What is Heredity? • Heredity is the passing of genetic material from parents to offspring • Gregor Mendel – an Austrian monk who began to study the inheritance traits of pea plants in the monestary’s garden • He studied seven characteristics of pea plants • A characteristic is a feature that has different forms • A trait is the different form of the characteristic • Mendel studied true – breeding plants – plants that will always produce offspring with a certain trait if allowed to self pollinate Mendel’s Findings • All the pea pods produced from the first generation were green • Mendel called the green pea pod trait the dominant trait • Mendel called the yellow pea pod trait the recessive trait • Mendel allowed this first generation of plants to self – pollinate • The second generation of pea pods produced ¾ green peas pods and ¼ yellow • Mendel showed that plants could still pass on the yellow trait even when it didn’t show up How Are Traits Inherited? • Genes are segments of DNA found in chromosomes that give instructions for producing a certain characteristic • All offspring have two versions of the same gene • The different versions are called alleles • These genes are represented by letter symbols • The capital letter means the allele is the dominant allele • The lower case letter means the allele is the recessive allele • An organism with two dominant or recessive alleles is homozygous for that trait • An organism with one dominant and one recessive alleles is heterozygous for that trait Genes Influence Traits • The combination of alleles that you inherited from your parents is your genotype • Your observable traits make up your phenotype • Some traits are dominant over others • The dominant allele contributes to the phenotype if one or two copies are present in the genotype • The recessive allele contributes to the phenotype only when two copies of it are present • If one chromosome in the pair has a dominant allele and the other contains a recessive allele, the phenotype is determined by the dominant allele • This shows complete dominance – one trait is completely dominant over another Many Genes/One Trait Many Traits/One Gene • Some characteristics such as eye color are the result of several genes acting together • Different combinations of alleles can result in different shades of eye color • Many traits do not have simple patterns of inheritance • Sometimes one gene influences more than one trait • A single gene causes the tiger to have white fur • The same gene influences the blue eye color • Many genetic disorders are linked to a single gene but affect many traits Environment Influences Traits • Environment can influence an organism’s phenotype • The gene for fur color in the arctic fox is affected by daylight • In winter, the daylight is short and fur turns white • In summer daylight is longer, fur color turns brown • Environment influences our growth • You may carry the gene for tallness, but without a healthy diet, you may not reach your full height • Traits that are learned are not inherited – bike riding Bending the Rules • Some human traits follow the pattern for complete dominance others do not • In incomplete dominance, each allele in a heterozygous individual influences the phenotype • The result is a blend of the phenotypes of the parents • Both alleles of the gene have some influence • Hair texture is an example of incomplete dominance -one straight hair allele and one curly hair allele will produce wavy hair Codominance • A trait that shows codominance has both of the alleles in a heterozygous individual contribute to the phenotype • Heterozygous individuals have both of the traits associated with the their two alleles • An example in humans is blood type • There are three alleles for blood type: A, B, and O • The A and B alleles are codominant • The blood type is AB Punnett Squares • One tool for understanding the patterns of heredity is a diagram called a Punnett square • A Punnett square is a graphic used to predict possible genotypes of offspring • Each parent has two alleles for a particular gene • An offspring receives one allele from each parent for a particular trait • A Punnett square shows all the possible allele combinations in the offspring • A Punnett square does not tell you what the exact results of a certain cross will be • It only helps you find the probability that a certain genotype will occur • Probability is the mathematical chance of a specific outcome in relation to the total number of possible outcomes • Probability can be expressed as a ratio • a ratio is written as 1:4 or ¼ and is read as “one to four” • Another way of expressing probability is percentage • A percentage states a certain outcome out of 100 Pedigree Charts • A pedigree is another tool used to study patterns of inheritance • It traces the occurrence of a trait through generations of a family • Pedigrees are useful in tracing a class of inherited disorders known as sex – linked disorders • Examples of sex – linked disorders are hemophilia and colorblindness Sex – Linked Disorders • Sex – linked disorders are associated with an allele on a sex chromosome • Many sex – linked disorders are caused by an allele on the X chromosome • Women have two X chromosomes so a woman can have one allele for a sex – linked disorder without having the disorder • A woman who is heterozygous for the trait is a carrier • Men have just one X chromosome • This single chromosome determines if the trait is present Cracking the Code • The genetic material in cells is contained in a molecule called DNA • DNA carries the information for cells to grow, divide and function • DNA is described as a code • A code is a set of rules and symbols used to carry information • Computers use a code of ones and zeros that is translated to letters, numbers and graphics • Many scientists over the world have contributed to our current understanding of DNA Structure of DNA • The structure of DNA is a twisted ladder shape called a double helix • The two sides of the ladder, called the backbone, are made of alternating sugars and phosphate groups • The rungs of the ladder are made of a pair of bases, each attached to one of the sugars • A base, a sugar and a phosphate group make up a building block of DNA known as a nucleotide Nucleotides • There are four different nucleotides in DNA • They are identified by their bases: adenine (A), thymine (T), Cytosine (C), and guanine (G) • Adenine always pairs with thymine (A – T) • Cytosine always pairs with guanine (C – G) • The order of the nucleotides in DNA is a code that carries information • Genes are segments of DNA that describe a different trait • Each gene reads a part of the code - this code stores information about which protein it should build – the protein built determines your trait Replication • Cells are able to make copies of DNA molecules through a process known as replication • Two strands of DNA separate • The bases on each side of the molecule are used as a pattern for a new strand • As the bases on the original are exposed, complementary nucleotides are added • When replication is complete, there are two identical DNA molecules • Replication occurs before cell divides so that each new daughter cell will have a complete set of instructions Mutations • Changes in the number, type, or order of bases on a piece of DNA are known as mutations • If a base is left out, it is known as a deletion • If a base is added, it is known as an insertion • The most common mutation happens when one base replaces another – substitution • Occurs due to random errors but can be influenced by chemical agents known as mutagens – ultraviolet light and cigarettes are examples • Mutations may be beneficial, harmful or neutral • A genetic disorder results from mutations that harm the normal function of a cell Protein Factory • When the cell uses DNA to build proteins, it only needs some of the information stored in the DNA molecule • Some of the information in the DNA is copied to a separate molecule called ribonucleic acid, or RNA • The copy is used to build the protein • RNA has a similar structure to DNA, but instead of thymine (T), RNA contains the base uracil (U) • Three types of RNA: messenger RNA, ribosomal RNA and transfer RNA • Each type has a special role