Patterns of Inheritance

Gregor Mendel

Back in the 19th century, an Austrian Monk named Gregor Mendel had a lot of time on his hands and so he decided to investigate how to grow pea plants, like the ones shown here.  He wanted to determine the inheritance of different traits and so he investigated how individual genes were inherited by breeding these plants and making very careful observations on the parents and offspring.  He bred 100s of generations of these plants. He noticed patterns of inheritance that he described in his two laws: The Law of Segregation and the Law of Independent Assortment. His work wasn’t really appreciated until 50 years after he published it. I don’t think that Mendel’s obscurity was due to the difficulty of his research. Instead, I think that many biologists weren’t ready for Mendel’s work; at least not until they understood chromosomes and the role that they play during meiosis and fertilization. That understanding didn’t come until the early 20th century.

Vocab

Let’s set up some terminology that will become very important for answering questions on genetics.  When students have a misunderstanding of those Punnett Squares, I find that it usually comes from the misunderstanding of a vocab term.  The questions is asking this, but you think it’s asking that because you don’t really know the definition of one of the words in the question.

OK, so, you and I are both humans, which means that we both have the same genes.  You might be thinking: Amy, you and I definitely have different genes because we are not identical twins.  Yeah, but the word gene just means location on a chromosome.  On my 3^rd chromosome, at a certain location, I have a gene for making me have two arms, let’s say.  You also have that gene at that same place.  In face, every human does, that’s what makes us human.  But, your gene says to make long arms and my gene says to make short stubby arms.  There are different versions of these genes.  These different versions are called alleles.  So, Mendel, looked at the gene for plant height and there were two variations of it or two alleles: one for making a tall plant and one for making a dwarf plant.  In this class, if you go to use the word gene, you probably should be using the word alleleDifferent versions of a gene that determine specific traits..  Get used to it.

You may recall that we established in an earlier module that your chromosomes are like socks in that they come in pairs.  Instead of left and right, you have a chromosome from Mom and one from Dad.  So, this means that for any one gene, or any one location on a chromosome, you would actually have two DNA sequences for that gene, not one, as shown in the picture above.  The combination of alleles, for any one gene, is called your genotype.  Here, we seems to have two alleles that are different, but, obviously, you can have two alleles that are the same, either the green pieces of DNA in the picture above or the orange pieces of DNA.  But, there is also a word we need to get to know and that is phenotype.

This is their phenotype.  It’s actually more than your physical appearance, like a widow’s peak or frecklesSmall, flat patches of increased melanin concentration in the skin., it’s your blood type, your propensity for cardiovascular disease, and much more.  A student once told me they remembered this because phenotype and physical appearance both begin with ph.  If your trait is genetic, or determined by your genes, you have a genotype or DNA on your chromosomes that will determine this phenotype.  For these plants here we have two phenotypes or appearances that are possible based on the alleles that exist: tall plants or dwarf plants.  So, we get the sense that genotype determines phenotype, and yes, it does.

Homologous chromosomes

Although Mendel couldn’t possibly have known it at the time, he was describing the behavior of homologous pairs of chromosomes during meiosis and fertilization. Remember that chromosomes are long pieces of DNA, each one typically containing hundreds of genes. Humans have 46 chromosomes; pea plants have 14. These chromosomes come in matched sets called homologous pairs. Humans have 23 homologous pairs of chromosomes and pea plants have 7 homologous pairs.  You have to think of homologous pairs like socks.  No one ever says: I have 46 socks.  Well, my husband might because he is one of those people that buys a 20-pack of socks that are all the same.  I, on the other hand, have pairs of socks that feel different and thus, I will not mix and match from the pairs.  SO, I would say that I have 23 pairs of socks.  This is how you refer to your chromosomes: 23 pairs.  This is like insurance, if one chromosome doesn’t work, hey, no problem, you have another one.  It’s like that with all the important organs: kidneys, eyes, lungs.  One goes bad, and the other is like: I got this. 

So, don’t you forget this.  Mendel didn’t have the benefit of knowing about chromosomes, but you do. 

Mendel’s Law of Segregation

I love this picture here.  I took it from an animation that is (sadly) no longer on the interwebs.  This illustrates the Law of Segregation.   Let’s look at Dad and Mom there.  Next to them is a picture of their 46 chromosomes, paired up into the homologous chromosomes.  We can see that Dad has 23 pairs and we can assume that the green chromosomes came from one parent and the red from another.  Mom has 23 pairs with one set blue from one parent and the other yellow set from the other parent.  When they go to reproduce, look, the father donates on of each pair.  Whether it’s green or red is just a 50/50 chance.  Same with Mom.  Note the combination of chromosomes and the colors for the baby.  This picture explains it in a way words can’t.

Mendel proposed that for each characteristic he studied, a pea plant had two alleles (or alternate forms of a gene), one obtained from the plant’s mother and one from the father. Mendel’s Law of Segregation states that these two alleles separate from each other when gametes (sperm or egg) are produced.   That is, each sperm or egg gets only one allele of a given pair.   In short, Dad could give the green or red, but not both.  What is the probability of giving green or red? 50/50.

Law of Independent Assortment

Now let’s talk about Mendel’s Law of Independent Assortment. Mendel stated that when more than one trait was considered during a genetic cross, the inheritance of each trait was independent of the other traits. For example, whether a pea plant had purple or white flowers had nothing to do with whether it was short or tall. Flower color and height were inherited independently. In factA statement based on direct observation that is repeatedly confirmed., he found that this relationship held for all seven traits that he studied. To put it in terms of chromosomes, each pair of alleles segregates independently of the other pairs during gamete A haploid reproductive cell (sperm or egg). formation.  Referring to this picture again, Dad will give green at chromosomes 1 and that has no bearing on whether or not he gives green at chromosomes 2.

This is like tossing two coins at once. One coin has a different allele for flower color on each side, and the second coin has a different allele for height on each side. Every time a gamete is formed by meiosis, the two coins are flipped and the outcome determines the genotype of the gamete. Neither coin affects the other, so any combination of heads and tails is equally likely.  This applies to human traits like freckles and a widow’s peak.  Whether or not you have freckles has nothing to do with whether or not your have a widow’s peak.

Mendelian traits

The Law of Segregation applies to both Mendel’s genetics and more complex genetic relationships.  Mendel’s principle of Independent Assortment works as long as you are referring to genes that are on separate chromosomes. Mendel developed his theories concerning inheritance by observing genes that have only two alleles and that these alleles have a dominant/recessive relationship.  This accounts for some easily observed traits in humans such as having a widow’s peak or not, having freckles or not, and having detached or attached earlobes. 

Phenotypes can be dominant or recessive.  How can you tell?  You can’t.  You are told.  The dominant-recessive relationship of genetics is predetermined and you will always be told which is dominant and which is recessive.  Questions on the lab will set you up with this information.  It will be your job to get this info from the word problems.  One more thing…dominant doesn’t always mean more common or more popular.  In fact, polydactyl, or having 6 digits on a hand or foot, is a dominant trait.  However, it is not more common.

Mendel’s pattern of inheritance is a simple one

Let’s recap before we end.  These are the rules for Mendel’s patterns of inheritance.  Just read them quickly here as a reminder before I do the next slide, which is about exception to Mendel’s rule.  There are always exceptions.

Exceptions

Some genes are located near each other on the same chromosome, a situation called gene linkage. Linkage is actually a big exception to Mendel’s Law of Independent Assortment, since the can be inherited because they are on the same chromosome.  We see this with pigmentation traits in humans.  We have some common pigmentation schemes like the light-complexioned, light hair, blue eyes of the Swedish and the dark pigmentation, dark eyes, and dark hair of African populations.  Yes, genes for pigmentation can be inherited together.  BUT…crossing over in meiosis I can break the linkage between genes.  Remember that crossing over was the exchange of DNA between homologous chromosomes in Meiosis I.  This exchange will happen for the one chromosome carrying pigmentation genes, exchanging maybe one of a few of the pigmentation genes. 

Like gene linkage, nearly all of the exceptions that geneticists have found to Mendel’s basicA solution with a pH above 7, having a lower concentration of H⁺ ions. laws are due to the behavior of chromosomes. One really interesting set of exceptions that you could explore further on your own is sex-linked traits. In such cases, genes located on the sex chromosomes or the X and Y chromosomes.  These chromosomes follow unusual patterns of inheritance, they are passed down in meiosis and fertilization like all other chromosomes, but the relationship between the X and Y chromosomes is just a little bit different than the dominant recessive relationship.  Think about the X and Y chromosomes for a minute; the X has a lot more DNA than the Y.  There are genes on the X chromosome that are not on the Y chromosome like the genes for hemophilia, a blood disease, or the genes for color-blindness.  Therefore the XX combination of females produces two chances to get a gene that gives you normal color-sight, but with the XY combination of men they only have one chance to get the normal color-sight allele.  This is why there are more men who are color-blind than women; it’s just the math of probability.

At first, such cases seemed like exceptions to Mendel’s laws, but further investigation revealed that the chromosomal basis of inheritance can explain these apparent exceptions as well.