Powers Mendelian Genetics Presentation

Mendelian Genetics

& Heredity

Unit 4 Biology

Background

• With the exception of gametes (aka egg

and sperm), all cells in your bodyare
diploid and contain two copies of each
chromosome.
– One copy from mom (in her egg)
– One copy from dad (in his sperm)

How many total chromosomes make up who

you are?

Background

• Gene: section of DNA that provides the instructions for

making a protein.

• Since a gene is a piece of a chromosome and we have two of every

chromosome, we have two copies of instructions to make every protein
in our bodies.

• Alleles: different versions of the same gene.

• We inherited a full set of chromosomes (containing genes) from each of our

parents, but we may not have inherited the same version of every gene.

- Ex. Mom gave you a gene for blue eyes and dad gave you a gene for brown eyes.

• Homologous Chromosomes: the matching

chromosomes from our mom and dad.

• They contain the same genes in the same locations, but may contain

different alleles.

Multiple Choice

A segment of DNA that codes for a trait

• gene

• centromere

• chromosome

• chromatid

Multiple Choice

an observable characteristic

• trait

• personality

• cell

• DNA

Multiple Choice

Genetic traits are determined by one or more

• genes

• organelles

• random chances

• cells

Multiple Choice

A version or variation of a gene

• trait

• gene

• allele

• DNA

Gregor Mendel

• Gregor Mendel– Austrian monk who used pea

plants to learn about genetics. He is known as the
“Father of Genetics”.

3 Laws of Inheritance:

– Law of Dominance
– Law of Segregation
– Law of Independent

Assortment

Mendel’s Experiments

In his experiments crossing pea plants he made sure that:

1.

He had control over breeding.
– Mendel used pea plants that

typically self-pollinate.

2.

He used only purebred
plants.
– Purebred = type of organism

whose ancestors are
genetically uniform.

3.

He observed only “either-or”
traits.
– Chose pea shape, pea

color, pod shape, pod color,
plant height, flower color.

Multiple Choice

What did Mendel call the two kinds of traits exhibited by the pea plants (one coming from the father, one from the mother)?

• Genesis

• Elective traits

• Alleles

• Quadrants

Multiple Choice

Another word for homozygous is...

• purebred

• hybrid

• heterozygous

• dominant

What is a Cross?

• Cross – mating of 2 organisms

P = parental generation
F1 = first generation
F2= second generation

Significance of Alleles

•

Alleles have assigned letters:

-

Uppercase for dominant (Ex. A)

-

Lowercase for recessive (Ex. a)

•

Remember, each individual has 2 alleles (one from
each parent) for every gene.

• Homozygous – 2 of

the SAME alleles.
– Ex. AA or aa

• Heterozygous – 2

DIFFERENT alleles.
- Ex. Aa

Punnett Squares

Name ____________________

Date __________ Period _____

1. Cross a heterozygous brown coat male mouse to a heterozygous brown coat

female mouse. (B= brown coat, b=white coat)

What is genotype ratio of this cross?
What is the phenotype ratio of this cross?

2. Cross a heterozygous pointy ear female dog to a homozygous floppy ear male

dog. (P=pointy ear, p=floppy ear)

What is the genotype ratio of this cross?
What is the phenotype ratio of this cross?

3. Cross a homozygous black fur bat to a homozygous white fur bat.

(F=black fur=white fur)

What is the genotype ratio of this cross?

What is the phenotype ratio of this cross?

4. Cross a heterozygous curly tribble with a straight hair tribble.

(C= curly, c= straight)

What is the genotype ratio of this cross?

What is the phenotype ratio of this cross?

5. Suppose that in space there exist creatures whose traits are inherited by

Mendel’s laws. You find that purple eyes, P, are dominant over yellow eyes, p.
Two purple-eyed creatures mate and produce six offspring. Four of them have
purple eyes and two have yellow eyes. What are:

The genotypes of the parents?
The phenotypes of the parents?
The genotypes of the offspring?

Multiple Choice

Tt is-

• homozygous dominant

• homozygous recessive

• heterozygous

• none of these

Multiple Choice

If a pea plant were homozygous recessive for height, how would its alleles be represented?

• TT

• Tt

• tt

• tT

Multiple Choice

Which of the following is an example of a heterozygous genotype?

• RR

• Rr

• rr

Law of Dominance

– Dominant = if present, allele will always have

that trait expressed (seen).

• Ex. If brown hair is dominant over blonde hair, b represents the

allele for blonde hair and B represents brown hair.

• If you inherited the B allele from one or both of your parents, you will

have brown hair. (whether you are BB or Bb = brown)

– Recessive = allele will only have that trait

expressed (seen) when the dominant allele is
NOT present.

• The only way to get blonde hair is to inherit the b allele from both

parents. (bb = blonde hair)

A dominant(strong) allele will express itself over

a recessive (weak) allele.

Genotype vs. Phenotype

• Genotype– the actual

alleles inherited.
– Ex: Genes that code for

flower color such as FF,
Ff, or ff

• Phenotype – the

physical
traits/characteristics
seen in an organism.
– Ex: Purple flowers

Multiple Choice

A recessive allele is only expressed when a dominant allele is not present

• Law of dominance

• Law of inheritance

Multiple Choice

The "Father of Genetics"

• Isaac Newton

• Gregor Mendel

• Albert Einstein

• Robert Hooke

Law of Segregation

When chromosomes separate in meiosis, each

gamete (egg or sperm) will receive only one

chromosome from each pair.

Example:

•If a man has alleles for
brown hair and blonde
hair, he is heterozygous
(Bb) for hair color.

•His sperm cells can
contain the allele for
brown hair (B) OR the
allele for blonde hair (b).

•Each sperm only gets
ONE of the alleles.

Multiple Choice

The law that states that when an organism is hybrid for a pair of contrasting traits, the dominant trait is the one that is shown. 

• Law of Dominance

• Law of Independent Assortment

• Law of of Segregation

• Law of Incomplete Dominance

Multiple Choice

If your genotype is Bb, then you can only pass a B or b to your child but you can't pass both.

• Law of Dominance

• Law of Segregation

• Law of Independent Assortment

Law of Independent

Assortment

The assortment of chromosomes for one trait
doesn‘t affect the assortment of chromosomes

for another trait.

This allows for any combination of

maternal and paternal

chromosomes to be passed on

because homologous chromosomes

line up RANDOMLY during

metaphase.

Multiple Choice

Law states that 1 allele is more dominant than the other. Recessive allele is only expressed when dominant is not present

• Law of dominance

• Law of inheritance

Multiple Choice

Law which states that different traits are inherited independently of each other

• Law of independent assortment

• Genetics law

Punnett Squares

• Punnett Square– a diagram that shows the probability

of inheriting traits from parents with certain genes.
– Monohybrid cross: a cross between two organisms looking at one trait.

Possible
genotypes
of offspring

PRACTICE

with Monohybrid Crosses

1.

In pea plants, round seeds are dominant to wrinkled seeds.
Cross a homozygous recessive plant with a homozygous
dominant plant.

1.

Cross two heterozygous plants.

1.

Cross a heterozygous plant with a plant that has wrinkled
seeds.

Dihybrid Crosses

Used when finding the possible genotypes for

offspring when considering two traits at the

same time.

Example #1: Cross two tomato plants that are both

heterozygous tall (Tt) / red (Rr). Note: the recessive traits in

tomatoes are short and yellow.

1. Write genotypes of parents: Mom = Dad =

1. Sort the alleles. (Ask yourself, what combinations could

go into the gametes during meiosis?)

Dihybrid Crosses

Example #1: Cross two tomato plants that are both

heterozygous tall (Tt) / red (Rr). Note: the recessive traits in

tomatoes are short and yellow.

3. Write these

combinations on the
sides of the Punnett
square. (In this case,
we will use the same
combinations on the top
and the side because the
mother and father had
the same genotype.)

3. Combine the alleles

from the top and left
to fill in the square.

4. Determine the

phenotypic ratio.

Dihybrid Crosses

Example #2: In peas, purple flowers are dominant to white, and

tall plants are dominant to short. A white and short plant is
crossed with a plant that is heterozygous for both traits.

1. Write genotypes:

Mother =
Father =

1. Sort the alleles.
2. Write these

combinations on the
sides of the Punnett
square.

3. Combine the alleles

to fill in the square.

4. Determine the

phenotypic ratio.

Dihybrid Crosses

Example #3: In humans, brown eyes are dominant to blue eyes,

and brown hair is dominant to blonde. Cross a homozygous

dominant brown-eyed blonde man, with a blue-eyed and

heterozygous brunette woman.

Using Probability

• Probability can be used to determine the

possibility of getting a specific genotype.
– This is especially helpful when looking at more

than 2 genes at a time.

• How to:

1.

Perform a monohybrid cross for each
individual gene and determine the odds of
getting each genotype in fraction form.

2.

Multiply the odds to determine the likelihood of
getting this and this and this and etc.

Using Probability

• Example:

– Erica has the genotype AABbcc for three different genes.
– Carlos has the genotype AaBbCc for those same three

genes.

– Determine the probability of Erica and Carlos having a

child who is heterozygous for all three genes (AaBbCc) .

Using Probability

• Example:

– Erica has the genotype AABbcc for three different genes.
– Carlos has the genotype AaBbCc for those same three genes.
– Determine the probability of Erica and Carlos having a child who

is heterozygous for all three genes (AaBbCc) .

Therefore, the probability of being AaBbCc = ½ x ½ x ½ = 1/8

½ Cc
½ cc

Incomplete Dominance & Nondominance

Ms. Cabrera