T - Needham.K12.ma.us

Transcript T - Needham.K12.ma.us

Genetics…

it’s all about

YOU!

Have you ever heard someone say… “You have your father’s eyes.” or “You’ve got your mother’s smile.” or “You look just like your grandfather.”

physical characteristics

We all know that we get certain

traits

from our parents or grandparents.

But have you ever wondered

HOW

it happens?

How

you got your dad’s eyes or your mom’s smile?

The answer to “How does it happen?” is…

GENETICS!

Heredity

the passing of traits from parents to offspring.

Genetics

the study of heredity.

Traits

are

physical characteristics, like eye color, hair color, freckles, or blood-type.

Who was the first person to study genetics?

An Austrian priest and science teacher named Gregor Mendel

Some Human Traits

Dimples mid-digit hair cleft chin Tongue-roller

nd el Me

Mendel was an Austrian monk and science teacher who was also responsible for the monastery’s garden.

During the years

1856-1863 ,

he conducted experiments with over

28,000 pea plants.

His experiments were the first large-scale, long-term scientific study of heredity ever done.

His work, published in 1865, was ignored at the time, because other scientists did not understand its importance.

However, Mendel’s experiments with pea plants were rediscovered in 1900.

He is now known as the “Father of Genetics” for his discoveries of inheritance patterns in pea plants.

flower bed staircase & flower bed a rno ustri B n A of ry i te Photos monas northern wall of the monastery’s garden Photos of Brno monastery in Austria beehives and apiary

Mendel’s choice of pea plants for his experiments was a good choice, for 2 reasons: 1. They reproduce quickly & have many offspring 2. They have many simple, either-or traits Mendel tested 7 pea plant traits:



seed shape



seed color



flower color



pod color



pod shape Inflated Pinched



stem height Short



flower position Side End

Pea Plants

In order to control which plants pollinated which plants, Mendel had to hand-pollinate every flower in the pea plants he was experimenting on.

purebred

purebred means that all the offspring have the same trait as the parents.

Short Short X X Short Short Short

In his first experiment, Mendel crossed (mated) a purebred tall pea plant with a purebred short pea plant.

Short X What do you think happened?

You would predict that half the offspring would be tall and half would be short, wouldn’t you?

Mendel’s first experiment

Or maybe you would predict that the offspring would all be medium height.

Both predictions are WRONG!!!

In every single experiment, Mendel found that ALL the offspring were… tall!!!

Short

X P generation (parents)

purebred tall purebred short

F 1 generation (children)

offspring are all tall

So what happened to the “short” trait? Did it disappear?

Here’s where Mendel showed true genius!

What he did next was… he crossed (mated) the tall offspring from the first experiment with each other.

Mendel’s second experiment

He was trying to find out if the “short” trait had really disappeared, or if it was still present in the tall pea plants, but was covered up somehow. The results from his second cross were truly amazing. What do you think happened?

In EVERY SINGLE EXPERIMENT, the offspring in the second cross were: X offspring of first cross (F 1 generation)

Short

offspring of second cross (F 2 generation)

3/4 (75%) tall

1/4 (25%) short

P1 Here’s a summary of Mendel’s experiments X “Parents” Here’s a (purebred summary of Mendel’s plants) experiments F1 X generation “Children” (hybrid tall plants) F2 generation “Grandchildren” 75% tall 25% short

Mendel tested his experiment again and again, and got the same results every time.

Then he tested he got the 6 other traits in the same way, and exact same results with those traits.

“Parents” P “Children” F 1 “Grand children” F 2 The F 1 generation plants were always 100% the dominant trait.

The F 2 generation plants were always 75% the dominant trait, and 25% the recessive trait.

Mendel’s actual experimental results

So, what’s going on here?

Mendel drew several conclusions:

The inheritance of each trait is determined by "factors" (now called genes) that are passed on from parents to offspring unchanged.

An organism inherits two factors, one from each parent, for each trait. 3.

One factor can “mask” or cover up another factor.

A trait may not show up in an individual but can still be passed on to the next generation.

Genes

Mendel realized that one factor in a pair was masking, or hiding, the other factor. For instance, in his first experiment, when he crossed a purebred tall plant with a purebred short plant, all offspring were tall.

Although the F1 offspring all had both tall and short factors, they only displayed the tall factor

. He concluded that

the tallness factor masked, or “covered up”, the shortness factor.

Today, scientists refer to the

“factors” that control traits

genes.

Genes

A gene is a section of a chromosome, which contains the instructions for a trait (Examples: plant height or flower color in pea plants, or hair color and blood type in humans)

Genes

All chromosomes come in pairs that are the same size, and have the same genes in the same locations. This is because an organism inherits 2 sets of chromosomes, one from the father and one from the mother.

Since the chromosomes come in pairs, the genes come in pairs too. Every organism has 2 of every gene in their chromosomes. These genes are called gene pairs.

•

Alleles different forms of a gene

For example:



If the gene is for tail color in critters, the 2 alleles would be “blue tail” or “orange tail”.

Chr.



If the gene is for flower color in pea plants, the 2 alleles would be “purple” or “white”.

Gene for tail color Gene for flower color Blue allele Orange allele

Alleles

•

different forms of a particular gene

What are some possible alleles for: ~ handedness? ~ hair color?

~ eye color?

~ hair texture?

~ dimples?

Dominant and Recessive

Dominant and Recessive Alleles

**Alleles Dominant: Alleles that cover up or hide recessive alleles .**

Recessive: alleles that are hidden or covered up by a dominant allele.

Which of the 2 tail color alleles in critters was dominant?

Homozygous and Heterozygous



homozygous: has 2 identical alleles for a trait (purebred)



heterozygous : has 2 different alleles for a trait (hybrid)

Homozygous or Heterozygous?

Homozygous or Heterozygous?

T T b b R r S S G g t t Y Y

Alleles

Homozygous or Heterozygous?

A b a B c D e c d e F F G h I G H I

Genetic Notation

Geneticists assign a letter to each allele for a trait. They use the first letter in the dominant trait. So, for the trait stem height, since tall is dominant, the letter “ T ” would be used. The dominant allele (tall) is abbreviated “T” and the recessive allele (short) is abbreviated “t”.

A purebred tall plant would be TT , and a purebred short plant would be tt .

A hybrid pea plant (like the tall plants in the F1 generation) would be because Tt, it has one dominant tall allele, and one recessive short allele.

Genetic Notation

Genotype

and

Phenotype



An organism’s genetic make-up

.

genotype is its (Examples: TT, Tt, tt)



An organism’s phenotype physical appearance.

is its (Examples: tall plant, round seed, purple flower)

Genotype TT Tt tt Phenotype tall plant tall plant short plant

What would be the phenotype genotypes?

for the following Genotype Gg Phenotype green pod gg GG yellow pod green pod pp Pp PP Yy RR rr SS white flower purple flower purple flower yellow seed round seed wrinkled seed side flower

What are the possible following genotypes phenotypes?

for the Phenotype end flower side flower green seed yellow seed round seed wrinkled seed tall plant short plant Possible Genotypes ss SS, Ss yy YY, Yy RR, Rr rr TT, Tt tt

Probability…

•

is the likelihood will happen.

(chance) that an event is expressed as a percentage or fraction.

Everyday examples of probability:

•

weather!

Lots of times we hear predictions like “80% chance of rain,” or “20% chance of snow.”

•

the lottery “There’s a 1 in 10 million chance your ticket will be a winner.”

•

genetics is all about probability

Calculating Probability

Probability is calculated using this formula: Actual outcomes x 100 = Probability % Possible outcomes Example A: If I flip a coin, the probability that it will show heads is 1 out of 2, or 50%.

Example B: If I roll a die, the probability that I will roll a 3 is 1 out of 6, or 16.7%.

Example C: If I roll a die, the probability that I will roll an even number is 3 out of 6, or 50%.

Multiple Probabilities

In multiple probabilities

(likelihood of 2 or more events happening at the same time)

you multiply the probability of one event times the probability of the second event.

Prob of Event 1 x Prob. of Event 2 = Multiple Probability

For example: the probability of flipping

two

coins and getting

2 heads

is: 1 x 1 = 1 2 2 4

or 25%

Probability Practice

1. What is the probability that, if I roll a pair of dice, I will get 2 sixes?

1 out of 36, or 2.8% 2. What is the probability that, if I flip a penny 10 times, I will get heads all 10 times?

1 ÷ 2 10 , or 1/1024 3. If I’m a pea plant, and my father is hybrid tall, what is the probability that I got a t gene from him?

1/2 4. If I’m a pea plant, and my mother is hybrid tall, what is the probability that I got a t gene from her?

1/2 5. If I’m a pea plant, and both my parents are hybrid tall, what is the probability that I will get a t gene from both of them?

1/2 x 1/2 = 1/4, or 25%

Probability

When flipping 2 pennies, there are 4 possible outcomes: H-H H-T

Same thing

T-H Probability 1/4 or 25% Probability Probability 1/4 or 25% 1/4 or 25% T-T Probability 1/4 or 25% 25% + 25% = 50% How does this relate to genetics?

In hybrid (heterozygous) crosses, the probabilities are the same as when flipping pennies!

Punnett Squares

Punnett squares are diagrams that show the probability that offspring will inherit a certain trait.

For example: this is a Punnett square for a cross between

two purebred (homozygous) tall pea plants .

T T T T T T T Genotype of offspring: 100% TT T T T T T Phenotype of offspring:

Two

purebred tall 100% tall

plants can only have

purebred tall

offspring.

How To Make Punnett Squares

How about a cross between two purebred short pea plants?

Short Short

t t t t t t t t t t t t Genotype of offspring: 100% tt Phenotype of offspring: 100% short

Two

purebred short

plants can only have

purebred short

offspring.

How about a cross between a purebred tall and a purebred short pea plant?

T T t t Tt Tt Tt Tt Genotype: 100% Tt Phenotype: 100% tall This is a Punnett square of Mendel’s first experiment. The result is all hybrid tall offspring.

Cross between 2 Hybrid (Heterozygous) tall plants T t T

T T T t

T t t t

Genotype: TT 25% Tt 50% tt 25 % Phenotype: Tall 75% Short 25% Look familiar? This is Mendel’s second cross…

Okay…how about a cross between a purebred tall and a hybrid tall plant?

T T Genotype: T T T T T 50% TT, 50% Tt t T t T t Phenotype: 100% tall How about a cross between a purebred short and a hybrid tall plant?

t t Genotype: T t T t T t 50% Tt, 50% tt Phenotype: t t t t 50% tall, 50% short

Punnett Square for Albinism in Humans

In the cross

, where

is a dominant allele for

Normal pigmentation

and

pigmentation (albinism)

, is a recessive allele for

there is a

¾_

probability the offspring will be

normal pigmentation

and

¼_

probability they will be

albino .

N n N NN N n n N n nn Albino child, USA Albino child, Tanzania

Dirk brings his family tree to class

Family Tree

Exploring a Pedigree

Pedigree: diagram that shows the presence of a trait in a family.

A carrier is a person who does not have the recessive trait, but does have the recessive gene. (They’re hybrid)

Pedigree of Queen Victoria of England

for the trait of hemophilia How many children did Victoria and Albert have?

How many were daughters, and how many were sons?

How many of Victoria’s daughters were carriers of the hemophilia gene?

How many of Victoria’s sons were hemophiliacs?

How many of her grandsons were hemophiliacs? How many of her great-grandsons?

Mermaid Tails

Pedigree Analysis is a Key Tool in Human Genetics Analyzing a pedigree is like puzzle-building – you try things (assigning potential genotypes) until the pieces fit, and you’re as certain as you can be about genotypes and inheritance patterns (autosomal vs. X-linked; dominant vs. recessive; complete, incomplete or co-dominance).

Pedigree Analysis

Shea Family Pedigree Blue/non-blue eyes

Shea Family Pedigree ADD/non-ADD

A Pedigree of a Dominant Human Trait

A Pedigree of a Recessive Human Trait

Note that the trait can appear in offspring of parents without the trait.

Heterozygotes (hybrids) who do not show the trait are termed carriers .

How many possible carriers are there on this pedigree?

Co-Dominance

One exception to the dominant / recessive pattern is

co-dominance.

dominant, and

In this pattern,

both traits both alleles are expressed.

are A capital letter represents one of the co-dominant alleles.

A different capital letter represents the other co-dominant allele.

In cattle,

red

and

white

coats are

co-dominant

offspring is called

roan

; it has

both

red and . The hybrid white hairs in its coat.

RR X WW red white R W ro an roan steer

Co-Dominance

In horses, gray horses (GG) are codominant to white horses (WW). The heterozygous horse (GW) is an appaloosa horse (a white horse with gray spots). Gray (GG)

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

X White (WW) A p p a l o o s a ( G W )

BLOOD TYPE:

an example of co dominance in humans



There are 4 blood types: A, B, AB, O



Blood type is determined by 2 factors in the blood

factors A and B

•If •If •If •If factor A is present, you are factor B is present, you are Type A

Type B

A and neither B factors are present, you are Type factor is present, you are Type O.

A B

• The A and B factors are co-dominant ; when both are present, both are expressed.

• Type O is recessive (needs two O genes to be present).

Blood Type Genotypes & Phenotypes

Phenotype Type A blood Genotype AA or AO Type B blood Type AB blood BB BO AB or Type O blood OO 1) What are the genotype and phenotype probabilities for the children of a man with Type A blood (homozygous) and a woman with type B blood the children of a man with Type O blood and a woman with Type AB blood?

1) What are the genotype and phenotype probabilities for the children of a man with Type A blood (homozygous) and a woman with Type B blood (homozygous)?

2) What are the genotype and phenotype probabilities for the children of a man with Type O blood and a woman with Type AB blood?

Incomplete Dominance

One thing Mendel didn’t realize was that there are some exceptions to the dominant / recessive pattern of inheritance that he observed in his pea plants.

One exception is: pattern, incomplete dominance.

In this one allele/trait does not completely dominate the other allele/trait

The result : the traits are blended

F 1 generation: offspring of red and white flowers are all pink (a blend of the 2 parents’ colors).

F 2 generation: offspring of pink plants are 25% red , 25% white , and 50% pink Incomplete dominance

Incomplete Dominance

Here’s what’s happening: R R W R W RW Red

crossed with

white

makes

pink

offspring.

W RW RW R W R RR W RW RW WW Pink

crossed with

pink

makes:

25% red 25% white 50% pink

Incomplete Dominance



A capital letter (P) represents one of the incompletely dominant alleles.



The same capital letter prime (P 1 ) represents the other incompletely dominant allele, so that the two do not get mixed up.

In humans, curly hair (HH) is incompletely dominant to straight hair (H 1 H 1 ).

A heterozygous human has (HH 1 ).

wavy hair

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Cross a person with curly hair with a person who has wavy hair.

Polygenic Inheritance

When a single trait is

determined by more than one gene

, we say that it is

polygenic

Height is a polygenic trait

Also: •

eye color

•

hair color

•

blood type

• •

Multiple Alleles

Eye color is determined by more than one gene Thus eye color appears to vary on an almost continuous scale from brown to green to gray to blue

•

Eye color is determined by three genes : one controls texture of the iris which refracts light to make blue, and a second which determines the amount of pigment, called melanin . When a small amount of melanin is present, blue or green eyes result, while brown & black eyes result from increasing amounts of melanin

Eye Color

Eye colors can range from the most common color, brown , to the least common, green . Rare genetic specialties can even lead to unusual eye colors: black, red , and violet . Eye color is an inherited trait influenced by more than one gene ( polygenic ).

Heterochromia (eyes that are different colors)

There are 3 genes that control eye color. One gene has Brown (B) and blue (b) alleles ( Brown is dominant over blue ). The 2nd gene also has 2 alleles: Green / hazel (G) and lighter color (g).

Green is dominant over the lighter-color allele.

Eye Color Calculator activity

•

Hair color

Hair color is determined by more than one gene

• • •

Thus hair color appears to vary on an almost continuous scale from black to brown to blond red to The brown and black pigment is melanin The red pigment is an iron containing molecule

Hair color

It is thought that hair color is controlled by two genes.

Black hair

Dark brown hair Brown hair Auburn hair Red hair Grey (gray) hair Blonde hair White hair One gene has 2 alleles: Brown (B) The 2nd gene has 2 alleles: and Non-red (N) blonde (b).

and red (n).

The combination of these two genes, plus environmental factors (and age), contributes to the many different shades of hair color in humans.

Skin Color

Skin color is determined by the amount and type of

melanin

, the pigment in the skin. Skin color is determined by 6 different genes, which accounts for the vast range of different skin colors in human beings.

link

Chromosomes, Genes and DNA

The structure and an actual picture of a chromosome. Human chromosome set from a skin cell.

Chromosomes

are around proteins.

long strands of DNA

, wrapped Humans have

chromosomes in every cell.

Genes

are

sections of chromosomes

Humans have 20,000 - 25,000 genes in every cell.

karyotype

Karyotype: a picture of all 46 of a person’s chromosomes, arranged in 23 pairs

Note that 22 of the 23 pairs of chromosomes are the same size, and have the same banding patterns.

The first 22 pairs of chromosomes are called “autosomal.” However, the 23 rd pair of chromosomes do not look alike at all.

The 23 rd pair of chromosomes are called the “sex” chromosomes, or “X and Y” chromosomes.

Make a karyotype How scientists read chromosomes

Human Chromosome 1

False-color photograph shows human chromosomes, with the Chromosome 1 pair highlighted in blue. Chromosome 1 contains nearly twice as many genes as the average chromosome and makes up eight percent of the human genetic code. It is packed with 3,141 genes and linked to 350 illnesses including cancer, Alzheimer’s, Parkinson’s disease, and a gene for a common form of cleft lip and palate.

X and Y Chromosomes

The 23rd pair of chromosomes in humans determines a person’s gender. A female has two X chromosomes; a male has 1 X and 1 Y chromosome.

Female: XX Male: XY X chromosome is much larger than the Y chromosome.

Which of these karyotypes shows a male, and which shows a female?

B A

Gender Determination

egg X girl boy X Y

Genetic abnormalities of the XY Chromosomes Klinefelter Syndrome XYY syndrome Triple X syndrome Turner Syndrome

Fragile X Syndrome

Fragile X Syndrome

Fragile X syndrome is caused by a mutation in the FMR1 gene , located on the X chromosome. The mutated gene cannot produce enough of a protein that is needed by the body's cells, especially brain cells, to develop and function normally. There are a variety of symptoms, including: Mental retardation, Hyperactivity, Short attention span, and Autism .

Click here to learn more about FXS

Down Syndrome

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Down Syndrome, or Trisomy 21, occurs when a person has 3 copies of Chromosome 21, instead of the normal 2 copies. Other Genetic Abnormalities

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Plants Apple Peas Onion 34 14 16 Potato Rice 48 24 Tomato 24 Corn 20 Click here to find out how many chromosomes other species have.

Animals Dog Frog Goldfish Horse Housefly 78 26 94 66 12 Human Mosquito 46 6 Mouse 40 Chicken 78

GENES BY THE NUMBERS GENES BY THE NUMBERS genes, the genes that are “turned on” in each cell vary depending on the cell’s function. These are the numbers of working genes in different parts of the body.

Brain White blood cell Liver Heart Pancreas Bone Colon Skeletal muscle Kidney Skin Thyroid Gland Eye Small Intestine Smooth muscle Esophagus Red blood cell 3195 2164 2091 1195 1094 904 879 735 712 629 584 547 297 127 76 8

in Some Genetics Numbers Some Genetics Numbers one human cell there are… 3 billion base pairs 1 billion codons 25, 000 genes 46 chromosomes 6.5 feet of DNA Number of people on Earth: 7 billion Number of people with exactly your DNA ….1

YOU !!!

Asexual Reproduction Asexual Reproduction



organism divides and produces an exact replica (clone) of itself. Examples: • bacteria, amoeba, yeast, algae no genetic material (DNA) is exchanged.

• no genetic diversity in the species (except for mutations) paramecium algae

amoeba

hydra (budding)

Sexual Reproduction



of an egg cell and a sperm cell.

Examples: humans, flowers, fish, frogs, birds, snakes



half the genetic material comes from the mother, and half from the father.



offspring is unique (no other organism on Earth has exactly the same DNA it has)



there is genetic diversity in the species, which helps ensure that a species will survive a widespread disease or environmental disaster.

frogs sperm fertilizing egg paramecia hoverflies

Twins and Multiple Births Twins and Multiple Births

There are two types of twins, and they occur in two different ways… Fraternal twins • not identical • two eggs are fertilized at the same time

Click here learn more to about twins

Identical twins • identical • formed when one fertilized egg divides Chances of multiple births Twins: 1 in 90 Triplets: 1 in 8100 Quadruplets: 1 in 729,000

first set of octuplets born

Multiple Births Multiple Births

alive, in Houston TX, in 1998

triplets identical quadruplets van Tol quintuplets Gosselin twins & sextuplets McCaughey septuplets

news on multiple births

Homologous chromosomes

•

Two chromosomes which contain the same genes but may contain different alleles* *Alleles are different forms of the same gene.

For example: the gene for eye color has many alleles: blue

green

brown

hazel

black

gray

etc.

nucleus

What’s Where?

cell chromosome gene Chromosomes are made out of… DNA.