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Ch. 14: Mendel and the Gene Idea. Introduction A. Heritable traits (brown eyes, green eyes, blue eyes) are passed down from parents to offspring. B. Blending hypothesis: Offspring should have a blend of parental traits. (Yellow + Blue = Green).

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Ch. 14:

Mendel and the Gene Idea

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  • Introduction

  • A. Heritable traits (brown eyes, green eyes,

  • blue eyes) are passed down from parents

  • to offspring.

  • B. Blending hypothesis: Offspring should

  • have a blend of parental traits.

  • (Yellow + Blue = Green)

 This blending hypothesis is incorrect.

  • Particulate inheritance: parental genes

  • retain their separate identities, and are

  • sorted and then passed on to future

  • generations.

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  • Gregor Mendel

  • A. An Augustinian monk who studied

  • the sciences and math. His studies

  • influenced his experimentation in genetics.

  • In 1857, Mendel began breeding garden

  • peas to study inheritance.

  • Peas were an ideal subject to study:

  • a. Many heritable traits (color, height,

  • pod shape, etc.)

  • b. Easy to control mating

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- Self-pollination

- Cross-pollination

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  • Mendel’s experiment:

  • 1. Mendel cross-pollinated to hybridize

  • two contrasting, true-breeding pea

  • varieties.

  • - True-breeding: P generation

  • - Offspring: F1 generation

  • Mendel then allowed for the F1

  • generation to self-pollinate to produce

  • the F2 generation.

  • Mendel quantitative analysis of the

  • F2 generation enabled him to come up

  • with two important principles of heredity:

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  • Law of segregation: two alleles for a

  • trait are packaged into separate gametes.

  • When the P generation cross-pollinated,

  • the F1 generation were all purple.

  • When the F1

  • generation

  • self-pollinated,

  • the F2

  • generation had

  • both purple

  • and white.

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The ratio between purple and white

flowers in the F2 generation were 3:1.

  • Mendel reasoned

    that though the

    F1 generation had

    no white flowers,

    they must have

    carried the

    heritable trait for

    white flower color.

  • Mendel said that the purple color gene

    was “dominant” and the white color

    gene was “recessive.”

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Example: Round v. Wrinkled pea seeds

F1 = 100% Round

F2 = 75% Round, 25% Wrinkled

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Ex. Purple and

white color genes

have different

DNA sequences.

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  • If two alleles inherited are different, then

  • the dominant allele will be expressed.

  • The recessive allele will have no effect.

  • The two alleles segregate during gamete

  • production; homologues will separate

  • when gametes are made.

  •  This is the Law of Segregation

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  • Mendel’s Law of Segregation accounts

  • for his observation of 3:1 in the F2

  • generation:

  • a.The F1 generation will create two kinds

  • of gametes = half will have the white

  • allele, half will have the purple allele.

  • During self-pollination, the gametes

  • will combine randomly to form four

  • combinations.

  • A Punnett Square can be made to

  • predict the results of a genetic cross:

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  • An organism with two different alleles

  • for a character is heterozygous for that

  • trait.

  • An organisms genetic makeup is called

  • its genotype.

  • An organisms physical traits is called

  • its phenotype.

  •  Two organisms can have the same

  • phenotype but different genotypes.

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The only way

to produce a

white offspring

is to have two





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  • The only way to know the genotype of

  • an organism with

  • a dominant traits

  • is by doing a

  • “test-cross.”

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  • Mendel did other experiments that

  • followed the inheritance of two different

  • characters in a dihybrid cross

  • (v. monohybrid).

  • Mendel studied seed shape and color:

  • Y = Yellow, y = Green

  • R = Round, r = wrinkled

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  • Mendel crossed true-breeding plants

  • that had yellow, round seeds (YYRR)

  • with true-breeding plants that has

  • green, wrinkled seeds (yyrr).

  • One possibility is that the two

  • characteristics could be transmitted as

  • a package.

  •  F1 = Yellow, Round

  •  F2 = 3 Yellow, Round:

  • 1 Green, Wrinkled

** However, this was not the results of

Mendel’s experiment.

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  • F1 = Yellow, Round

    Their gametes can have four

    combinations: YR, Yr, yR, yr

Therefore, the ratios for the F2

generation is:

F2 = 9 Yellow, Round (Yy, Rr):

3 Yellow, Wrinkled (Yy, rr):

3 Green, Round (yy, Rr):

1 Green, Wrinkled (yy, rr)

** Whenever Mendel did a dihybrid cross,

he always got the 9:3:3:1 ratio. This

can be explained as the result of the

“Law of Independent Assortment.”

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  • Mendel chose to study pea characteristics

  • that had simple genetic; each phenotypic

  • characteristic was determined by one gene.

  • However, in most cases, the relationship

  • between phenotype and genotype is

  • rarely simple.

  • Incomplete

  • Dominance: F1 hybrids (heterozygotes)

  • show a intermediate phenotype:

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P = Red, White

F1 = 100% Pink

F2 = 25% Red

50% Pink

25% White

  • Mendel’s peas

    showed that


    and homozygous

    dominant plants

    had the same

    phenotype. This is

    complete dominance.

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  • Codominance: two alleles affect the

  • phenotype in distinguishable ways.

  • Example: Blood types A, B, AB, and O

  • Tay-Sach’s Disease

  • Because an allele is dominant does not

  • necessarily mean that it is more common

  • in a population than the recessive allele.

  • Example: Polydactyly is a dominant gene.

  • However, the recessive allele is far more

  • prevalent.

  • Multiple Alleles: Most genes have more

  • than one allele in a population.

  • 1. Example: Blood alleles A, B, and O

  • with four possible phenotypes for blood

  • type.

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  • A type blood = genotype AA or AO.

  • A type blood has oligosaccharides on

  • the surface of the RBCs. A type blood

  • also produces antibodies against B

  • type blood.

  • B type blood = genotype BB or BO.

  • B type blood has oligosaccharides on

  • the surface of RBCs. B type blood

  • also produces antibodies against A

  • type blood.

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  • Matching compatibility for blood type

  • is crucial for transfusions.

  • - O type blood = universal donor

  • - AB type blood = universal recipient

  • Pleiotropy: Genes have multiple effects.

  • Example: Sickle Cell gene has multiple of

  • effects.

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  • Epistasis: a gene on one locus can alter

  • the effects of another gene on a different

  • locus.

  • Example: Coat color in mice is determined

  • by two genes.

  • The epistatic gene, determines whether

  • or not pigment will be deposited into

  • hair.

  • Presence of pigment ( C ) is dominant

  • to no presence ( c ).

  • Pigment color black (B) is dominant to

  • brown pigment (b).

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  • A cross between

  • two mice that

  • are heterozygous

  • for both genes

  • (Cc, Bb) follows

  • the law of

  • independent

  • assortment:

However, the

ratio will be

9 black:3 brown:

4 white

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  • Polygenic Inheritance: an additive effect

  • of two or more genes on a single

  • phenotypic character.

  • 1. Example: Human skin color is due to

  • more than 3 separate genes. For

  • simplicity sake, however, we will

  • consider just 3 genes for determining

  • skin color.

  • A, B, and C are the 3 different genes,

  • all incompletely dominant to the

  • alleles (a, b, and c).

  • An individual with AABBCC genotype

  • will be very dark. An individual with

  • AaBbCc genotype will have an

  • intermediate shade.

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  • A cross between

  • two individuals

  • with AaBbCc

  • genotypes

  • would result

  • in a bell-

  • shaped

  • curve,

  • called a

  • normal

  • distribution.

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  • The environmental impact on phenotype:

  • 1. Example: Nutrition can influence height.

  • Exercise influences build. Sun exposure

  • influences skin color. Practice improves

  • intelligence tests.

  •  Identical twins?

  • The products of a genotype can be a

  • wide range of variation. This phenotypic

  • range due to the environment is called

  • the norm of reaction.

Norm of reaction:

colors of hydrangea

flowers, range from

blue to pink, depending

on the acidity of soil.

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  • While peas are an easy subject to study

  • genetics, humans are not.

  • 1. Human generation span is too long.

  • 2. Parents produce few offspring.

  • 3. Breeding experiments is socially

  • unacceptable.

  • Pedigree analysis reveals Mendelian

  • patterns in human inheritance.

  • 1. Phenotypic information is gathered from

  • as many members of a family across

  • generations.

  • The information can then be mapped

  • onto a family tree.

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  • If an individual has no widow’s peak

  • but both his/her parents have a

  • widow’s peak, we can predict that both

  • parents are heterozygous.

  • A pedigree can help us understand the

  • past and predict the future.

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  • Thousands of genetic disorders can be

  • inherited as recessive traits.

  • Genetic disorders can range from mild

  • to life-threatening.

  • Heterozygous individuals are

  • phenotypically normal but are “carriers”

  • of the disorder.

  • Most individuals that are born with a

  • disorder are born to carriers with normal

  • phenotypes.

  • a. Two carriers have ¼ probability of

  • having a child with the disorder, ½

  • probability of having carriers, and ¼

  • free.

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  • Cystic fibrosis: 1 in 2,500 caucasians

  • of european descent. 1 in 25

  • caucasians are carriers. CF is caused

  • by a defective Cl- membrane protein

  • that causes build up of mucus in the

  • lungs, pancreas, & digestive system.)

  • Tay-Sachs: 1 in 3,600 births in

  • Ashkenazic Jews. T-S is caused by

  • a defective enzyme that cannot break

  • down certain lipids in the brain. This

  • causes brain degeneration.

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  • Sickle-Cell Anemia:

  • 1 in 400 African

  • Americans. It is

  • caused by a

  • substitution in one

  • amino acid of the

  • hemoglobin protein.

  • When oxygen levels

  • are low in blood, red

  • blood cells deform

  • into a sickle shape.

This sickling can cause a number of results.

This sickling has pleiotropic effects.

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The non-sickling allele is incompletely

dominant to the sickle-cell allele.

Heterozygous individuals are carriers

and can suffer some symptoms of

the disease under blood oxygen

stress. Both normal and abnormal

hemoglobins are synthesized.

Individuals that are heterozygous

are also resistant to malaria, a

parasite that spends part of its life

cycle inside RBCs.

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  • Achondroplasia, a form of dwarfism,

  • has an incidence of one case in

  • 10,000 people.

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  • A lethal dominant allele can be passed

  • down to generations if the onset of

  • the disease is later in life.

  • Example: Huntington’s Disease, a

  • degenerative brain disorder; onset

  • bt. 35-45 years of age.

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-Offspring born to a parent who has

the allele for Huntington’s disease has

a 50% chance of inheriting the

disease and the disorder.

-Molecule geneticists have recently

discovered that the gene for HD is

found on the tip of chromosome #4.

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  • Albinism is a rare condition that is inherited as a recessive phenotype in many animals, including humans.

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  • Many hospitals have genetic counselors

  • that can provide information to

  • prospective parents who are concerned

  • about a family history of a specific

  • disease.

  • Using Mendelian probability (Punnett

  • Squares), one can determine the risks

  • of passing on lethal genes.

  • Tests determine in utero if a child has a

  • disorder. One technique is called an

  • amniocentesis.

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-can be done after 14-16 weeks of


-fetal cells are extracted and karyotyped.

-other disorders can be detected from

chemicals in the amniotic fluid.

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  • Chorionic villus sampling (CVS) can

  • allow faster karyotyping and can be

  • performed as early as the eighth to

  • tenth week of pregnancy.

-Fetal tissue is extracted from the

chrionic villi of the placenta.

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  • Ultrasound and fetoscopy, allow fetal

  • health to be assessed visually in utero.

-Both fetoscopy and amniocentesis

cause complications in about 1% of

cases. They can cause maternal

bleeding or fetal death.

-These techniques are usually reserved

for cases in which the risk of a genetic

disorder or other type of birth defect is

relatively great.

  • After the results of a test are revealed,

    the parents must face the difficult decision

    of terminating the pregnancy or preparing

    to care for a child with a genetic disorder.

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-1 in 10,000-15,000 births

-causes a build-up of the amino acid

phenylalanine, and its derivative

phenypyruvate in the blood to toxic


-this build-up can cause mental


-if the genetic test is given at birth, a

child can be given a special diet low in

phenylalanine, which usually promotes

normal growth.