I spent quite a bit of time studying corn genetics trying to figure out why a given trait kept doing strange disappearing acts. I finally located some information on the web that helped to understand. I'm going to try to reduce it to simple terms so others can grasp the way it works. If you want to look up some of the detailed steps, Meiosis, Mitosis, Gametophyte, Pollen, Egg, and Oogenesis are some good words to look up for meaning. To make this understandable, I am not going to talk about exceptions. Several of the statements below are true most of the time, but there will be times that they are not true because more than one gene is involved in a given effect.
1. Production of the embryo starts with a single megasporocyte. This is a massive cell that develops from a regular diploid cell near the surface of the cob. This cell goes through the preliminary stages for division by fully duplicating both sets of chromosomes. (has 2 complete centromere joined copies of the dna of the maternal plant, i.e. fully duplicated and ready to divide). The megasporocyte then divides and divides again (this is meiosis which reduces the total number of chromosomes per cell) to form 4 haploid megaspores. Here is where it gets tricky. Three of the megaspores disintegrate and disappear. One of them begins to divide again ( this is mitosis which results in duplication of a complete cell) first forming 2 duplicate haploid cells, then divides again forming 4 duplicate haploid cells, then divides again forming 8 duplicate haploid nuclei but they are in only 7 cells because two of the nuclei are held in a single large cell that will become the endosperm.
2. The protoembryo now contains 7 cells with 8 nuclei of which one is a germinal cell, one has 2 nuclei and will become the endosperm, two are synergids which attract the pollen nucleus, and three are antipodal and inactive. All nuclei have a single identical set of chromosomes that are chosen more or less at random from among the double set that makes up the nuclear dna in the maternal corn plant. If the plant has 1 gene for yellow kernels and another for white kernels, then the ovules on the cobs on that stalk will more or less have equal numbers of white gene and yellow gene protoembryos.
3. One of the single nucleus cells formed in the protoembryo can be fertilized by pollen. This cell will become the future plant when the corn kernel germinates and grows. One large cell has two nuclei and can be fertilized to become the endosperm of the developing kernel. The endosperm cells will form the part of the kernels that you can see. They have absolutely nothing to do with the genetics of the resulting embryo which will grow, but they have everything to do with the kernel of corn you will see on the cob.
4. In the tassels, a similar process (meiosis to turn one megasporocyte into 4 haploid pollen cells) forms each pollen grain with only 1 set of chromosomes. When pollen lands on the silk, a tube develops down the silk to the ovule. As the pollen grain moves down the silk, it splits into two identical sperm cells, one moves to the embryo and fertilizes it, the other merges with the endosperm cell giving it 3 complete sets of chromosomes. The two synergid cells are vital to the process of pollination. Once pollination is complete, they degenerate and disappear.
Now lets look at the genetics. The embryo is diploid which means it has 2 sets of chromosomes, one from the mother (ovule), and one from the father (pollen). We can figure out the genetics fairly easy because there are only two sets of chromosomes. Using the yellow/white example, if an embryo has two genes for yellow, then if you grow that kernel into a plant, it will produce yellow corn seed unless some other dominant color comes from the pollen plant. If it has two genes for white, then the offspring will always have at least one gene for white, but it may be masked by a gene for a color such as yellow or red because these colors are dominant. If the embryo has one yellow and one white chromosome, then the offspring could be either color depending on the pollen parent. And this is important, the corn ear will have a mix of colors, some with white and some with yellow.
The endosperm is a different ballgame entirely. It is triploid. It has 3 sets of chromosomes. You have to do a different kind of math to understand it. For example, the maternal plant may have two genes for red kernels. If the endosperm receives a sperm from pollen that has yellow, then the resulting endosperm will have some yellow and some red making it look yellowish red. The genetics will be red/red/yellow meaning that there are three chromosomes, two have genes for red and one has a gene for yellow. While this example is pretty cut and dried, the effect of having different combinations in the endosperm can be crucial in sweet corn breeding. For example, the "dull" gene can make a kernel of corn slighly sweeter all the way up to supersugary depending on whether the endosperm contains one du gene or three du genes.
Now lets see what we will find if we make a cross with corn using some diverse genetics. Lets say we grow some white dent corn and we cross it with a yellow sweet corn. We can use either variety as the female parent in this example. Lets say we use the sweet corn as the female and let the white dent corn produce pollen. The sweet corn kernels will be fertilized by a dent corn which happens to have the gene to produce starch. Therefore when we save the seed from our sweet corn ear, it will not look like sweet corn. It will look more like dent corn with normal starch formation.
Lets plant some of our F1 seed which genetically has a chromosome for sweet paired with a chromosome for dent and another chromosome for white paired with a chromosome for yellow. How will these segregate? We grow the corn to maturity letting it self-pollinate. The resulting kernels will have embryo's that are in a ratio of 1:2:1 or one yellow/yellow, 2 yellow/white, and 1 white/white. But we CAN'T SEE this by looking at the endosperm, it is just in the embryo. The endosperm will have the following combinations.
1 - white/white pollinated by white - w/w/w
1 - yellow/yellow pollinated by white - y/y/w
1 - white/white pollinated by yellow - w/w/y
1 - yellow/yellow pollinated by yellow - y/y/y
You can usually tell a slight difference in the kernels because the colors will grade from white to pale yellow, medium yellow, and to deep yellow.
What about sweet vs dent? The same type genetics apply.
1 - sweet/sweet/sweet - s/s/s(shriveled sweet kernel)
1 - sweet/sweet/dent - s/s/d(looks dent)
1 - dent/dent/sweet - d/d/s(looks dent)
1 - dent/dent/dent - d/d/d(looks dent)
Notice that this time you can't really tell the difference between the kernels because the gene for dent completely masks the su gene for sweet corn. So what do we see on the ear of corn for the above? 3/4 of the kernels will be dent and 1/4 will be sweet. Of the 1/4 that are sweet, 3 will be shades of yellow and one will be white in a ratio of 1:1:1:1. Of the 3/4 that are dent, 1/3 will be homozygous dent and will breed true, but 2/3 will have a hidden sweet gene that can show up in future growouts. The color genes will follow the 1:1:1:1 ratio same as for sweet kernels.
There are 4 commonly used genes in sweet corn breeding and a few others that can be used.
se - chromosome 2 commonly called Sugar Enhanced
sh2 - chromosome 3 commonly called Shrunken 2
su - chromosome 4 commonly called Sugary
bt - chromosome 5 commonly called Brittle
du is a minor gene that can be used to enhance sweetness but is rarely used. It is on chromosome 10.
DarJones
Links to help understand what is happening:
books.google.com/books?id=qtboPf9eeUkC&pg=PA109&lpg=PA109&dq=meiosis+polar+nuclei&source=bl&ots=Ri94SOsP-c&sig=xG0iI1Om--5qlyRwKnG9Lbi1QuM&hl=en&ei=QIHBTOq0H9j9lAe63pjPDA&sa=X&oi=book_result&ct=result&resnum=9&ved=0CDUQ6AEwCA#v=onepage&q=meiosis%20polar%20nuclei&f=falseThis one is about paramutation. Really really odd!
www.biology-online.org/articles/purple-corn-rna-break-genetic.htmlAnd here are the transposons. From the description, Cherokee Squaw is an excellent example of transposons in action.
www.carolina.com/category/teacher+resources/classroom+activities/corn+genetics+hip+hop+genes.doThis one will keep you reading for hours.
www.maizegdb.org/Here is a patent that makes fabulous reading once you understand the terminology.
patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4,630,393.PN.&OS=PN/4,630,393&RS=PN/4,630,393
This one is a sweet corn breeders handbook.
www.globalsciencebooks.info/JournalsSup/images/0706/IJPB_1%281%2927-30o.pdfThis discussion of gama grass teosinte maize hybrids is an excellent resource.
www.maiscoltura.it/maydica/articles/51_315.pdfA reasonably up to date monograph about corn's origins. Still has quite a few errors repeated from years past, but overall gives an excellent picture of corn today. Click the "preview" link to see the book.
books.google.com/books?id=eDJ3NjHh8H8C&printsec=frontcover&source=gbs_atb#v=onepage&q&f=falseThis one is a phenomenally good discussion of the origin of corn from a genetic basis.
www.bio-nica.info/biblioteca/Buckler2005MaizeOrigins.pdfHere is a good general discussion of corn vs teosinte morphology and origin.
sfmatheson.blogspot.com/2007/10/they-selected-teosinteand-got-corn.htmlAnd this link will get you to an excellent resource covering maize and its relatives.
teosinte.wisc.edu/Here is an outstanding link covering the cross of maize and tripsacum and resulting genetic segregation.
www.genetics.org/cgi/reprint/78/1/493.pdf
