G. Mendel and his experiment, Law of Segregation
Long before modern technology could show that children inherit genes or traits (ex: she has her mom's eyes), Mendel (a monk) has already realized that organisms pass down something to their offsprings. Although he did not completely understand what is passed down or thought of names for them, he performed experiments and came upon theories that were truly ahead of his times. He came up with 2 laws, his 1st law is discussed below:
1) LAW OF SEGREGATION
1) Organisms inherit 2 copies of each gene, one from each parent.
2) Organisms donate only one copy of each gene in their gametes. Thus, the 2 copies of each gene segregate, or separate, during gamete formation.
The Experiment
Mendel pollinated (used pollens to "mate" them) a type of flower that was pure purple and pure white. This means that they were purebreds. These flowers have been purple and white in colors for many generations in his monastery, where he lived. From these flowers, the 1st generation of offsprings were produced (F1 Gen.).
These 1st generations all looked purple. Mendel pollinated these 1st generations together and found that about 3/4 of the offsprings (F2 Gen.) were purple HOWEVER about 1/4 of them were white!!! He did the same experiment multiple times and the results were almost similar. There was always a 3:1 ratio. There was always another trait that shows up even if the parents flowers both had the same color. This led Mendel to conclude that the color WHITE did not disappear in the F1 Gen. and that it was somehow passed on to the F2 Gen. So, the F1 and F2 Gens were what is called hybrids. From these experiments, he theorize that the same could be connected to human traits (remember, he did not know anything about genes at that time); that traits from parents do not go away but some traits may be hidden more than other traits. (so even if a child looks alot like one parent, it does not mean that the other parents traits are not there, it is just "hidden" and can still be passed on). The parents each would pass on 1 copy of the traits to the offsprings to give 2 copies total in the offsprings.
Based on his experiments, he can predict the probabilities of what trait will appear compared to other traits.
1) LAW OF SEGREGATION
1) Organisms inherit 2 copies of each gene, one from each parent.
2) Organisms donate only one copy of each gene in their gametes. Thus, the 2 copies of each gene segregate, or separate, during gamete formation.
The Experiment
Mendel pollinated (used pollens to "mate" them) a type of flower that was pure purple and pure white. This means that they were purebreds. These flowers have been purple and white in colors for many generations in his monastery, where he lived. From these flowers, the 1st generation of offsprings were produced (F1 Gen.).
These 1st generations all looked purple. Mendel pollinated these 1st generations together and found that about 3/4 of the offsprings (F2 Gen.) were purple HOWEVER about 1/4 of them were white!!! He did the same experiment multiple times and the results were almost similar. There was always a 3:1 ratio. There was always another trait that shows up even if the parents flowers both had the same color. This led Mendel to conclude that the color WHITE did not disappear in the F1 Gen. and that it was somehow passed on to the F2 Gen. So, the F1 and F2 Gens were what is called hybrids. From these experiments, he theorize that the same could be connected to human traits (remember, he did not know anything about genes at that time); that traits from parents do not go away but some traits may be hidden more than other traits. (so even if a child looks alot like one parent, it does not mean that the other parents traits are not there, it is just "hidden" and can still be passed on). The parents each would pass on 1 copy of the traits to the offsprings to give 2 copies total in the offsprings.
Based on his experiments, he can predict the probabilities of what trait will appear compared to other traits.
Meiosis
Here are terms you will need to know:
1. Mitosis
2. Meiosis
3. somatic (body) cells
4. germ cells
5. sex cells
6. diploid (2n)
7. haploid (n)
8. gametes
9. fertilization
10. homologous chromosomes
11. crossing over
12. genetic variation
13. autosomes
14. sex chromosomes
15. zygote
16. spermatogenesis
17. oogenesis
18. genetic variation
Meiosis: A division process that reduces the number of starting chromosomes in half. For example, in humans, there are always 46 chromosomes. Meiosis will reduce this number so that each cell, at the end, will have only 23 chromosomes.
In our body, when we are talking about reproduction, there is only one type of cell that carries the genetic information and have the ability to pass that on, they are called sex cells (or gametes). Any other cells are called Somatic cells (or body cells). Somatic cells include cells of your skin, liver, heart, anywhere in your body. These somatic cells have 46 chromosomes and divide mitotically (identical copies). For example, when skin cells die, more skin cells will divide themselves to replace the ones lost...all are identical copies. And even though somatic cells have the same 46 genetic information, they cannot be passed down. Sex cells, however, can pass on the genetic information. In Sex cells, however, they are produced from a process called Meiosis that reduces the chromosome # in half? Why? Because one of these cells will combine with another sex cell of the opposite sex to produce a baby. The process of meiosis will produce 4 gametes (sperm or egg), each with only 23 chromosomes. When 2 gametes combine (sperm combines with egg), a zygote is produced. A zygote is an early stage of embryo or baby. The zygote is just one cell with 46 chromosomes (remember 23 from sperm and 23 from egg) or 23 pairs of chromosomes. Pairs 1-22 are called autosomes and pair 23 is called sex chromosome. All the traits are on pairs 1-22. Pair 23 determines sex: XX gives a female and XY gives a male. This one cell will divide through Mitosis for the next 9 months.
1. Mitosis
2. Meiosis
3. somatic (body) cells
4. germ cells
5. sex cells
6. diploid (2n)
7. haploid (n)
8. gametes
9. fertilization
10. homologous chromosomes
11. crossing over
12. genetic variation
13. autosomes
14. sex chromosomes
15. zygote
16. spermatogenesis
17. oogenesis
18. genetic variation
Meiosis: A division process that reduces the number of starting chromosomes in half. For example, in humans, there are always 46 chromosomes. Meiosis will reduce this number so that each cell, at the end, will have only 23 chromosomes.
In our body, when we are talking about reproduction, there is only one type of cell that carries the genetic information and have the ability to pass that on, they are called sex cells (or gametes). Any other cells are called Somatic cells (or body cells). Somatic cells include cells of your skin, liver, heart, anywhere in your body. These somatic cells have 46 chromosomes and divide mitotically (identical copies). For example, when skin cells die, more skin cells will divide themselves to replace the ones lost...all are identical copies. And even though somatic cells have the same 46 genetic information, they cannot be passed down. Sex cells, however, can pass on the genetic information. In Sex cells, however, they are produced from a process called Meiosis that reduces the chromosome # in half? Why? Because one of these cells will combine with another sex cell of the opposite sex to produce a baby. The process of meiosis will produce 4 gametes (sperm or egg), each with only 23 chromosomes. When 2 gametes combine (sperm combines with egg), a zygote is produced. A zygote is an early stage of embryo or baby. The zygote is just one cell with 46 chromosomes (remember 23 from sperm and 23 from egg) or 23 pairs of chromosomes. Pairs 1-22 are called autosomes and pair 23 is called sex chromosome. All the traits are on pairs 1-22. Pair 23 determines sex: XX gives a female and XY gives a male. This one cell will divide through Mitosis for the next 9 months.
Mitosis vs. Meiosis Process
Students should be familiar with Mitosis because it is covered in 7th grade. So, I will briefly discuss about Mitosis and in more details about Meiosis.
Mitosis is a duplication process (like a copying machine) that produces identical cells every time. So, if you start with 46 chromosomes, the copied cells will also have 46. These copied cells are called daughter cells. We call these cells diploid cells having 2n (the n=the chromosome number from one parent). For humans, our n=23; so in a diploid cell 2xn or 2x(23)= 46. Remember that Mitosis only occurs in somatic (body) cells, NOT in sex cells.
Meiosis is reduction process that reduces the number of chromosomes in half. So, if you start with 46 chromosomes, you will end up 23. This is because of the second part of Meiosis (called Meiosis II). The end result of Meiosis gives 4 gametes, capable of being sperms or eggs, depending if they are in a male or female organism. The gametes contain 23 unique chromosomes that are different from each other AND different from the original (parent) chromosomes. Because there are only 23, we call these cells...these gametes... haploids having n (n=23). For humans, haploid cells would be 1xn=23 or 1x23=23.
Mitosis, the process:
1. start with 46 chromosomes in somatic(body) cells.
2. 46 chromosome doubles during Prophase. So, now we have 92 chromosomes.
3. all 92 chromosomes line in the middle during Metaphase.
4. 96 chromosomes are pulled apart to both sides (left and right) during Anaphase
5. cell membrane starts to form to create 2 cells, each with 46 chromosomes and identical to each other...called daughter cells during Telophase.
Meiosis, the process:
1. Start with 46 chromosomes (or 23 pairs) in germ cells. (germ cells are located in the reproductive organs-testes or ovaries-and are the only cells that can go through meiosis.)
2. During Prophase I, several events happen:
: the 46 chromosomes duplicate. So now, we have 92 chromosomes.
: chromosomes from mom and dad pair up. We call these Homologous chromosomes. Homologous chromosomes (comes in pairs) have similar genes but maybe different alleles (ex: gene would be eye color and alleles can be that mom has blue eyes and dad has brown eyes).
: Once the homologous chromosomes pair up, crossing over occurs. Crossing over is an event that exchanges pieces of chromosomes of mom and dad across one another so that some of mom's DNA cross over to dad's and vice versa. Once this event occurs, the chromosomes are no longer like the parents' and no longer identical to each other.
3. The 46 pairs of chromosomes (92 total) align in the middle during Metaphase.
4. During Anaphase, these pairs are pulled apart. (This looks like it is similar to Mitosis, but it is NOT. Although you pull apart the 92 chromosomes to have 46 in each cell, the chromosomes are not identical to eachother like they are in Mitosis).
5. A second division process occurs called Meiosis II. As a result, during Anaphase II, the 46 chromosomes are pulled apart to result in 23 chromosomes in each cell. And in Telophase, you get 4 sex cells or gametes at the end, each with 23 chromosomes that are unique and not identical. These gametes are the sperms or the egg.
Meiosis Animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html
Meiosis Animation 2: http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
Here are terms that you will need to know:
1) gene
2) allele
3) homozygous
4) heterozygous
5) dominant
6) recessive
7) genotype
8) phenotype
6) traits
7) cross
DOMINANT/ RECESSIVE alleles or traits.
These traits are controlled by just one gene (or a pair of alleles). Traits such as widow's peak or no widow's peak,green or yellow in peas, etc. Notice that these traits only have TWO choices...either being a dominant or recessive trait. **Note: You will be asked to solve problems that are about hair color, height, or eye colors. Notice that these traits DO NOT just have two choices...people are not all of the same height. In reality, hair, height, eyes and many more are controlled by multiple alleles/genes to give you the many different variations of brown or green or height. BUT at the beginning, we will ask you to solve these problems as simple Dominant/Recessive Punnett square problems.
Sample Dominant/Recessive Punnett Square Problem
Both parents are Heterozygous for green crossed together. (talk it out: Since heterozygous consists of a capital and lower case letter and the look is green, I know that G must code for green and g must code for a second color.)
Mitosis is a duplication process (like a copying machine) that produces identical cells every time. So, if you start with 46 chromosomes, the copied cells will also have 46. These copied cells are called daughter cells. We call these cells diploid cells having 2n (the n=the chromosome number from one parent). For humans, our n=23; so in a diploid cell 2xn or 2x(23)= 46. Remember that Mitosis only occurs in somatic (body) cells, NOT in sex cells.
Meiosis is reduction process that reduces the number of chromosomes in half. So, if you start with 46 chromosomes, you will end up 23. This is because of the second part of Meiosis (called Meiosis II). The end result of Meiosis gives 4 gametes, capable of being sperms or eggs, depending if they are in a male or female organism. The gametes contain 23 unique chromosomes that are different from each other AND different from the original (parent) chromosomes. Because there are only 23, we call these cells...these gametes... haploids having n (n=23). For humans, haploid cells would be 1xn=23 or 1x23=23.
Mitosis, the process:
1. start with 46 chromosomes in somatic(body) cells.
2. 46 chromosome doubles during Prophase. So, now we have 92 chromosomes.
3. all 92 chromosomes line in the middle during Metaphase.
4. 96 chromosomes are pulled apart to both sides (left and right) during Anaphase
5. cell membrane starts to form to create 2 cells, each with 46 chromosomes and identical to each other...called daughter cells during Telophase.
Meiosis, the process:
1. Start with 46 chromosomes (or 23 pairs) in germ cells. (germ cells are located in the reproductive organs-testes or ovaries-and are the only cells that can go through meiosis.)
2. During Prophase I, several events happen:
: the 46 chromosomes duplicate. So now, we have 92 chromosomes.
: chromosomes from mom and dad pair up. We call these Homologous chromosomes. Homologous chromosomes (comes in pairs) have similar genes but maybe different alleles (ex: gene would be eye color and alleles can be that mom has blue eyes and dad has brown eyes).
: Once the homologous chromosomes pair up, crossing over occurs. Crossing over is an event that exchanges pieces of chromosomes of mom and dad across one another so that some of mom's DNA cross over to dad's and vice versa. Once this event occurs, the chromosomes are no longer like the parents' and no longer identical to each other.
3. The 46 pairs of chromosomes (92 total) align in the middle during Metaphase.
4. During Anaphase, these pairs are pulled apart. (This looks like it is similar to Mitosis, but it is NOT. Although you pull apart the 92 chromosomes to have 46 in each cell, the chromosomes are not identical to eachother like they are in Mitosis).
5. A second division process occurs called Meiosis II. As a result, during Anaphase II, the 46 chromosomes are pulled apart to result in 23 chromosomes in each cell. And in Telophase, you get 4 sex cells or gametes at the end, each with 23 chromosomes that are unique and not identical. These gametes are the sperms or the egg.
Meiosis Animation: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html
Meiosis Animation 2: http://www.sumanasinc.com/webcontent/animations/content/meiosis.html
Here are terms that you will need to know:
1) gene
2) allele
3) homozygous
4) heterozygous
5) dominant
6) recessive
7) genotype
8) phenotype
6) traits
7) cross
DOMINANT/ RECESSIVE alleles or traits.
These traits are controlled by just one gene (or a pair of alleles). Traits such as widow's peak or no widow's peak,green or yellow in peas, etc. Notice that these traits only have TWO choices...either being a dominant or recessive trait. **Note: You will be asked to solve problems that are about hair color, height, or eye colors. Notice that these traits DO NOT just have two choices...people are not all of the same height. In reality, hair, height, eyes and many more are controlled by multiple alleles/genes to give you the many different variations of brown or green or height. BUT at the beginning, we will ask you to solve these problems as simple Dominant/Recessive Punnett square problems.
Sample Dominant/Recessive Punnett Square Problem
Both parents are Heterozygous for green crossed together. (talk it out: Since heterozygous consists of a capital and lower case letter and the look is green, I know that G must code for green and g must code for a second color.)
Punnett Squares: calculations to predict the likelihood of inheriting particular traits.
Here are terms that you will need to know:
1) gene
2) allele
3) homozygous
4) heterozygous
5) dominant
6) recessive
7) genotype
8) phenotype
6) traits
7) cross
DOMINANT/ RECESSIVE alleles or traits.
These traits are controlled by just one gene (or a pair of alleles). Traits such as widow's peak or no widow's peak,green or yellow in peas, etc. Notice that these traits only have TWO choices...either being a dominant or recessive trait. **Note: You will be asked to solve problems that are about hair color, height, or eye colors. Notice that these traits DO NOT just have two choices...people are not all of the same height. In reality, hair, height, eyes and many more are controlled by multiple alleles/genes to give you the many different variations of brown or green or height. BUT at the beginning, we will ask you to solve these problems as simple Dominant/Recessive Punnett square problems.
Sample Dominant/Recessive Punnett Square Problem
Both parents are Heterozygous for green crossed together. (talk it out: Since heterozygous consists of a capital and lower case letter and the look is green, I know that G must code for green and g must code for a second color.)
1) gene
2) allele
3) homozygous
4) heterozygous
5) dominant
6) recessive
7) genotype
8) phenotype
6) traits
7) cross
DOMINANT/ RECESSIVE alleles or traits.
These traits are controlled by just one gene (or a pair of alleles). Traits such as widow's peak or no widow's peak,green or yellow in peas, etc. Notice that these traits only have TWO choices...either being a dominant or recessive trait. **Note: You will be asked to solve problems that are about hair color, height, or eye colors. Notice that these traits DO NOT just have two choices...people are not all of the same height. In reality, hair, height, eyes and many more are controlled by multiple alleles/genes to give you the many different variations of brown or green or height. BUT at the beginning, we will ask you to solve these problems as simple Dominant/Recessive Punnett square problems.
Sample Dominant/Recessive Punnett Square Problem
Both parents are Heterozygous for green crossed together. (talk it out: Since heterozygous consists of a capital and lower case letter and the look is green, I know that G must code for green and g must code for a second color.)
Percentage %:
Genotype GG= 1/4= 25% chance offsprings will inherit GG Gg= 2/4 or 1/2= 50% chance offsprings will inherit Gg gg= 1/4= 25% chance offsprings will inherit gg Phenotype Green (GG and Gg)= 3/4 or 75% chance offsprings will look Green other color (gg)= 1/4 or 25% chance offsprings will look another color |
Mendel and Punnett Squares
A long time later, when genes were discovered as the inheritance factor, Mendel's work became a great contribution to the genetics world. Looking back at Mendel's experiments, the purebreds are the homozygous alleles and hybrids are the heterozygous alleles. His calculations are the Punnett Squares that we use today. And yes, the ratios of the traits would still come out 3:1......But for traits that are controlled by 1 gene (having 2 choices). Traits that are controlled by multiple genes/alleles, did not follow his Laws.
Punnett Squares- Incomplete, Codominance, Multiple Alleles, Polygenic traits, and Sex-linked
Mendel's experiments and observations were only on limited organisms so his theories do not explain ALL of the patterns of traits that exist.
Incomplete Dominance: when 2 traits mix to get a 3rd trait. For example, Red allele with White allele will give a Pink trait.
Example:
R= Red flower color
W= White flower color
RW= Pink flower color
* We usually use capital letters because these traits are dominance and there is no recessive trait.
Incomplete Dominance: when 2 traits mix to get a 3rd trait. For example, Red allele with White allele will give a Pink trait.
Example:
R= Red flower color
W= White flower color
RW= Pink flower color
* We usually use capital letters because these traits are dominance and there is no recessive trait.
Co Dominant: when 2 traits BOTH show up at the same time. For example, when Red and White both show up to give spots.
Multiple Alleles: when a trait is controlled by one gene but with MORE THAN TWO ALLELES.
Example: Blood types are of one gene, but that one gene has multiple alleles: A, B, AB, O, and more.
Polygenic Traits: when a trait is controlled by more than one gene.
Example: Height, eye color, hair color are all controlled by more than one gene.
Sex-linked genes: Genes that are linked to the 23rd pair of human chromosomes. Although the 23rd pair of chromosomes (called sex chromosomes) determine the gender of the offspring, there are also many genes/disorders that are related to the X or the Y chromosome of this 23rd pair.
X-linked: these genes are located on the X chromosome.
Y-linked: these genes are located on the Y chromosome....although there are many disorders located on the X than the Y chromosome.
Example: Blood types are of one gene, but that one gene has multiple alleles: A, B, AB, O, and more.
Polygenic Traits: when a trait is controlled by more than one gene.
Example: Height, eye color, hair color are all controlled by more than one gene.
Sex-linked genes: Genes that are linked to the 23rd pair of human chromosomes. Although the 23rd pair of chromosomes (called sex chromosomes) determine the gender of the offspring, there are also many genes/disorders that are related to the X or the Y chromosome of this 23rd pair.
X-linked: these genes are located on the X chromosome.
Y-linked: these genes are located on the Y chromosome....although there are many disorders located on the X than the Y chromosome.