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Heredity

Vocabulary

Heredity

Big IdeasHeredity and Reproduction Understand and explain that every organism

requires a set of instructions that specifies its traits, that this hereditary information (DNA) contains genes located in the chromosomes of each cell, and that heredity is the passage of these instructions from one generation to another.

Heredity

Is the passing of physical characteristics, or traits, from parents to offspring

Genetics is the study of heredity Parents are referred to as the P generation The first offspring are referred to as F1

The offspring of the F1 generation are the F2 generation

Note: F stands for “filial”, or son

Gregor Mendel, a monk living in the mid nineteenth century, discovered the principles of heredity through his experiments breeding pea plants and noticing which traits were passed from parent to offspring

Mendel’s work was unknown during his lifetime, but was rediscovered during the early 1900s

Mendel is known as the Father of Genetics

Heredity

Different forms of the same gene are called alleles

During fertilization, sperm from the male parent and eggs from the female parent join to form new offspring

An organism’s traits are controlled by the alleles it receives from its parents

Heredity

In the system of complete dominance, alleles can be dominant or recessive

A dominant allele’s traits always show up in the offspring A recessive allele’s traits is hidden whenever the

dominant allele is present Heterozygous means that the two inherited alleles are

different. An organism with heterozygous traits is called a hybrid

Homozygous means that the two inherited alleles are the same (either both dominant or both recessive). An organism that is homozygous is called a purebred

Heredity

Traiteye coloring

Dominantbrown eyes

Recessivegrey, green, hazel, blue

hair dark hairnon-red haircurly hairwidow's peak

blonde, light, red red hairstraight hairnormal hairline

facial features dimplesfreckles

no dimplesno freckles

Dominant versus recessive traits in humans

Heredity

Using Punnett squares to predict inherited traits– A Punnett square shows how dominant

and recessive traits combine– Capital letters stand for dominant traits– Lowercase letters stand for recessive traits– Example: Brown eyes (B) are dominant

over blue eyes (b)

Heredity

Using Punnett squares – If one parent is a hybrid

brown-eyed (Bb), and the other parent is a pure blue-eyed (bb), this is how the Punnett square would look.

– First, write the alleles of one parent on the side and the other on the top of the square

b b

B

b

Heredity

Using Punnett squares– Rewrite each allele

straight across or straight down

– By convention, capital letters go first

– Remember, B (brown eyes) dominates b (blue eyes). What you will see is the offspring’s genotype, or actual genes. Let’s see what the eye colors are

b b

B Bb Bb

b bb bb

Mother’s traits

Fat

her’s

tra

its

Heredity

Using Punnett squares– Remember, B (brown

eyes) dominates b (blue eyes). So, in this Punnett square, the possible offspring (on average) will be 50% brown and 50% blue.

– The actual appearance of the organism is its phenotype.

b b

B BbBrown

BbBrown

b bbBlue

bbBlue

Heredity

Using Punnett squares– Let’s try again with two

parents who are both hybrids for eye color

– First, write the alleles of on parent on the side and the other on the top of the square

B b

B

b

Heredity

Using Punnett squares– Rewrite each allele

straight across or straight down

– By convention, capital letters go first

– Remember, B (brown eyes) dominates b (blue eyes).

– Now lets see what the eye colors are

B b

B BB Bb

b Bb bb

Heredity

Using Punnett squares– Remember, in humans,

B (brown eyes) dominates b (blue eyes). So the possible offspring (on average) will be 75% brown and 25% blue.

b b

B BBBrown

BbBrown

B BbBrown

bbBlue

Heredity

Using Punnett squares– Another way to look at

this is that the offspring, on average, will be 25% homozygous dominant (BB), 25% homozygous recessive (bb) and 50% heterozygous (Bb)

b b

B BBBrown

BbBrown

B BbBrown

bbBlue

Heredity

Using Punnett squares– Let’s try some Punnett

square problems– Write them on your own

paper– In humans, brown eyes (B)

is dominant over blue eyes (b). One parent is pure brown eyes, and the other parent is pure blue eyes, create the Punnett square to determine the average of each eye color

Heredity

Using Punnett squares– A brown-eyed man

marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage.

Heredity

Using Punnett squares– In fruit flies, gray body

color is dominant (G), and ebony body color is recessive (g). What happens when a pure gray fruit fly has offspring with an pure ebony fruit fly?

Heredity

Using Punnett squares– In a species, tall (T) is

dominant and short (t) is recessive. If a pure short parent (tt) mates with a tall parent (TT or Tt), what would the percentages of tall and short offspring be?

– You will need to draw two Punnett squares to answer this one

Heredity

Using Punnett squares– There is a second system

you need to know, called co- dominance

– In this system, more than two alleles exist.

– For example, with blood types, there are A alleles, B alleles and b alleles (recessive)

Heredity

Using Punnett squares– “Could a man with type B

blood and a woman with type AB produce a child with type O blood?"

– The woman must be AB, and the man may be either BB or Bb. Draw the two Punnett squares and see if any of the offspring could have type O (bb).

Note: the only possible outcomes for blood type are Bb (Type B), bb (Type O), Ab (Type A) and AB (Type AB)

Heredity

Using Punnett squares– There is a third system

you need to know, called incomplete dominance

– In this system, neither trait dominates, the offspring’s traits are a mixture of both.

– For example, red ( R) and white (W) would combine to form pink (RW)

R R

W RW RW

W RW RW

Chromosomes and Inheritence

Chromosomes exist in pairs– In the early 1900s, Walter Sutton, an American

geneticist, looked at chromosomes in grasshoppers– His hypothesis was that chromosomes are the key

to understanding how traits are passed from parents to offspring

– Sutton discovered that sex cells in grasshoppers (sperm and egg cells) only had half the number of chromosomes as regular body cells


Chromosomes exist in pairs– In an organism’s body cells, chromosomes exist in

pairs– In an organism’s sex cells, chromosomes exist

alone– These sex cells are called sperm in males, ova or

egg cells in females– During fertilization, chromosomes in sperm and egg

cells merge to form a cell with chromosomes in pairs One half of each pair came from the male parent One half of each pair came from the female parent

Chromosomes and Inheritance

How do sex cells form?– Meiosis is the process through which sex cells are

formed– During meiosis, chromosome pairs separate into

two cells– The sex cells that form later have only half as many

chromosomes as the other cells in the organism– Note: females have two X chromosomes– Note: males have one X and one Y chromosome


Before meiosis


After meiosis Sex cells

Sexual versus asexual reproduction

Reproduction– During sexual reproduction, two parents

contribute different DNA – This results in a new organism with traits of both

parents– Sex cells are formed through the process of

meiosis


Reproduction– During asexual reproduction, one parent

produces an identical offspring– New cells are created through the process of

mitosis


Advantages Disadvantages

Asexual Can quickly produce many offspring

No variation in offspring, may not survive in a new environment

Sexual Variation in offspring, some may survive if environment changes

Finding mate, waiting for offspring to develop

Classwork 1 - Heredity

1. Different forms of the same gene are called

2. The study of heredity

3. The “Father of Genetics”

4. Define dominant allele

5. Define recessive allele

6. Define Heterozygous

7. Define Homozygous

Classwork 1 – Heredity (continued)

8. In __________ dominance, neither trait dominates, and the offspring’s traits are a mixture of both.

9. In __________ dominance, dominant triats mask recessive traits

10. In __________, more than two alleles are present

Human Genetics

Pedigree: A family tree that shows the presence or absence of a trait according to family relationships over several generations

Human Genetics

Humans inherit 23 pairs of chromosomes (46 total): 23 from Mom and 23 from Dad

There is no noticeable genetic differences between different “races”

There is no gene for alcoholism. There IS a gene that, when not properly functioning, gives a greatly increased likelihood that the person will develop alcoholism IF certain circumstances occur (like the person drinks alcohol)– A dysfunction on the gene BRAC1 causes one type of

breast cancer. That does not mean that everyone with BRAC1 is destined to have breast cancer. Also, there are plenty of people without any dysfunction on BRAC1 that have breast cancer.

Human Genetics

Some human diseases are caused by genetic disorders– Hutchinson's Disease (dominant)– Sickle Cell (recessive)– Cystic Fibrosis (recessive)

Some diseases are X-linked (only found on the X chromosome)– Hemophilia

Some diseases are caused by having an extra chromosome– Down’s syndrome (extra chromosome #21)

Sickle Cell Disease

An altered hemoglobin protein (red blood cells) allows the red blood cells to change shape when under pressure or stress (like during exercise).

This recessive disorder has remained in constant levels in the population because it makes the person immune to malaria

Sickle Cell Disease

An altered hemoglobin protein A homozygous person for the disease frequently dies younger than normal.

A heterozygous person for the disease can lead a normal life and be immune to malaria; the heterozygous person has both sickle shaped red blood cells and round shaped red blood cells. What type of dominance is shown?

X-linked diseases

If the defective gene only occurs on the x chromosome then it is called an x-linked disease. In this case, women may be unaffected carriers while all men will be affected (as they only have one x chromosome). In pedigrees, these conditions are passed from mother to son.

-Ex. Color blindness and male pattern baldness (not diseases but still x-linked)

X-linked diseases

An example of an x-linked disease is Hemophilia. In this condition, the person affected has platelets that do not function normally. When cut, the hemophiliac will bleed without clotting. These individuals used to always die young but new modern technology has allowed them to live longer lives.

About 1/6000 people in America has hemophilia

Classwork 2 – Human Genetics

1. Define pedigree2. Humans inherit __ pairs of chromosomes, half from

each parent This gene causes one type of breast cancer This genetic disorder affects red blood cells In this x-linked disease, the person affected has

platelets that do not function normally, which means they may bleed to death without clotting.

Even though her great-great grandmother Queen Victoria was a carrier of hemophilia, the children of this queen do not have the genetic disorder

Genetics

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