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Classical Genetics. Gregor Mendel. The Work of Gregor Mendel. Genetics : the scientific study of heredity, or how traits are passed from one generation to the next Gregor Mendel’s Peas - Mendel was an Austrian monk who is credited as the “father of genetics.”. The Work of Gregor Mendel.

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classical genetics

Classical Genetics

Gregor Mendel

the work of gregor mendel
The Work of Gregor Mendel

Genetics: the scientific study of heredity, or how traits are passed from one generation to the next

Gregor Mendel’s Peas

- Mendel was an Austrian monk who is credited as the “father of genetics.”

the work of gregor mendel3
The Work of Gregor Mendel

- Mendel began working with pea plants that were true-breeding, meaning that if allowed to self pollinate, they would only produce offspring identical to themselves

-ex. Tall plants produce tall plants, green seeded plants produce green seeded plants

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The Work of Gregor Mendel

Genes and Dominance

- Mendel studied 7 different plant traits. Traits are characteristics such as height or flower color.

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The Work of Gregor Mendel

Seed Color



Seed Shape

Seed Coat


Flower Position

Pod Color

Plant Height






















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The Work of Gregor Mendel

- Mendel called the original plants the P (parent) generation. The offspring were the F1 (first filial) generation .

- the offspring of crosses between parents of different traits are called hybrids

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The Work of Gregor Mendel

Mendel\'s F2 Generation












11 1 the work of gregor mendel
11-1 The Work of Gregor Mendel

From his experiments, Mendel drew two conclusions

1) biological inheritance is determined by factors that are passed from one generation to the next. Today we call these factors genes.

- the different forms of a gene are called alleles

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The Work of Gregor Mendel

ex. gene (trait) is eye color

alleles (forms) are blue, brown, green, etc.

2) the second conclusion is called the principle of dominance

- the principle of dominance states that some alleles are dominant and others are recessive

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The Work of Gregor Mendel


- Mendel saw that some forms of traits would disappear during the F1 generation, but reappear during the F2 generation

- he explained this by showing that the alleles segregated during the formation of gametes, or sex cells.

section quiz
Section Quiz

Gametes are also known as

  • genes.
  • sex cells.
  • alleles.
  • hybrids.
section quiz13
Section Quiz

The offspring of crosses between parents with different traits are called

  • alleles.
  • hybrids.
  • gametes.
  • dominant.
section quiz14
Section Quiz

In a cross of a true-breeding tall pea plant with a true-breeding short pea plant, the F1 generation consists of

  • all short plants.
  • all tall plants.
  • half tall plants and half short plants.
  • all plants of intermediate height.
section quiz15
Section Quiz

If a particular form of a trait is always present when the allele controlling it is present, then the allele must be

  • mixed.
  • recessive.
  • hybrid.
  • dominant.
probability and punnett squares
Probability and Punnett Squares

Genes and Probability

- the likelihood that an event will occur is called probability

ex. a coin landing on heads has a probability of 50%

- the principle of probability can be used to predict the outcome of genetic crosses

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Probability and Punnett Squares

Punnett squares

- the gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square

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Probability and Punnett Squares

Organisms that have two identical alleles for a particular trait are called homozygous.

ex. TT

Organisms that have two different alleles for a particular trait are called heterozygous

ex. Tt

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Probability and Punnett Squares

The physical characteristics of an organism are called its phenotype

ex. tall plant

The genetic makeup of an organism is its genotype

ex. TT or Tt

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Section Quiz

Probability can be used to predict

  • average outcome of many events.
  • precise outcome of any event.
  • how many offspring a cross will produce.
  • which organisms will mate with each other.
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Section Quiz

Compared to 4 flips of a coin, 400 flips of the coin is

  • more likely to produce about 50% heads and 50% tails.
  • less likely to produce about 50% heads and 50% tails.
  • guaranteed to produce exactly 50% heads and 50% tails.
  • equally likely to produce about 50% heads and 50% tails.
section quiz24
Section Quiz

Organisms that have two different alleles for a particular trait are said to be

  • hybrid.
  • heterozygous.
  • homozygous.
  • recessive.
section quiz25
Section Quiz

Two F1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall?

  • 100%
  • 50%
  • 0%
  • 25%
exploring mendelian genetics
Exploring Mendelian Genetics

Independent assortment

- the principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes

ex. Seed color does not depend on seed shape

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Exploring Mendelian Genetics

Beyond dominant and recessive alleles

- some alleles are neither dominant nor recessive , and many traits are controlled by multiple alleles or multiple genes

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Exploring Mendelian Genetics

Incomplete dominance or Codominance

- the heterozygous phenotype is somewhere between the two homozygous phenotypes

ex. red flower + white flower = pink

Punnett square animation

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Exploring Mendelian Genetics

Multiple alleles

- multiple alleles contribute to the phenotype ex. Blood type (i, IA, IB)

Type O = i i

Type A = IA IA or IA i

Type B = IB IB or IB i

Type AB = IA IB

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Exploring Mendelian Genetics

Polygenic traits

- two or more genes control the outcome of a particular trait

ex. Skin color in humans is controlled by the combinations of 4 different genes

section quiz35
Section Quiz

In four o\'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o\'clock plants have

  • pink flowers.
  • white flowers.
  • half white flowers and half red flowers.
  • red flowers.
section quiz36
Section Quiz

Mendel\'s principles apply to

  • pea plants only.
  • fruit flies only.
  • all organisms.
  • only plants and animals.
human heredity
Human Heredity

Human Chromosomes

A typical human body cell contains 46 chromosomes

- 44 autosomes and 2 sex chromosomes

Chromosomes are arranged in a karyotype, a picture of the chromosomes

human heredity39
Human Heredity

Males and females differ in the sex chromosomes they contain

- males are XY

- females are XX

All egg cells carry a single X chromosome. However, half of all sperm cells carry a X chromosome while half carry a Y chromosome.

human heredity41
Human Heredity

Human Traits

Scientists use a pedigree to help study how traits are passed from one generation to the next.

Pedigree: a chart which shows the relationships within a family

Pedigree animation

human heredity42
Human Heredity

Square = Male

Circle = Female

Parental line

Marriage line

carrier of the trait

not shaded = does not express the trait

shaded = express the trait

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Human Heredity

Since most human traits are polygenic (many genes), they can not be traced to a single parent

Many genes have been discovered through the study of genetic disorders

- they can be dominant or recessive

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Human Heredity

In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder

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Human Heredity

Cystic Fibrosis

- caused by a recessive allele on chromosome 7

- causes digestive and respiratory problems

- only half of people with CF live into their 20s

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Human Heredity

Sickle Cell Disease

- causes hemoglobin to be less soluble

- changes the shape of red blood cells; become bent into the sickle-shape

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Human Heredity
  • Malaria and the Sickle Cell Allele

Regions where malaria is common

Regions where the sickle cell allele is common

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Section Quiz

A chromosome that is not a sex chromosome is know as a(an)

  • autosome.
  • karyotype.
  • pedigree.
  • chromatid.
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Section Quiz

An individual with a blood type phenotype of O can receive blood from an individual with the phenotype

  • O.
  • A.
  • AB.
  • B.
human chromosomes
Human Chromosomes

Human genes and chromosomes

- on each chromosome there is the information for several hundred genes

ex. Gene for ALS is on 22nd chromosome

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Human Chromosomes

Sex-linked genes: genes located on the X or Y chromosome

ex. colorblindness, hemophilia, muscular dystrophy

Males have only 1 X chromosome and cannot be heterozygous for the trait. Females can be carriers (heterozygous).

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Human Chromosomes


(normal vision)

Normal vision





(normal vision)


(normal vision)

Mother (carrier)





human chromosomes54
Human Chromosomes

Chromosomal disorders

The most common error occurs when homologous chromosomes fail to separate during meiosis. This is called nondisjunction.

human chromosomes55
Human Chromosomes

Homologous chromosomes fail to separate

Meiosis I:


Meiosis II

human chromosomes57
Human Chromosomes

If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result.

- Down syndrome results from a nondisjunction of the 21st chromosome

- Turner’s syndrome (XO) and Klinefelter’s syndrome (XXY)