The Role of DNA in The Trait of Hair Texture
Hi! My name is Shennendoah. The physical trait I have chosen to describe is hair texture. In my case, my hair is curly, which is incompletely dominant (HH).
The Basics
A phenotype is when a trait and is expressed physically or biochemically. A phenotype is determined by pairs of genes, because you get one gene (in the pair) from your mom and one from your dad. The pairs of genes represent the genotype for the trait (think "type of gene" for Genotype). People can be recognized (curly hair) by their phenotypic traits.
The rules of dominance and recessiveness: You only need one copy of a dominant gene in order to see a dominant phenotypic trait, but you need two copies of a recessive gene in order to see a recessive phenotypic trait. As such, a dominant phenotypic trait could be represented by a homozygous dominant (AA), or heterozygous (Aa) genotype. However, a recessive phenotype will always be represented by a homozygous recessive genotype (aa). Also, dominant genes are represented by capital letters and recessive genes are represented by lower case letters. The dominant gene is always written first in a heterozygous individual (Aa) not (aA).
Incomplete dominance (as in the case of my curly hair) is a special case when the heterozygous phenotype is intermediate, instead of being completely governed by a dominant gene. When incomplete dominance occurs, the phenotype of the heterozygous individual is intermediate between the phenotype of a homozygous dominant and homozygous recessive individual. Snapdragon flowers is another an example of this: homozygous dominant flowers are red, homozygous recessive flowers are white, and heterozygous flowers are pink.
In the case of my hair texture, my phenotype is curly. Some traits are determined by dominant genes, it's kind of difficult to distinguish between Homozygous dominant and Heterozygous. Testing and pedigree analysis (looking at the traits of your family members) is one way to determine. In my family, my mom has straight (blonde) hair. My dad had wavy brown (now it's peppered grey) hair. My younger brother has wavy brown hair like my dad. I do know that I carry a dominant gene.
According to a study by the Queensland Institute of Medical Research in Brisbane, Australia (Volume 12, Number 5), only 13% of Caucasians have type IV "Curly" hair.
Biologically speaking, the amount of curliness is related to the amount of hair keratins (protein in the hair) and cell type in the hair fiber, while the number of mesocortical cells (they release dopamine) decrease as the curl intensifies (dopamine is the 'feel good' neurotransmitter chemically produced by the brain). Thibaut et al., 2007.
The Basics
A phenotype is when a trait and is expressed physically or biochemically. A phenotype is determined by pairs of genes, because you get one gene (in the pair) from your mom and one from your dad. The pairs of genes represent the genotype for the trait (think "type of gene" for Genotype). People can be recognized (curly hair) by their phenotypic traits.
The rules of dominance and recessiveness: You only need one copy of a dominant gene in order to see a dominant phenotypic trait, but you need two copies of a recessive gene in order to see a recessive phenotypic trait. As such, a dominant phenotypic trait could be represented by a homozygous dominant (AA), or heterozygous (Aa) genotype. However, a recessive phenotype will always be represented by a homozygous recessive genotype (aa). Also, dominant genes are represented by capital letters and recessive genes are represented by lower case letters. The dominant gene is always written first in a heterozygous individual (Aa) not (aA).
Incomplete dominance (as in the case of my curly hair) is a special case when the heterozygous phenotype is intermediate, instead of being completely governed by a dominant gene. When incomplete dominance occurs, the phenotype of the heterozygous individual is intermediate between the phenotype of a homozygous dominant and homozygous recessive individual. Snapdragon flowers is another an example of this: homozygous dominant flowers are red, homozygous recessive flowers are white, and heterozygous flowers are pink.
In the case of my hair texture, my phenotype is curly. Some traits are determined by dominant genes, it's kind of difficult to distinguish between Homozygous dominant and Heterozygous. Testing and pedigree analysis (looking at the traits of your family members) is one way to determine. In my family, my mom has straight (blonde) hair. My dad had wavy brown (now it's peppered grey) hair. My younger brother has wavy brown hair like my dad. I do know that I carry a dominant gene.
According to a study by the Queensland Institute of Medical Research in Brisbane, Australia (Volume 12, Number 5), only 13% of Caucasians have type IV "Curly" hair.
Biologically speaking, the amount of curliness is related to the amount of hair keratins (protein in the hair) and cell type in the hair fiber, while the number of mesocortical cells (they release dopamine) decrease as the curl intensifies (dopamine is the 'feel good' neurotransmitter chemically produced by the brain). Thibaut et al., 2007.
Background Information on Hair
Hair is basically made up of keratin. Keratin basically coils itself into a cord-like, very stable filament, which makes hair strong. Hair grows inside the skin in the hair follicle (this is the only "living" part of the hair) and extends beyond the skin's surface (this is the part that is considered "dead"). The smooth, little muscles (erector pili) that are attached to the hair follicles (in mammals), when contracted, cause "goosebumps"!
Goosebumps aren't something we consciously control, they're involuntary contractions (like feeling cold) that stimulate the sympathetic nervous system responsible for the fight or flight response.
Hair is made up of three parts: the medulla, cortex, and cuticle. Melanin is found in the cortex. The shape of the cortex and the shape of the fiber is based on the straightness or curliness of the hair. Round fibers result in straighter hair, while oval and irregularly-shaped fibers result in wavy or curly hair. The cuticle is the part that covers the cortex, and the lipids that cover the hair actually help the hair repel water. The average diameter of a human hair is between 17 to 180 µm.
Goosebumps aren't something we consciously control, they're involuntary contractions (like feeling cold) that stimulate the sympathetic nervous system responsible for the fight or flight response.
Hair is made up of three parts: the medulla, cortex, and cuticle. Melanin is found in the cortex. The shape of the cortex and the shape of the fiber is based on the straightness or curliness of the hair. Round fibers result in straighter hair, while oval and irregularly-shaped fibers result in wavy or curly hair. The cuticle is the part that covers the cortex, and the lipids that cover the hair actually help the hair repel water. The average diameter of a human hair is between 17 to 180 µm.
The Trait of Curly Hair and its Heritability Pattern
As you can see from my school picture on the left, I have really curly hair. A heritable trait is basically a trait we show that resembles our parents' traits, except in my case, neither of my parents have curly hair. So, how can a child of non-curly haired parents end up with curly hair? For the answer, we need to look to Gregor Mendel (1822 - 1884), the Austrian monk who published his findings on pea plants he bred and established some early rules governing the inheritance of traits.
For more information on Gregor Mendel, check out: http://anthro.palomar.edu/mendel/mendel_1.htm
I also found an interactive Pea Experiment where you can breed your own hybrid pea plants! Check it out: http://www.sonic.net/~nbs/projects/anthro201/exper/
Before Mendel noticed that some peas were producing yellow peas and others green, basic concepts on genetics and heritability had already been around for quite some time. The Ancient Greeks believed that males and females both passed on fluids to their offspring that accounted for gender. In the Middle Ages, the theory of spontaneous generation (living organisms could develop from non-living substances) was already accepted (like when flies come out of meat or fleas from mud and dirt). Of course, Francesco Redi (1626 - 1697) in his Experiments on the Generation of Insects (1668) put meat into jars to later disprove this theory. The jars that were covered did not produce maggots, the uncovered jars, exposed to air, metamorphosized and became flies. Anton Van Leeuwenhoek (1632 - 1723), "The Father of Microbiology", using his handcrafted microscopes, and was the first to observe and describe bacteria, protists, sperm cells, blood cells, and much more. His ideas opened up the concept of in utero, which basically means that development or reproduction was occurring inside the body (uterus).
Essentially, Mendelian genetics helps us understand how traits from one generation to the next can produce visibly different (phenotypic) traits due to the patterns of inheritance that allow for variance. Mendel calculated the genotypic and phenotypic ratios of his peas offspring (yellow vs. green) and developed the law of independent assortment, which explains how you can get a number of different outcomes when you breed differently-colored of peas.
For more information on Gregor Mendel, check out: http://anthro.palomar.edu/mendel/mendel_1.htm
I also found an interactive Pea Experiment where you can breed your own hybrid pea plants! Check it out: http://www.sonic.net/~nbs/projects/anthro201/exper/
Before Mendel noticed that some peas were producing yellow peas and others green, basic concepts on genetics and heritability had already been around for quite some time. The Ancient Greeks believed that males and females both passed on fluids to their offspring that accounted for gender. In the Middle Ages, the theory of spontaneous generation (living organisms could develop from non-living substances) was already accepted (like when flies come out of meat or fleas from mud and dirt). Of course, Francesco Redi (1626 - 1697) in his Experiments on the Generation of Insects (1668) put meat into jars to later disprove this theory. The jars that were covered did not produce maggots, the uncovered jars, exposed to air, metamorphosized and became flies. Anton Van Leeuwenhoek (1632 - 1723), "The Father of Microbiology", using his handcrafted microscopes, and was the first to observe and describe bacteria, protists, sperm cells, blood cells, and much more. His ideas opened up the concept of in utero, which basically means that development or reproduction was occurring inside the body (uterus).
Essentially, Mendelian genetics helps us understand how traits from one generation to the next can produce visibly different (phenotypic) traits due to the patterns of inheritance that allow for variance. Mendel calculated the genotypic and phenotypic ratios of his peas offspring (yellow vs. green) and developed the law of independent assortment, which explains how you can get a number of different outcomes when you breed differently-colored of peas.
Mendel's Genetics
A really nice website that explains Mendel's Genetics can be found at http://anthro.palomar.edu/mendel/mendel_1.htm . I like this website because it explains the basic principles of genetics, cross pollination, and the principles of segregation and independent assortment.
As I explained on the Understanding Traits page, heredity is the transmission of traits from one generation to the next. Physical traits are basically observable characteristics that are determined by certain sections or segments of DNA called genes. Multiple genes group together to form chromosomes, which are in the nucleus of the cell. Every cell (except eggs and sperm) in our bodies have two copies of each gene. This is because the mother and the father both give a copy when they combine. The genetic material that our parents have gets copied every time a cell divides, this way, all the cells have the same DNA. Genes have all the information needed for the cell to build proteins. That's what gives us specific physical traits.
Everyone inherits a single copy of every gene from each of our parents. Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set. This is called meiosis. When cells divide, which is necessary in reproduction, meiosis produces gametes (sperm) and egg cells (in fungi, meiosis creates spores). Meiosis starts when one diploid cell that has two copies of each chromosome (one from the mom and one from the dad) produces four haploid cells, which have one copy of each chromosome. This creates a mixture and ensures that offspring are genetically different from their parents. This is where we get genetic diversity and part of the reason I have curly hair and my mother, for example, does not.
On the DNA page, we looked at the basics of DNA, which kind of looks like a twisted ladder called the double helix. The rungs on the DNA ladder are called base pairs (these are the important ones that give instructions). These base pairs can break apart, which causes the sides of the helix to unravel. This is what allows DNA to copy itself, and to act as the instructions.
All the cells in our body do not exhibit all the traits we have, but they still have the same DNA. One of the best sites I've found that explains how this works is on the National Institute of General Medical Sciences: http://publications.nigms.nih.gov/insidethecell/chapter4.html .
Basically, not all cells in an organ go through mitosis at the same time. Some of the cells need to continue doing their jobs: the heart muscle cells need to contract and pump blood, our intestinal cells need to absorb the food we eat, our thyroid glands need to send out hormones, etc. So, when one cell divides, the neighboring cells keep working so that the body continues to function.
As I explained on the Understanding Traits page, heredity is the transmission of traits from one generation to the next. Physical traits are basically observable characteristics that are determined by certain sections or segments of DNA called genes. Multiple genes group together to form chromosomes, which are in the nucleus of the cell. Every cell (except eggs and sperm) in our bodies have two copies of each gene. This is because the mother and the father both give a copy when they combine. The genetic material that our parents have gets copied every time a cell divides, this way, all the cells have the same DNA. Genes have all the information needed for the cell to build proteins. That's what gives us specific physical traits.
Everyone inherits a single copy of every gene from each of our parents. Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set. This is called meiosis. When cells divide, which is necessary in reproduction, meiosis produces gametes (sperm) and egg cells (in fungi, meiosis creates spores). Meiosis starts when one diploid cell that has two copies of each chromosome (one from the mom and one from the dad) produces four haploid cells, which have one copy of each chromosome. This creates a mixture and ensures that offspring are genetically different from their parents. This is where we get genetic diversity and part of the reason I have curly hair and my mother, for example, does not.
On the DNA page, we looked at the basics of DNA, which kind of looks like a twisted ladder called the double helix. The rungs on the DNA ladder are called base pairs (these are the important ones that give instructions). These base pairs can break apart, which causes the sides of the helix to unravel. This is what allows DNA to copy itself, and to act as the instructions.
All the cells in our body do not exhibit all the traits we have, but they still have the same DNA. One of the best sites I've found that explains how this works is on the National Institute of General Medical Sciences: http://publications.nigms.nih.gov/insidethecell/chapter4.html .
Basically, not all cells in an organ go through mitosis at the same time. Some of the cells need to continue doing their jobs: the heart muscle cells need to contract and pump blood, our intestinal cells need to absorb the food we eat, our thyroid glands need to send out hormones, etc. So, when one cell divides, the neighboring cells keep working so that the body continues to function.
Gene Expression
The video below gives a great visual explanation of DNA replication and expression.
Ribosomes inside the cell make the necessary proteins for the cell to go on living. The synthesis process (making the proteins) is more complex that DNA duplication, but the process is similar.
Understandably, before the ribosome can make a protein, it needs DNA instructions. The ribosome sends out an enzyme into the nucleus to "rip apart" or "unzip" the portion of DNA it needs. Then, a nucleic acid called mRNA (messenger rubonucleic acid) bonds to one of the unzipped strands (the sense strand), kind of like the replication process. This is called transcription. RNA is just like DNA except that it uses ribose sugar instead of deoxyribose sugar as its backbone. RNA also doesn't use the base thymine (T); instead, it uses uracil (U). The copied mRNA strand goes back to the ribosome to make the protein. There's actually another type of RNA, called transfer RNA, that collects all the free amino acids and carries them back to the ribosome. This process is called translation.
Ribosomes inside the cell make the necessary proteins for the cell to go on living. The synthesis process (making the proteins) is more complex that DNA duplication, but the process is similar.
Understandably, before the ribosome can make a protein, it needs DNA instructions. The ribosome sends out an enzyme into the nucleus to "rip apart" or "unzip" the portion of DNA it needs. Then, a nucleic acid called mRNA (messenger rubonucleic acid) bonds to one of the unzipped strands (the sense strand), kind of like the replication process. This is called transcription. RNA is just like DNA except that it uses ribose sugar instead of deoxyribose sugar as its backbone. RNA also doesn't use the base thymine (T); instead, it uses uracil (U). The copied mRNA strand goes back to the ribosome to make the protein. There's actually another type of RNA, called transfer RNA, that collects all the free amino acids and carries them back to the ribosome. This process is called translation.
Punnett Square (Will my kids have curly hair?)
The best way to know whether or not my kids would have curly hair is to use the Punnett Square graphical method invented by the early 20th century English geneticist, Reginald Punnett. The technique he used is what we now call the Punnett Square. It's a simple graphical way to figure out all the potential combinations of genotypes that can occur in children, using the genotypes of their parents. It also shows us the odds of each of the offspring genotypes that can occur.
A good website that walks you through the basics of setting up and using a Punnett square is: http://anthro.palomar.edu/mendel/mendel_2.htm
However, my favorite explanation is actually by Khan Academy. Check out his video below.
A good website that walks you through the basics of setting up and using a Punnett square is: http://anthro.palomar.edu/mendel/mendel_2.htm
However, my favorite explanation is actually by Khan Academy. Check out his video below.
Genetics
Basically, half of our father's genetic information and half of our mother's genetic information is passed onto our offspring. At the gene level, we get about 51.3% of our genetic material from mom (the rest from our dad's). The mitochrondrion DNA (the energy producing organelles floating freely in the cell cytoplasm) comes directly from our mom's because she donates the entire egg cell (the mitochondria is already inside). If you count genes, we get 19,324 from our mom and 18,425 genes from our dad (the Y chromosome has fewer chromosomes than the X chromosome), that's 51.19% from mom. Then if you add in the number of mitochrondrial genes we get from our moms, we can add 72 genes to her number, giving us 51.28% from mom. The percentage of DNA in the is so small, that's it's still pretty much a 50-50 split.
Since I mentioned above that curly hair is an incomplete dominance, you can use the analogy in the video above with the red and white flowers to predict whether or not my children will have curly hair. When you draw the Punnett Square, my kids could inherit straight hair (50% chance) or curly hair (50% chance). Remember, incomplete dominance means you can have blending of the traits. So, a red and white flower, when mixed, could result in pink. This is similar to blending straight hair (my mom) and wavy hair (my dad), you can get curly hair (incomplete dominance).
So, basically, while I have curly hair, one of the phenotypes I have that would result in straight hair as a trait explains how I could pass on genetic material to my offspring without having the trait myself.
Since I mentioned above that curly hair is an incomplete dominance, you can use the analogy in the video above with the red and white flowers to predict whether or not my children will have curly hair. When you draw the Punnett Square, my kids could inherit straight hair (50% chance) or curly hair (50% chance). Remember, incomplete dominance means you can have blending of the traits. So, a red and white flower, when mixed, could result in pink. This is similar to blending straight hair (my mom) and wavy hair (my dad), you can get curly hair (incomplete dominance).
So, basically, while I have curly hair, one of the phenotypes I have that would result in straight hair as a trait explains how I could pass on genetic material to my offspring without having the trait myself.
Changes in DNA (Mutation)
Basically, everyone is different. This means, we have different DNA. Subtle differences are called polymorphisms (many forms). Even though polymorphisms are the result of mutation in the gene, geneticists only refer to changes as mutations when it is not part of the normal differences between people.
In mutation, you can have a simple copying error when the DNA replicates itself. Every time a cell divides, all of its DNA is duplicated so that the two new cells have a full set of DNA. Other changes such as sunlight, cigarette smoke, and radiation can cause mutations. Fortunately, our cells are programed to catch and repair most of the changes that occur during DNA replication and even from environmental damage. Unfortunately, as we get older, DNA repair doesn't work as well and our DNA changes a lot.
When you spend too much time in the sun, skin cells can change from sun exposure. This is not passed onto our children.
This is not the case with diseases, which run in families. When there's errors in the DNA of cells that produce the eggs and sperm. These are called germline mutuations and can be inherited.
Any change to the instructions can alter the gene's meaning and change the protein that is made, or even how or when a cell makes that protein. There are many ways to change a gene, just like there's a million chances there will be typo's in this website. Here's a fun example of a Point Mutation:
A point mutation is a simple change in one base of the gene sequence. This is the same as changing one letter in a sentence. Here, we'll change the "c" in cat to an "h":
Original: The fat cat ate the wee rat.
Point Mutation: The fat hat ate the wee rat.
There are other types of mutations (frame-shift, deletion, insertion, inversion, etc.) and some mutations that don't change the protein but rather where and how much of a protein is made, but I think the first example is sufficient enough to imagine how many types of mutations could occur.
In the Khan Academy video on Variation in a Species (below), he explains some of the environmental factors that could prohibit some people from reproducing. If one dominant group (let's say curly haired people) ended up reproducing more because straight-haired people mostly died out (for some strange reason), and then were not liked by the future curly hair generations, this could affect your children (if they had straight hair) in a negative way (not being liked). Of course, if you had curly haired children, they might end up ruling the world (in this scenario). Yeah, I know, wishful thinking! lol
In mutation, you can have a simple copying error when the DNA replicates itself. Every time a cell divides, all of its DNA is duplicated so that the two new cells have a full set of DNA. Other changes such as sunlight, cigarette smoke, and radiation can cause mutations. Fortunately, our cells are programed to catch and repair most of the changes that occur during DNA replication and even from environmental damage. Unfortunately, as we get older, DNA repair doesn't work as well and our DNA changes a lot.
When you spend too much time in the sun, skin cells can change from sun exposure. This is not passed onto our children.
This is not the case with diseases, which run in families. When there's errors in the DNA of cells that produce the eggs and sperm. These are called germline mutuations and can be inherited.
Any change to the instructions can alter the gene's meaning and change the protein that is made, or even how or when a cell makes that protein. There are many ways to change a gene, just like there's a million chances there will be typo's in this website. Here's a fun example of a Point Mutation:
A point mutation is a simple change in one base of the gene sequence. This is the same as changing one letter in a sentence. Here, we'll change the "c" in cat to an "h":
Original: The fat cat ate the wee rat.
Point Mutation: The fat hat ate the wee rat.
There are other types of mutations (frame-shift, deletion, insertion, inversion, etc.) and some mutations that don't change the protein but rather where and how much of a protein is made, but I think the first example is sufficient enough to imagine how many types of mutations could occur.
In the Khan Academy video on Variation in a Species (below), he explains some of the environmental factors that could prohibit some people from reproducing. If one dominant group (let's say curly haired people) ended up reproducing more because straight-haired people mostly died out (for some strange reason), and then were not liked by the future curly hair generations, this could affect your children (if they had straight hair) in a negative way (not being liked). Of course, if you had curly haired children, they might end up ruling the world (in this scenario). Yeah, I know, wishful thinking! lol