Not completely understanding what was occurring he stated to do what he could. Count.
He did nearly 20,000 crosses and started to notice a predictable pattern or Probability (table 12.1) or percent chance that a trait would “be observed” in any future generation.
Mendels noticed that Dominent purple plants still could have offspring that are white ressive flowers! That’s not very “dominent”. Becuase traits seemed from mixes parents carrying latent traits, Mendel created
Phenotype: The observable trait expressed by an organism.
Genotype: The underlying actual genetic make up of an organism.
Since there were two possibiliites they were two genes being inherited for “a” singular trait, thus gene variants of the same trait. He called these alleles.
Go back to Meiosis. Single copies from each parent in an offspring = two copies, but an organism can only “use” one copy, or read from one genetic instruction to function and “carry” the other to be inherited.
All these “combos” greated two outcomes.
Homozygous- having two identicle alleles.
Heterozygous- having two different alleles.
We know know there are great variances, but for us we’ll keep it “simple”.
Remember what a zygote is from our cell division chapters?
Lets apply the vocab
PO hybridization of: a Purple homozygous phenotype
a White homozygous phenotype
F1 Generation produces all purple offspring.
F2 Generation is a cross between only F1 hybridized plants.
Results are approximately 3 purple to 1 white offspring,
a 3:1 ratio.
He did this nearly 20,000 times.
The ratios stayed consistent! He stated to “predict”…
Medel startd to predict the outcome of known parents offsprings for multiple of generations. The emperical evidence mounted!
To do this a handy Punnet Square was invented by R. Punnett. We now know that the genes Mendel was tracking were now called “autosomes”, meaning that they are not “sex related” genes, or specific to male or female biology. More on this later…
Single trait cross is a ”mono-” = 1, monohybrid.
Note: two separate offspring with same heterozygous Yy, thus the observed is 3 yellow, ~75 % and 1 green, ~25%
Later self F2 crossing shows that 50% were Yy.
A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.
Great! Neat. Mendel and future genetics research about inheritable traits is awesome… I love my Labrodoodle. Thanks Mendel!
So what does that all mean?
Predictable frequencies result in genotype ratios we call
Pedigree- a known inheritance pattern
Human Genetic counlsing: risks of disorders? (not “disease”)
Human Genetic counlsing: risks of disorders?
The son of a woman who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected. A daughter will not be affected, but she will have a 50 percent chance of being a carrier like her mother.
And oh down the ”genetic rabbit hole” we can follow Allice and Wonderland of genetics.
Genetics as a science gets complicated-
There are alternatives and further layers of complexity beyond a simple monohybrid cross.
incomplete dominance- the expression (what’s seen= phenotype) of two contrasting alleles.
Codominance- both alleles for teh same characteristic are simultaneiously expressed in a heterozygote.
Wild type- denotes the most common phoneotype or genotype in a population
Lethality- if an inheritable trait is expressed it is lethal, or the allele is dysfuntional in providing the organism with biological funtion(s) and the organisms dies.
Sex linked traits.
In humans, the sex chromosomes carry information that directs a growing fetus to develop as a male or a female.
Male if the Y chromosome is present
Female if there is no Y chromosome
Human Sex determination depends on the sex chromosome inherited from the father.
The sex of the offspring is determined in a variety of ways in other species.
A variety of methods are used for sex determination in animal and plant species including:
The presence or absence of sex chromosomes
The number of chromosome sets
Mendel… What a guy!
Law of segregation: that paired unit factors (we now call genes) must segregate equally into gametes (sperm/eggs) such that offspring haen an equal likelihood (% chance ever cross of parents) to inheret either factor (gene).
“segregate equally” into “eggs/sperm” = describing meosis!
Humans: boy or a girl? It’s 50/50 everytime.
Law of independent assortment: genes do not influence each other with regard to the sorting of alleles into gametes (eggs/sperm). Every possible combination of alleles for every gene is equally likely to occur.
(law, not a theory).
We have learned that there are exceptions now: “Linkage” and much more… Take a genetics class
Law: An empirical generalization; a statement of a biological principle that appears to be without exception at the time it is made, and has become consolidated by repeated successful testing; rule
Theory: The grandest synthesis of a large and important body of information about some related group of natural phenomena
Chapter 13 – Chromosomes.
Mendel talked about genes and traits, but later in the 1900 with more advanced microscopes it was discovered that what is “inherited” is not individual traits or “genes” but larger packets of DNA with ALL the genes in them. These were called Chromosomes.
Chromosomal Theory of Inheritances is what we consider today. It has gone on to explain and help organize species as per their DNA make up.
It has also given Science the ability to create chromosomal maps. It’s the beginning of “Modern Biotechnology” covered in chapter 17.
This karyotype is of a female human. Notice that homologous chromosomes are the same size, and have the same centromere positions and banding patterns. A human male would have an XY chromosome pair instead of the XX pair shown. (credit: Andreas Blozer et al) (fig 13.5)
Learning objectives: Tying together the vocab
Control the “Crosses” of traits. You can “count” the outcomes:
Thus conceptualize and predict ”probability” of genes passed into generations
Probability creates creates the concept of dominant and recessive genes (traits) and allows us to predict a % concept of “alleles”.
Mixing alleles forms concepts of the genetic mix or “genotype” = homozygous, heterozygous (of a zygote)
Genotype and Probability (% chance) results in the inheritble qualities of each trait’s phenotype being “observed” and this is due to it’s “dominance or recesive” as concept of “what is present” in the genotype.
Combining genotype and phenotype= What is seen (phenotype) in living organisms (genotype- inheritable DNA) that sexually reproduce, usually by the cellular process of division called Meiosis and Mitosis (for some single celled organisms).
Chapter Learning Objectives
Single to multicellular
Basics of Genetics
Importance of Genetics and genetic diversity in understanding Evolution by Natural Selection.
See the info represented differently
(not from our book)
Some of this will be covered
in next chapter about DNA
Cancer: Unrestrained cell growth and division in multicellular organisms. (unregulated cell division) …can lead to tumors… …the second leading cause of death in the United States! Cancer cells have several features that distinguish them from normal cells, including…
Cancer cells have several features that distinguish them from normal cells, including:
1. They lose their “contact inhibition.” That is, most normal cells divide until they bump up against other cells or collections of cells (called tissues). At that point, they stop dividing. Cancer cells, however, ignore the signal that they are at high density and continue to divide.
2. They can divide indefinitely. Normal cells can divide approximately 50 times. After that point, they may continue living but they lose the ability to divide. Cancer cells, on the other hand, never lose their ability to divide and continue to do so indefinitely, even in the presence of conditions that normally would halt the cell cycle prior to the initiation of a cell division.
3. Cancer cells have reduced “stickiness.” Cells are normally held together by adhesion molecules, proteins within cell membranes. And cancer cells, too, usually group together, forming a tumor. But the membranes of cancer cells tend to have reduced adhesiveness, causing them to stick to each other less than do non-cancerous cells.
Segregation: two copies of each gene but put only sperm one copy in each or egg.
A dominant trait masks the effect of a recessive trait.
Each parent puts a single set of instructions for a particular trait into every sperm or egg.
The instruction set is called a gene.
The trait observed in an individual depends on the two copies (alleles) of the gene it inherits from its parents.
Here’s where Mendel’s meticulous and methodical experiments paid off. First, he started with true-breeding white-flowered plants. Then he got some true-breeding purple-flowered plants. He wondered: Which color wins out when the white-flowered plant is crossed with the purple-flowered plant? The answer was definitive: purple wins. All of the offspring were purple, every time. For this reason, Mendel called the purple-flower color trait dominant, and he considered the white-flower color trait to be the recessive trait. In general, a dominant trait masks the effect of a recessive trait when the individual carries both the dominant and the recessive versions of the instructions for the trait.
Observing an individual’s phenotype is not sufficient for determining its genotype.
The outward appearance of an individual is called their phenotype.
Underlying the phenotype is the genotype.
This is an organism’s genetic composition.
Things are not always as they appear. Take skin coloration for example. Humans and many other mammals have a gene that contains the information for producing melanin, one of the chemicals responsible for giving our skin its coloring (An albino giraffe stands out from its peers). Unfortunately, there is also a defective, non-functioning version of the melanin gene that is passed along through some families. An individual who inherits two copies of the defective version of the gene cannot produce pigment and has a condition known as albinism, a disorder characterized by little or no pigment in the eyes, hair, and skin. It is impossible to tell whether a normally pigmented individual carries one of the defective alleles just by looking—one would need to do a genetic analysis to discover this information.
The inability to deduce an individual’s genetic makeup through simple observation is a general problem in genetics: Physical appearances don’t always exactly reflect the underlying genes. A normally pigmented individual may have two copies of the pigment-producing allele or they may have only one copy. In either case the individual will look the same. The outward appearance of an individual is called their phenotype. A phenotype includes features visible to the naked eye such as flashy [correct word?] coloration, height, or the presence of antlers. A phenotype also includes less easily visible characteristics such as the chemicals that an individual produces to clot blood or digest lactose. An individual’s phenotype even includes the behaviors it exhibits.
Underlying the phenotype is the genotype. This is an organism’s genetic composition. We usually speak of an individual’s genotype in reference to a particular trait. For example, an individual’s genotype might be described as “homozygous for the recessive allele for albinism.” Another individual’s genotype for the melanin gene might be described as “heterozygous.” Occasionally, the word genotype is also used as a way of referring to all of the genes that the individual carries.
How do we analyze and predict the outcome of crosses?
Genotypes: Homozygous Dominant & Heterozygous
Assign symbols to represent the different variants of a gene.
Generally, we use an uppercase letter for the dominant allele and lowercase for the recessive allele.
It is not always possible to determine an individual’s genotype from its phenotype.
A recessive allele’s effects may be masked by a dominant allele.
Genetic analysis makes use of clever experiments and Punnett squares.
We can trace the possible outcomes of a cross using a Punnett square.
In above picture, we illustrate the cross between a true-breeding pigmented individual, PP, with an albino, pp. On the top of the square we list, individually, the two alleles that one of the parents produces, and on the left side of the square we list the two alleles that the other parent produces. We split an individual’s two alleles (remember segregation) up because, although the individual carries two alleles, there is only one of the alleles in each sperm or egg cell that they produce.
In the four sections of the Punnett square, we enter the genotypes of the possible offspring resulting from our cross. In the cross illustrated in Figure 7-11a, every possible offspring would be heterozygous and would be normally pigmented because it receives a dominant allele from the pigmented parent and a recessive allele from the albino parent.
In Figure 7-11b, we trace the cross between two heterozygous individuals. Note that each parent produces two kinds of gametes, one with the dominant allele and one with the recessive allele. This cross has four possible outcomes: one quarter of the time the offspring will be homozygous dominant (PP), one quarter of the time the offspring will be homozygous recessive (pp), and the remaining half of the time the offspring will be heterozygous (Pp). Phenotypically, three-quarters of the offspring will be normally pigmented (PP or Pp) and one quarter will be albino (pp).
Any gamete produced by an individual heterozygous for a trait has a 50% probability of carrying the dominant allele and a 50% probability of carrying the recessive allele.
Chance is important in genetics.
Probability has a central role in genetics for two reasons:
The first is a consequence of segregation (each gamete receives only one of the two copies of each gene).
The second reason is that fertilization, too, is a chance event (impossible to know which allele goes into which gamete).
All of an individual’s sperm or eggs are different.
Any of these gametes may be the gamete involved in fertilization.
If a male is heterozygous for albinism (Aa) and a female is homozygous for albinism (aa), what is the probability that their child will be homozygous for albinism (aa)?
In the albinism cross, two events also must occur to produce a homozygous recessive offspring. First, the mother’s gamete must carry the recessive allele (“a”, the only type of allele she carries as an albino) and, second, the father’s gamete must carry the recessive allele (“a”). The overall event of a homozygous recessive offspring is 1 * 0.5 or 0.5. This is a general rule when determining the likelihood of complex events occurring: If you know the probability of each component that must occur, you simply multiply all of them together to get the overall probability of that complex event occurring (Figure 7-12 Using probability to determine the chance of inheriting albinism).
Similarly, it can be possible to determine whether a trait is carried on the sex chromosomes or on one of the non-sex chromosomes (i.e., the autosomes). Traits that are controlled by genes on the sex chromosomes are called sex-linked traits. Recessive sex-linked traits, for example, appear more frequently in males than females, whereas dominant sex-linked traits appear more frequently in females. These patterns may become obvious only upon inspection of a large pedigree.
The complete version… You don’t need to know the steps in details.
So what’s the big deal? Why go through all this…
FIGURE 6-20 Meiosis: generating reproductive cells, step by step.
In species with two sexes, females produce the larger gamete and males produce a smaller gamete.
Male and female gametes both end up with just one copy of each chromosome
Why the size difference in the male/female gamete?
Female Oocyte (egg) Male Sperm
Unequal distribution of cytoplasm results in one large egg
The first division occurs just as described above, except that in telophase 1 as the cell divides, instead of forming two identically sized cells, the division is lopsided. The genetic material is evenly divided, but nearly all of the cytoplasm goes to one of the cells and almost none goes to the other. Then, in the second meiotic division, there is again an unequal division of cytoplasm. As in the first division, one of the cells gets nearly all of the cytoplasm and the other gets almost none. The net result of meiosis in the production of eggs is one large egg and three small cells with very little cytoplasm, which degrade almost immediately and never function as gametes.
Unequal distribution of cytoplasm results in one large egg.
Mitosis leads to duplicate cells.
Parent cells daughter cells
The number of somatic cells that must be replaced by mitosis every day is huge.
The rate at which mitosis occurs varies dramatically.
About 40,000 skin cells shed per minute.
Cells lining your small and large intestines replace themselves every 3 weeks.
a Red Blood cell last about 6 weeks. ~ 5 million per/replace
The ultimate result of mitosis and cytokinesis is the production of two genetically identical cells.
Mitosis occurs in just four steps. It cannot begin, however, until after an important event occurs during the previous portion of the cell cycle, interphase. During the synthesis portion of interphase, all of the chromosomes replicate. Mitosis then begins with 1) the condensing of the chromosomes, which during interphase are all stretched out and stringy. 2) Next, all of the duplicated and condensed pairs of chromosomes move to the center of the cell. 3) Each chromosome is pulled apart from its duplicate. 4) And finally, new cell membranes form around each complete set of chromosomes and the cytoplasm duplicates as well. Where once there was one cell, now there are two (Figure 6-10 A simplified introduction to mitosis).
Eukaryote chromosomes are linear (usually). So why does DNA wrapped up into chromosomes they look like the letter “X” in pictures? Because they do! Because there are copies (alleles)
A chromosome and its identical replicated copy, joined at the centromere.
At this point, each chromosome looks like the letter X. This appearance is misleading; Chromosomes are not actually X-shaped. They are linear. Each X consists of two identical linear DNA molecules—a chromosome and its identical, replicated copy—joined at the centromere.
Each of the identical DNA molecules is called a chromatid; together, the two are called sister chromatids.
The reason for the X shape in most photos of chromosomes is that the only time the chromosome is coiled tightly and thus thick enough to be seen (and photographed) is during cell division.
Four steps to Mitosis: Review in Book. I only want the “summery of each step”.
1. Mitosis: Prophase stage. Because a lot of room is required for the duplicated chromosomes to separate, membrane around the nucleus is dismantled and disappears near the end of this first stage of mitosis. At the same time, a structure called the spindle is assembled. The spindle, which is part of the cell’s cytoskeleton, can be thought of as a group of parallel threads stretching across the cell between its two ends, or poles—where the threads connect at each pole to a structure called the centriole.
These threads (known as spindle fibers) pull the sister chromatids to the middle of the cell and will eventually be used to pull the chromatids apart as cell division proceeds.
2. The chromosomes congregate at the cell center. After condensing, the chromatids appear to move aimlessly around the cell, but eventually they purposefully move toward the center. Eventually, all of the pairs of sister chromatids (the Xs) convene in the center. Called metaphase, this is like half-time of meiosis. They all line up in an orderly fashion, straddling the center, at what is called the metaphase plate. The chromatids are at their most condensed during this part of mitosis.
3. The chromatids separate and move in opposite directions. At end of metaphase, the pairs of sister chromatids are all simultaneously pulled apart by the spindle fibers. From each pair of sister chromatids, one strand of DNA is pulled in one direction and the other, identical strand is pulled in the opposite direction. This process is called anaphase and it leads to one full set of the chromosomes going to one side of the cell and another identical full set going to the other side. These chromosome sets will eventually reside in the nucleus of each of the two new daughter cells that result from this cell division.
4. New nuclear membranes re-form around the two complete chromosome sets. Finally, since two full and identical sets of chromosomes are collected at either end of the cell, the cell can now divide into two genetically identical cells. In this last step, called telophase, the chromosomes begin to uncoil and fade from view, the nuclear membrane is reassembled, and the cell begins to pinch into two.
In a process called cytokinesis, the cell’s cytoplasm is also divided into approximately equal parts with some of the organelles going to each of the two new cells. When cytokinesis is complete, the two new daughter cells, each with an identical nucleus, enter interphase and begin the business of being a cell.