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DNA and Sex Determination in Humans By the end of this lesson, you will never look at this family the same. Chromosome: A threadlike structure of acids and protein found in the nucleus of most living cells; carries genetic information Genes: Characteristics of an organism that are inherited from their biological parents; made up of DNA. Different “stretches” of DNA are genes that give us our characteristics. DNA: Stands for deoxyribonucleic acid. The chemical that genes are made up of. This is the chain of ‘links’ that determines how the different cells in your body will function. To summarize: DNA is in genes, and genes are on chromosomes. Genes Vs. Chromosomes Vs. DNA Evidence for DNA • Freidrich Miescher discovered the substance of DNA in 1868, while researching the nucleus of fish sperm. He did not know it’s purpose. • Scientists predicted that DNA held the information of inheritance, but they weren’t sure how. • Using X-ray diffraction, Rosalind Franklin discovered the structure of DNA as a double helix in 1951. She was not noted for this discovery until her death in 1958. Rosalind Franklin The Human Cell Nearly every cell in the human body contains 46 chromosomes inside of its nucleus: 23 from your mother, and 23 from your father. The exception to this rule is the sex cells or gametes (i.e. the sperm and the egg), which have 23 chromosomes. Why might this be? If they each had 46 chromosomes, then when they combine there would be a baby with 92 chromosomes, which would be disastrous. (It would NOT be a super baby). All of the chromosomes can be seen on a Karyotype Information about • • • A karyotype is the arrangement of an organism’s chromosomes from one body cell Karyotypes are made using the amniotic fluid (fluid in the amniotic sac surrounding the fetus) from a pregnant female. Karyotypes show – the number of chromosomes – the sex of the individual – large errors in chromosome structure who you are is contained in these Typical male karyotype XY chromosomes Typical female karyotype Two X chromosomes Sex Chromosomes • Sex chromosomes determine the gender of an individual • Like the rest of your chromosomes, you get one of these from each of your parents • They control the primary and secondary sexual characteristics of humans • Consist of an X chromosome and Y chromosome How do parents pass on genes to their children? • Traits are passed on from both parents. • You have some traits from your mom and some from your dad. • Because traits are coded for by DNA, you have some of your mom’s DNA and some of your dad’s DNA. But your DNA is unique to you; no one else has exactly the same DNA as you (unless you are a twin). Is it biologically beneficial to be different from our parents? Yes, in theory… A child must be similar enough to its parents to survive in a similar environment, but have the chance to be different enough to survive in a changing environment. It is a benefit to be genetically different from our parents, in case we must survive in an environment that is different from the one they live in. With different genes, we may be better prepared. People in drastically different environments have slightly different genes. Can you think of any examples? How are our traits a combination of our parents traits? • Traits are not mixed when they are passed on. – For example, if your mom has blue eyes and your dad has yellow eyes, you will not have green eyes. You will either have blue or yellow eyes. • Some traits are determined by dominance. – If you have two genes in you that code for the same trait, one trait will win over the other. The trait that “wins” is the dominant gene. The trait that does not get expressed is the recessive gene. • Some traits, like height, are determined by genes from both parents. There is not one dominant gene for these traits. Dominant Traits: -attached or unattached earlobes -widow’s peak hairline Combination Traits: -height -hair color Let’s take a breather and see what the common traits in this classroom are. Dominant Traits Each person has two genes for every trait. • Dominant physical characteristics will show up unless both genes code for the recessive trait. Dominant trait: Unattached earlobes, “U” Recessive trait: Attached earlobes, “A” If your genes code: UU UA AA You will have: Unattached Unattached Attached Attached “A” Unattached “U” Evidence for Dominance • In the late 1800’s, Gregor Mendel, a monk, was studying pea plants. • He noticed that when he bred two plants with different traits, their offspring did not show a mix of characteristics, but instead showed either characteristic. + = = or Predicting Physical Characteristics Genes of Parent 1: A Genes of Parent 2: • When a scientist knows the characteristics of the parents, he or she can predict the characteristics of the offspring using a Punnett-Square • Genes of both parents are labeled on the outside of the big square. • Each combination of the parent’s genes in the small squares is a possible combination for the offspring. B A AA AB B AB BB Offspring could be: AA, AB, AB, or BB Probability (P) • By knowing which genes the parents have, you can figure out the probability those parent’s have of having a child with a certain trait. If the parents are both dominant (DD): D D D DD DD D DD DD There is a 100% chance that the child will have the dominant trait. P = 4/4 If the parents are both dominant but also carry the recessive gene (DR): D R D DD DR R DR RR D = dominant R = recessive P = Probability It’s a fraction! There is a 75% chance that their child will have the dominant trait. P = 3/4 Try It Dominant Trait: Straight Thumb (S) Recessive Trait: Hitchhiker’s Thumb (H) If Parent 1 has Hitchhiker’s Thumb (HH) And Parent 2 does not (SH)… Parent 1: S H H SH HH S What probability is there that a child will have hitchhiker’s thumb (the recessive Trait)? 3 out of 4 = 3/4 H HH HH H *Dominant traits are not always the most common. Designer “Genes” Alleles - two forms of a gene (dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) copyright cmassengale 18 More Terminology Genotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white) copyright cmassengale 19 Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid copyright cmassengale 20 FACT OR FAKE: Our bodies are made up of cells. Our cells are constantly being replaced by new cells, so in a way we’re a different person now than we were 7-10 years ago. (Mostly) Yes, it’s true that individual cells have a finite life span, and when they die off they are replaced with new cells. There are between 50 and 75 trillion cells in the body.... Each type of cell has its own life span, and when a human dies it may take hours or day before all the cells in the body die. (Forensic investigators take advantage of this vaguely morbid fact when determining the cause and time of death of homicide victims.) However, red blood cells live for about four months, while white blood cells live on average more than a year. Skin cells live about two or three weeks. Colon cells have it rough: They die off after about four days. Sperm cells have a life span of only about three days, while brain cells typically last an entire lifetime (neurons in the cerebral cortex, for example, are not replaced when they die). Once DNA replicates, how do cells replicate and divide? • • The body grows by one cell dividing into two, and those two diving into four. Skin cells live for many days, blood cells which carry oxygen live for months, nerve cells last a lifetime. One cell dividing into two One Cell Let’s Listen to a Robot Explain Mitosis and Meiosis: “Normal” Cells Divide by the Process of Mitosis Cell with a single copy of DNA DNA replicates to form chromosomes (two copies of DNA) Chromosomes line up in the middle of the cell Chromosomes are split. Half of each chromosome travels to either end of the cell. The cell divides to form two new cells with their own DNA “Sex Cells” divide by the Process of Meiosis Original cell with 46 chromosomes Some DNA can detach from one chromosome and switch with DNA from the chromosome that holds similar genes. This is called crossing over. After two cell divisions, four cells with 23 chromosomes each are formed. How do sex cells form new individuals? • Sex cells in females are eggs. • Sex cells in males are sperm. • One sex cell from the mother (an egg) merges with one sex cell from the father (a sperm) to form a gamete. This cell continues to divide, until it forms a complete organism. • The individual grows through mitosis (i.e. a single cell keeps dividing as a human being forms). A Growing Human One cell divides into an identical cell in mitosis One cell divides into four new ones in meiosis Mitosis VS. Meiosis • Mitosis – Occurs in the creation of normal cells – One division – Forms 2 cells – Newly formed cells have 46 chromosomes – Results in growth • Meiosis – Occurs only in the process of producting of sex cells (called gametes) – Two divisions – Forms 4 cells – Newly formed cells have 23 chromosomes – Allows for procreation Atypical Meiosis • Meiosis occurs correctly thousands of times throughout life – sometimes it can go wrong! Atypical Meiosis • Nondisjunction: – When homologous chromosomes do not separate during meiosis which produces gametes with the wrong number of chromosomes So what happens???!!! • During fertilization, if one of the gametes has the wrong number of chromosomes the zygote will have either too much or too little genetic material Down Syndrome • Common disorder that results from a zygote with 47 chromosomes • Down Syndrome is one of the few genetic disorders where an individual can survive with an extra chromosome. • They have three copies of chromosome 21 Turner Syndrome • Occurs when a female is born with only 1 X chromosome – Females with turner syndrome cannot reproduce Klinefelter Syndrome • Males that are born with two X chromosomes and one Y chromosome – Males with Klinefelter Syndrome produce less sex hormone and cannot father a child Alzheimer Disease • Individuals with Alzheimer disease lose the ability to create and keep memories. • This genetic disease is thought to be due to a combination of environmental and genetic effects. • Scientists do not know which genes affect Alzheimer Disease. • They hypothesize that a certain protein that affects the structure of nerves is over produced. This destroys nerves in the brain that are responsible for storing memories. Affected individual MRI of the Brain Cancer • Like Alzheimer disease, cancer is caused by both genetic mutations and environmental effects. – Environmental effects, such as excess sun exposure, are thought to be a partial cause of genetic mutations. • There are many types of cancer, but they all involve un-controlled cell growth. Lance Armstrong, 7 time winner of the Tour de France, Testicular cancer Bob Marley, Nancy Regan, Former First Lady, reggae musician, Died of skin cancer Breast cancer in 1981 Sexual Reproduction “Life Hacks” Cool, Outside of the Box Idea OR A Threat to Humanity? Artificial Insemination (Animal Version) A veterinarian collects sperm from male animal & inserts into a female animal of the same sex. This helps animals to conceive more quickly & more often. Used extensively in agriculture to help breed animals with desirable traits. Ex. Dairy Cows, also used in Zoos to maintain or increase the population of endangered species. With humans, this can work in a similar fashion through the use of sperm banks. In Vitro Fertilization Process that builds on artificial insemination Both sperm & eggs are collected from animals/humans with desired traits Fertilization occurs in a lab, Petri dish Once eggs are fertilized the embryos can be inserted into the female. These females are surrogate mothers, because they are not genetically related to the embryo. Eugenics “Improving human genetic qualities” • Eugenics is a social philosophy which advocates the improvement of human hereditary traits through various forms of intervention. Governments and Eugenics in the 20th Century • • • • • • Genetic screening Birth control Marriage restrictions Segregation (racial and mentally ill) Forced abortions or pregnancies Genocide Abortion • Sometimes used if it’s discovered early that the child will be born with severe health issues. • Procedure is more difficult and dangerous the longer the fetus has developed in the womb. Cloning an adult animal Cloning a single adult animal, especially a mammal, is very complicated. The most famous animal clone is Dolly the sheep, who was born on 5th July 1996. Dolly was not the first animal clone, but the first mammal to be cloned from an adult cell. Did Dolly have a successful, happy, healthy life? That’s for this class to research... What is nuclear transfer? Dolly was created using a technique called nuclear transfer. In this technique, the nucleus (i.e. DNA) from a body cell of the adult (a somatic cell) is removed. This nucleus is then inserted into an egg cell that has had its own nucleus removed. An electric shock makes the egg cell divide and develop like a normal fertilized egg. The important difference is that it only contains the DNA from one, rather than two, animals. Why couldn’t the DNA be taken from a sperm or egg cell? Because they only contain half the genes of the animal. But wait… what about that A&W Burger Family from earlier?