Genetics

Changes since Mendels work….

S7L3 - Students will recognize how biological traits are passed on to successive generations.

The Standard:

a. Explain the role of genes and chromosomes in the process of inheriting a specific trait.

b. Compare and contrast that organisms reproduce asexually and sexually (bacteria, protists, fungi, plants & animals).

c. Recognize that selective breeding can produce plants or animals with desired traits.

DeVries – reproduced Mendel’s work, and discovered that mistakes can and do happen in the duplication of DNA. He named these mistakes MUTATIONS. A mutation is any change in a gene or chromosome.

Mutations can cause a change in the trait, or phenotype from what it normally would have been.

Comes from Latin word meaning “change”

A Mutation that occurs in a body (somatic) cell, like a skin cell, will not be passed on to the organism’s offspring.

However, a mutation that occurs in a sex cell, it can be passed on to an offspring and affect it’s phenotype

Mutations

• a change or error in the DNA of a cell• Can be caused by chemicals, x-rays, errors in

division• can be helpful or harmful or no effect

http://abcnews.go.com/widgets/mediaViewer/image?id=10178458

Feet of a 6 year old boy whose genetic mutation caused the formation of 16 toes on each foot.

An example of a mutation is when chromosomes don’t separate correctly during meiosis, which ends up with too many or too few chromosomes, or extra segments of chromosomes.

Mutations can be harmful, or beneficial, or neither, or have no observable effect at all.

Harmful – causes albinism, or down syndrome

Helpful – antibiotic resistance in bacteria.

Neither – extra toes on house cats

Disorders• A disease or condition created by a change in the

genetic make up away from the original.• Can be

–X-linked (male patterned baldness, color-blindness)

–Recessive (cystic fibrosis, sickle cell anemia)

–Dominant (Huntington’s, Marfans)

Genetic Disorders

• Sometimes results from errors during division. • Certain chemicals , x-rays, and radioactive

substances can also cause mutations or disorders.

• Many mutations are not harmful.• Incorrect number of chromosomes is usually

fatal. • Trisomy 21 is an example of a disorder. Extra

21st chromosome. Individuals can be short, have learning disabilities, and have heart problems. Many can lead normal lives if they have no severe health complications.

Sex-determination

• Eggs contain 1 X chromosome• Sperm contain either an X or Y

–XX = female–XY = male

genotypes: Man XY, Woman XX phenotypes: Male, Female

X Y

X

X

XX

XYXX

X Y

• Color blindness:

Usually affects males more often, males do not have another x chromosome to offset gene.

Therefore color blindness affects more males (8 percent) than females (0.4 percent). Do you see anything in A?

A

Major type is red/green color blindness, Other types include blue/yellow and total Absence of color sight.

http://colorvisiontesting.com/ishihara.htm#return%20to%20the%20top

Sex-linked disorders• An allele inherited on a sex chromosome

– Example: Color blindness- cannot see certain colors like red and green, on x chromosome

• More likely to show up in males

• In a Punnett Square we use a superscript on the X chromosome to indicate affected alleles

–XC (dominant normal site) and Xc (recessive colorblind)

genotypes: Man Xc Y, Woman Xc XC phenotypes: color blind Male, normal sighted Female

Xc Y

Xc

XC

Xc Xc

XC YXcXC

Xc Y

A colorblind man marries a normal sighted woman carrying the colorblind allele. What are the chances that their offspring will be color blind?

Recessive genetic disorders

• occur when both parents have the recessive allele

• Cystic fibrosis-build up of thick mucus in lungs and digestive system

• Hemophilia- blood does not clot • Sickle cell anemia-clogs blood vessels

Genetic Counseling• Helps couples understand their chances of having

children that have a specific disorder. • Counselors use tools to help explain the probability

of a genetic disorder occurring.• Explains help that is available

– physical therapy aids in mucus removal (cystic fibrosis)– Education and job training (helps adults with down

syndrome live and work independently)

Tools Used for Genetic Counseling

• Genetic counselors and scientists use three tools in their work:– Punnett Squares – Karyotypes– Pedigrees

You are familiar with Punnett Squares, now a brief look at the other tools…..

Karyotypes

A picture showing all of the chromosomes of a cell

A tool very useful in diagnosis of chromosomal disorders like Down Syndrome

A karyotype is a picture showing the arrangement of a full set of human chromosomes.

Pedigrees

• A chart or “family tree” that traces which members of a family have a particular trait like hemophilia or color blindness.

Genetic Disorders - examples• Cystic fibrosis: Recessive disorder that produces

thick mucus which builds up, blocking lungs and making it hard to breathe. Also impairs digestion. (Death usually before 20 )

• Sickle-cell Anemia: Recessive disorder in which red blood cells are shaped like a sickle or crescent moon. Causes blockages, severe pain, and organ damage.

Albinism:

Producing little or no pigment (melanin) in eyes, skin or hair. One in 17,000Vision problems are common

Some recessive disorders include:• phenylketonuria (PKU), • growth hormone deficiency• Werner's syndrome (juvenile muscular

dystrophy),• Tay-sachs (attacks the nervous tissue)• Autism. • And many others….

Review of Sex-linked Disorders

• An allele inherited on a sex chromosome is called a sex-linked gene.

• They can be either recessive or dominant• Examples include:

• Hemophilia: This disorder is caused by the absence of a protein (clotting factor) responsible for the clotting of blood. It affects males primarily, although it has been known to occur in females.

Sex-linked Disorders• Duchenne's muscular dystrophy :(DMD) mostly young boys shows by age three or fourCharacterized by wasting leg and pelvic muscles.

Usually wheelchair-bound by the age of 12Death typically before the age of 20

And Now just a bit extra…. Time allowing of course…and no

notes required..!

Dominant Genetic Disorders

• About 2000 types have been identified Examples include:• Huntington’s Disease This disorder is characterized by progressive

involuntary, rapid, jerky motions and mental deterioration. It usually appears in affected individuals between the ages of 30 and 50, and leads to dementia and eventual death in about 15 years.

Dominant Genetic DisordersOther examples:

• Polydactyly (extra toes or fingers)

• Achondroplasia (a type of dwarfism)

• Marfan syndrome, also called arachnodactyly, is

characterized by long, thin arms, legs, and fingers. These people also tend to be stoop-shouldered. They

have a high incidence of eye and heart problems. Abraham Lincoln is

believed to have had Marfan syndrome.

Incomplete dominance

• When neither allele is dominant over the other

• The phenotype produced is a blend of the two alleles– Ex. Horses -- brown chestnut + white cremello =

cream palomino– White chicken + black chicken = gray– Red four o’clock flower + white four o’clock flower

= pink four o’clock flowers

Multiple alleles

• A trait controlled by more than 2 alleles• Produces more than 3 phenotypes for that

trait– Ex. Blood types: A, B, O– Possible phenotypes: AA,AO,OO, BB, AB,BO

Incomplete Dominance

• What about when red flowered plants crossed with white flowered plants create pink ones? How does that happen?

Some alleles share dominance by blending their characteristics.This is called incomplete dominance.

Examples are carnations and Four o’clock plants.

Let’s do one: Red flowers crossed with white flowers. Then cross two of their offspring…..

Incomplete Dominance

• Restated: Production of a phenotype that is intermediate ( a blending) between two homozygous parents.

RR WW

RW

RR RW RW WW

Co – Dominance

The alleles are neither dominant nor recessive. Therefore, both alleles appear in the offspring. Examples: feather colors in chickens, sickle cell anemia in humans.

Sooo, What about when white chickens crossed with black chickens create chickens with both black and white feathers? How does that happen?

Some alleles share dominance by BOTH appearing in the offspring in original form.

Let’s try a co-dominant cross….

Cross Purple flowers (PP) with white flowers (WW) to create Purple and White striped flowers…

PP

P

W

W

P

PW PW

PW PW

WW

PW

Polygenic inheritance

• A combination of many genes acts together to produce a trait – eye color, height, hair– skin color is controlled by 3-6 gene pairs

Multiple Alleles

Blood TypingAlleles of Blood Types

Blood Type Combination of Alleles

A IA IA or IAio

B IB IB or IB io

AB IAIB

O io io

Type O, being recessive will always be masked or hidden by either one of the other two (types A or B).Type A and type B are co-dominant to each other and will both appear in the phenotype.

• Let’s try a blood typing example: A woman with type AB marries a man who is

heterozygous type A. What are their genotypes? What are the possible genotypes, and phenotypes of their offspring?

Woman: IAIB

Man: IAio IA

IAIAIA

IA IB

IB

IB

IA

ioioio

Genotypes of Offspring: IAIA, Iaio, IAIB, and IBio

Phenotypes of Offspring: A, AB, B

io

io

io

io

io

io

io

io io

io

io

io

100

0

0

0

Let’s Practice

IA

IA

IA

IA

IA

IB

IBIB

IB IB

0

100

0

0

____% O

____% A

____% B

____% AB

Father is type A, homozygous; Mother is type B, homozygous

IA

IB

IA

IB

IA

IA

io

IB

ioio

io IB

25____% O

____% A

____% B

____% AB

Father is type A, heterozygous; Mother is type B, heterozygous

io

io

25

25

25

With Mendel’s genetics we learned about alleles that are dominant or recessive. But….more was learned…Let’s look at an (incomplete) time line first!

1866 – Mendel’s work is published

1900 – DeVries, Correns, von Tscherack independently rediscover Mendel’s work which leads to it becoming more widely accepted as valid.

1886 – DeVries – discovers and describes the process of Mutation

1950 – Chargoff had the idea of base pairs in DNA (guanine always with cytosine and thymine always with adenine) He also noted that every organism has different ratios of these base pairs. His work ultimately helped discover the shape of DNA.

1952 – Rosalind Franklin took x-ray diffraction photographs of DNA which greatly helped to discover the shape of the DNA molecule.

1953 – Watson and Crick discovered the true shape of the DNA molecule

1988 – Human Genome Project begins

2003 – Human Genome Project is complete, analysis will take many years