MendelianGeneticsMendelianGenetics

Chapter 8

Genetics is the study of how traits are passed from parents to offspring.

Male Female

I. Patterns of Inheritance

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These traits are the characteristics that make an organism what it is. Traits can be for:

wings

proteins

legs

tongues

hair color, to name a few…

The study of genetics started with an Austrian monk named Gregor Mendel (1822-1884).

Mendel is known as the Father of Genetics.

Mendel used pea plants to study how traits Mendel used pea plants to study how traits are inherited. He observed the traits of are inherited. He observed the traits of tallness and color of sweet pea plants. He tallness and color of sweet pea plants. He fertilized the flowers of the peas to produce fertilized the flowers of the peas to produce offspring with different traits. From this, he offspring with different traits. From this, he made up the basic laws of genetics, called made up the basic laws of genetics, called Mendelian traitsMendelian traits..

•Some were Some were hybridshybrids (also called (also called heterozygousheterozygous), or ), or crossbreedscrossbreeds (organisms (organisms that receive different that receive different forms of a genetic trait forms of a genetic trait from each parent).from each parent).

•Some of the peas were purebred, or have the same genetic traits from both their parents.

Some of the traits in peas that Mendel studied

Mendel hypothesized that each trait is controlled by a distinct “factor.” These factors are now known as genesgenes, or segments of DNA , or segments of DNA that carry the code for a specific trait; genes that carry the code for a specific trait; genes are dark bands found on the are dark bands found on the chromosomechromosome, or , or rod-shaped structures found inside the nucleus rod-shaped structures found inside the nucleus of a cell.of a cell.

Cell - Nucleus - Chromosome - DNA Strand - Gene Code

Most organisms have two copies of every gene, one from each parent.

Ex. Humans have 46 chromosomes: 23 from each parent (sperm and egg)

Some other organisms’ chromosome counts compared to humans.

The different forms of the pair of genes are known as alleles.

One from mom and one from dad.

Difference between a pair of alleles and just any two genes on a pair of chromosomes.

DOMINANT GENES•If an organism has two different alleles for a trait, say height, only one is expressed, or visible. This is known as the dominant gene.

•Symbols for dominant genes are always capitalized (ex. D, B, T, R).Symbols for dominant genes are always capitalized (ex. D, B, T, R).

Which color is dominant in this family?

Examples of dominant traits in humans.

Recessive Genes

•The recessive gene is a weaker gene that does NOT show even though they are present unless there is no dominant gene present.

•Symbols for recessive genes are always lowercased (ex. d, b, t, r).

Here the recessive genes are only expressed 1 out of 4 times. 3 out of 4 times, the dominant gene hides it.

Examples of recessive traits in humans.

Here’s an example:

- alleles for height

T = the allele for tallness

t = the allele for shortness

If an organism was Tt or TT, then the dominant allele would be expressed and the recessive allele would not. The organism would be tall.

If the organism was tt, then the recessive alleles would be expressed and the organism would be short.

II. Principles of InheritanceIf an organism is either TT or tt (the same case letter), then that individual is homozygous (homo- means “the same”)- homozygous dominant- homozygous dominant = HH, BB, TT ( = HH, BB, TT (Pure Dominant)Pure Dominant)

- homozygous recessive - homozygous recessive = tt, rr, hh, bb = tt, rr, hh, bb (Pure Recessive)(Pure Recessive)

- Many pets and domesticated animals are bred for their homozygous traits.

If an organism is Tt (different case letters), then that individual is called heterozygous (hetero- means “different”)

Also called: Hybrid, Carrier, Crossbreed)

Some animals bred for… …their heterozygous traits.

What is the difference between Genotype and Phenotype?

Phenotype (“pheno” means “physical”) A. This term refers to a physical trait that can be seen. (Blue eyes or Type A blood, would be examples.) Genotype (“geno” means “genetic”) A. This term refers to an organism’s genetic (DNA) make-up for a trait. (Such as BB, Bb, and bb.)

The examples TT, Tt, or tt are known as an individual’s The examples TT, Tt, or tt are known as an individual’s genotypegenotype, , or the actual genetic makeup of an organism; or the actual genetic makeup of an organism; type of genestype of genes. ex . ex - Aa, AA, aa- Aa, AA, aa

If the genotype were TT or Tt, for example, then their outward appearance would be tallwould be tall. This is known as phenotypephenotype, or the way an offspring , or the way an offspring appears or looks; a physical appears or looks; a physical descriptiondescription. .

blondeblonde

talltall

albinoalbinofrecklesfreckles

If the genotype were tt, then their phenotype would be short. Get it?!

Short plants

Actor, Warwick Davis

Mendel’s discoveries of inheritance were lost until 1903.

Then, Walter S. Sutton outlined the chromosome theory of heredity.

Sutton observed stained cells through a microscope and witnessed chromosomes for the first time.

Stop.

Gregor Mendel’s studies produced three major laws known as Mendel’s Laws.

1. The Law of Segregation

- gene pairs separate when gametes (sperm & egg) form

- half the gametes contain one gene, half the other

2. The Law of Independent Assortment

- gene pairs segregate into gametes randomly and independently of each other

3. The Law of Dominance

- the dominant allele is expressed and the recessive allele can be hidden

Offspring are all red. Red is dominant over white.

Complete Dominance A. The dominant allele has information and it is expressed, even if there is only one copy. Geneticists call this “100% penetrance”. Incomplete Dominance A. Information from both alleles is expressed in the cell. Neither phenotype is completely penetrant, therefore the heterozygous phenotype appears “blended”. (Red + White = Pink)

Dominance

Codominance A. Both alleles are expressed in the cell. They are both equally present in terms of phenotype, i.e. black and white coats in animals, AB blood type in humans.

Dominance continued…

Punnett Square – grid work to determine probabilities for each pregnancy, mating, crossing, or pairing. Each

new mating requires a NEW Punnett Square.

Father’s Alleles Go Here

Moth

er’

s A

llele

s G

o

Here

III. Genetics and Predictions

Example:

If a mother is Pure Dominant for Brown Eyes (BB) and a father is Hybrid for brown eyes (Bb), the Punnett Square can help you find the possible eye color of the offspringDad’s

Alleles

B b

Mom’s Alleles

B

B B

BB

BB

B b

b

The offspring has a 50% chance of being Pure Dominant, a 50% chance of being hybrid, and a 0% chance of being recessive.

Sex-Linked Traits – attaches to an X chromosome; female is usually a carrier only (she will possibly pass on bad trait, but will not get it herself); the reason is that she has 2 “X” chromosomes and one is probably normal. The male only has one “X” chromosome, so he is usually affected with the condition.

Male:XY

Female:

XX

Examples:

Color Blindness

Hemophilia

Duchenne muscular dystrophy

Example #1:

A normal mother and a father with color blindness have a child. What are the chances that the child will have color blindness?

Xc Y

X

X XX

X XXc

Xc

Y

Y

Example #2:

A carrier mother and a father with color blindness have a child. What are the chances that the child will have color blindness?

X

Xc

Xc Y

XcXc

XcX X

Xc Y

Y

There is no chance that a child will have color blindness.

There is a 50% chance that a child will have color blindness.

One process scientists use to locate genes is called cytogenetic mapping. After breaking open nuclei, scientists fix the chromosomes on a slide, and then stain the chromosomes to see the bands. A complete set of banded chromosomes is also called a karyotype. The banding pattern is unique for each chromosome and allow to pair the chromosomes together and identify abnormalities.

XX – Karyotype Female (normal)

XY – Karyotype male (normal)

Bellwork: Punnett Square Problems

1.A pure dominant father and a pure recessive mother have a child. What are the chances of the baby having blonde hair? ( Hair color : B=dominant, b= recessive) 2.A heterozygous black rabbit and a white rabbit cross. What percentage of the offspring will be black?3.A homozygous brown dog mates with a heterozygous brown dog. What are the chances of the offspring being homozygous?4.A pure recessive white flower crosses with a pure dominant red flower. How many off spring will be homozygous?5.Two tall parents produce a short offspring. How can you explain this? Show your work.

Down syndrome:Causes cognitive impairments and developmental delays ranging from mild to serious. Effects one in every 800 infants.

Symptoms include:• Flattened facial features • Protruding tongue • Small head • Upward slanting eyes• Unusually shaped ears • Poor muscle tone • Broad, short hands • Single crease in the palm • Relatively short fingers • Excessive flexibility

Genetic Disorder

caused by “Trisomy

21”

Actor Chris Burke from “Life Goes On”

XXX – Multi X Female

Around one woman in 1,000 has three or more X-chromosomes. Most 47, XXX women are normal but some may have mild cognitive delays. With each extra X the impairments increase. For instance, a woman with 49, XXXXX will have severe disabilities.

Caused by trisomy (or greater) of the X chromosome

XXY – Klinefelter’s Syndrome• XXY – male with extra X chromosome• Extreme or non-apparent depending on the person• Can have affects on sex characteristics

For example:

– Enlarged breasts– Wide hips– Voice not breaking– Narrow shoulders– Long arms or legs– testicular atrophy – infertility

XYY – “super male”• Male with extra Y chromosome• More physically active • Delayed mental maturation• Learning Difficulties• More aggressive• Lower IQ• ADD or ADHD

XO – Turner’s syndrome • Turners Syndrome: a genetic defect in women in which there is only one

X chromosome instead of the usual two. • Affected women are infertile. They have female external genitalia but no

ovaries and therefore no menstrual.• Characteristically they are short in stature and have variable

developmental defects which may include webbing of the neck.

Actress Geri Jewell fro “Facts of Life”

Tay-Sachs disease • progressive destruction of nerve cells in the brain and spinal cord.• Infants appear normal until the age of 3 to 6 months• lose motor skills such as turning over, sitting, and crawling• develop seizures• vision and hearing loss• mental retardation• Paralysis• Rarely live beyond 5 years

Prader-Willi 7 genes are deleted from chromosome #15• mild cognitive delays • crossed eyes • small hands and feet• growth delay• short stature • become extremely obese due to incorrigible eating.

“floppy” child – muscle hypotonia

Patau's syndrome (Trisomy-13)

• tiny head • abnormal limbic system• tiny eyes • one cerebral hemisphere • a single eye

Stillborn term infant with Patau's syndrome. The baby has no eyes, no nose opening, and an elongated bulb hanging from forehead.

Cri Du Chat Syndrome “Cat’s Cry”

• deletion of part of chromosome 5• 1 in 50,000  live births.• distinctive facial features• shortened lifespan • Cognitive delays  

William's Syndrome • deletion of a part of chromosome 7 • Causes difference in the protein Elastin • Elf-like features: wide smile, large ears• Disorders with circulatory system • Affinity to music

William's Syndrome is a rare genetic defect, affecting approx. 1/20,000 births

Stem cell research

?

Behavioral Genetics

Your genes determine how you will behave.

Forensics DNA can

prove innocence or guilt in many

criminal cases.

Gene Testing

Can determine if a fetus has

genetic disorders

Gene therapy

is a technique

for correcting defective

genes

Genetic Discrimina

tion ?

Genetic Engineering

Food, crops, animals,

HUMANS?

Cloning

?

Regeneration

?

Ethical Issues with Gene Research