in haploid cells called
gametes.
Each gamete contains one complete set of chromosomes, or half of the
genetic content of the original cell.
These resultant haploid cells can fuse with other haploid cells of the opposite
sex or mating type during fertilization to create a new diploid cell, or zygote.
Thus, the division mechanism of meiosis is a reciprocal process to the joining
of two genomes that occurs at fertilization.
Because the chromosomes of each parent undergo genetic recombination
during meiosis, each gamete, and thus each zygote, will have a unique
genetic blueprint encoded in its DNA.
In other words, meiosis and sexual reproduction produce genetic variation.
Meiosis uses many of the same biochemical mechanisms employed during
mitosis to accomplish the redistribution of chromosomes.
There are several features unique to meiosis, most importantly the pairing
and genetic recombination between homologous chromosomes.
Chapter 10 Review: Meiosis
1. What type of cell undergoes meiosis?
Gamete cells
or
Somatic cells
2. For each of the following state if the cell is haploid or diploid.
Sperm cell =
Liver cell =
Egg cell = Stomach cell =
3. If the diploid number in a liver cell is 52, how many chromosomes are there
in the egg of this organism? _________
4. During meiosis, the chromosome number:
a) is doubled
b) is reduced
c) remains the same
5. What are the stages of meiosis called?
Meiosis I: _________________, _____________________,
_________________, ____________________/cytokinesis
Meiosis II: _________________, _____________________,
_________________, ____________________/cytokinesis
6. Which of the following best describe the term
“crossing over”?
a.) An exchange of information between
two homologous chromosomes
b.) A molecular interaction between two
sister chromatids
c.) A molecular interaction between two
non-sister chromatids
d.) A separation of two sister chromatids
.
7. Crossing-over can be found in the stage of
a.) Prophase I
c) Anaphase I
b) Prophase II
d) Anaphase II
10. A sperm cell is a (gamete, zygote), and is (haploid, diploid).
24. How does Meiosis differ from Mitosis?
Figure 2
Snow Day 2: Mendelian Genetics
A. Gregor Mendel
The essence of genetics or the study of heredity comes from the Austrian
monk Gregor Mendel. In the 1800s this Catholic priest worked his monastery
garden and raised all the fruit and vegetables needed. As he had a science
background, he started studying how traits were passed down from parent
(P generation) to the offspring (F1 generation) and so on. Using hundreds of
pea plants he discovered the basics of genetics, which is why we today call it
Mendelian genetics.
1. What are the offspring of the F1 generation referred to as?
2. Why did Gregor Mendel choose pea plants to study as opposed to
potatoes or tomatoes?
3. Briefly describe the essence of Mendel’s first experiments with two
purebred pea plants. Use the trait pea color for your description.
B. Mendelian Laws
Through Mendel’s many years and hundreds of pieces data, he threw out
many old ideas about heredity and came up with four hypotheses that
turned into two laws that still hold true today. The first hypothesis stated
that individuals have two copies of their genes, one from each parent.
The second hypothesis says that there exist two different versions of the
same gene represented by letters. We now call those versions alleles.
The third hypothesis states that if two different alleles occur together,
one may be expressed while the other is not. We say one is dominant
and the other is recessive. His fourth and final hypothesis states that
when gametes are formed, alleles for each trait separate independently
during meiosis. From these hypotheses which have been proven true time
and again, we now have two laws that can be attributed back to Mendel’s
research. The first law is called the law of segregation and it says that
T
t
Y
y
because each individual has two different alleles, it can produce two different
types of gametes. If the gene is represented by the letter R, it can produce
R allele or r allele, which represents different forms of the same trait. The
second law is called the law of independent assortment and it states that
genes for different traits are inherited independently of each other. For
example, if a person has gene A and gene B on the same chromosome, they
are both inherited without being tied to the other.
4. What is Mendel’s first law?
5. Explain the first law in terms of plant height (T).
6. What is Mendel’s second law?
7. Explain the second law in terms of both height and pea color (T and
Y).
C. General Rules
Alleles are said to be dominant or recessive. A dominant allele expresses
(shows) itself even if there is only one. For example, if the trait is eye color,
brown is dominant. Therefore, one B allele will make eye color brown. A
recessive allele is only expressed when no dominant allele is present. Blue
eyes are recessive to brown eyes so the only time blue eyes are expressed is
when there are two as in bb. Dominant traits determine the allele used. For
example, if grey is the dominant skin color gene in aliens, white is recessive.
The allele used to represent both colors is g for grey. To differentiate grey
and white, G is grey since it is dominant and g is white since it is recessive.
If the combination of the two alleles is such that both alleles are dominant, it
is said to be homozygous dominant. If both alleles are recessive, it is said
to be homozygous recessive. If the two alleles are different, it is said to
be heterozygous.
8. If red fur is dominant over blue fur, what are the alleles for the
different furs?
9. If yellow peas are dominant over green peas, what are the alleles for
the different color peas?
10.
What would the alleles for a heterozygous grey alien be?
11.
What would the alleles for a homozygous recessive skinned alien
be? What color is it?
D. Punnett Squares and Probability
Punnett squares are a method in which all the possible
offspring types are determined based on the parents’
genes. The genes of individuals (represented by alleles)
are called genotypes. The physical appearance or
phenotype of an individual is a result of what the
genotype determines. For example, if freckles are
dominant over no freckles, the genotype Ff would have the
phenotype of having freckles. The parents’ genotypes determine what
possible alleles are given to the offspring. The allele type varies and
according to the laws of segregation and independent assortment, two
different genes with different alleles separate completely and recombine in
four possible gamete combinations. An easy way for students to remember
how to find the possible gametes, the acronym FOIL (first, outside, inside,
last) is often used. For example, if the parent genotype is AaBb, the four
possible gametes are AB, Ab, aB, and ab. Using a Punnett square, the
gametes are combined in such a way as to determine all the possible
genotypes. A ratio of the number of genotypes is gathered by adding up all
the same genotypes and comparing them to the others using a colon
between the numbers. A ratio of the phenotypes of the offspring is gathered
in a similar manner.
12.
What is a genotype?
13.
What is a phenotype?
14.
If freckles are dominant over plain cheeks, and cleft chin is
dominant over a smooth chin, what would the genotype of a parent be
who is heterozygous freckled and heterozygous cleft?
15.
What are the possible gametes of the father?
16.
Using a Punnett square, what are the possible offspring of the
parents if they both are heterozygous for freckles and cleft chin?
17.
What are the genotypic ratios and phenotypic ratios of the
offspring of those two parents?
Snow Day 3- Pedigree
A pedigree is a type of family tree that traces a particular trait that runs in
an entire family. Circles represent females while squares represent males.
Lines connecting two individuals horizontally represent marriage or a
coupling in which offspring were produced. Vertical lines emanating from the
horizontal connector line represent the offspring of the coupling. An
individual family member with the trait is shaded a dark or different color.
An individual who carries the trait (heterozygous and phenotype of the
dominant trait) is half-shaded. Each generation has a Roman numeral and
each individual of that generation has Arabic numbers.
18.
According to the pedigree on the right,
individual II-2 is what sex?
I
1
2
2
3
4
1
2
II
1
III
19.
According to the pedigree, individual I-2 is what
sex?
20.
If the trait being traced is brown eyes, what phenotype is
individual II-3?
21.
What is the phenotype of individual III-1?