Mendel and Heredity Chapter 8. The Origins of Genetics The passing of traits from parents to offspring is called heredity. The scientific study of heredity

Mendel and Heredity

Chapter 8

The Origins of Genetics

• The passing of traits from parents to offspring is called heredity.

• The scientific study of heredity began more than a century ago with the work of an Austrian monk named Gregor Johann Mendel.


• Mendel was the first to develop rules that accurately predict patterns of heredity.

• The patterns that Mendel discovered form the basis of genetics, or the branch of biology that focuses on heredity.

• Mendel carried out his experiments using a garden pea plant.


• Mendel counted the number of each kind of offspring and analyzed the data

• Mendel’s method was on the cutting edge of research at the time.


• The garden pea is a good subject for studying heredity for several reasons:- Has many traits that have two clearly different forms that are easy to tell apart.

*Flower color- purple or white*Seed color- yellow or green*Seed Shape- round or wrinkled*Pod color- green or yellow*Pod Shape- round or bumpy*Flower position- side or top*Plant height- tall or short


• The mating can be easily controlled because the male and female reproductive parts are enclosed within the same flower.

–Can fertilize itself.

–You can transfer the pollen to another flower on a different plant.


• The pea plant is small, grows easily, matures quickly, and produces many offspring.

–Results can be obtained quickly, and there are plenty of subjects to count.


• Mendel’s experiments were monohybrid

crosses.

• A monohybrid cross is a cross that

involves one pair of contrasting traits.

• For example: crossing a plant with purple

flowers and a plant with white flowers is a

monohybrid cross.


• Mendel carried out his experiments in three steps:–Step 1: He allowed each variety of

garden pea to self pollinate for several generations.

–This method ensured that each variety was true breeding for a particular trait.


• These true breeding plants served as the parental generations in Mendel’s experiment.

• The parental generation, or the P generation, are the first two individuals that are crossed in a breeding experiment.


• Step 2: He cross pollinated two P generation plants that had contrasting forms of a trait (like purple flowers and white flowers).

• He called the offspring of this cross the first

filial generation or the F1 generation.

• He then examined and recorded the number

of F1 plants expressing each trait.


• Step 3: Mendel allowed the F1

generation to self pollinate.

• He called the offspring of the F1

generation plants the second filial

generation, or the F2 generation.

• Each plant was examined and counted the number expressing each trait.

Results

• Each F1 plant showed only one form of the trait. The contrasting form of the trait had disappeared.

• When allowed to self pollinate, the missing trait reappeared in some of the plants in the F2 generation.

Results

• Mendel put his findings in a ratio.

• He found the same 3:1 ratio of plants expressing the contrasting traits in the F2 generation.

Mendel’s Theory

• Before Mendel’s experiments, many people thought offspring was a blend of their parents.

• Example: If the dad was tall and the mom was short, then the child will be medium height.

• Mendel’s results did not support this.

Mendel’s Theory

• The four hypotheses Mendel developed were based directly on the results of his experiments.

• These now make up the Mendelian theory of heredity, which forms the foundation of genetics.

Mendel’s Hypotheses

• 1. For each inherited trait, an individual has two copies of the

gene- one from each parent.


• 2. There are alternative versions of the gene.

• Example: The gene for flower color in peas can exist in a “purple” version or a “white” version.

• Today, the different versions of a gene are called its alleles.


• An individual receives on allele from each parent.

• Each allele can be passed on when the individual reproduces.


• 3. When two different alleles occur together, one of them may be

completely expressed, while the other may have no observable effect on the

organisms appearance.


• Mendel described the expressed form of the trait as dominant.

• The trait that was not expressed when the dominant form of the trait was present was described as recessive.


• Dominant alleles are indicated by writing the first letter of the trait as a capital letter.

• Example: In pea plants, purple flower color is a dominate trait and is written as “P”.


• Recessive alleles are indicated by using the letter of the dominant trait, but the letter is lower case.

• Example: White flower color is recessive and is written as “p”.


• Example: If a plant has both purple and white alleles for flower color but blooms purple flowers, then purple is the dominate form of the trait; while white is the recessive form.

• The plants alleles look like (Pp).

• Note that the dominant form of the trait is written first, followed by the lower

case letter for the recessive form of the trait.


• 4. When gametes are formed, the alleles for each gene in an individual

separate independently of one another. Thus, gametes carry only one allele for

each inherited trait. When gametes unite during fertilization, each gamete

contributes one allele.


• Each parent can contribute only one of the alleles because of the way gametes are produced during the process of meiosis.

• Figure 8.5 page 164.

Modern Terms

• Geneticists have developed specific terms and ways of representing an individual’s

genetic makeup.

Modern Terms

• If the two alleles of a particular gene present in an individual are the same, the individual is said to be homozygous for that trait.

• Example: A plant with two white flower alleles (pp) is homozygous for flower color.

Modern Terms

• If the alleles of a particular gene present in an individual are different, the individual is heterozygous for that trait.

• Example: A plant with one purple flower allele (P), and one white flower allele (p) is heterozygous for flower color (Pp).

Modern Terms

• In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed.

• So, if it is Pp then the plant will have purple flowers.

Modern Terms

• Example: Freckles (F) is dominant, while no freckles (f) is recessive.

• If a person is heterozygous for freckles (Ff) then the person will have freckles, but is a carrier of the recessive gene.

Modern Terms

• The set of alleles that an individual has is called its genotype.

• The physical appearance of a trait is called a phenotype.

Modern Terms• Example:

• If Pp is the genotype of a pea plant, its phenotype is purple flowers.

• If pp is the genotype of a pea plant, its phenotype is white flowers.

Modern Laws

• Mendel’s ideas are often referred to as the Laws of Heredity.

• The Law of Segregation: States that two alleles for a trait segregate (separate) when gametes are formed.

Modern Laws

• The Law of independent assortment:

– Mendel conducted a dihybrid cross to determine whether the inheritance of one trait (like plant height) influenced the inheritance of a different trait (like flower color).

– He found that for the pairs of traits he studied, the inheritance of one trait did not influence the inheritance of any other trait.

Modern Laws

• Example: The alleles for plant height separated independently of

the alleles for flower color.

Studying Heredity

• Breeders must be able to predict how often a trait will appear when two animals are crossed (bred).

• One simple way of predicting the expected results of the genotypes of phenotypes in a cross is the use a Punnett square.

Studying Heredity

• A Punnett square is a diagram that predicts the expected outcome of a genetic cross by considering all possible combinations of gametes in the cross.

• Named for the inventor, Reginald Punnett, the simplest Punnett square consists of four boxes inside a square.

Studying Heredity

• The possible gametes that one parent can produce are written along the top of the square. (dominate traits labeled first).

• The possible gametes that the other parent can produce are written along the left side of the square.

• Fill each square with the letter from the top and on the side of the box.

Crosses

Punnett squares can be used to predict the outcome of a

monohybrid cross.

Crosses

• Can predict the results of a monohybrid cross between two pea plants that are both heterozygous (Yy) for seed color.– ¼ of the offspring would be expected to have

the genotype YY.– 2/4 would be expected to have the genotype

Yy.– ¼ of the offspring would be expected to have

the genotype yy.

Crosses

• Since the Y allele is dominant over the y allele:

– ¾ of the offspring would be Yellow.

– ¼ of the offspring would be Green.

Crosses

• Animal breeders and horticulturists are not always certain what

characteristics will turn up in the offspring, but they can use the

predictions from Punnett squares to cross individuals that they know will be most likely to produce offspring

with the desired phenotypes.

Determining unknown genotypes

• How might a horticulturist determine whether a pea plant

with a dominant phenotype, such as yellow seeds, is homozygous

(YY) or heterozygous (Yy)?


• The horticulturist could perfom a test cross, or a cross in which an individual whose phenotype is dominant, but whose genotype is not known.

• Example: A plant with yellow seeds but of unknown genotype (Y?) is test crossed with a plant with green seeds (yy).


• If all of the offspring produce yellow seeds, the genotype of the unknown plant must be YY.

• If half of the offspring produce yellow seeds and half produce green seeds, the genotype of the unknown plant must be Yy.


• In reality, if the cross produces even one plant that produces

green seeds, the genotype of the unknown parent plant is likely to

be heterozygous.

Probabilities

• Probability calculations can be used to

predict the results of genetic crosses.

• Probability is the likelihood that a

specific event will occur.

• Can be expressed in words, as

decimals, percentages, or as fractions.

Probabilities

• Probability can be determined by the following formula:

• Probability= # of one kind of possible outcome

Total number of all possible outcomes

Probabilities

• Consider the possibility that a coin

tossed into the air will land on heads

(one possible outcome).

• The total number of all possible

outcomes is two- heads or tails.

• Thus, the probability that a coin will land

on heads is ½.

Probabilities

• If a pea plant has two alleles for seed color, that individual can contribute either allele (yellow or green) to the gamete it produces.

• The probability that a gamete will carry the allele for green seed color is ½.

• The probability that a gamete will carry the allele for yellow seed color is ½.

Probabilities

• Since two parents are involved in a genetic cross, both parents

must be considered when calculating the probability of the

outcome of a genetic cross.

Probabilities

• The allele carried by the gamete from the first parent does not depend on the allele carried by the gamete from the second parent.

• The outcomes are independent of each other.

Probabilities

• To find the probability that a combination of two independent events will occur, multiply the separate probabilities of the two events.

• Thus, the probability that a nickel and a penny will both land on heads is

½ x ½ = ¼

Pedigree

• How would you find out the chances of passing the trait to your children?

• Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations.

Pedigree

• Pedigrees are helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the

disorder.

Pedigree

• Carriers are individuals who are heterozygous for an inherited disorder but do not show symptoms of the disorder.

• Carriers can pass the allele for the disorder to their offspring, resulting in the child having the disorder.

Pedigree

• In the genetic disorder albinism, the body is unable to produce an enzyme necessary for the production of melanin.

• Without melanin, an organism’s surface coloration may be milky white and its eyes may be pink.

Autosomal or sex linked

• If a trait is autosomal, it will appear in both sexes equally. (autosome is a chromosome other than X or Y).

• If a trait is sex linked, it is usually seen only in males.

• A sex linked trait is a trait whose allele is located on the X chromosome.

Sex linked

• Most sex linked traits are recessive.

• Because males have only one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex linked condition.

Sex linked

• A female who carries a recessive allele on one X chromosome will not exhibit the condition if there is a dominant allele on her other X chromosome.

• She will express the recessive condition only if she inherits two recessive alleles.

Sex linked

• In other words, a female’s chances of inheriting and

exhibiting a sex linked condition are significantly less than males!

Dominant or Recessive

• If the trait is autosomal dominant, every individual with the trait will have a parent with the trait.

• If the trait is autosomal recessive, an individual with the trait can have one, both or neither parent exhibiting the trait.

Heterozygous or Homozygous

• If individuals with autosomal traits are homozygous dominant (YY) or heterozygous (Yy), their phenotype will show the dominant characteristic.

• If individuals are homozygous recessive (yy), their phenotype will show the recessive characteristics.

Heterozygous or Homozygous

• Two people who are heterozygous carriers of a

recessive mutation will not show the mutation, but they can produce children who are

homozygous for the recessive allele.

Traits

• Most of the time, traits, such as hair color in horses, display more complex patterns of heredity than

the simple dominant recessive patterns.

Traits

• When several genes influence a trait, the trait is said to be a polygenic trait.

• The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes.

• Determining the effect of any one of these genes is difficult.

Traits

• Due to independent assortment and crossing over during meiosis, many different combinations appear in offspring.

• Examples: polygenic traits in humans include eye color, height, weight, and hair color.

Intermediate Traits

• In some organisms, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance.

• Example: Incomplete dominance in plants occurs when a snapdragon with red flowers is crossed with a snapdragon with white flowers to produce a pink snapdragon.

Intermediate Traits

• A child of straight haired parents and a curly haired parent will have wavy hair.

• Straight and curly hair are homozygous dominate traits!!

• Wavy hair is heterozygous and is intermediate between straight and curly hair.

Codominance

• For some traits, two dominant alleles are expressed at the same time.

• When two dominant alleles are

expressed at the same time, both

forms of the trait are displayed, this is

a phenomenon called codominance.

Codominance

• For example: a roan (mixed) coat in horse that consist of red and white hairs.

Multiple Alleles

• Genes with three or more alleles are said to have multiple alleles.

• For example: in the human population, the ABO Blood Groups are determined by three alleles: IA, IB, i

Multiple Alleles

• The i allele means that neither carbohydrate is present.

• The IA and IB alleles are both dominant over i, which is recessive.

• When IA and IB are both present in the genotype, they are codominant.

Multiple Alleles

• Combinations of the three different alleles can produce four different blood types- A, B, AB, O

• O is the universal donor, and AB is the universal receptor.

Blood Types

• IAIA and IAi will give you Type A blood.

• IBIB and IBi will give you Type B blood.

• IAIB will give you Type AB blood.

• ii will give you Type O blood.

Traits influenced by environment

• An individual’s phenotype often depends on conditions in the environment.

• Hydrangea flowers of the same genetic variety range in color from blue to pink, depending on the acidity of the soil.

• Acidic soil will bloom blue flowers.• Basic soil will bloom pink flowers.


• The artic fox has enzymes that make pigments in warm temperatures. So in the summer the fox’s coat is reddish brown.

• During the winter, the enzymes do not function and the coat is white.


• In humans, height is influenced by the environment (nutrition).

• Exposure to the sun, an external environmental condition, alters skin color.

Mutations

• In order for a person to develop and function normally, the proteins encoded by his or her genes must function precisely.

• Changes in genetic material are called mutations.

• Mutations are rare because cells have efficient systems for correcting errors, but when they do not occur they have harmful effects.

Mutations

• Harmful effects produced by inherited mutations are called

genetic disorders.

Sickle Cell Anemia

• An example of a recessive genetic disorder is sickle cell anemia, a condition caused by a mutated allele that produces a defective form of the protein hemoglobin.

• Hemoglobin transports oxygen and gives blood the red color.

Sickle Cell Anemia

• In sickle cell anemia, the defective form of hemoglobin causes many red blood cells to bend into a sickle shape.

• It can rupture easily, resulting in less oxygen being carried by the blood.

• These cells also tend to get stuck in blood vessels and can cause a clot.

Hemophilia

• Another recessive genetic disorder is

hemophilia, a condition that impairs

the blood’s ability to clot.

• It is a sex linked trait.

• More than a dozen genes code for

the proteins involved in blood clotting.

Hemophilia

• A mutation on one of these genes on the X chromosome causes the form of hemophilia called hemophilia A.

Huntington’s Disease

• Huntington’s disease is a genetic disorder caused by a dominant allele located on an autosome.

• Symptoms- mild forgetfulness and irritability- appear in victims in their 30’s and 40’s.

Huntington’s Disease

• HD causes loss of muscle control, uncontrollable physical spasms, severe mental illness, and eventually death.

• Most people who have the HD allele do not know they have the disease until after they have had children.

• It is unknowingly passed on from one generation to the next.

• Would you want to know if you had a genetic disorder,

or could pass one on to your children?

Detecting Genetic Disorders

• Most disorders cannot be cured, although progress is being made.

• A person with family history of genetic disorders may wish to undergo genetic counseling before becoming a parent.


• Genetic counseling is a form of medical guidance that informs people about genetic problems that could affect them or their offspring.

• Therapy is available to treat a genetic disorder if it is diagnosed early enough.


• Gene technology may soon make it possible for scientists to correct certain genetic disorders by replacing defective genes with copies of healthy ones, a technique called gene therapy.

• This is being used by scientists seeking cures for many genetic disorders, including cystic fibrosis and muscular dystrophy.

Documents

Mendel and Heredity Chapter 8. The Origins of Genetics The passing of traits from parents to offspring is called heredity. The scientific study of heredity