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