Georgia Standards:1. Explain the relationship between changes in DNA and the appearance of new traits. 2. Explain the relationship between changes in DNA and the appearance of new traits.

Essential Questions:

•How do you predict the probability of various genotypes inherited and the expressed phenotypes?

•How does meiosis generate variation in offspring?

Gregor Mendel: Father of Genetics

• Genetics is the scientific study of heredity.

• Gregor Mendel (1860’s) an Austrian Monk, was interested in figuring out how heredity was determined in plants and animals.– used pea plants– quantitative approach to

collect data.

• Mendel studied seven different pea plant traits.– Seed shape & color, pod

shape & color, flower height & position and seed coat color

• A trait is a specific characteristic, such as seed color or plant height, that varies from one individual to another.

Gregor Mendel’s Experiment:• Mendel crossed plants with each of the seven

contrasting traits and studied their offspring.

• Mendel called each original pair of plants the P (parental) generation. – These peas were true-breeding (self-pollination),

meaning that if they were allowed to self-pollinate, they would produce offspring identical to themselves.

Gregor Mendel’s Experiment:• He called the offspring of the

P-generation, the F1, or “first filial,” generation. Filius is the Latin word for “son.”

– These pea plants were cross pollinated.

– In cross-pollination, male sex cells in pollen from the flower on one plant fertilize the egg cells of a flower on another plant.

• The offspring of crosses between parents with different traits are called hybrids.

• The F2 generation was allowed to self-pollinate

• Out of 929 F2 Generation plants, 705 were purple and 224 were white.– Ration of 3 purple to 1 white

How do we find the results of a genetic cross?

• The principles of probability can be used to predict the outcomes of genetic crosses.

• The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. (See note sheet)

Gregor Mendel’s Conclusions:

• Genes and Dominance– Mendel’s first conclusion was that biological

inheritance is determined by factors (genes) that are passed from one generation to the next.

• Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms.

• The different forms of a gene are called alleles

Punnett Squares• Organisms that have two identical alleles for a

particular trait—TT or tt in this example—are said to be homozygous

• Organisms that have two different alleles for the same trait are heterozygous (Ex: Tt)

• Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait.

Punnett Squares• All of the tall plants have the same

phenotype, or physical characteristics.

• They do not, however, have the same genotype, or genetic makeup.

Gregor Mendel’s Conclusions:

• Mendel’s second conclusion is called the principle of dominance, which states that some alleles are dominant and others are recessive.

– Alleles that are expressed are considered DOMINANT.

• An organism with a dominant allele for a particular form of a trait will always have that form.

– Alleles that are present in the genotype, but not expressed in the phenotype are considered RECESSIVE.

• An organism with a recessive allele for a particular form of a trait will have that form only when the dominant allele for the trait is not present.

Gregor Mendel’s Conclusions:

•  Segregation of Alleles:– When each F1 plant flowers, the two alleles are

segregated from each other so that each gamete carries only a single copy of each gene.

– Therefore, each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness.

Law of Segregation:• During gamete

formation, alleles are segregated from each other so that each gamete carries only a single copy of each allele.

– Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness.

– The alleles are paired up again when gametes fuse during fertilization.

Did this mean that the two dominant alleles would always stay together?

• (Law of Independent Assortment)

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

Principle of dominance

• Some alleles are dominant and others are recessive.

• The importance of Mendel’s work on heredity was not discovered until 30 years later.

• Scientist soon realized that CHROMOSOMES are the carriers of heredity.

A Summary of Mendel’s Principles• Genes determine

inheritance of biological characteristics.

• Genes are passed from parents to offspring

• Some forms of the gene may be dominant and others may be recessive.

• In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each during gamete formation.

• The alleles for different genes are assorted independently of one another.

Pop Quiz: Use Book- Chp. 10.2

1. What are dominant and recessive alleles?2. What happens to alleles during segregation? 3. What did Mendel conclude determines

biological inheritance? 4. Describe how Mendel cross-pollinated pea

plants. 5.   Describe how Mendel self-pollinated pea

plants. 6. Give an example of the law of independent

assortment.

Probability and Punnett Squares

• The principles of probability can be used to predict the outcomes of genetic crosses.

• The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. (See note sheet)

Punnett square• This Punnett square

shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

• Genotype: 25% TT, 50% Tt, 25%tt (1:2:1)

• Phenotype: 75% Tall, 25% short (3:1)

A Dihybrid Cross: F1• Dihybrid Cross:

– Shows inheritences of two traits at once

• F1 Mendel crossed plants that were homozygous dominant for round yellow peas with plants that were homozygous recessive for wrinkled green peas. All of the F1 offspring were heterozygous dominant for round yellow peas.

• The cross did not indicate whether genes assort, or segregate, independently.

Did this mean that the two dominant alleles would always stay together? Or would they “segregate

independently.”• A combination of alleles

were produced that were not found in either parent– This means that genes for

different traits can segregate independently during the formation of gametes. (Independent Assortment)

Dihybrid Cross Problem• Determine the genotype and phenotype

ratios and percents for a F2 generation dihybrid cross of two plants; one that is homozygous recessive for short, purple leaves and the other that is homozygous dominant for tall, orange leaves.

• Height (H-tall, h-short)• Color (C-orange, c-purple)

concluded that

which is called the

which is called the

Peaplants

GregorMendel

Law ofDominance

Law ofSegregation

“Factors”determine

traits

Some alleles are dominant,

and some alleles are recessive

Alleles are separated during gamete formation

Concept Map

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experimented with

genes for different traits can segregate independently during the formation of gametes

which is called the

Law ofIndependent Assortment

Exceptions to Simple Dominance

Does the segregation of one pair of alleles affect the segregation of another pair of alleles?

• For example, does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color? Must a round seed also be yellow?

Exceptions to the Laws:• Some alleles are neither dominant nor

recessive, and many traits are controlled by multiple alleles or multiple genes.

• incomplete dominance occurs when one allele is not completely dominant over another

• In incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes.

Incomplete Dominance • Some alleles are neither

dominant nor recessive. • In four o’clock plants, for

example, the alleles for red and white flowers show incomplete dominance.

• Heterozygous (RW) plants have pink flowers—a mix of red and white coloring

Incomplete Dominance Practice

• In Mendel's experiments, if the gene for tall (T) plants was incompletely dominant over the gene for short (t) plants, what would be the result of crossing two Tt plants?

• A. 1/4 would be tall; 1/2 intermediate height; 1/4 short • B. All the offspring would be tall. • C. 1/2 would be tall; 1/4 intermediate height; 1/4 short. • D. All the offspring would be intermediate. • E. 1/4 would be tall; 1/4 intermediate height; 1/2 short.

Incomplete Dominance Practice• Disappearance of parental

phenotypes in the F1 generation

• A genetic cross of inbred snapdragons with red flowers with inbred snapdragons with white flowers resulted in F1-hybrid offspring that all had pink flowers. When the F1 plants were self-pollinated, the resulting F2-generation plants had a phenotypic ratio of 1 red: 2 pink: 1 white. The most likely explanation is:

• A. pink flower color is epistatic to red flower color.

• B. pink flowers are the result of a blending of the red and white genotypes.

• C. flower color is due to 2 or more complementary genes.

• D. heterozygous plants have a different phenotype than either inbred parent because of incomplete dominance of the dominant allele.

• E. flower color inheritance in snapdragons does not behave as a Mendelian trait.

Codominance • In codominace, both

alleles contribute to the phenotype of the organism.

• For example, in cattle the allele for red hair is codominant with the allele for white hair. – Cattle with both alleles

are roan, or pinkish brown, because their coats are a mixture of both red and white hairs.

• In certain varieties of chickens, the allele for black feathers is codominant with the allele for white feathers.– Heterozygous

chickens appear speckled with black and white feathers.

Codominant alleles: The human ABO markers

• Human blood type is determined by codominant alleles. There are three different alleles, known as IA, IB, and i. The IA and IB alleles are co-dominant, and the i allele is recessive.

• The possible human phenotypes for blood group are type A, type B, type AB, and type O. Type A and B individuals can be either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively).

• A woman with type A blood and a man with type B blood could potentially have offspring with which of the following blood types?

• A. type A • B. type B • C. type AB • D. type O • E. all of the above

Codominant Practice: Predicting human blood types

• What are the possible blood types of the offspring of a cross between individuals that are type AB and type O? (Hint: blood type O is recessive)

• A. AB or O • B. A, B, or O • C. A or B • D. A, B, AB, or O • E. A, B, or AB

Sickle Cell Disease 

• Sickle cell disease is a common genetic disorder inherited in a codominant fashion, and mainly found in African Americans.

• Sickle cell disease is characterized by the bent and twisted shape of the red blood cells

• These sickle-shaped red blood cells are more rigid than normal cells and tend to get stuck in the capillaries, the narrowest blood vessels in the body.

• As a result, blood stops moving through these vessels, damaging cells and tissues beyond the blockage.

• Sickle cell disease produces physical weakness and damage to the brain, heart, and spleen. In some cases, it may be fatal.

Sickle Cell Disease • Hemoglobin is the protein

that carries oxygen in the blood.

• Mutation: the amino acid valine in place of glutamic acid.

• As a result, the abnormal hemoglobin is somewhat less soluble than normal hemoglobin. Blood gets stuck in cappillaries.

Why do so many African Americans carry the sickle cell allele?

• Most African Americans can trace their ancestry to west central Africa.

• Malaria, a serious parasitic disease that infects red blood cells, is common in this region of Africa.

• People who are heterozygous for the sickle cell allele are generally healthy and are resistant to malaria.

Multiple Alleles• Many genes have

more than two alleles and are therefore said to have multiple alleles.

• This does not mean that an individual can have more than two alleles. It only means that more than two possible alleles exist in a population.

• One of the best-known examples is coat color in rabbits. – Full color, albino,

himalayan, chinchilla

• A rabbit’s coat color is determined by a single gene that has at least four different alleles.

• ABO blood groups have three forms of alleles.

Complex Inheritance and Human Heredity

Coat Color of Rabbits

Light gray

Dark gray Himalayan

Albino

Chinchilla

11.2 Complex Patterns of InheritanceChapter 11

Phenotype(Blood Type Genotype

Antigen on Red Blood Cell

Safe Transfusions

To From

Section 14-1 Blood Groups

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Epistasis:

• The result of one allele hiding the effects of another allele.

• Ex: Labrador’s coat color can vary from yellow to black

Complex Inheritance and Human Heredity

EpistasisVariety is the result of one allele hiding the

effects of another allele.

No dark pigment present in fur Dark pigment present in fur

eebbeeB_ E_bb E_B_

11.2 Complex Patterns of InheritanceChapter 11

Polygenic Traits • Many traits are produced

by the interaction of several genes.

• Traits controlled by two or more genes are said to be polygenic traits.

• Polygenic traits often show a wide range of phenotypes.

– Ex: skin color, eye color, height, and fingerprint pattern

Polygenic Traits:

• Most traits are controlled by two or more genes and are, therefore, called polygenic traits.

• Each gene of a polygenic trait often has two or more alleles.

• As a result, one polygenic trait can have many possible genotypes and even more possible phenotypes.

EX: height (A bell-shaped curve is also called a normal distribution)

Complex Inheritance and Human Heredity

Environmental Influences

Environmental factors

Diet and exercise

Sunlight and water

Temperature

11.2 Complex Patterns of InheritanceChapter 11

Checkpoint Point Questions:1. Explain what independent assortment means.

2. Describe two inheritance patterns besides simple dominance.

3. What is the difference between incomplete dominance and codominance?

4. Identify examples of polygenic traits and multiples alleles. (one example for each)