14 1 human heredity - mr. taubitz's science...

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14–1 Human Heredity 14-1 Human Heredity

14–1 Human Heredity

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Human Chromosomes

Human Chromosomes

Cell biologists analyze chromosomes by looking at

karyotypes.

Cells are photographed during mitosis. Scientists then

cut out the chromosomes from the photographs and

group them together in pairs.

A picture of chromosomes arranged in this way is

known as a karyotype.


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Human Chromosomes

Two of the 46 human chromosomes are known as

sex chromosomes, because they determine an

individual's sex.

• Females have two copies of an X chromosome.

• Males have one X chromosome and one Y

chromosome.

The remaining 44 chromosomes are known as

autosomal chromosomes, or autosomes.


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Human Chromosomes

All human egg cells carry a single X

chromosome (23,X).

Half of all sperm cells carry an X

chromosome (23,X) and half carry a Y

chromosome (23,Y).

About half of the zygotes will be 46,XX

(female) and half will be 46,XY (male).

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Human Chromosomes

Males and females are

born in a roughly 50 : 50

ratio because of the

way in which sex

chromosomes

segregate during

meiosis.


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Human Traits

Human Traits

In order to apply Mendelian genetics to humans,

biologists must identify an inherited trait controlled

by a single gene.

They must establish that the trait is inherited and

not the result of environmental influences.

They have to study how the trait is passed from

one generation to the next.

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Human Traits

Pedigree Charts

A pedigree chart shows the relationships within a

family.

Genetic counselors analyze pedigree charts to

infer the genotypes of family members.

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Human Traits

A circle

represents

a female.

A horizontal line

connecting a male and

a female represents a

marriage.

A shaded circle or square

indicates that a person

expresses the trait.

A square

represents

a male.

A vertical line and a

bracket connect the

parents to their

children.

A circle or square

that is not

shaded indicates

that a person

does not express

the trait.


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Human Traits

Genes and the Environment

Some obvious human traits are almost impossible to associate

with single genes.

Traits, such as the shape of your eyes or ears, are polygenic,

meaning they are controlled by many genes.

Many of your personal traits are only partly governed by

genetics.

Human Genes

The human genome includes tens of thousands of genes.

In 2003, the DNA sequence of the human genome was

published.


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Human Genes

* In a few cases, biologists were able to identify

genes that directly control a single human trait

such as blood type.

Blood Group Genes

Human blood comes in a variety of genetically

determined blood groups.

A number of genes are responsible for human

blood groups.

The best known are the ABO blood groups and the

Rh blood groups.


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Human Genes

The Rh blood group is determined by a single gene

with two alleles—positive and negative.

The positive (Rh+) allele is dominant, so individuals

who are Rh+/Rh+ or Rh+/Rh are said to be Rh-

positive.

Individuals with two Rh- alleles are said to be Rh-

negative.

ABO blood group

• There are three alleles for this gene, IA, IB, and i.

• Alleles IA and IB are codominant.


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Human Genes

Individuals with alleles IA and IB produce both A and

B antigens, making them blood type AB.


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Human Genes

The i allele is recessive.

Individuals with alleles IAIA or IAi produce only the A

antigen, making them blood type A.


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Human Genes

Individuals with IBIB or IBi alleles are type B.


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Human Genes

Individuals who are homozygous for the i allele (ii)

produce no antigen and are said to have blood

type O.


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Human Genes

Recessive Alleles

The presence of a normal, functioning gene is

revealed only when an abnormal or nonfunctioning

allele affects the phenotype.

Many disorders are caused by autosomal recessive

alleles.


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Human Genes

Dominant Alleles

The effects of a dominant allele are expressed

even when the recessive allele is present.

Two examples of genetic disorders caused by

autosomal dominant alleles are achondroplasia

and Huntington disease.


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Human Genes

Codominant Alleles

Sickle cell disease is a serious disorder caused by

a codominant allele.

Sickle cell is found in about 1 out of 500 African

Americans.


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From Gene to Molecule


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.

Cystic Fibrosis

• Cystic fibrosis is caused by a recessive

allele.

• Sufferers of cystic fibrosis produce a thick,

heavy mucus that clogs their lungs and

breathing passageways.


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The most common

allele that causes

cystic fibrosis is

missing 3 DNA

bases.

As a result, the amino

acid phenylalanine is

missing from the

CFTR protein.


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Normal CFTR is a

chloride ion channel in

cell membranes.

Abnormal CFTR

cannot be transported

to the cell membrane.


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The cells in the

person’s airways are

unable to transport

chloride ions.

As a result, the

airways become

clogged with a thick

mucus.


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Sickle Cell Disease

Sickle cell disease is a

common genetic

disorder found in

African Americans.

It is characterized by

the bent and twisted

shape of the red blood

cells.


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Hemoglobin is the protein in red blood cells that

carries oxygen.

In the sickle cell allele, just one DNA base is

changed.

As a result, the abnormal hemoglobin is less soluble

than normal hemoglobin.

Low oxygen levels cause some red blood cells to

become sickle shaped.

People who are heterozygous for the sickle cell allele

are generally healthy and they are resistant to

malaria.


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There are three phenotypes associated with the

sickle cell gene.

An individual with both normal and sickle cell alleles

has a different phenotype—resistance to malaria—

from someone with only normal alleles.

Sickle cell alleles are thought to be codominant.


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Malaria and the Sickle Cell Allele

Regions where malaria is

common Regions where the sickle

cell allele is common

- or -

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14–1

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14–1

A chromosome that is not a sex chromosome is

know as a(an)

a. autosome.

b. karyotype.

c. pedigree.

d. chromatid.

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14–1

Whether a human will be a male or a female is

determined by which

a. sex chromosome is in the egg cell.

b. autosomes are in the egg cell.

c. sex chromosome is in the sperm cell.

d. autosomes are in the sperm cell.

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14–1

Mendelian inheritance in humans is typically studied by

a. making inferences from family pedigrees.

b. carrying out carefully controlled crosses.

c. observing the phenotypes of individual humans.

d. observing inheritance patterns in other animals.

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14–1

An individual with a blood type phenotype of O

can receive blood from an individual with the

phenotype

a. O.

b. A.

c. AB.

d. B.

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14–1

The ABO blood group is made up of

a. two alleles.

b. three alleles.

c. identical alleles.

d. dominant alleles.

14 1 human heredity - mr. taubitz's science...

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