Genetics Part I - Napa Valley College 9 - Genetics... · Genetics Biology 105 Laboratory 9 ... Autosomal Dominant Disorders Autosomal dominant disorders are disorders controlled by

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Genetics

Biology 105

Laboratory 9

Chapter 20

Copyright © 2009 Pearson Education, Inc.

Note:

This information will be presented during lab

time but covered on the lecture exam.

You will be given a homework assignment with

genetics problems. The homework is due on

Tuesday, and similar content will be on the lab

exam.


History – Gregor Mendel and his Pea Plants

When Mendel lived, no one knew about DNA

or meiosis.

It was known that offspring inherited traits

from the parents, but no one knew how…

It was thought that whatever the genetic

material was, it would be blended to produce

the offspring.

But nature did not seem to follow this rule!



Problem!

If the genetic material was blending to

produce offspring, then why is the foal not

gray?

Even though people could see this in nature

and in agricultural breeding programs, they

still believed in the blending theory.


Charles Darwin

Charles Darwin was one of the few scientists

who did not believe in the blending theory.

Darwin believed that individuals in a

population show variation.

The traits that give you an edge to survive will

be passed on to your offspring.

The traits are not blended, but instead the

traits will be seen more or less often

depending on how advantageous they are for

the individual.

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More About Mendel’s Work…

Just before Darwin presented his theory, Mendel

started to work on his experiments with peas.

Mendel was a monk and a scientist.

He studied both math and botany.

He was raised on a farm and was aware of

agricultural practices and research.

He was well-known for breeding new varieties of

fruits and vegetables.

Mendel believed that sperm and eggs contained

“units” of information (= traits).

He used pea plants to prove his theory.


Pea Plant Characteristics

Pea shape – smooth or wrinkled

Seed color – yellow or green

Pod shape – smooth or wrinkled

Pod color – yellow or green

Flower color – purple or white

Flower position – low on stem or at tip

Stem length – tall or short

Mendel made three key decisions in his

experiments.

- use of purebred plants

- control over breeding

- observation of seven

“either-or” traits


Terminology

Traits – distinguishing characteristics that

vary between individuals and are inherited.

Parental generation (P): the parents in

Mendel’s experiments.

These plants are “pure” – they only produce

plants with their own phenotype when they are

self-pollinated.

First filial generation (F1): the offspring of the

parental generation (= children).

Second filial generation (F2): the offspring of

the first filial generation (= grandchildren).


• Mendel used pollen to fertilize selected pea plants.

Mendel controlled the

fertilization of his pea plants

by removing the male parts,

or stamens.

He then fertilized the female

part, or pistil, with pollen from

a different pea plant.

– P generation crossed to produce F1 generation

– Interrupted the self-pollination process by

removing male flower parts


• Mendel allowed the resulting plants to self-pollinate.

– Among the F1 generation, all plants had purple

flowers.

– F1 plants were all heterozygous.

– Among the F2 generation, some plants had

purple flowers and some had white.


• Mendel observed patterns in the first and second

generations of his crosses.

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Results from Mendel’s Experiments

P generation were pure purple bred to pure white.

F1 generation were all purple flowers!

F2 generation had both purple and white flowers (3:1 ratio).

Somehow the white trait had “hidden” for a generation.


Dominant and Recessive

The trait that is expressed is the dominant trait.

The trait that is hidden, or not expressed, is

recessive.

In this example, the purple trait is the dominant

trait over the recessive white trait.


DNA and Genes

Gene: the specific part of DNA that tells a cell

how to make a specific protein.

There are pairs of chromosomes.

Pairs of homologous chromosomes contain the code

for the same proteins.

In a pair of homologous chromosomes, each one

contains the same genes.

These different forms of the same genes are called

alleles.

Allele = any alternative form of a gene occurring at a

specific place on a chromosome.


Each chromosome in the pair of chromosomes

has an allele.

If both the chromosomes in the pair have the

same allele form, then they are homozygous

for that trait.

Just for that trait! Other alleles on the DNA can be

different.

If the chromosomes in the pair have different

alleles, they are heterozygous for this trait.


Allele for

yellow is

“Y”

Allele for

green is

“y”

A yellow

pea can

be YY or

Yy

A green

pea is yy


Dominant and Recessive

Dominant alleles are written with a capital

letter.

Recessive alleles are written with a lowercase

letter, and you always use the same letter that

you chose for dominant.

In Mendel’s peas, the allele for yellow peas is

written as “Y” and the allele for green peas is

written as “y”.

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Genotype and Phenotype

Phenotype – physical features of the organism.

Genotype – the genetic makeup that

determines the phenotype.

The phenotype for a yellow pea is yellow color,

and the genotype is YY or Yy.

The phenotype for a green pea is green color,

and the genotype is yy.


Punnett squares illustrate genetic crosses.

The Punnett square is a grid system for

predicting all possible genotypes resulting

from a cross.

The axes represent

the possible gametes

of each parent.

The boxes show the

possible genotypes

of the offspring.

• The Punnett square

yields the ratio of

possible genotypes

and phenotypes.

Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc.

Punnett Square – Pea Example

Remember the yellow allele is dominant (Y)

and green allele is recessive (y).

In Mendel’s experiment, the parental

generation (P) self-fertilized and only produced

the same kind of plant.

So the plants with yellow peas are YY and the

plants with green peas are yy.


When P generation were cross-fertilized

(yellow pea plants with green pea plants), they

produced only yellow plants.

The offspring of the P generation (= F1) were

all Yy genotype and yellow phenotype.

y y

Y Yy Yy

Y Yy Yy

Yello

w

Green


Y y

Y YY Yy

y Yy yy

F2 Generation

The F1 peas were planted, and the plants

self-fertilized to produce the F2 generation.

Yellow

Yello

w

Phenotype of the

offspring: on

average, 3 peas

should be yellow

(YY or Yy) and 1

pea should be

green (yy).

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Q: You breed homozygous purple plants with homozygous

white plants (P generation), and the result is offspring that

are all purple (F1). Which allele is dominant?

1. Purple

2. White

Q: What are the genotypes of the P

generation?

1. PP and pp

2. Pp and pp

3. All Pp

4. All PP


Q: What are the genotypes of the F1

generation?

1. PP and pp

2. Pp and pp

3. All Pp

4. All PP


Practice Problems...


X-Linked Inheritance in Humans

Remember that X and Y are the sex

chromosomes.

Genes that are located on the X

chromosomes are called X-linked or sex-

linked.

X-linked recessive traits are mainly seen in

males.

Why?


X-Linked Inheritance in Humans

Males only have one X chromosome, so if

there is a trait on the X chromosome and they

inherit the recessive allele, then they will

express the trait.

If females inherit one recessive allele, they

have a chance to also inherit the dominant

allele on their other X chromosome.

There are many disorders that are X-linked:

color blindness, hemophilia, muscular

dystrophy


X-Linked Color Blindness

Color-blind people can not distinguish between

certain shades of greens and reds.

In color blindness, the proteins in pigments that

absorb green and red light are controlled by

DNA on the X chromosome.

There are two alleles for this – the dominant

allele produces the proteins, and the recessive

allele does not.

This disorder is a recessive disorder, which

means that a person will only express it if they

are homozygous for the recessive allele.

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X-Linked Traits

When drawing Punnett Squares:

Normal X = X

X carrying the trait = Xb

Normal female: XX

Female carrier: XbX

Color-blind female: XbXb

Normal male: XY

Color-blind male: XbY


Question!

Red-green color blindness is due to a sex-

linked recessive allele on the X chromosome.

Two parents with normal vision produce a

color-blind son.

Draw a Punnett square to show how this

happened:


Q: What is the chance the couple could have

a color blind daughter?

1. 1/4

2. 2/4

3. 3/4

4. 0/4 (none)


Autosomal Genetic Disorders

Sex-linked disorders are only those disorders

that are controlled by the X or Y

chromosomes.

All other chromosomes are autosomal (=

body) chromosomes – they control the

autosomal disorders.

There are autosomal recessive disorders, and

autosomal dominant disorders.


Autosomal Recessive Disorders

Autosomal recessive disorders: disorders controlled by DNA in any of the autosomal chromosomes.

These are recessive disorders, so they are only expressed if the person is homozygous for the allele.

Examples include: sickle-cell anemia, cystic fibrosis, albinism, phenylketonuria

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Sickle Cell Anemia

The protein in blood

cells that carries the

oxygen is hemoglobin.

Sickle cell disorder is

due to a point mutation

in the DNA that controls

the formation of the

hemoglobin protein,

causing misshapen red

blood cells.


A carrier is a person who does not express a

recessive disorder but carries the allele for the

disorder (heterozygous).

What is the chance that two carriers will

produce a child with sickle cell anemia?

To determine the chances that two carriers will

pass on the disorder, draw a Punnett square!


The child will

have:

1 in 4 chance

(25%) of having

the disorder

50% chance of

being a carrier

25% chance of

being “normal”


The Benefits of Sickle Cell

There is an

overlap of where

sickle cell anemia

is found in the

highest numbers

and where malaria

is found:


Sickle Cell Anemia

People who are homozygous for the disorder

experience severe symptoms, but people who

are heterozygous (carriers) show few, if any,

symptoms.

But carriers of sickle cell anemia have greater

protection from malaria symptoms.

People who are heterozygous are able to

produce both the normal and abnormal

hemoglobin proteins.

The abnormal protein protects against the effects

of malaria.


Autosomal Dominant Disorders

Autosomal dominant disorders are disorders controlled by the autosomal chromosomes.

These disorders will be expressed when the person has one or two alleles for the disorder.

The allele for the disorder is dominant over the normal allele.

Examples include: Huntington’s disease, cholesterolemia, ACHOO syndrome

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Huntington’s Disease

Huntington’s disease: degenerative disease that affects the cerebral cortex region of the brain.

Initial symptoms include abrupt, jerky movements.

These symptoms typically develop in middle age.

Dementia occurs late in the disease.

Caused by a repeat of just three bases of the DNA on chromosome 4.


Huntington’s Disease

Huntington’s is an autosomal dominant disorder.

In a Punnett square, the Huntington’s allele is written “H” and the normal allele is “h”.

A person with the genotype Hh or HH will develop Huntington’s.

People with hh will not develop the disease.

People with Huntington’s do not usually show symptoms until after they have reproduced…


Q: Would you want to know if you have the

Huntington’s allele?

1. Yes

2. No


Practice Problems…


Incomplete Dominance

In peas, everything worked out neatly!

In other plants like snapdragons it does not work

so cleanly...

If you cross a red snapdragon with a white

snapdragon, you get all pink snapdragons!

Red is dominant, but not completely dominant.

RR = red

rr = white

Rr = pink

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Co-Dominance

Another type of dominance is called co-

dominance.

Here, both the alleles are expressed in

heterozygous organisms.


Blood Type Co-Dominance

What does your blood type refer to?

Refers to the type of proteins on the surface of the

blood cells.

There are two main proteins on blood cells: A

and B.

Neither type is dominant over the other.


Type A has only A proteins

Type B has only B proteins

Type AB has A and B

proteins

O Type has no proteins


Genotype Phenotype Blood Type

AB A & B proteins AB

AA A proteins A

BB B proteins B

OO No proteins O

BO B proteins B

AO A proteins A


Q: If the parents have type A (homozygous) and type

O blood, what is the phenotype of the offspring?

1. Type A

2. Type B

3. Type AB

4. Type O

Q: If the parents have type A (homozygous) and type B

(homozygous) blood, what is the phenotype of the offspring?

1. Type A

2. Type B

3. Type AB

4. Type O

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Q: If the parents have type AB and type O blood, what

are the phenotypes of the offspring?

1. Type A

2. Type B

3. Type AB

4. Type O

5. Type A and B

6. Type AB, B and A


Practice Problems…


Genes and the Environment

The phenotype is not completely controlled by

genes.

Think about the height of a person:

Part of what determines the height of a person is

genetic, but diet also plays a roll.

The color of Hydrangea flowers depends on

the soil:

Acidic soil produces blue flowers, and basic soil

produces pink flowers .


Aneuploidy

Aneuploidy are disorders where a person has

an abnormal number of chromosomes.

Everyone should have 23 pairs of

chromosomes (or 46 total chromosomes).

Gametes should have 23 total chromosomes,

so when two gametes join, the offspring will

have 46 chromosomes.


Aneuploidy


Down Syndrome

Down syndrome is caused by aneuploidy.

People with Down syndrome have three of the

21st chromosome, instead of the normal two

chromosomes.

Down syndrome results in mental retardation,

developmental delays, and other lifelong

health problems.

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Changes in the Chromosome Structure

During Meiosis I when crossing-over occurs,

or at other times during Meiosis, the

chromosomes may be damaged.

Examples of types of damage include:

deletion, inversion, translocation, and

duplication.

Causes can include: radiation, chemicals or

mere chance.


Important Concepts for Lecture Exam

X-linked inheritance: how are traits passed to

offspring on the X chromosome?

Why are males more likely to be affected?

Example: color blindness

Autosomal disorders – differences between

recessive and dominant autosomal disorders,

and examples of recessive and dominant

autosomal disorders.

The benefits of sickle cell anemia.


Important Concepts for Lecture Exam

What is aneuploidy and how does it happen?

Example: Down syndrome

What are the different types of chromosomal

structural changes (e.g. deletion, inversion,

translocation, and duplication) and what

happens to the chromosome in each of these

cases?


Definitions

For both lecture and lab exams:

Phenotype, genotype, parental (P) generation,

first filial (F1) generation, X-linked, autosomal

disorder, autosomal chromosome, heterozygous,

homozygous, recessive, dominant

For lecture exam only: aneuploidy


Important Concepts for Lab Exam

Given the genotype or phenotypes of the parents,

be able to determine the chance that: (1) the

offspring will inherit a disorder, (2) the offspring

will be a carrier, or (3) the offspring will not be

either.

You should be able to do this for “regular”, X-linked,

recessive autosomal, dominant autosomal,

incomplete dominance, and co-dominance.

You should be able to solve genetics

problems similar to those on the homework.

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Optional Information of Interest…

The following slides will not be tested on

either the lecture or lab exam!


Genetic Counseling

What options do couples have to try to prevent having a baby with a birth defect?

The first option is going to a genetic counselor who will help them draw a family tree of genetic traits:

The counselor may recommend getting tested to see if they are carriers for certain traits (if the test is available).


Genetic Screening

Genetic screening is the process of analyzing

blood or skin to search for a particular genotype.

In order to locate this faulty gene, scientists

search for variations in pieces of DNA called

"markers".

Markers have already been found for various

diseases, including Huntington’s disease, sickle

cell anemia, cystic fibrosis, Tay-Sachs disease,

Duchenne muscular dystrophy, and hemophilia.

With such markers it becomes theoretically

possible to screen individuals of every age, from

infants to adults, even babies before birth. Copyright © 2009 Pearson Education, Inc.

Types of Genetic Screening

1. Carrier screening:

Analysis of an adult to determine if they have a

gene or a chromosome abnormality that may cause

problems for offspring or the person screened.

2. Prenatal screening:

Performed when a fetus is at risk for various

identifiable genetic diseases or traits.

Examples: amniocentesis

3. Newborn screening:

Analyzes blood or tissue samples during early

infancy to detect genetic disorders for which early

intervention can avert serious health problems.


Newborn Screening

Should all newborns be screened for all the

known diseases?

Who should pay for the tests?

Would it be worth it to test all babies in order

to save a few babies?

Would the health care money be better spent

on pre-natal care?


4. Pre-Implantation Genetic Screening

What if you and your partner are both carriers

for a disorder, but you really want a child?

What is the chance that you will have a baby

with the disorder? Or a baby who will be a

carrier?

What if you (or your partner) are 43-years-old

and are having trouble conceiving, and want to

make sure your baby does not have Down

syndrome or another genetic disorder?

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Pre-Implantation Genetic Diagnosis

Many couples use in vitro fertilization

techniques to become pregnant.

The woman is given hormone injections so

she will produce more than one egg.

These eggs are collected and sperm is added

to the eggs outside the woman’s body.

The developing embryos are then placed into

the woman’s uterus to develop.

You can test the embryos for many disorders,

including Down syndrome.



Things to Consider:

Not all the embryos produced in the clinics will be implanted.

What do you do with the surplus embryos?

If multiple embryos implant, would you keep them all?

Where would you draw the line?

What would you do with the embryos you do not use?

Which factors would you use to “select” your offspring (e.g. gender)?


Ethical Dilemmas

Science gives us the tools to diagnose certain

disorders, but it is up to us to decide the ethical

use of these tools.

Couples need to decide what risks they are

willing to live with when conceiving, and what

to decide in the event of a bad outcome of a

prenatal test.


Would you want to be told that you have the allele that is responsible for Huntington’s disease, or another disorder that has no cure?

Would you want to know if you are a carrier for an autosomal recessive disorder?

Would you want to have prenatal screening done, even if you know you will not abort the fetus no matter what the results?

What should the criteria be for deciding which children receive newborn screening?

Do you think that pre-implantation diagnosis is a good idea? What are some things you might want to consider before performing this procedure?

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Genetics Part I - Napa Valley College 9 - Genetics... · Genetics Biology 105 Laboratory 9 ... Autosomal Dominant Disorders Autosomal dominant disorders are disorders controlled by