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