Journal•What is genetics? What is heredity?

Mendelian Genetics

Where does an organism get its unique characteristics?

• An individual’s characteristics are determined by factors that are passed from one parental generation to the next.

• Every living thing has a set of characteristics inherited from its parent or parents.

• The delivery of characteristics from parent to offspring is called heredity.

• The scientific study of heredity, known as genetics, is the key to understanding what makes each organism unique.

Genetics•The science of

heredity

Heredity• The set of characteristics an

organism receives from its parents

• Not only the science of humans but also of many other organisms

Gregor Mendel•Studied heredity carefully & objectively using pea plants

“Father of Genetics”

• The modern science of genetics was founded by an Austrian monk named Gregor Mendel.

• Mendel was in charge of the monastery garden, where he was able to do the work that changed biology forever.

•

– Mendel carried out his work with ordinary garden peas, partly because peas are small and easy to grow. A single pea plant can produce hundreds of offspring.

– Today we call peas a “model system.”

• Scientists use model systems because they are convenient to study and may tell us how other organisms, including humans, actually function.

What did Mendel know?• Mendel knew that the male part of each

flower makes pollen, which contains sperm—the plant’s male reproductive cells.

• Similarly, Mendel knew that the female portion of each flower produces reproductive cells called eggs.

–During sexual reproduction, male and female reproductive cells join in a process known as fertilization to produce a new cell.

– In peas, this new cell develops into a tiny embryo encased within a seed

Pea Plants can reproduce through:

1. Cross-pollination

2. Self-pollination

Self-Pollination• Pollen from a flower fertilizes the

egg cells on the same flower• A plant grown from a seed

produced by self-pollination inherits all of its characteristics from the single plant that bore it. In effect, it has a single parent.

• Offspring have one parent

True-Breeding Stock• Always passes its characteristics

to the next generation• Produce offspring with identical

traits to themselves• Ex. True-breeding stock of pea

plants always produces tall plants with green pods

Cross-Pollination• Pollen from one plant fertilizes eggs

from another plant

• Offspring have 2 parents

a.k.a. crossThis process, known as cross-

pollination, produces a plant that has two different parents.

Trait• A specific characteristic of an

individual and may vary from one individual to another

• A characteristic that distinguishes one individual from another

Mendel experimented using cross-pollination.

Mendel Studied 7 Traits

• Seed shape

• Seed color• Seed coat

color

• Pod shape• Pod color• Flower

position• Plant height

Hybrid• When two plants cross or

cross pollinate the offspring is called a hybrid

• HybridOffspring of parents with different characteristics

• Ex. Tall plant X short plant

F1 Generation•The hybrids•F stands for filius meaning son in latin

P Generation•True breeding generation

•“Parents”

•Mendel crossed the F1 generation among themselves

F2 Generation•Result of a cross between two F1 hybrids

•From the F1 X F1 crosses Mendel discovered several rules concerning how traits were transferred from generation to generation

Genes• The name given to different

traits of an organism• The unit that determines traits;

the traits of an organism

• Ex. If a plant is tall, it has a gene for tallness.

•Mendel concluded from each of the traits that each pea plant must contain two traits—one from each parent

Alleles•Different forms of a gene for a specific trait

•Ex. The gene that determines height in pea plants has 2 alleles; one that produces a tall plant & one that produces a short plant

• Some genes have only 2 alleles while others have dozens of different alleles

• Alleles are represented by either uppercase or lowercase letters

Two Types of Alleles•Dominant Allele•Recessive Allele

Dominant Allele

•Allele that will be expressed

• Ex. If a plant has one allele for tallness & one for shortness, then the plant will be tall because the dominant allele is the allele for tallness & that will be expressed by the plant

•Expressed by Uppercase Letters–Ex. Dominant allele for tallness: “T”

Recessive Allele• Allele that will not be expressed• An organism with a recessive

allele for a trait will exhibit that form only when the dominant allele for that trait is not present.

• Represented by lowercase letters

• If a plant has a tallness allele & an allele for shortness, the plant will be tall, the shortness allele would be recessive.

• The plant would be tall not short.

•Example: Recessive Allele for tallness “t”

Homozygous Alleles• Have 2 alleles for the same trait

• The organism will have a pair of identical alleles—either two dominant or two recessive

•Homozygous Tall—TT •Homozygous short—tt •TT—homozygous dominant

• tt—homozygous recessive

Heterozygous Alleles

•Have 2 opposite alleles•The organism will have one dominant & one recessive allele

•Tt—heterozygous tall; the plant will be tall, but will carry one dominant & one recessive allele

• Naming & labeling all the different kinds of alleles & gametes allowed Mendel to do something called a test-cross to determine how organisms would look after mating—called the crosses Punnett Squares

Punnett Squares

Punnett Square• A chart that illustrates Mendel’s test-crosses

between organisms• Punnett squares allow you to predict the

genotype and phenotype combinations in genetic crosses using mathematical probability.

• Allows one to determine how gene traits will result after two parent alleles have crossed

Phenotype• The form of the trait that an

organism displays• A plant can express a phenotype

for either tallness or shortness no matter what pair of alleles it has. It may be homozygous tall ( TT ) or heterozygous tall ( Tt ), but the phenotype will still be “tall”

•Think P for physical—what you see the organism has

Genotype•An organism’s genetic composition

• It will give the actual gametes that make up the genetic trait.

• A plant can express a phenotype for either tallness or shortness

• More specific than the phenotype

•Tells which type of tall or short plant

• It will determine if a plant is homozygous tall or heterozygous tall

–The genotype of an organism is inherited, whereas the phenotype is formed as a result of both the environment and the genotype.

– Two organisms may have the same phenotype but different genotypes.

Mendel’s Laws (acquired from Test-crossing)

• Law of Unit Characteristics• Law of Segregation• Law of Independent Assortment

Law of Unit Characteristics

• There are units in a cell that are responsible for traits, and these units come in pairs (otherwise known as alleles)

•Each offspring he believed received one allele from each parent.

•Each gamete carries an allele.

Law of Segregation

• States that the alleles for a trait separate when gametes are formed. The allele pairs are then randomly united at fertilization

Law of Independent Assortment

• The principle of independent assortment states that genes for different traits can segregate independently during gamete formation.

• Genes for different traits can segregate independently during the formation of gametes

•Helps account for the many genetic variations observed in plants, animals, and other organisms

Monohybrid Cross

• a genetic cross containing ONE TRAIT

• (ex. The trait for tallness)

How To Make a Punnett Square for a One-Factor Cross

– Write the genotypes of the two organisms that will serve as parents in a cross.

– In this example we will cross a male and female birds that are heterozygous for large beaks. They each have genotypes of Bb.

– Bb and Bb

Determine what alleles would be found in all of the possible gametes that each parent could produce.

– Draw a table with enough spaces for each pair of gametes from each parent.

– Enter the genotypes of the gametes produced by both parents on the top and left sides of the table.

Fill in the table by combining the gametes’ genotypes.

– Determine the genotypes and phenotypes of each offspring.

– Calculate the percentage of each. In this example, three fourths of the chicks will have large beaks, but only one in two will be heterozygous

Example of a Monohybrid Cross

• In pea plants, round seeds ( R ) are dominant to wrinkled ( r ). In a genetic cross of two plants that are heterozygous for the seed shape trait, what are the phenotypic and genotypic ratios?

Parents: Rr X Rr

RR Rr

Rr rr

R rR

r

Genotypes:

¼ RR, 2/4 Rr, ¼ rr

Phenotypes:

¾ Round, ¼ wrinkled

Dihybrid Cross•Crosses with 2 traits•From these crosses Mendel came up with the Law of Independent Assortment

Example • In pea plants, round ( R )are dominant

to wrinkled(r). Also tall plants (T) are dominant to short (t). In a genetic cross of 2 plants in which one plant is heterozygous round, heterozygous tall (RrTt) and a second plant is homozygous recessive wrinkled, homozygous recessive short (rrtt), what are the phenotypic and genotypic ratio?

Journal

In humans widow’s peak (W) is dominant over straight hairline (w). A heterozygous man for this trait marries a woman who is also heterozygous.

• a. List possible genotypes of their offspring.• b. List the phenotypic ratio for their

children.

Exceptions to the Rules

• Incomplete Dominance• Codominance• Pleiotropy• Epistasis• Polygenic Traits

Incomplete Dominance

•The appearance of F1 hybrids that appear half way in between the two parents

Example• If a red flowering plant were

mated to a white flowering plant one would expect red or white to be the colors of the offspring. In this case all of the offspring were pink in color.

• A cross between red-flowered snapdragons and white-flowered snapdragons produces offspring with pink flowers. Let R = red and W = white flowers. Suppose a pink-flowered plant is crossed with a pink-flowered plant, what are the genotypes and phenotypes of the offspring?

Codominance•Both alleles of a gene are expressed without a recessive

Example• If white feathers & black

feathers were both dominant traits in a chicken, then a heterozygous chicken (one with an allele for white & black feathers) will have both feather colors

Pleiotropy• When a single gene influences

multiple phenotypic traits

• A new mutation in a gene will have an effect on all traits simultaneously

Epistasis• The interaction between two or

more genes to control a single phenotype

• Occurs when more than one gene is needed to control one trait

Polygenic Traits• Traits determined by multiple

alleles for a characteristic

• Ex. Skin color—there are 6 genes responsible