Wednesday, January 19, 2022

Introducing Human Genetics

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Identify patterns of inheritance to predict genotype/phenotype and solve punnett square problems.

Article includes ideas, images, and content from Troy Smigielski (2022-01)

Introducing Human Genetics:
Sex-linked Inheritance
(Pleased to meet you! See what I did, there?)

In previous articles, we have looked closely at genetics. In this and a few more that will follow, we will apply those to humans as a specific case of some additional general genetics principles.

Recall that…

  • Genetic information is carried by DNA.

  • DNA is organized into structures called chromosomes.

  • Chromosomes carry information for specific traits in sections called genes.

  • Genes having different versions or variations of a trait are alleles.

  • Some traits are dominant, some are recessive, some are incompletely dominant, and some are codominant.

  • Some traits require information from more than one gene; these are polygenic.

  • Humans have 46 chromosomes organized into 23 pairs; on chromosome from each pair comes from the mother and the other comes from the father.
We should probably repeat some of that, but with pictures!

Humans  have 23 pairs of each chromosome. That is to say, there are two each of the 23 different chromosomes that are paired up. That makes 46; 23 come from the mother and 23 come from the father.


In other words, humans have two copies of each chromosome. This makes us diploid organisms.

Wooo, that's a cool picture! What's it called?

Biologists use diagrams called karyotypes to map out chromosomes of an organism. For example, there is not a human.




To create a karyotype, biologists take pictures of cells in stages of mitosis. Then, they organize the chromosomes by size and shape. Something like the following…



So, in other words, it's magic?

A karyotype can give you three major pieces of information:

  1. How many chromosomes an organism has
  2. Sex of the organism
  3. Presence of a chromosomal disorder
How can that bunch of dark splotches tell what the sex of an organism is?

Sex/gender is all about the 23rd pair of chromosomes.

The 23rd chromosome pair of a male is XY.
The 23rd chromosome pair of a female is XX.

To indicate an organism biologically, scientists use the number of chromosomes followed by XX or XY for sex. For example, a human male would be 46,XY and a human female would be 46,XX.

The X and Y chromosomes are called sex chromosomes because they determine sex. In humans, these are found on the 23rd pair.

All chromosomes that do not determine sex are called autosomes. In humans, these are the first 22 pairs. Most genes are located somewhere on an autosome.



Determining The Sex of Offspring

So, genetics… all that… genes… chromosomes… all of this must let us figure things out. For instance, what is the probability that a baby will be a male? A female?

50/50. 

How do we know that? 

What two sex chromosomes does mom have? X and X…

What two sex chromosomes does dad have? X and Y…

We can find probabilities of the offspring being a certain sex through Punnett squares. Males will either pass on an X or a Y chromosome. Females will pass on an X chromosome no matter what.


Looking at the Punnett square, two of the boxes have XX and two have XY. Hence, half will be mail and half will be female.

Since only the male can pass the Y, then ultimately, the father ultimately the sex of the child.

Somebody should have told Henry XIII… Just saying…

Sex-linked Traits and Disorders

Although most traits are carried on the autosomes, some traits and disorders are carried on the sex chromosomes. These are called sex-linked traits.

Sex-linked traits: traits that are carried on the sex chromosomes.

Other traits that are carried on Chromosomes #1-22 are called autosomal traits. Sex-linked traits and disorders are typically carried on the X chromosome because it is larger and has more space to carry genes. The Y chromosome is shorter and does not carry as many genes. Most sex-linked traits are recessive.


So… Think… If the recessive trait is on the X chromosome, since the male has only one of them, it will be expressed; there's no "competing trait" on the Y chromosome. In the female, there is a chance that the other X chromosome will carry the dominant trait, so the recessive trait won't be expressed.

Let's look at this idea with another image…
Source, 2022-01-21

In the image above, the Y chromosome does not have the genes labeled A, B, and C. (This is an example and does not necessarily model real genes exactly.) Therefore, whatever traits are controlled by those genes will come through in the child. The Y chromosome has no alternative to offer; the gene of the X chromosome will be expressed.


Example: Color Blindness

Many people can think of someone who is color blind. If you can, are they male or female? You probably answered that they were male. Why?

Sex-linked traits and disorders are more common in males. Why is that?

Since men only have one X chromosome, whatever is on it will be expressed. Since women have two X chromosomes, they get “two chances”.


Example: Hemophilia

Hemophilia is a recessive, sex-linked disorder. Both males and females can have it.

In the image to the right, the father has the recessive trait on the X chromosome. He has no allele for that trait on the Y chromosome. The female has one X with, and one X without the trait. 

Therefore, the presence of hemophilia is determined by which of th X chromosomes the female passes.

The four possible outcomes are shown in among the daughters and sons.

There are numerous sex-linked disorders that come from X chromosome. Thus, these disorders can only be passed on from the mother. Three of them are shown in the image below.



Some Examples of Sex-linked Traits:

  • Red-green colorblindness
  • Male Pattern Baldness
  • Hemophilia
  • Duchenne Muscular Dystrophy


Predicting The Inheritance of Sex-linked Traits

We can do Punnett squares to determine the likelihood that offspring will inherit a sex-linked trait.

Below, the gene H is indicated as being part of the X chromosome. The H is the dominant, "unaffected" trait and the h is the recessive "affected" trait.



If we were to cross a normal male with a colorblind female, the results would be predictable using a Punnett square. It would show both the chances for sons and daughters to have the condition. 

This is the Punnett square for the above cross:



Note that the female carries the recessive c gene for color blindness on each of the X chromosomes. The male does not have gene for color blindness at all on the Y chromosome and has the dominant gene (not color blind) on his X chromosome.

Hence, the daughters will always get the "not color blind" X chromosome from the father (and the recessive c "color blind" gene from the mother. So, 0% of the daughters will be color blind.

The sons will get the Y chromosome from the father which does not have a gene for color blindness at all. Thus, the Xc chromosome from the mother (she has 2) will always be expressed. Therefore, 100% of the sons will be color blind.


Origins of Sex-linked Trait Understanding


The classic example of X-linked inheritance is eye color of fruit flies (Drosophila melanogaster).

Thomas Hunt Morgan was working with fruit flies when he noticed that most of the flies that had white eyes were males. (Fruit flies can have either red or white eyes.) 

This told him that eye color must be carried on the X chromosome because one is much more common in males. This also told him that red eyes are dominant to white eyes.

Red eyes are the wild type which means they are normal and more common. White eyes are the mutant type which means they are abnormal and less common.

Using Punnett squares, different combinations of different crosses can be examined. Where R is the dominant (red) trait and r is the recessive (white) trait, we get this:

Female Key:
XRXR = red eyes
XRXr = red eyes
XrXr = white eyes

Male Key:
XRY = red eyes
XrY = white eyes

Crossing a heterozygous female with a white-eyed male would result in:




50% of females have red eyes.

50% of males have red eyes.

This process can be repeated, of course, for any variation of the cross.


Non-typical Genetic Outcomes

There are occasions when the normal process does not take pace just right. Normally, the process of meiosis results in four haploid gametes.




Sometimes, the separating process does not go as expected. When chromosomes separate incorrectly during Meiosis, it is called non-disjunction.

That might look something like this (instead of the illustration above).



This leads to aneuploidy which is when one of the gametes has the incorrect number of chromosomes.




Aneuploidy can occur in both the autosomes and in the sex chromosomes. There are a numerous kinds of aneuploidy caused by nondisjunction. A few of them are illustrated below:


Monosomy
is when the offspring only received one copy of a chromosome.
Trisomy is when the offspring received three copies of a chromosome.




Down syndrome is when an individual received three copies of chromosome #21. This is also known as Trisomy 21.




Klinefelter’s syndrome is when an individual receives XXY chromosomes; this results in the male phenotype with enlarged breasts.




Turner's syndrome is when an individual receives one X chromosome; this results in the female phenotype with underdeveloped breasts and degenerated ovaries.



Interactions Of Chromosomes

The human body only needs one X chromosome. Women have 2 X chromosomes, so they inactivate one of them. The inactivated chromosome is called a Barr body.

The presence or absence of the disorder is determined as soon as you are created. X-inactivation happens after zygote formation, and some scientists think that a type of decision making process may occur at the cellular level.

A cat with different colors is very likely a female. This is because the fur color gene is on the X chromosome. At different places on the cat’s body, different X chromosomes are inactivated. Therefore, there are different colors in different spots. 

For a male to have different colors, he would have to have XXY.

Wednesday, January 12, 2022

Law of Independent Assortment and Non-Mendelian Genetics

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Identify patterns of inheritance to predict genotype/phenotype and solve punnett square problems.

Article includes ideas, images, and content from Troy Smigielski (2022-01)

Law of Independent Assortment
and Non-Mendelian Genetics
(Onward and upward?)

So in previous articles, we explored genetics, many concepts of which grew out of Gregor Mendel's research.



Beyond the importance of how traits pass, he discovered something else.

Gregor Mendel also found out that genes are distributed to gametes independently from one another. He called this Law of Independent Assortment. This means that the version passed on of Gene A has no effect on the probability of getting either version of Gene B.

For example, just because a person has brown hair does not mean he or she will definitely have green eyes. Nor does it mean that the eye color will definately be green. Hair color does not determine eye color.


This is because the hair color gene is distributed (assorted) independently from the eye color gene. The gene for any one trait can be passed on with any combination of genes for other traits.

The  Law of Independent Assortment greatly increases genetic variation because it allows for any possible combination of available genes. Combined with the crossover process in meiosis, independent assortment results in the vast range of trait variations genetic diversity that can be observed.

Mendel came to the understanding of independent assortment while doing his famous experiment with pea plants.


In additions to the color of the flower bloom, pea plants exhibit many other traits (including those pictured above). The dominant variations of the trait are at the top.

So, crossing the purebred (homozygous) varieties resulted in heterozygous plants in the F1 generation. When Mendel crossed those heterozygous plants, he ended up with a lot of variations in the F2 generation.

If he looked at any one trait, in the F2 generation, the predicted 1:2:1 genotype and 3:1 phenotype was observed. But many of the other traits would be mixed in.

This is because genes for the various traits are inherited separately from each other (Law of Independent Assortment).

So, would it be possible for a cross between AaYy (axial; yellow) x AaYy (axial; yellow) to produce an axial, green-seed plant?

In order to have an axial plant, there must be at least one dominant "A" gene (AA or Aa). To have green seeds, requires that both of the genes be recessive (yy).

Can that happen?

REMINDER: Each parent will contribute one allele for each gene.

Yes; it is possible because you could inherit an “A” from either parent resulting in the genotype “AA” (axial). 

You could also inherit a “y” from both parents resulting in the genotype “yy” (green). 

This is possible because these genes are inherited separately from each other (Law of Independent Assortment).

We can visualize that with two punnett squares. Because the genes assort independently, you can do two independent Punnett squares to find out if this is possible.


What if you wanted to examine two traits at the same time using a single Punnett square? Is that possible? 



Dihybrid Cross Punnett Squares


It is possible to set up two or more traits in one Punnett square. There's a process…

Recall that to look at one trait, the genotype of each parent is written, one at the top horizontally and one at the side vertically. (See above examples.)

When looking at more than one trait, the genotype will take on a form like…

AaYa

AAyy

AaYY


Now, the same process is used… but with an added level of complexity.


To set up the square, write out the genotype for both, one at the top and one along the side.

In the above illustration, we have two homozygous parents:

AaYy

Number the genes 1 to 4…

1  2  3  4
A a  Y  y

Going along the top, you will put in the genes in this order:

1 and 3

1 and 4

2 and 3

2 and 4

Do the same thing down the side to get what is shown in the image above.

Then, you fill in each box much like with a one trait Punnett square. Take the genes from the top and side and order them into the boxes. You would end up with this:


A dihybrid Punnett square is an effective way to examine the offspring with consideration to two trait at the same time.

Law of Independent Assortment Recap

The Law of Segregation states that alleles separate independently during gamete formation.

Law of Independent Assortment states that genes separate independently during gamete formation.


Non-Mendelian Inheritance

Not all patterns of inheritance follow Mendelian Laws of dominance. There are other other systems by which traits can be passed from the parents to the offspring.

Incomplete dominance is where one allele does not show complete dominance over the other; they meet in the middle. Instead, of seeing the dominant trait, the traits of each allele can be thought to blend.

The most common example of this if crossing a red flower with a white flower and all offspring have pink flowers.

Though the homozygous parents are red and white, the heterozygous offspring are neither. They are a blend of the two.


Codominance is where both alleles appear in the phenotype; they are both expressed. A common example of this is blood type. 
 
If blood type can be homozygous A or homozygous B, the heterozygous version of this is AB. Both traits are present.


You can think of incomplete dominance as blending of one version of the trait with the other. Red wasn't quite dominant enough to fully show up… it partly showed up as a blend. Dominance was incomplete.

For codominance, think of a team with co-captains or a club with co-presidents. You would say Bob and Mary are the co-chairs of our club. Codominance means that both of the versions of the trait show up. The flower is red and white, both. Co = and.


Polygenic traits are traits (physical appearance) that are affected by more than one gene. In actuality, most traits are polygenic. Common examples include height, skin color, eye color, and hair color.

To explain a little more, a trait like height might be made up of… let's say four genes. Each gene would appear as two alleles (versions) of the trait. Height would be the result of all of the genes combined.

Suppose someone's genotype for height looked like…

AaBBccDd

Those four genes would work together to establish the genetic potential for height. Of course, environmental factors such as nutrition would further influence height.

Someone with AAbbCcdd would have a different genetic potential for height.
Someone with aaBbccDD would also have a different genetic potential for height.

In polygenic traits (remember, most traits are polygenic), several genes mutually have a say in the ultimate phenotype that is produced.

Monday, January 10, 2022

Mendelian Genetics And Probability

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Identify patterns of inheritance to predict genotype/phenotype and solve punnett square problems.

Article includes ideas, images, and content from Troy Smigielski (2022-01)

Mendelian Genetics And Probability
(Why does this make me think of Star Wars?)

To get started, let's reprise some of the content from the overview page


Gregor Mendel is a famous scientist who studied the genetics of pea plants. 

He coined the terms “dominant” and “recessive” to describe alleles.

How?

Over the course of seven years, Mendel conducted several experiments that led him to this conclusion.

Check out the overview page for more information on what he found out!

What does this tell us?

Mendel concluded that alleles must separate independently from each other in meiosis so that each gamete (reproductive cell) only carries one allele.

He called this the Law of Segregation.

This is essentially confirming how meiosis works.



Punnett Squares

These days, we can do Mendel's experiment using a Punnett square. Check this out!

Punnett square is a diagram used to predict the genotype and phenotype of offspring.


To do a Punnett Square, put the genotype of one parent at the top and the other parent down the side. Then, make the matches of the alleles. 

In the square to the left of the image, there is a capital P in each box, so all of the flowers expressed the purple trait. 

In the Punnett Square to the right, mixing two Pp flowers allowed for one box to have the pp genotype resulting in a phenotype of being white.

Okay… now, on to using this to make predictions…


Probability and Ratios

Suppose you cross the flowers like Mendel did… What is the probability that you will get a white flower? Obviously, you'd need to know the genotype of the parent flowers, but you also need to know how to go from some "this out of that" number thing, to percentages.

Probability is the odds of a certain event happening. It can be expressed as a "this out of that" chance or as a percentage.
  • What is the probability of a coin flip landing on heads?
  • What is the probability of a coin flip landing on heads four times in a row?
  • What is the probability of a coin flip landing on heads if the previous 6 flips landed on tails?
These are questions that understanding probability will allow you to answer.

Say you flip a coin 6 times. How many would you expect to land on heads? On tails? Since there are only two sides to a coin, it has to come up heads or tails. And it makes sense that either one could come up.

So you'd expect to get 3 heads and 3 tails.

A ratio is a way of expressing the number of one thing compared to the number of another thing. For instance, in water, there is a fixed ratio of hydrogen and oxygen atoms: 2:1. Bicycles have a ratio of 2 tires for every 1 handlebars. Skateboards have a ratio 4 wheels per 1 deck. Birds have 2 wings per 1 beak. Birds also have a ratio of 2 wings per 2 feet.

Usually, you want to reduce your numbers to "lowest terms" so you'd say birds have a 1 to 1 ratio of beaks to wings.

So, when you flip a coin, you can expect to get the same number of heads and tails. Thus, the expected ratio is what you should get in a theoretical situation.

But, what actually happens does not always follow what is expected. It is possible to flip a coin 6 times and get heads 5 times. The actual ratio is a report of what really happened in an actual trial situation.

While some things are bound to fixed ratios (like bicycles having a 2 to 1 wheel to handlebar ratio), other things are free to vary. 


Some things are constrained to a fixed ratio by design (bicycles) or laws of nature (water). 

Other things tend to follow a fixed ratio, but are free to vary. Like flipping a coin.

Where things only tend to follow a fixed ratio, it is possible to get surprising results. It is certainly possible to flip a coin and get "heads" 10 times in a row. However, it is unlikely. Or another way to say that is this:

"The probability is very low."

The more times you flip a coin, the less effect "luck" will have on the out come. You might get "lucky" and flip "heads" three times. Maybe four. But, you are equally lucky to get "unlucky" and flip "tails" the same number of times.

In the long run, the more times you flip the coin, the closer to a 1 to 1 ratio you'll get.

Here, it's worth pausing to add that ratios can be written with a colon. So, 1 to 1 would be written as 1:1.

We will be working with probabilities and ratios when we do Punnett squares, and the same concept applies.

When crossing parents with different genotypes, the more children you have, the closer to the expected ratio you will get.

The diagram used by geneticists to predict gene combinations and probabilities is called a Punnett square. These are commonly used to test for the presence of genetic disorders.




Stem Cells and Research into Genetic Disorders

  • Cystic fibrosis is a disorder that causes the body to produce thick, sticky mucus that can clog up the lungs.
  • Huntington’s disease is a brain disorder that causes uncontrollable movements and loss of thought, and it generally appears around age 30-40.
  • Hemochromatosis is a disorder where too much iron is absorbed in your body.

Scientists are beginning to investigate the use of stem cells to treat some of these disorders and other neurological (CNS or PNS) disorders such as Alzheimer’s disease. 

Stem cells that can develop into any kind of bodily cell are called pluripotent cells. 

The cells that have found their place and function in an organism are referred to as differentiated cells.


Stem cells divide to create division of labor (to divide the workload of the body). A stem cell can differentiate into specific types of cells.

Example of differentiated cells: Neuron


A neuron is a cell of the nervous system. 

The elongated, branched structures of these cells enable them to receive and transmit electrical messages quickly.



Example of differentiated cells: Muscle


Muscle cells are responsible for producing force and motion. 

These cells have filaments that move past each other to change the size of the cell, allowing it to contract and relax.



Example of differentiated cells: Blood


Red blood cells transport oxygen. 

They are disc-shaped, thinner in the center than at the edges. This shape maximizes the surface area available to transport oxygen and allows the cells to pass through the narrowest blood vessels.


Okay… back to Punnett Squares!

As we were saying…

The diagram used by geneticists to predict gene combinations and probabilities is called a Punnett square. These are commonly used to test for the presence of genetic disorders.


If a child has the genotype “ee”, he/she will suffer from sickle-cell anemia.

If both parents are heterozygous for the sickle-cell anemia trait, what are the chances that their child will have the disease? Will not have the disease?

If a child has the genotype “ee”, he/she will suffer from sickle-cell anemia. That is to say that the expressed trait, the phenotype would be the presence of sickle-cell anemia.

So, that will look like this:



So, the phenotype of the offspring would occur in an expected Dominant:recessive ratio of 3:1.

From the Punnett square, you can also find a genotype ratio. Since there are three possible combinations in the above, you can find a ratio of EE:Ee:ee (Think of the colons as "to" so EE to Ee to ee)

The genotype ratio of EE:Ee:ee is 1:2:1.


Now… keep this in mind!

There is a difference between genotype and genotype ratio. Genotype is the genetic makeup of an organism (ex: Ee). Genotype ratio shows the probability of an organism being “EE”, “Ee”, or “ee.” 

The same holds true for phenotype and phenotype ratio.

Two parents are heterozygous for dwarfism (Tt). Since dwarfism is a dominant trait, both parents are dwarfs. Do a Punnett square and determine the probabilities for their children.





Take a moment and figure out the genotype and phenotype ratios. 

Genotype ratio = TT:Tt:tt

Phenotype ratio = Dominant:Recessive


Spoiler alert!
A heterozygous cross will always have a genotype ratio of 1:2:1 and a phenotype ratio of 3:1.

A botanist is crossing a homozygous tall plant (TT) with a heterozygous tall plant (Tt) for fun. A Punnett square can determine the probabilities for the plant children.


Tall stem is dominant, T and short stem is recessive, t…



Looking at the genotype possibilities, you can count:

TT = 2
Tt = 2
tt = 0

Out of 4 plants, then, 50% were TT and 50% were Tt

There are only 2 possible phenotypes (tall or not tall). 

Let's do that ratio thing again!

Genotype ratio = TT:Tt:tt

Phenotype ratio = Dominant:Recessive



The genotype ratio of TT : Tt : tt is 2:2:0.

The phenotype ratio of Tall : not tall is 4:0.

For any cross of  homozygous dominant and heterozygous, the above ratios will be present.

Another one!

One parent is homozygous dominant (RR) for a widow’s peak while the other is homozygous recessive (rr). Since widow’s peaks are a dominant trait, one parent has one while the other does not. Do a Punnett square and determine the ratios for their children.



And that looks like…





The genotype ratio of RR : Rr : rr is 0:4:0

The phenotype ratio of Widow's Peak : No Widow's Peak is 4:0.




Wednesday, January 5, 2022

Genetics Overview

Biology Index

https://www.google.com/search?q=DNA

Where are we going with this? The information on this page should increase understanding related to this standard:  Identify patterns of inheritance to predict genotype/phenotype and solve punnett square problems.

Article includes ideas, images, and content from Troy Smigielski (2021-11)

Genetics Overview
(At least I've heard of this word…)

Genetics is the scientific study of heredity and analyzes genes. Genes produce traits, which are functions or characteristics of an organism.

What type of biomolecule is responsible for storing genetic information? 

You will probably recall that genetic information genetic information is stored in nucleic acid biomolecules, specifically in DNA (deoxyribonucleic acid). 

DNA is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses (Source 2022-01).

Specific sequences of DNA that are passed onto offspring are called genes.


Traits in an organism come from the genes in the DNA, which is organized into chromosomes. Each chromosome carries multiple genes in specific segments of DNA. 

To say that differently, "A gene is a region of DNA that encodes function. A chromosome consists of a long strand of DNA containing many genes. A human chromosome can have up to 500 million base pairs of DNA with thousands of genes" (Source, 2021, 12)

Genes are kept side by side in an organized fashion on structures called chromosomes.




Humans have 46 chromosomes and each one has different genes on it. We are half mom/half dad, but each gene produces only one trait.

How does 50% mom and 50% dad get put into one gene?

Each chromosome contains one form of a gene called an allele


Let's take a moment to more better understand the allele.

Think about the chromosome on which is the DNA code for hair color… That chromosome has a section of the code that contains the hair color code. The sections of chromosome code are called genes. 

So, if you are curious to know, the gene for human hair color is called MC1R. This gene is located on chromosome 16. 
 
So, there are different versions of the hair-color gene. Let's go with black, brown, blond, red and blond, just to simplify our thinking.

The allele is the version of the gene, either red, blond, brown, or black. 
 
In other words, the different variants of a gene are called alleles.


Each gene you have has two alleles; one allele from mom and one allele from dad for every single gene that we have. Sometimes they are the same; sometimes they are different.

If mom gives us an allele coding for brown hair and dad gives us an allele coding for blonde hair, how do we figure out which one wins?

How Traits Are Passed On

Genes contain 2 alleles for traits which are either dominant and/or recessive. It the trait show up in the offspring, it is said to be expressed.

A dominant allele is one that will be expressed if present (capital letter). A recessive allele is one that will not be expressed unless there is no dominant allele present (lowercase letter). In order for a recessive allele to produce a trait, there must be no dominant trait passed on to the offspring.

Okay, let's do an example. Humans have a trait known as a widow's peak.


It is a dominant trait, so the allele for it is represented as a "W." The recessive trait of not-having-a-widow's peak is represented as a "w."

So, here's the thinking process for deciding what trait will be expressed:

• If an organism has at least one dominant allele for a gene, the organism will express the dominant trait.

WW or Ww --> Widow's Peak present

• Even if an organism has one recessive allele for a gene, the organism will still express the dominant trait!

• However, if an organism has two recessive alleles for a gene, the organism will express the recessive trait.

ww --> Widow's Peak not present

The only way to express a recessive trait is to have two recessive alleles. If you have one dominant allele, you will express the dominant trait.




How about we classify genes with some fancy words?
That should be really nice! Science is very specific with its words!

A gene that has two of the same alleles is called homozygous (ex: RR or rr).

A gene that has two different alleles is called heterozygous (ex: Rr).


You will recall that the prefix "homo" means "same." Homeostasis means to keep everything unchagned. Homogenized milk is processed so that all of the molecules are nearly exactly the same size.

The prefix "hetero" means "different." Where the root "geneous" means "origin" or "made up of" a heterogeneous substance is made up of distinctly different things.

So, homozygous means made up of the same versions of the alleles.

And, heterozygous means made up of different versions of the alleles.


Being heterozygous is also referred to as being a carrier. Even though the trait is not expressed, it is carried recessively.

There are times when being a carrier, even if the recessive trait is not desirable, is advantageous. 


Sickle Cell is a dangerous, recessive trait. However, a person who is a carrier for sickle-cell anemia is also resistant to malaria! 
 
In the following image, on the bottom row, the two in the middle are resistant to malaria, but do not have the disease. The one on the left side of the image does NOT have resistance to malaria.





Genotypes and Phenotypes
Oh… so more fancy terms!

The genotype is the genetic makeup of a gene (ex: Bb).

The phenotype is the physical, observable characteristic of an organism (ex: brown hair).

Genotype causes phenotype! Your genes cause you to look a certain way.



Mendelian Genetics

Gregor Mendel is a famous scientist who studied the genetics of pea plants. 

He coined the terms “dominant” and “recessive” to describe alleles.

How?

Over the course of seven years, Mendel conducted several experiments that led him to this conclusion.

Mendel wanted to know how traits are passed to offspring. To do this, he crossed a purebred purple flower with a purebred white flower.


(Purebred refers to a homozygous organism (PP or pp).)

He hypothesized that one color was a dominant trait while the other was a recessive trait, but he wasn’t sure which was which.

After crossing, he found that all of the offspring had purple flowers. This means that purple is dominant. This means purple flowers have the genotype PP or Pp while white flowers have the genotype pp.


This thought of dominant and recessive made sense, but what happened to the white flowers? Would that color ever come back in this family line?

To find out if the recessive allele was still there, Mendel self-crossed the offspring.

After self-crossing, 1 flower was white and the remaining 3 were purple. The recessive allele was still present!!



In genetics, the first generation of offspring is denoted by F1 and the second generation is F2.


What does this tell us?

Mendel concluded that alleles must separate independently from each other in meiosis so that each gamete (reproductive cell) only carries one allele.

He called this the Law of Segregation.

This is essentially confirming how meiosis works.






Punnett Squares

These days, we can do Mendel's experiment using a Punnett square. Check this out!

A Punnett square is a diagram used to predict the genotype and phenotype of offspring.


To do a Punnett Square, put the genotype of one parent at the top and the other parent down the side. Then, make the matches of the alleles. 

In the square to the left of the image, there is a capital P in each box, so all of the flowers expressed the purple trait. 

In the Punnett Square to the right, mixing two Pp flowers allowed for one box to have the pp genotype resulting in a phenotype of being white.