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Saturday, March 28, 2009
Friday, March 27, 2009
Identical Vs Non-identical twins
IDENTICAL TWINS
Sex Determination
Sex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). The details of some sex-determination systems are not yet fully understood.
XX/XY sex chromosomes
The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in human beings and most other mammals, although at least one monotreme, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and it also lacks the SRY gene. Several Arvicolinae (voles and lemmings) and some other rodents are also noted for their unusual sex determination systems.
In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness. The XY sex chromosomes are different in shape and size from each other unlike the autosomes, and are termed allosomes.
The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in human beings and most other mammals, although at least one monotreme, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and it also lacks the SRY gene. Several Arvicolinae (voles and lemmings) and some other rodents are also noted for their unusual sex determination systems.
In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness. The XY sex chromosomes are different in shape and size from each other unlike the autosomes, and are termed allosomes.
FEMALE
EXERCISE 3.3 (Mechanism of trait inheritance)
The figure below shows a monohybrid cross between two plants.
R- gene for red flower (dominant)
r-gene for white flower (recessive)
parental phenotype: red flower X white flower
parental genotype :
Meiosis--------------------
Gametes :
Fertilization:--------------
F1 generation
Genotype :
Phenotype :
Genotype ratio :
Phenotype ratio :
When two F1 plants are crossed:
parental phenotype: red flower X red flower
parental genotype :
Meiosis--------------------
Gametes :
Fertilization:--------------
F2 generation
Genotype :
Phenotype :
Genotype ratio :
Phenotype ratio :
Mendelian Inheritance
Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. They were initially derived from the work of Gregor Mendel published in 1865 and 1866 which was "re-discovered" in 1900, and were initially very controversial. When they were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.
Mendel's Laws
The principles of heredity were written by the Augustinian monk Gregor Mendel in 1865. Mendel discovered that by crossing white flower and purple flower plants, the result was not a hybrid offspring. Rather than being a mix of the two, the offspring was purple flowered. He then conceived the idea of heredity units, which he called "factors", one which is a recessive characteristic and the other dominant. Mendel said that factors, later called genes, normally occur in pairs in ordinary body cells, yet segregate during the formation of sex cells. Each member of the pair becomes part of the separate sex cell. The dominant gene, such as the purple flower in Mendel's plants, will hide the recessive gene, the white flower. After Mendel self-fertilized the F1 generation and obtained the 3:1 ratio, he correctly theorized that genes can be paired in three different ways for each trait; AA, aa, and Aa. The capital A represents the dominant factor and lowercase a represent the recessive.
Mendel stated that each individual has two factors for each trait, one from each parent. The two factors may or may not contain the same information. If the two factors are identical the individual is called homozygous for the trait. If the two factors have different information, the individual is called heterozygous. The alternative forms of a factor are called alleles. The genotype of an individual is made up of the many alleles it possesses. An individual's physical appearance, or phenotype, is determined by its alleles as well as by its environment. An individual possesses two alleles for each trait; one allele is given by the female parent and the other by the male parent. They are passed on when an individual matures and produces gametes: egg and sperm. When gametes form, the paired alleles separate randomly so that each gamete receives a copy of one of the two alleles. The presence of an allele doesn't promise that the trait will be expressed in the individual that possesses it. In heterozygous individuals the only allele that in expressed is the dominant. The recessive allele is present but its expression is hidden.
EXERCISE 3.2 (The principles and mechanism of inheritance)
1. Dominant genes are genes that show their effects wherever they are present.
2. Recessive genes are genes that only show their effects in the absence of dominant genes.
3. Find the words in the puzzle that describe the traits given below:
a) The ability to roll the tongue
b) The type of ear lobes
c) The types of hair
d) The way of using hand
Use blue pen to mark the dominant traits and black pen/pencil to mark the recessive trait.
2. Recessive genes are genes that only show their effects in the absence of dominant genes.
3. Find the words in the puzzle that describe the traits given below:
a) The ability to roll the tongue
b) The type of ear lobes
c) The types of hair
d) The way of using hand
Use blue pen to mark the dominant traits and black pen/pencil to mark the recessive trait.
ACTIVITY (Dominant trait Vs Recessive trait)
ACTIVITY: Observing and identifying dominant and recessive traits among human being.
METHOD:
METHOD:
1.Students formed four groups in the class.
2.The following traits among group members are observed.
a) The ability to roll the tongue
b) The type of ear lobes
c) The types of hair
d) Being right-handed or left-handed
3.The numbers of students who have particular traits are recorded.
4.The results are tabulated.
RESULTS:
DISCUSSION:
There are _______ people who can roll their tongue than those who cannot.
There are _______ people with free ear lobes than those who have attached ear lobes.
There are _______ people with curly hair than those with straight hair.
There are _______ right-handed people than left-handed people.
CONCLUSION:
____________ traits are more common in comparison with ____________ traits.
Dominant Vs Recessive
Dominant trait
A dominant trait refers to a genetic feature that hides the recessive trait in the phenotype of an individual. A dominant trait is a phenotype that is seen in both the homozygous AA and heterozygous Aa genotypes. Many traits are determined by pairs of complementary genes, each inherited from a single parent. Often when these are paired and compared, one allele (the dominant) will be found to effectively shut out the instructions from the other, recessive allele. For example, if a person has one allele for blood type A and one for blood type O, that person will always have blood type A. For a person to have blood type O, both their alleles must be O (recessive).
When an individual has two dominant alleles (AA), the condition is referred to as homozygous dominant; an individual with two recessive alleles (aa) is called homozygous recessive. An individual carrying one dominant and one recessive allele is referred to as heterozygous.
The term "recessive allele" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in homozygous genotypes (organisms that have two copies of the same allele) and never in heterozygous genotypes. Every diploid organism, including humans, has two copies of every gene on autosomal chromosomes, one from the mother and one from the father. The dominant allele of a gene will always be expressed while the recessive allele of a gene will be expressed only if the organism has two recessive forms.[2] Thus, if both parents are carriers of a recessive trait, there is a 25% chance with each child to show the recessive trait.
A dominant trait refers to a genetic feature that hides the recessive trait in the phenotype of an individual. A dominant trait is a phenotype that is seen in both the homozygous AA and heterozygous Aa genotypes. Many traits are determined by pairs of complementary genes, each inherited from a single parent. Often when these are paired and compared, one allele (the dominant) will be found to effectively shut out the instructions from the other, recessive allele. For example, if a person has one allele for blood type A and one for blood type O, that person will always have blood type A. For a person to have blood type O, both their alleles must be O (recessive).
When an individual has two dominant alleles (AA), the condition is referred to as homozygous dominant; an individual with two recessive alleles (aa) is called homozygous recessive. An individual carrying one dominant and one recessive allele is referred to as heterozygous.
A dominant trait when written in a genotype is always written before the recessive gene in a heterozygous pair. A heterozygous genotype is written Aa, not aA.
Recessive trait
The term "recessive allele" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in homozygous genotypes (organisms that have two copies of the same allele) and never in heterozygous genotypes. Every diploid organism, including humans, has two copies of every gene on autosomal chromosomes, one from the mother and one from the father. The dominant allele of a gene will always be expressed while the recessive allele of a gene will be expressed only if the organism has two recessive forms.[2] Thus, if both parents are carriers of a recessive trait, there is a 25% chance with each child to show the recessive trait.
The term "recessive allele" is part of the laws of Mendelian inheritance formulated by Gregor Mendel. Examples of recessive traits in Mendel's famous pea plant experiments include the color and shape of seed pods and plant height.
Friday, March 20, 2009
Mitosis
- Mitosis is a process whereby a cell divides into two identical cells, each having exactly the same number and kind of chromosomes as the parent cell.
- Mitosis occurs in somatic cells.
- In animals, mitosis takes place in all body part.
- In plants, mitosis occurs at the tips of the shoots and roots.
MITOSIS
- The phases in mitosis:
- Each chromosome become shorter and thicker.
- Each chromosome doubles to become a pair of chromatids joined at the centromere
- Chromosomes line up at the cell equator.
- Chromosome are pulled to the opposite poles at the cell.
- Two identical daughter cells are formed.
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