Genetics Overview PDF

Heredity and Genetics Chromosomes and Genes Genes are heritable units of DNA found along chromosomes. Your DNA sequence contains the information for all of your traits, and more. Sequences of DNA comprise genes, heritable units found on your chromosomes. Humans have approximately 25,000...

Heredity and Genetics Chromosomes and Genes Genes are heritable units of DNA found along chromosomes. Your DNA sequence contains the information for all of your traits, and more. Sequences of DNA comprise genes, heritable units found on your chromosomes. Humans have approximately 25,000 genes. Pairs of homologous chromosomes have the same types of genes, but each chromosome can contain different molecular forms of genes, called alleles. In the activity below, drag the appropriate alleles to each gene family. blonde red Hair Color Eye Color brown blue hazel brunette Dominant and Recessive Alleles Some alleles are not expressed if others are present. You possess two alleles, or copies, of every gene in your genome. You inherited these alleles from your mom and your dad. Let's look at an inherited trait that's easy to notice – your earlobes! Take a quick look in the mirror and see which type of Detached earlobe (E) Attached earlobe (e) earlobe you have. Are they attached to the side of your head, as in the image on the right, or are they detached from the side of your head and dangling a little bit, as in the image on the left? The earlobe gene has two different alleles. The allele for detached earlobes, shown by an uppercase letter E, is a dominant allele. The allele for attached earlobes, represented by a lowercase letter e, is a recessive allele. A dominant allele is the form of a gene that controls, or masks, the expression of another allele that is paired with it. A recessive allele, which is always represented by a lowercase letter, is the form of an allele that is not expressed if a dominant form is present. Two alleles are needed for a recessive phenotype to be expressed. Description: Description: Description: Detached Detached Attached earlobe earlobe earlobe Genetic Genetic Genetic combination: combination: combination: EE Ee ee Genotypes and Phenotypes Your combination of alleles determines your genotype and phenotype. The unique combination of alleles — EE, Ee, or ee — determines your genotype. There are homozygous and heterozygous genotypes. If you have two of the same allele, such as two dominant (E) alleles or two recessive (e) alleles, then you are homozygous, since both alleles are the same. If you received a dominant allele (E) from one parent and a recessive (e) allele from your other parent, then you are heterozygous. This woman has dimples when she smiles. If dimples are a dominant trait, what possible genotypes could she have? Predicting Genetic Outcomes Probability is the likelihood of an event occurring. What is the likelihood that a trait, such as your mother's curly hair or your father's earlobes, will be passed from one generation to the next? The chance that you will inherit any particular trait is determined by probability, which is the measure of the likelihood of an event. Just like genetic events involving inheritance, common daily events have a certain probability of occurring. You can predict whether a coin toss will result in "heads" or "tails" before the referee tosses the coin at the beginning of a game, or how likely you are to roll a six on a die. You'll learn how to calculate probability using dice and a game of chance on the next page. Probability If you flip a coin, how often will it come up heads? ____% of the time. Every time you flip a coin, there is a ____% chance of getting heads. The results of previous flips do / do not change the outcome of future attempts. With only 10 flips, the results may not be exactly 50%. As the sample size increases, what happens to the results? Punnett Squares and Monohybrid Crosses Punnett squares can be used to predict genotype and phenotype ratios. In order to illustrate the probability of passing on specific alleles to offspring, a tool called the Punnett square is used to predict genotypes and phenotypes based on genetic inheritance. In a Punnett square, the genotypes of each parent are entered into the diagram. The grid is then filled in to predict the genotypic ratio, which is the probability of each genotype occurring in the offspring. The resulting genotypes determine phenotypes, and the phenotypic ratios. Study the example on the next slide to see how a Punnett square works. The gene for flower color can be expressed as two phenotypes: purple and white. In this example, the allele for purple flowers is dominant over white flowers. In this example, two homozygous parents were crossed. This results in offspring that all have the heterozygous genotype Aa. Since purple is dominant, what will all the offspring look like? All the offspring have the same heterozygous genotype, Aa. Because all the offspring have a dominant and recessive allele, only the dominant allele will be expressed in the phenotype, and all the offspring will have purple flowers. The ratio of phenotypes will be four purple to zero white, or four to zero. Using Punnett Squares Punnett squares are useful for determining ratios. Many traits are inherited as dominant and recessive traits: seed color, seed shape, and flower color are three such genes. The unique allele combination determines the phenotype. Now practice predicting genotype and phenotype ratios using a Punnett square in the following activity. Be sure to indicate the dominant allele(s) in each box before the recessive allele(s). Step 1: Enter the parents’ alleles into What will be the phenotype the top and sides, one allele per box. ratio if you cross two plants Step 2: In each gray box, combine the with genotypes RR and rr? allele letters from the top of the column and left of the row. Capital letters Y = yellow y = green should be listed first. R = round r = wrinkled Step 3: Determine the genotype and A = purple a = white phenotype ratios. More Punnett Square Examples Genotype ratio: What will be the phenotype ratio if you cross a Phenotype ratio: homozygous white plant and a heterozygous plant? Y = yellow y = green R = round r = wrinkled A = purple a = white What will be the phenotype Genotype ratio: ratio if you cross two plants that are heterozygous yellow? Phenotype ratio: Y = yellow y = green R = round r = wrinkled A = purple a = white Testcross A testcross can be used to determine an unknown genotype. How do you determine the genotype of an individual with a dominant phenotype? In a testcross, an organism with an unknown dominant genotype is crossed with a plant of known recessive genotype. The outcome of the cross is then analyzed to determine the genotype of the original unknown organism. If you were given a pea plant with a white flower, what do you know about the genotype of the plant? If you have a purple pea plant flower, what do you know about its genotype? Genetic Crosses with Two Traits Punnett squares can be used to predict the outcomes of dihybrid crosses. So far you have investigated how to track the inheritance of one trait. However, individuals possess many genes and express many different traits. You can also use a Punnett square to keep track of a cross involving two different traits, known as a dihybrid cross. A dihybrid Punnett square allows you to demonstrate the law of independent assortment, which states that allele pairs of different genes separate, or "assort," independently of one another during gamete formation. Therefore, whether or not you possess the trait of attached earlobes will not affect the angle of your thumb. Each allele transmitted independently of the other. Mendelian Genetics Problems The rules of probability can be used to solve Mendelian genetics problems. quickly and analyze the results of a dihybrid cross, researchers often use fruit flies (Drosophila), since they have easily recognizable phenotypic traits, such as body color and wing shape. Fruit flies also reproduce very quickly and are easy to maintain in a laboratory environment. The wild-type (normal) fruit flies exhibit phenotypes determined by the inheritance of at least one dominant allele. The mutants (non-wild-type) express phenotypes determined by the inheritance of two recessive alleles. Wild type Mutant Trait (dominant allele) (recessive allele) Body color Beige Ebony (black) Wing shape Normal wings Miniature wings (vestigial) Mendelian Genetics Problems Wild type Mutant Trait (dominant allele) (recessive allele) Body color Beige Ebony (black) Wing shape Normal wings Miniature wings (vestigial) 1. What is the probability of a heterozygous wild-type female fruit fly producing an egg with the dominant allele for body color? 2. What is the probability of a heterozygous wild-type female fruit fly producing an egg with the dominant allele for wing shape? 3. Based on the video, which probability rule should you use to determine the chance of getting both wild-type alleles in the gamete? 4. What is the probability of the wild-type female fruit fly producing an egg with wild-type alleles for body color and wing shape? Non-Mendelian Traits Non-Mendelian Traits Traits that don't follow Mendel's patterns are non-Mendelian traits. Inheritance patterns are often more complex than predicted by simple Mendelian genetics. There are other traits, called non-Mendelian traits, which do not follow Mendel's patterns of heredity. Traits controlled by more than one gene, alleles that are not clearly dominant, and genes with more than two alleles all lead to non-Mendelian patterns. For example, consider a polygenic trait, a trait controlled by more than one gene. These traits result from the additive — or quantitative — effects of two or more genes on a single phenotype. There can be from 5 to 100 genes involved! In humans, eye color, height, and skin color are all examples of quantitative traits. Another Non-Mendelian Trait Genes found on the same chromosome are linked genes. Humans have 20,000 to 25,000 genes that are located on 23 pairs of chromosomes. Genes that are located on the same chromosome, called linked genes, tend to be inherited together because the chromosome is passed along as a unit. During the process of crossing over in meiosis, genes may not be able to separate from each other. The probability that any two linked genes will move as a unit is a function of the distance between them. The closer the genes are, the greater the likelihood of linkage and the smaller the chance of crossing over. The percentage of crossing over is also known as the recombination frequency. Recombination refers to the effects of the crossing-over event during meiosis, when sections of DNA from the mother's chromosomes switch places with sections of DNA from the father's chromosomes. Linked Genes Let's analyze a genetic linkage map listing the relative positions of hypothetical genes b, cn, and vg along a chromosome. 1. According to the diagram above, which of the following pairs of genes has the greatest probability of being inherited together? 2. Predict why and how two genes that are located very far apart on a chromosome can give the appearance of independent assortment. Sex-Linked Traits Can you read the number? If so, you are not color-blind. Color blindness is a sex-linked trait in humans. X-linked traits are more common in males. So far, the offspring in the crosses have not been referred to as male or female. However, some genes are located on the sex chromosomes and may be inherited differently in daughters and sons. One example is color blindness. A trait that shows up differently in men and women, such as color blindness, is referred to as a sex-linked trait. Why are more men than women color-blind? The gene that causes color blindness is on the X chromosome. Color blindness is a recessive trait, which means that a person must possess at least one recessive allele to be color-blind. Women possess two X chromosomes, so they would need to possess two recessive alleles to be color-blind. Men possess one X chromosome, and they therefore only need to possess one recessive allele to be color-blind. Color vision genotypes: XNY = male, normal vision; XnY = male, color-blind vision XNYN, and XNYn are not correct genotypes. XNXN and XNXn are female genotypes, normal vision (the heterozygous genotype is a carrier); a female with XnXn is color-blind.