Genetics
Background:
Homunculus in Sperm One question that has always intrigued us humans is Where did we come from? Long ago, Hippocrates and Aristotle proposed the idea of what they called pangenes, which they thought were tiny pieces of body parts. They thought that pangenes came together to make up the homunculus, a tiny pre-formed human that people thought grew into a baby. In the 1600s, the development of the microscope brought the discovery of eggs and sperm. Antonie van Leeuwenhoek, using a primitive microscope, thought he saw the homunculus curled up in a sperm cell. His followers believed that the homunculus was in the sperm, the father planted his seed, and the mother just incubated and nourished the homunculus so it grew into a baby. On the other hand, Regnier de Graaf and his followers thought that they saw the homunculus in the egg, and the presence of semen just somehow stimulated its growth. In the 1800s, a very novel, radical idea arose: both parents contribute to the new baby, but people (even Darwin, as he proposed his theory) still believed that these contributions were in the form of pangenes.
Modern genetics traces its beginnings to Gregor Mendel, an Austrian monk, who grew peas in a monastery garden. Mendel was unique among biologists of his time because he sought quantifiable data, and actually counted the results of his crosses. He published his findings in 1865, but at that time, people didnt know about mitosis and meiosis, so his conclusions seemed unbelievable, and his work was ignored until it was rediscovered in 1900 by a couple of botanists who were doing research on something else. Peas are an ideal organism for this type of research because they are easy to grow and it is easy to control mating.
We will be looking at the sorts of genetic crosses Mendel did, but first, it is necessary to introduce some terminology:
Monohybrid Cross and Probabilities:
A monohybrid cross is a genetic cross where only one gene/trait is being studied. P stands for the parental generation, while F1 and F2 stand for the first filial generation (the children) and second filial generation (the grandchildren). Each parent can give one chromosome of each pair, therefore one allele for each trait, to the offspring. Thus, when figuring out what kind(s) of gametes an individual can produce, it is necessary to choose one of the two alleles for each gene (which presents no problem if they are the same).
Purple Pea Flower White Pea Flower For example, a true-breeding purple-flowered plant (the dominant allele for this gene) would have the genotype PP, and be able to make gametes with either P or P alleles. A true-breeding white-flowered plant (the recessive allele for this gene) would have the genotype pp, and be able to make gametes with either p or p alleles. Note that both of these parent plants would be homozygous. If one gamete from each of these parents got together to form a new plant, that plant would receive a P allele from one parent and a p allele from the other parent, thus all of the F1 generation will be genotype Pp, they will be heterozygous, and since purple is dominant, they will look purple. What if two individuals from the F1 generation are crossed with each other (PpPp)? Since gametes contain one allele for each gene under consideration, each of these individuals could contribute either a P or a p in his/her gametes. Each of these gametes from each parent could pair with each from the other, thus yielding a number of possible combinations for the offspring. We need a way, then, to predict what the possible offspring might be. Actually, there are two ways of doing this. The first is to do a Punnett square (named after Reginald Crandall Punnett). The possible eggs from the female are listed down the left side, and there is one row for each possible egg. The possible sperm from the male are listed across the top, and there is one column for each possible sperm. The boxes at the intersections of these rows and columns show the possible offspring resulting from that sperm fertilizing that egg. The Punnett square from this cross would look like this:
Note that the chance of having a gamete with a P allele is and the chance of a gamete with a p allele is , so the chance of an egg with P and a sperm with P getting together to form an offspring that is PP is =, just like the probabilities involved tossing coins. Thus, the possible offspring include: PP, ( Pp + pP, which are the same (Pp), since P is dominant over p), so = Pp, and pp.
Another way to calculate this is to use a branching, tree diagram:
Note, again, that the chance of Pp is +=. A shorter way of telling how many PP, Pp, and pp could be expected, would be to say that there is a 1:2:1 genotype ratio (that comes from the , , and , above, and by the way, notice that they add up to , so we know we have accounted for everything). The chance of getting at least one dominant allele (either PP or Pp) necessary for purple color (this can be written as P) is +=, so we could say that theres a 3:1 phenotype ratio. These two ratios are classic genotype and phenotype ratios for a monohybrid cross between two heterozygotes.
Mendels Four-Part Theory:
Based on his data, Mendel came up with a four-part theory of how genetics works:
Some special cases:
(Rh factor, by the way, is a totally separate gene with Rh+ [R] and Rh [r] alleles [actually, that gene also has multiple alleles, but the vast majority of people are positive or negative for one particular allele called D]. In the U. S., about 85% of the population is Rh+ [RR and Rr] and 15% Rh [rr], thus the chances of someone being O [having both ii and rr] would be 45% 15% = 6.75%. The rarest blood type in the U. S. would be AB, about 0.45% of the population.]
This is a cross where two traits/genes are under consideration. For example, in peas if R = round, so r = wrinkled, and Y = yellow, so y = green, in a cross between RRYY rryy, the gametes must have ONE ALLELE FOR EACH GENE, so in this case, RRYY could produce gametes with one R AND one Y, or RY, and rryy could produce gametes with one r AND one y, or ry. The F1 would get RY from one parent and ry from the other, thus would all be RrYy. Note that it is necessary to keep the alleles for the same gene together and put the dominant allele (capital letter) first for EACH GENE. In calculating what the F2 generation would be, you must first figure out what gametes (eggs or sperm) each parent can make. It is very important to remember that gametes must have ONE ALLELE FOR EACH GENE, so figure out the possibilities this way:
Thus, each parent could make four kinds of gametes, so the Punnett square would be 44 cells.
This would give the following possible offspring:
Thus the genotype ratio is 1:2:1:2:4:2:1:2:1 and the phenotype ratio is 9:3:3:1. Notice the shorthand used to represent the phenotypes. Since both RR and Rr will look round, rather than writing round pea seeds, we can use R to say its got at least one R, so itll be round.
Try This:
On your own, try IAiRr IBiRr, a cross involving both the ABO blood group and Rh factor. Note, a little later, we will discuss what those blood groups actually are/do.
Genotype and Phenotype Are Not the Same:
It is important to understand the difference between genotype and phenotype. For example, for most of the genes we will be discussing, an organism with the genotype of, say, BB and an organism who is Bb both have at least one dominant allele for that gene, and thus, would both express/show/be the dominant phenotype. If, for example, this was a gene for human eye color, then B would represent the dominant allele which codes for make brown eyes, and b would represent the recessive allele which codes for blue eyes (technically, more like, we dont know how to make brown, so blue is the default). Thus, people whose genotypes are either BB or Bb both have instructions for make brown, so the phenotypes of both are brown eye color.
As another example where many people get confused, an individuals sex is a phenotype, not a genotype! We can talk of a person as having either two X chromosomes (XX) or one X and one Y chromosome (XY). Those are, essentially, genotypes, and there are also a few people who have genotypes such as X (also called XO), XXX, or XXY. Those X and Y chromosomes contain/consist of a number of genes, and factors such as what alleles a person has for each of those genes, how those alleles are expressed, and how that gene expression affects/influences various body processes will all come together to produce that phenotype which we call a persons sex. In humans, if all those alleles are expressed in what we like to think of as being normal, then, usually, X, XX, and XXX are expressed as a female phenotype (with X and XXX producing some other physical characteristics considered to be typical for those genotypes), while the result of how the XY combination is expressed usually results in what we refer to as a male phenotype.
However, while uncommon, it is entirely possible that due to a mutation in some gene, somewhere, that codes for some enzyme or hormone, a person with 2 X chromosomes (XX) can have a male phenotype; can, clearly and unambiguously, be male. Similarly, while also not very common, it is also possible, due to a mutation in some gene, somewhere, that codes for some hormone or enzyme, that a person with an X and a Y chromosome (XY) can have a female phenotype; can, clearly and unambiguously, be female. Interestingly, because of differences in how the genes/alleles are expressed, the XXY combination typically results in a male in humans but results in a female in fruit flies.
Our culture, our way of thinking, is so locked into having/needing to choose between male and female as the only two options, that while in the unambiguous cases just mentioned where a persons expressed phenotype obviously fits our preconception of maleness or femaleness even if their genotype/chromosomes are different from what we might think (and of which we would not even be aware unless we were that persons doctor and maybe not even then), on the other hand, people whose bodies dont exactly and neatly fit into one of those two categories are lumped together in a group and labeled as intersex. Typically, at birth, their parents are advised by medical personnel to choose whether they wish to bring this child up as a boy or a girl, and may even be pressured into having cosmetic surgery performed on the child to make the child look more like the chosen sex assignment, yet it frequently happens as the child grows up, due to the influence of internal factors such as hormones, etc., that he or she does not feel like the sex which the doctors assigned/labeled at birth. On the other hand, if parents try to be more neutral and let the child make that choice when and if the child decides to do so, that tends to expose the child to a lot of ridicule from classmates and even other adults.
Pedigrees:
Sample Pedigree In pedigrees, a circle represents a female and a square represents a male. Filled-in vs. open symbols are used to distinguish between two phenotypes for the gene in question, and a half-filled symbol may be used to designate a carrier (a heterozygous individual who has a recessive allele for some gene, but is not showing that phenotype). Here is a sample pedigree for eye color. If the people with filled-in (dark) symbols have brown eyes and those with open (light) symbols have blue eyes, can you figure out the genotypes of the people marked with *?
Genetic Basis of Behavior, Polyploids:
Some further notes on genetics: We tend to think of genes that control what an organism looks like, etc., but genes can also control behavior of animals. For example, bird songs and other courtship rituals are under genetic control. The most successful competitors live and mate and pass on their genes. On a different subject, many of our horticultural plant varieties are polyploid plants. Typically, like us, plants are diploid. Horticulturists have figured out ways to manipulate plants and make triploid or tetraploid plants. Typically these plants are larger and/or have bigger or more ruffled flowers and/or larger seeds. While triploid plants are usually sterile (with three sets of chromosomes they have trouble doing meiosis), tetraploid plants are usually fertile and can reproduce. I believe I read somewhere that the wheat we eat is actually a hexaploid, resulting in seeds that are quite a bit larger than its grass-like ancestor.
References:
Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.
Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5th Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3rd Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.
See the original post:
Genetics - Biology
- Genetics - National Geographic Society - March 28th, 2025
- Genetics: Introduction, law of inheritance and Sex Determination - BYJU'S - March 28th, 2025
- Genetics, ecology and evolution of phage satellites - Nature.com - March 28th, 2025
- As a geneticist, I will not mourn 23andMe and its jumble of useless health information | Adam Rutherford - The Guardian - March 28th, 2025
- Rare loss-of-function variants in HECTD2 and AKAP11 confer risk of bipolar disorder - Nature.com - March 28th, 2025
- With 23andMe filing for bankruptcy, what happens to consumers genetic data? - The Conversation Indonesia - March 28th, 2025
- A genetic tree as a movie: Moving beyond the still portrait of ancestry - Phys.org - March 28th, 2025
- Genetic mutations linked to Marek's disease in chickens identified - Phys.org - March 28th, 2025
- 23andMe is looking to sell customers genetic data. Heres how to delete it - CNN - March 28th, 2025
- Horses Pulled Off a Genetic Trick Only Viruses Were Thought to Use - SciTechDaily - March 28th, 2025
- CONSUMER ALERT: Warning 23AndMe Customers That Their Private Genetic Data May Be at Risk - Office of the Attorney General for the District of Columbia - March 28th, 2025
- A new study reveals the genetic change that made horses so athletic - KUOW News and Information - March 28th, 2025
- "Mystery ancestors" gave humans 20% of our current DNA, but who were they? - Earth.com - March 28th, 2025
- Correcting the Mutation Behind a Genetic Eye Disease - The Scientist - March 28th, 2025
- Your DNA is safe here: The AncestryDNA Genetic Test Kit is only $39 now - New York Post - March 28th, 2025
- 23andMe Is Bankrupt. Heres What You Need to Know About Your Genetic Data. - The Wall Street Journal - March 28th, 2025
- Commentary: 23andMe files for bankruptcy, putting its hoard of personal health information at risk - Los Angeles Times - March 28th, 2025
- DNA Microscopy Creates 3D Maps of Life From the Inside Out - SciTechDaily - March 28th, 2025
- Eugenics Must Be Included in Genetics Curriculum: Prof - Mirage News - March 28th, 2025
- 11-minute video on human genetics can make people more accepting of others, reveals new study - Hindustan Times - February 24th, 2025
- Advancing Cancer Genetic Testing to Improve Prevention and Patient Treatment - The Scientist - February 24th, 2025
- Environmental factors, lifestyle choices have greater impact on health than genes, study finds - ABC News - February 24th, 2025
- Study finds lifestyle, environment have greater impact on lifespan than genetics - CBS Boston - February 24th, 2025
- Safeguard repressor locks hepatocyte identity and blocks liver cancer - Nature.com - February 24th, 2025
- Mass spectrometry-based mapping of plasma protein QTLs in children and adolescents - Nature.com - February 24th, 2025
- The Avestagenome Project and TIGS Sign Strategic Alliance to Advance Research in Rare Genetic Disorders - The Tribune India - February 24th, 2025
- Researchers make breakthrough discovery after studying genetics of trees: 'There is a need for proactive conservation' - MSN - February 24th, 2025
- iPSCs and iPSC-derived cells as a model of human genetic and epigenetic variation - Nature.com - February 24th, 2025
- Beyond genetics: The biggest factors that influence health and aging - Earth.com - February 24th, 2025
- Genetic diversity and dietary adaptations of the Central Plains Han Chinese population in East Asia - Nature.com - February 24th, 2025
- How a uniquely human genetic tweak changed the voices of mice - NPR - February 24th, 2025
- Genetic evidence identifies a causal relationship between EBV infection and multiple myeloma risk - Nature.com - February 24th, 2025
- Genetic markers of early response to lurasidone in acute schizophrenia - Nature.com - February 24th, 2025
- Bupa to offer first genetic test for disease prediction in UK - The Times - February 24th, 2025
- Advancing Therapeutic Knowledge of Genetic Influence in ALS: Matthew B. Harms, MD - Neurology Live - February 24th, 2025
- Association of dietary carbohydrate ratio, caloric restriction, and genetic factors with breast cancer risk in a cohort study - Nature.com - February 24th, 2025
- Evaluation of polygenic scores for hypertrophic cardiomyopathy in the general population and across clinical settings - Nature.com - February 24th, 2025
- Familiar autism-linked genes emerge from first analysis of Latin American cohort - The Transmitter: Neuroscience News and Perspectives - February 24th, 2025
- Almost 90% of people would agree to genetic testing to tailor medication use, survey finds - Medical Xpress - February 24th, 2025
- Largest Genetic Study of Bipolar Disorder Identifies 298 Regions of the Genome That Increase Risk for the Condition - Mount Sinai - January 27th, 2025
- Study Sheds Light On The Origin Of Earth Lifes Genetic Code - Astrobiology News - January 27th, 2025
- Largest study on the genetics of bipolar disorder to date gives new insights into the underlying biology - Medical Xpress - January 27th, 2025
- Genetic Swiss Army Knife: New Tool For Gene Editing And Therapy - Forbes - January 27th, 2025
- Uhm Ji-won says the power of genetics is undeniable with Hyun Bin and Son Ye-jin's son - - January 27th, 2025
- Integrative proteogenomic analysis identifies COL6A3-derived endotrophin as a mediator of the effect of obesity on coronary artery disease -... - January 27th, 2025
- Genetic analysis reveals the genetic diversity and zoonotic potential of Streptococcus dysgalactiae isolates from sheep - Nature.com - January 27th, 2025
- Eight psychiatric disorders share the same genetic causes, study says - Medical Xpress - January 27th, 2025
- Exploring genetic associations and drug targets for mitochondrial proteins and schizophrenia risk - Nature.com - January 27th, 2025
- Predictive Genetic Testing and Consumer Genomics Market - GlobeNewswire - January 27th, 2025
- Evolution without sex: How mites have survived for millions of years - EurekAlert - January 27th, 2025
- Our Understanding of Rules that Produce Lifes Genetic Code May Require a Revision - DISCOVER Magazine - January 27th, 2025
- Personalized therapy for rare genetic diseases: Patient-derived organoids offer new hope - Medical Xpress - January 27th, 2025
- The One Thing That's More Important for Longevity Than Your Genes - Parade Magazine - January 27th, 2025
- Complete recombination map of the human genome created - Medical Xpress - January 27th, 2025
- Evidence of genetic determination of annual movement strategies in medium-sized raptors - Nature.com - January 27th, 2025
- Genetic study of Alaska red king crabs suggests species is more diverse and resilient to climate change - Global Seafood Alliance - January 27th, 2025
- Smartwatches reveal insights into psychiatric illnesses and genetic links - Medical Xpress - January 27th, 2025
- Unlocking the Blueprint of Human Life With a Revolutionary DNA Map - SciTechDaily - January 27th, 2025
- Largest Genetic Study of Bipolar Disorder Identifies Nearly 300 Risk-Associated Genome Regions - Inside Precision Medicine - January 27th, 2025
- Genetic Discrimination Is Coming for Us All - The Atlantic - November 16th, 2024
- Family connection: Genetics of suicide - WNEM - November 16th, 2024
- Study links heart shape to genetic risk of cardiovascular diseases - News-Medical.Net - November 16th, 2024
- Genetic architecture of cerebrospinal fluid and brain metabolite levels and the genetic colocalization of metabolites with human traits - Nature.com - November 16th, 2024
- Genetic connectivity of wolverines in western North America - Nature.com - November 16th, 2024
- Toward GDPR compliance with the Helmholtz Munich genotype imputation server - Nature.com - November 16th, 2024
- Leveraging genetic variations for more effective cancer therapies - News-Medical.Net - November 16th, 2024
- Bringing precision to the murky debate on fish oil - University of Arizona News - November 16th, 2024
- International experts gathered in Tashkent to tackle rare disease for Uzbekistan - EurekAlert - November 16th, 2024
- Mercys Story: Living life with 22q, a genetic condition - WECT - November 16th, 2024
- Cold case with ties to Houghton County solved through genetic genealogy after 65 years - WLUC - November 16th, 2024
- 23andMe customer? Here's what to know about the privacy of your genetic data. - CBS News - November 16th, 2024
- Single-cell RNA analysis finds possible genetic drivers of bone cancer - Illumina - November 16th, 2024
- Multi-trait association analysis reveals shared genetic loci between Alzheimers disease and cardiovascular traits - Nature.com - November 16th, 2024
- With 23andMe Struck by Layoffs, Can You Delete Genetic Data? Here's What We Know - CNET - November 16th, 2024
- Genetic testing firm 23andMe cuts 40% of its workforce amid financial struggles - The Guardian - November 16th, 2024
- Genetic study solves the mystery of 'selfish' B chromosomes in rye - Phys.org - November 16th, 2024
- Genetic changes linked to testicular cancer offer fresh insights into the disease - Medical Xpress - November 16th, 2024
- Eating less and genetics help you to live longer, but which factor carries the most weight? - Surinenglish.com - November 16th, 2024
- We must use genetic technologies now to avert the coming food crisis - New Scientist - November 16th, 2024
- NHS England to screen 100,000 babies for more than 200 genetic conditions - The Guardian - October 6th, 2024