Tour of the basics
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Heredity is the passing of traits from parents to offspring. Traits are determined by genes, which are located on chromosomes and provide instructions for traits. During reproduction, each parent contributes half of their chromosomes to the offspring randomly, ensuring each child receives a unique combination of traits. Some traits are determined by a single gene, while most involve multiple genes and environmental influences. Traits can be physical, behavioral, or a predisposition to certain medical conditions. Genes provide the basic instructions, but environmental factors also help shape expressed traits.
Tour of the basics
- 1. What is Heredity? WHAT IS HEREDITY? Why do children look like their parents? Why do brothers and sisters resemble each other? This is because we “inherit”traits from our parents. The passing oftraits from parents to child is the basis of heredity. Where exactly are our traits? Our genes encode the instructions that define our traits. Each of us has thousands of genes, which are made ofDNA and reside in our chromosomes. The environment we grow up and live in also helps define our traits. For example, while person’s genes may specify a certain hair color, exposure to chemicals or sunlightcan change thatcolor. How do we get traits from our parent? Humans have two complete sets of23 chromosomes. (2 x 23 = 46 total). Whenparentsconceivea child,they eachcontribute one completesetto the child.In this way, parents pass genes to the child. Every child receives halfof its chromosomes from the mother and halffrom the father. This transfer takes place atconception, when the father’s sperm cell joins with the mother’s egg cell. While mostcells in our bodies contains two sets of chromosomes (2 x 23 = 46), sperm and egg cells each have only one set(23). When they join, they create a single cell, called a “zygote” (ZY-goat), which has two sets of chromosomes (46). This cell will divide, ultimately developing into a child. Each parentcontributes one complete set ofchromosomes to the child. This set can contain chromosomes from both of the parent’s two sets. The only rule is that the child must receive exactly one of each chromosome
- 2. Since the parents contribute chromosomes randomlyto each newchild, every child inherits a unique setof chromosome. As a result, every child will have a unique combination of traits. Some will resemble the mother, and some will resemble the father. Still others will be unique, a productofnew combination ofchromosomes.
- 3. Whatwill happen when one of these babies grows up and starts a family? She will contribute a mixture of her own chromosomes to each new child. This is how some traits are passed through many generations. What is a Trait? WHAT IS A TRAIT? A trait is a notable feature or quality in a person.Each ofus has a differentcombination oftraits that makes us unique. Traits are passed from generation to generation. We inherit traits from our parents, and we pass them on to our children. What types of traits exist? PhysicalTraitsarecharacteristicsofone’sphysicalmakeup.Theseincludehaircolor,eye colorandheight. BehavioralTraits are characteristics of the way one acts. A sheepdog’s herding instinct and a retriever’s desire to fetch are good examples ofbehavioral traits. Predispositionto a Medical Condition. An increased risk ofgetting a certain type ofdisease is also a type of trait that can bepassedfrom parentto child. Some examplesofsuchdiseasesincludesicklecell anemia, cystic fibrosis, heartdisease, cancer, and certain types ofmental illness. What defines our traits? The instructions encoded in our genes play a role in defining traits. Butthe non-genetic, or “environmental” influences in our lives are just as important in shaping our traits. Sometimes these environmental factors can even change a trait! Let’s see some examples.
- 4. Physical Traits Genetics: Our genes determine our natural hair color. Environment: Exposure to sun or hair dyes can easily change thatcolor. Behavioral Traits Genetics: People breed retrievers to chase things and bring them back. Environment: You can train a retriever to instead roll over and “play dead”when you toss a ball. Predisposition to a Medical Condition Genetics: A person may be born with an increased risk ofheartdisease. Environment: Eating healthy foods and exercising can reduce this risk. How are our traits determined? Try to bend your thumb backwards atthe joint. Some people can form at least a 45 degree which is called “hitchhiker’s thumb.” Other people have straight thumbs which do not bend in this way. Which one do you have? Scientist describe the setof genetic information for each form as an allele (uh-LEEL). We can describe the straight thumb allele with an “H” and the hitchhiker’s thumb allele with an “h.” Each of us has two alleles for the thumb extension trait. As a result, we all have one ofthese combinations:
- 5. You can probably guess the traitin people who have two ofthe same allele: Those with H + H will have straight thumbs. Those with h + h will have hitchhiker’s thumbs. Scientists use the word “homozygous” (HO-mo-ZY-gus)to describe having two of the same allele for a trait. But what aboutpeople who have one of each allele, or H + h? Will they have one straight thumb and one hitchhiker’s thumb? No. when two different alleles are present, they interact. For the thumb extension trait, the H allele makes the h allele. People with the H + h allele combination will have straight thumbs. In this case, the H allele is called “dominant,” and the masked h allele is “recessive.” Scientists use the word “heterozygous” (HET-er-oh-ZY-gus)to describe having two different alleles for a trait.
- 6. How are traits inherited? We’ve seen how the thumb extension traitshows up in one person. But how is this trait passed from parentto child? The mother and the father each have two alleles for the thumb extension trait. When they have a child, they pass one oftheir two alleles to the child. The child’s trait is determined by the alleles she receives from her parents.
- 7. Each child from these parents can receive a differentcombination ofalleles. When the first child grows up and has children ofher own,she will contribute one ofher two alleles to each child. Are all traits inherited this simply? No. thumb extension is a well-defined physical trait. Most traits are more complex and cannotbe tracked through generations this easily. Alleles can also work together to produce incomplete dominance. Forexample,crossing a red carnation plantwith a white one can produce pink carnations.
- 8. What is DNA? WHAT IS DNA? Let’s examine a group ofcells in your inner ear. They help you supportthe function ofhearing. How these cells know that their role is to support hearing instead of something else, like making your heartbeat? Instructions providing all of the information necessary for a living organism to grow and live reside in the nucleus ofevery cell. These instructions tell the cell whatrole it will play in your body. Whatdo these instructions look like? The instructions came in the form of a molecule called DNA. DNA encodes a detailed set of plans, like blueprint, for building different parts of the cell. How can a molecule hold information? The DNA molecule comes in the form of twisted ladder shape scientist call a “double helix.” The ladder’s rungs are built with the four-letter DNA alphabet: A,C,T and G. These alphabet pieces join together according to special rules. A always pairs with T, and C always pairs with G. How can only four letter tell the cell whatto do?
- 9. What is a Gene? WHAT IS A GENE? Genes are instruction manuals for our bodies. They are the directions for building all the proteins that make our bodies function. Genes are made ofDNA. One strand of ourDNA contains many genes.All of these genes are needed to give instructions for how to make and operate all parts of our bodies. Forexample,bloodcontainsred bloodcells that transportoxygenaroundourbodies.Thecells use a protein called “hemoglobin” (HEE-mo-GLO-bin) to capture and carry the oxygen. Of our 25,000 genes,only a few contain the instructions for making hemoglobin proteins. The remaining genes contain the instructions for making other parts ofour bodies.
- 10. If ourhemoglobin gene is normal,the hemoglobin protein works fine. Butifthe instructions in that gene are changed,ormutated,changesinthe hemoglobinproteincouldresult. Onesuchmutation causesadisorder called sickle cell anemia. Genescontain instructions for building proteins,which are involved in all sorts ofthings. Hemoglobinprotein is just one example.Other proteins such as the enzymes that produce pigment in your eyes and keratin, responsible for growing hair and nails, are also produced by genes. What is a Protein? WHAT IS A PROTEIN? Proteinsare the machinesthatmakeall living things function, from viruses to daffodils, spidersto sealions, and everything in between.
- 11. How do proteins work in the body? Our bodies are made up of about100 trillion cells! Each ofthose cells is responsible for a specific job. Every cell contains thousands of different proteins, which work together as tiny machines to run the cell. You can think of proteins as parts of a car engine – each part looks different, and they all di separate jobs to make the engine run. Ouch! This person is hurting! Let’s take a closer look atwhat’s happening, Ournerve networksare madeupofindividualcells arrangedend-to-end,likea telephone line, to transmit the pain signal. The receivingend ofeach cellin the line contains specialproteins on its surface, called receptor proteins. Receptor Proteins are responsible for picking up the signal and passing it along to the next cell.
- 12. Did you notice that these nerve cells aren’t round? Instead, they’ve grown branches, which help them communicate with their neighbors in the line. Special proteins called structural proteins help cells extend these branches and hold them in place. Structural Proteins are like bricks, stacking together to form column-like supports that give the cell its shape. These proteins are just two of thousands thathelp the nerve cell do its job.Every cell in your bodyis just as complex! In real life, proteins are lot smallerthan this. Even with mostpowerful electron microscope,you would have trouble seeing them! Scientists use computer programs to model protein structure and function. Cells use the information encoded in their genes,which are a sort of protein library, as the blueprint for making proteins. Each gene in the DNA encodes information abouthow to make an individual protein. When a cell needs to make a certain protein, specialized machinery within the cell’s nucleus reads the gene and then uses thatinformation to produce a molecular message in the form of RNA, a molecule very similar to DNA. RNA moves from the nucleus into the cytoplasm of the cell. Once there, the cell’s protein-making machinery, the ribosome, reads the message and produces a protein that exactly matches the specifications laid out in the gene.
- 13. Once made, the protein travels to the part of the cell where it is needed and begins to work. Each step in making a protein itself requires the work ofhighly specialized proteins. Can you see why a cell needs thousands ofdifferentproteins to function? What is a Chromosome? WHAT IS A CHROMOSOME? Each cell in our body contains a lot of DNA. In fact, if you pulled the DNA from a single human cell, and stretched it out, it would be three meters long. That’s aboutas long as a car! How does all ofthat DNA fit into a cell? The DNA is packaged into compactunits called chromosomes. The packaging ofDNA into a chromosome is done by several steps, starting with the double helix ofDNA. Then the DNA is wrapped around some proteins. These proteins are packed tightly together until they form a chromosome.Chromosomes are efficientstorage units for DNA. How many chromosome does one cell hold? The correct answerto this depends on whetheryou’re a fish or a fly, or a human. Each human cellhas 46 chromosomes.All the DNA is organized into two sets of 23 chromosomes.We get genetic material from both of our parents – that’s why children look like both their mom and dad. Whatcan we learn from looking atour chromosomes?
- 14. Look at this set of chromosomes.You can see that matching chromosomes have been lined up in pairs – one each from mom and dad. Although the DNA double helix is too small to see, chromosomes can be viewed with a microscope, as in this picture. There are two sex chromosomes that determine whether you are male or female. In the picture the sex chromosomes are labeled X and Y. The set of chromosomes in this picture are from a male – you can tell because females do nothave a Y chromosome. Instead, they have two X chromosomes. Not all living things have 46 chromosomes,like humans.Mosquitos,for instance, have 6. Onions have 16. Carp have 104.