Chapter: Heredity

Chapter: Heredity

Table of ContentsTable of Contents

Section 3: Biotechnology

Section 1: Genetics

Section 2: Genetics Since Mendel

Inheriting Traits

• Eye color, nose shape, and many other physical features are some of the traits that are inherited from parents.

• An organism is a collection of traits, all inherited from its parents.

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Inheriting Traits

• Heredity (huh REH duh tee) is the passing of traits from parent to offspring.

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What is genetics?

• Generally, genes on chromosomes control an organism’s form and function.

• The different forms of a trait that a gene may have are called alleles (uh LEELZ).

• When a pair of chromosomes separates during meiosis (mi OH sus), alleles for each trait also separate into different sex cells.

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What is genetics?

• Every sex cell has one allele for each trait. • The study of how traits are inherited through

the interactions of alleles is the science of genetics (juh NE tihks).

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Mendel—The Father of Genetics• Gregor Mendel began

experimenting with garden peas in 1856.

• Mendel made careful use of scientific methods, which resulted in the first recorded study of how traits pass from one generation to the next.

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Mendel——The Father of Genetics

• Mendel was the first to trace one trait through several generations.

• He was also the first to use the mathematics of probability to explain heredity.

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Genetics in a Garden

• Each time Mendel studied a trait, he crossed two plants with different expressions of the trait and found that the new plants all looked like one of the two parents.

• He called these new plants hybrids (HI brudz) because they received different genetic information, or different alleles, for a trait from each parent.

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Genetics in a Garden • An organism that always produces the same

traits generation after generation is called a purebred.

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Genetics in a Garden

• Tall plants that always produce seeds that produce tall plants are purebred for the trait of tall height.

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Dominant and Recessive Factors

• In his experiments, Mendel used pollen from the flowers of purebred tall plants to pollinate by hand the flowers of purebred short plants.

• This process is called cross-pollination.

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• He found that tall plants crossed with short plants produced seed that produced all tall plants.

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• Mendel called the tall form the dominant (DAH muh nunt) factor because it dominated, or covered up, the short form.

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• He called the form that seemed to disappear the recessive (rih SE sihv) factor.

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Using Probability to Make Predictions

• Probability is a branch of mathematics that helps you predict the chance that something will happen.

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Using Probability to Make Predictions

• Mendel also dealt with probabilities.

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• One of the things that made his predictions accurate was that he worked with large numbers of plants.

• He studied almost 30,000 pea plants over a period of eight years.

• By doing so, Mendel increased his chances of seeing a repeatable pattern.

Punnett Squares

• How could you predict what the offspring would look like without making the cross?

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• A handy tool used to predict results in Mendelian genetics is the Punnett (PUH nut) square.


Punnett Squares

• In a Punnett square, letters represent dominant and recessive alleles.

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Punnett Squares 11

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• An uppercase letter stands for a dominant allele.

• A lowercase letter stands for a recessive allele.

Punnett Squares

• They show the genotype (JEE nuh tipe), or genetic makeup, of an organism.

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• The way an organism looks and behaves as a result of its genotype is its phenotype (FEE nuh tipe).

Alleles Determine Traits

• Most cells in your body have two alleles for every trait.

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• These alleles are located on chromosomes within the nucleus of cells.

Alleles Determine Traits

• An organism with two alleles that are the same is called homozygous (hoh muh ZI gus).

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• An organism that has two different alleles for a trait is called heterozygous (he tuh roh ZI gus).

Making a Punnett Square 11

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11Section CheckSection Check

Question 1

How did Gregor Mendel use his knowledge of mathematics in his study of heredity in pea plants?

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11Answer

Mendel was the first person to use the mathematics of probability to explain heredity. Probability is the branch of mathematics that helps you predict the chance that something will happen.

Section CheckSection Check

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11Question 2

According to this diagram, if meiosis proceeds correctly, how many alleles of a particular gene can a female pass on to her offspring?


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11Answer

Although she has two alleles of each gene, a mother can pass only one allele to her offspring. Meiosis separates alleles so that eggs have only one allele for each gene. The new individual then gets one allele from the mother and the other from the father.


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11Question 3

Mendel crossed pea plants that were pure-bred for yellow seeds with plants that were pure-bred for green seeds. All the offspring of this cross had yellow seeds. Based on these results, which form of color was recessive and which was dominant?


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11Answer

Green seed color was recessive and yellow seed color was dominant. Mendel called the form that seemed to disappear (green in this case) recessive and the form that covered up (yellow in this case) dominant.


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Incomplete Dominance• When the offspring of two homozygous

parents show an intermediate phenotype, this inheritance is called incomplete dominance.

22Genetics Since MendelGenetics Since Mendel

• Examples of incomplete dominance include the flower color of some plant breeds and the coat color of some horse breeds.

Multiple Alleles

• Many traits are controlled by more than two alleles.


• A trait that is controlled by more than two alleles is said to be controlled by multiple alleles.

Multiple Alleles

• Traits controlled by multiple alleles produce more than three phenotypes of that trait.


• Blood type in humans is an example of multiple alleles that produce only four phenotypes.

• The alleles for blood types are called A, B, and O.

Multiple Alleles • When a person inherits

one A allele and one B allele for blood type, both are expressedphenotype AB.


• A person with phenotype A blood has the genetic makeup, or genotypeAA or AO.

Multiple Alleles

• Someone with phenotype B blood has the genotype BB or BO.


• Finally, a person with phenotype O blood has the genotype OO.

Polygenic Inheritance• Polygenic (pah lih JEH nihk) inheritance

occurs when a group of gene pairs acts together to produce a trait.


• The effects of many alleles produces a wide variety of phenotypes.


Polygenic Inheritance• Your height and the color of your eyes and

skin are just some of the many human traits controlled by polygenic inheritance.


• It is estimated that three to six gene pairs control your skin color.

• The environment also plays an important role in the expression of traits controlled by polygenic inheritance.

Impact of the Environment

• Your environment plays a role in how some of your genes are expressed or whether they are expressed at all.


• Environmental influences can be internal or external.

Impact of the Environment

• Although genes determine many of your traits, you might be able to influence their expression by the decisions you make.


• For instance, if some people at risk for skin cancer limit their exposure to the Sun and take care of their skin, they might never develop cancer.

Human Genes and Mutations

• Occasionally errors occur in the DNA when it is copied inside of a cell.


• Such changes and errors are called mutations.

• Not all mutations are harmful. They might be helpful or have no effect on an organism.

Chromosome Disorders

• Every organism has a specific number of chromosomes.


• However, mistakes in the process of meiosis can result in a new organism with more or fewer chromosomes than normal.

Chromosome Disorders

• If three copies of chromosome 21 are produced in the fertilized human egg, Down’s syndrome results.


• Individuals with Down’s syndrome can be short, exhibit learning disabilities, and have heart problems.

Recessive Genetic Disorders

• Many human genetic disorders, such as cystic fibrosis, are caused by recessive genes.


• Some recessive genes are the result of a mutation within the gene.

• Many of these alleles are rare.


• Such genetic disorders occur when both parents have a recessive allele responsible for this disorder.


• Because the parents are heterozygous, they don’t show any symptoms.


• If each parent passes the recessive allele to the child, the child inherits both recessive alleles and will have a recessive genetic disorder.



• Cystic fibrosis is the most common genetic disorder that can lead to death among Caucasian Americans.


• In most people, a thin fluid is produced that lubricates the lungs and intestinal tract.

• People with cystic fibrosis produce thick mucus instead of this thin fluid.


• The thick mucus builds up in the lungs and makes it hard to breathe.


• This buildup often results in repeated bacterial respiratory infections.

Sex Determination

• Each egg produced by a female normally contains one X chromosome.


• Males produce sperm that normally have either an X or a Y chromosome.

Sex Determination

• When a sperm with an X chromosome fertilizes an egg, the offspring is a female, XX.


• A male offspring, XY is the result of a Y-containing sperm fertilizing an egg.

Sex-Linked Disorders • An allele inherited on a sex chromosome is

called a sex-linked gene.


• Color blindness is a sex-linked disorder in which people cannot distinguish between certain colors, particularly red and green.

Sex-Linked Disorders

• This trait is a recessive allele on the X chromosome.


• Because males have only one X chromosome, a male with this allele on his X chromosome is color-blind.

• A color-blind female occurs only when both of her X chromosomes have the allele for this trait.

Pedigrees Trace Traits

• A pedigree is a visual tool for following a trait through generations of a family.


• Males are represented by squares and females by circles.

Pedigrees Trace Traits • A completely filled circle or square shows

that the trait is seen in that person.


• Half-colored circles or squares indicate carriers.

• People represented by empty circles or squares do not have the trait and are not carriers.

Using Pedigrees

• A pedigree is a useful tool for a geneticist.


• When geneticists understand how a trait is inherited, they can predict the probability that a baby will be born with a specific trait.

Using Pedigrees • Pedigrees also are important in breeding

animals or plants.


• These organisms are bred to increase theiryield and nutritional content.


Question 1

Why is color blindness a sex-linked trait?

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22Answer

This trait is sex-linked because the alleles for this trait are carried on the X-chromosome, one of the sex chromosomes. Color-blindness is caused by a recessive allele and because males get only one X-chromosome, they are more likely to be color-blind than females.


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22Question 2

In Himalayan rabbits, dark-colored fur is only found on cooler parts of the rabbits’ bodies. This is an example of _______.


A. how sex-linked conditions change coat colorB. the risk of cancer in rabbits with light furC. the impact of internal environment on gene expressionD. what hybrid rabbits look like

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22Answer

The correct answer is C. The alleles for dark fur color are controlled by the internal temperature of the rabbits. These alleles are expressed only at lower temperatures.


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22Question 3

If an individual has three copies of chromosome 21, what condition will result?


A. color blindnessB. cystic fibrosisC. Down’s syndromeD. Hemophilia

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22Answer

The correct answer is C. Down’s syndrome occurs when there are three copies of chromosome 21 instead of the usual two.


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Genetic Engineering

• Through genetic engineering, scientists are experimenting with biological and chemical methods to change the arrangement of DNA that makes up a gene.

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• Genetic engineering already is used to help produce large volumes of medicine.

Recombinant DNA • Recombinant DNA is made by inserting a

useful segment of DNA from one organism into a bacterium.

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• This method is used to produce human insulin, human growth hormone, and other chemicals by bacteria.

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Gene Therapy • In gene therapy, a normal allele is placed in a

virus.

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• The virus then delivers the normal allele when it infects its target cell.

• The normal allele replaces the defective one.


Gene Therapy

• Scientists are conducting experiments that use this method to test ways of controlling cystic fibrosis and some kinds of cancer.

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• Gene therapy might be a method of curing several other genetic disorders in the future.


Genetically Engineered Plants

• Selecting plants with the most desired traits to breed for the next generation is called selective breeding.

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• Recent advances in genetics have not replaced selective breeding.


Genetically Engineered Plants • Genetic engineering can produce

improvements in crop plants, such as corn, wheat, and rice.

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• One type of genetic engineering involves finding the genes that produce desired traits in one plant and then inserting those genes into a different plant.



Question 1

What is it called when scientists use biological and chemical methods to change the arrangement of DNA in a gene?

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Answer

This is called genetic engineering. This process has been used to make large volumes of medicines and research is being conducted to find many other ways to use these techniques.

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Question 2What does this diagram illustrate?

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A. Bacteria cells can produce human insulin.B. Bacteria genes are put in human cells.

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C. People with diabetes are given the gene for insulin.D. The insulin gene is inserted in people with diabetes.

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The correct answer is A. Genetic engineering can be used to insert the human gene for insulin into bacterial cells. The bacterial cells then produce human insulin. This insulin can be used to treat people who have diabetes.

Answer

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Using genetic engineering to replace defective alleles in people with genetic diseases is called _______.

Question 3

A. gene therapyB. Mendelian genetics C. pedigree analysisD. recombinant DNA

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The correct answer is A. Often; a virus is used to deliver the normal allele to the patient. Scientists have been conducting experiments to cure many different genetic diseases, including cystic fibrosis, in this way.

Answer

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Chapter: Heredity

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