UNIT 7:GENETICS & HEREDITY

What is Heredity?

• Heredity is the passing of traits from parent to offspring.

• Genes on chromosomes control the traits that show up in an organism.

• Genes are separated during meiosis into the egg and sperm.

• The different forms of a trait that a gene may have are alleles.

• The study of how traits are inherited is genetics.

Genetic Traits Examples

Who was Gregor Mendel?• Austrian monk, known as

“The Father of Genetics”• 1856 - Studied pea plants• Examined the color, shape

and other physical characteristics of plants

• Discovered that plants received genetic information from its parents passed down through generations.

Mendel’s Findings

What are Dominant & Recessive Alleles?

• Dominant alleles will cover up, or dominate a recessive allele.• Represented by a capital letter (B)• Only need to inherit one dominant allele for trait to appear

• Recessive alleles can be masked by a dominant allele.• Represented by a lowercase letter (b)• Need to inherit two recessive alleles for the trait to appear

How do Alleles Determine Traits?

• Organisms inherit an allele from each parent to make an observable trait.

• Homozygous: Two of the same allele (example: BB, bb)• BB would be “homozygous dominant”• bb would be “homozygous recessive”

• Heterozygous: Two different alleles (example: Bb)

What are Genotype & Phenotype?

• Genotype: the genetic make-up of an organism (represented by letters, Aa, Dd, Rr, RR, DD, etc.)

• Phenotype: The way an organism looks or behaves as a result of its genotype (example: red hair, brown eyes, etc.)

What is a Punnett Square?

• A Punnett square is a tool used to predict results of genetic crosses.

How do I Solve a Punnett Square Problem?

1. Determine the genotypes of the parent organisms

2. Write down your "cross" (mating)

3. Draw a Punnett square

4. Split the letters of the genotype for each parent & put them outside of the Punnett square

5. Determine the possible genotypes of the offspring by filling in the Punnett square

6. Answer what the problem is asking for based on the results of your Punnett square

Punnett Square Example 1Mom is homozygous for having freckles (F) and dad (f) has none. Having freckles is dominant over not having freckles. What percentage of their children will have freckles?

The parents’ homozygous genotypes are:Mom = FF

Dad = ff

The parents’ homozygous genotypes are:Mom = F F

Dad = f f

SPLIT the parents’ genotypes across the top and the sides of the Punnett square.

F F

f

f

Fill in the Punnett square by dragging down and across the parent genotypes.

Final Answer:Since all of the children are Ff, 100% will have freckles.

F F

f

f

Ff Ff

Ff Ff

Punnett Square Example 2

Both mom and dad are heterozygous (Ff) and have freckles. What percentage of their offspring will NOT have freckles?

Their heterozygous genotypes are:Mom = Ff

Dad = Ff

The parents’ heterozygous genotypes are:Mom = F f

Dad = F f

F f

F

f

Final Answer:There is a 25% (1 out of 4) chance of their child NOT having freckles.

F f

F

f

FF Ff

Ff ff

Do Mendel’s Observations Apply to all Genes?• No… some genes are neither dominant nor

recessive.• Incomplete dominance: the inherited alleles from

the parents are blended together in the offspring.• Codominance: both alleles are expressed in the

offspring.• Multiple alleles: when a trait is controlled by more

than two alleles• Polygenic inheritance: when a group of gene

pairs act together to produce a trait.

What is Incomplete Dominance?• When two parents are homozygous for different forms of

trait, the offspring shows an intermediate phenotype.• Use two different capital letters to represent alleles. • Example: four-o’clock plants

A red 4-o’clock flower is crossed with a white, resulting in offspring with all pink flowers. What would be the results of a Pink and White flower?

R W

W

W

RW WW

RW

WW

RW x WWOffspring:RW: pink 50%WW: white 50%

What is Codominance?• Prefix “Co” means together.• When two parents are homozygous for different forms of a

trait, the offspring will express BOTH inherited alleles together.

• Use two different capital letters to represent alleles.• Examples:

In chickens, black and white feather color are codominant. What would be the resulting genotypes and phenotypes of a cross between two black and white feathered chickens?

B W

B

W

BB BW

BW

WW

BW x BWOffspring:BB: black 25%BW: black and white 50%WW: white 25%

What are Multiple Alleles?• You can only inherit 2 alleles, but more than that may

exist in the population.• Example: Human Blood Type• 3 alleles exist for blood type: A, B, and O. • A and B are both codominant• O is recessive

What is Polygenic Inheritance?• Polygenic inheritance occurs when a group of gene

pairs acts together to produce a trait• Produces a wide variety of phenotypes• Examples in humans include height, hair, skin and eye color

What is a Genetic Disorder?• Caused by DNA

mutations• Caused by mistakes in

meiosis (abnormal chromosome number)

• Caused by inheriting a dominant or recessive allele for the disorder

• Caused by inheriting a dominant or recessive allele for the disorder on the X-Chromosome (sex-linked)

Chromosomal Genetic Disorders

• Happens when the incorrect number of chromosomes is inherited from parents.

• Examples: Down Syndrome, Turner’s Syndrome, Klinefelter’s Syndrome

Dominant & Recessive Genetic Disorders

• Offspring must inherit two copies of recessive allele for recessive genetic disorder to appear.• Carriers carry recessive allele but do not show symptoms

• Examples: Cystic Fibrosis, Albinism, Tay-Sachs Disease

• Offspring only need to inherit one dominant allele for a dominant genetic disorder to appear.• No carriers • Examples: Huntington’s Disease, Marfan Syndrome

How is Gender Determined?• Chromosomes that determine sex in humans are XX in

females and XY in males.• Females produce eggs with only one X chromosome.• Males produce sperm with either an X or a Y

chromosome.• Father’s sperm determines sex of the offspring

Sex-Linked Genetic Disorders • Some genes are inherited on the X or Y chromosome,

called sex-linked genes.• Color-blindness is a sex-linked disorder caused by a

recessive allele on the X chromosome. • Sex-linked genetic disorders are more common in

males.

What is a Pedigree?• A pedigree is a diagram that follows a trait

through generations of a family. Like a “family tree”

Pedigree Example

What is Genetic Engineering?

• Changing the arrangement of DNA that makes up a gene

• Using a bacterium or a virus, abnormal DNA can be replaced with normal DNA. Can be used to treat genetic disorders.

• Genetically Modified Organisms (GMOs) are plants and other food organisms where the DNA has been altered.