Heredity Genetics is how genes control what we inherit Inheriting Traits • Heredity: the passing of traits from parents to offspring • What is Genetics? – Genes control an organism’s form (shape, size, etc.) and function (how everything works) – Different forms of a gene are Alleles example: a gene will code for dimples or no dimples Mendel: Father of Genetics Science + Math Gardener • Began experimenting with pea plants in 1856 • 1st to trace one gene through several generations • 1st to use mathematics of probability to explain heredity • Not validated until 1900 Genetics: the study of how traits are inherited through interactions of alleles – Example: gene in female sex cell may code for dimples, gene in male sex cell may code for no dimples – the interaction of these genes determines if you will have dimples or not D is dimples and d is no dimples. A pair of chromosomes with the alleles DD separate during meiosis. After sister chromatids separate during Meiosis II, two male sex cells (sperm) have the alleles D and D. The two female sex cells (eggs) have the alleles d and d. DD dd male D D d d female Fertilization results in a zygote inheriting one allele from each sex cell (male and female). How these alleles interact determines which trait will be expressed in the offspring (dimples, D, or no dimples, d?) One allele is dominant, and that is the allele that is expressed (that physically occurs in the organism). Dimples is represented by an uppercase letter, so it is dominant, and if an organism carries Dd, then you will observe dimples. Genetics in a Garden: purebred • An organism that always produces the same traits generation after generation is called a purebred, which means that it only carries alleles for one trait (like DD for dimples). Therefore, there is only one trait that can possibly be expressed, and only one trait that can be passed on to offspring. (If you cross two plants purebred for green seeds, then the 100% of offspring will have green seeds.) It takes many generations of breeding to produce a purebred. • Purebred green: GG Purebred yellow: yy Dominant and Recessive • When two purebred plants are crossed, the dominant gene is expressed (it dominates, or covers up, the recessive gene). G is the allele for green, and y is yellow. gg YY male g g Y Y female fertilization Yg Yg Both offspring look yellow. Do they still carry the green gene? Genetics in a Garden • Each time Mendel studied a trait, he crossed two plants with different expressions of the trait and found that the new plants all looked like one of the two parents. (ex: a plant with smooth seeds crossed with wrinkled seeds) • He called these new plants hybrids (HI brudz) because they received different genetic information, or different alleles, for a trait from each parent. Purebred Yellow: (after many generations) GG yy Cross-pollination: breeding two plants purebred for different traits on the same gene (alleles) Hybrid: has characteristic Gy from one of two different parents Alleles determine traits Most cells have two alleles for every trait (one from each parent) – Homozygous cell: has two alleles that are the same GG – Heterozygous cell: has two alleles that are different Gy Genotype: genetic makeup Phenotype: the way an organism looks and behaves as a result of the genotype GG yy male G G y y female fertilization Gy Gy Genotype: green and yellow Phenotype: green Dominant and Recessive Factors • Mendel called the tall form the dominant factor because it dominated, or covered up, the short form. Dominant (W) • He called the form that seemed to disappear the recessive factor. Recessive (s) What are the possible genotype(s) for the phenotype “widow’s peak”? What are the possible genotype(s) for the phenotype “straight hairline”? Punnett Squares • How could you predict what the offspring would look like without making the cross? • A handy tool used to predict results in Mendelian genetics is the Punnett square. • The Punnett square shows the mathematical probability that specific allele combinations will be inherited; and therefore, that specific traits will be expressed. Punnett Squares • In a Punnett square, letters represent dominant and recessive alleles. • An uppercase letter stands for a dominant allele. • A lowercase letter stands for a recessive allele. If Y is dark yellow and y is light yellow, then what is the % chance of an offspring being dark yellow? Light yellow? Section Check 1 Question 2 According to this diagram, if meiosis proceeds correctly, how many alleles of a particular gene can a female pass on to her offspring? Section Check 1 Answer Although she has two alleles of each gene, a mother can pass only one allele to her offspring. Meiosis separates alleles so that eggs have only one allele for each gene. The new individual then gets one allele from the mother and the other from the father. Section Check 1 Question 3 Mendel crossed pea plants that were pure-bred for yellow seeds with plants that were pure- bred for green seeds. All the offspring of this cross had yellow seeds. Based on these results, which form of color was recessive and which was dominant? Section Check 1 Answer Green seed color was recessive and yellow seed color was dominant. Mendel called the form that seemed to disappear (green in this case) recessive and the form that covered up (yellow in this case) dominant. Genetics Since Mendel 2 Incomplete Dominance • When the offspring of two homozygous parents show an intermediate phenotype, this inheritance is called incomplete dominance. • Examples of incomplete dominance include the flower color of some plant breeds and the coat color of some horse breeds. Genetics Since Mendel 2 Multiple Alleles • Many traits are controlled by more than two alleles. • A trait that is controlled by more than two alleles is said to be controlled by multiple alleles. Genetics Since Mendel 2 Multiple Alleles • Traits controlled by multiple alleles produce more than three phenotypes of that trait. • Blood type in humans is an example of multiple alleles that produce only four phenotypes. • The alleles for blood types are called A, B, and O. Genetics Since Mendel 2 Multiple Alleles • When a person inherits one A allele and one B allele for blood type, both are expressedphenotype AB. • A person with phenotype A blood has the genetic makeup, or genotypeAA or AO. Genetics Since Mendel 2 Multiple Alleles • Someone with phenotype B blood has the genotype BB or BO. • Finally, a person with phenotype O blood has the genotype OO. Polygenic Inheritance • A group of gene pairs works together to control one trait. Your height and the color of your eyes and skin are just some of the many human traits controlled by polygenic inheritance. • It is estimated that three to six gene pairs control your skin color. • The environment also plays an important role in the expression of traits controlled by polygenic inheritance. Impact of the Environment • Although genes determine many of your traits, you might be able to influence their expression by the decisions you make. • For instance, if some people at risk for skin cancer limit their exposure to the Sun and take care of their skin, they might never develop cancer. Human Genes and Mutations • Occasionally errors occur in the DNA when it is copied inside of a cell. • Such changes and errors are called mutations. • Not all mutations are harmful. They might be helpful or have no effect on an organism. Recessive Genetic Disorders • Many human genetic disorders, such as cystic fibrosis, are caused by recessive genes. • Some recessive genes are the result of a mutation within the gene. • Many of these alleles are rare. Recessive Genetic Disorders • Such genetic disorders occur when both parents have a recessive allele responsible for this disorder. • Because the parents are heterozygous, they don’t show any symptoms. The parents are carriers of the disorder. Recessive Genetic Disorders • If each parent passes the recessive allele to the child, the child inherits both recessive alleles and will have a recessive genetic disorder. Recessive Genetic Disorders • Cystic fibrosis is the most common genetic disorder that can lead to death among Caucasian Americans. • In most people, a thin fluid is produced that lubricates the lungs and intestinal tract. • People with cystic fibrosis produce thick mucus instead of this thin fluid. • This buildup often results in repeated bacterial respiratory infections. • The thick mucus builds up in the lungs and makes it hard to breathe. Sex Determination • Each egg produced by a female normally contains one X chromosome. • Males produce sperm that normally have either an X or a Y chromosome. Sex Determination • When a sperm with an X chromosome fertilizes an egg, the offspring is a female, XX. • A male offspring, XY is the result of a Y-containing sperm fertilizing an egg. Sex-Linked Disorders • An allele inherited on a sex chromosome is called a sex- linked gene. • Color blindness is a sex- linked disorder in which people cannot distinguish between certain colors, particularly red and green. Sex-Linked Disorders • This trait is a recessive allele on the X chromosome. • Because males have only one X chromosome, a male with this allele on his X chromosome is color-blind. • A color-blind female occurs only when both of her X chromosomes have the allele for this trait. Pedigrees Trace Traits • A pedigree is a visual tool for following a trait through generations of a family. • Males are represented by squares and females by circles. Why is it important to distinguish between males and females when tracing color blindness through a family? Pedigrees Trace Traits • A completely filled circle or square shows that the trait is seen in that person. • Half-colored circles or squares indicate carriers. • People represented by empty circles or squares do not have the trait and are not carriers. How do you describe the genotype of an individual who is a carrier of a trait? Using Pedigrees • A pedigree is a useful tool for a geneticist. • When geneticists understand how a trait is inherited, they can predict the probability that a baby will be born with a specific trait. Using Pedigrees • Pedigrees also are important in breeding animals or plants. • These organisms are bred to increase their yield and nutritional content. Chromosome Disorders • Every organism has a specific number of chromosomes. • However, mistakes in the process of meiosis can result in a new organism with more or fewer chromosomes than normal. Chromosome Disorders • If three copies of chromosome 21 are produced in the fertilized human egg, Down’s syndrome results. • Individuals with Down’s syndrome can be short, exhibit learning disabilities, and have heart problems. 02/01/10 • How do two alleles of a gene result in one expressed trait? What determines which alleles are inherited? • If you cross a green pea plant and a yellow pea plant, what are the possible alleles that the offspring carries? • If the green pea plant and the yellow pea plant produce 500 offspring and they are all green, what hypothesis can you make about the dominance, or strength, of the green and yellow alleles?
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