Genetics

Genetics Notes

Who is Gregor Mendel?

Principle of Independent Assortment – Inheritance of one
trait has no effect on the inheritance of another trait

“Father of Genetics”

Traits

• Genetics – study of how traits are passed from parent

to offspring

• Traits are determined by the genes on the

chromosomes. A gene is a segment of DNA that
determines a trait.

• Chromosomes come in homologous pairs, thus genes

come in pairs.
Homologous pairs – matching genes – one from female
parent and one from male parent

• Example: Humans have 46 chromosomes or 23 pairs.

One set from dad – 23 in sperm
One set from mom – 23 in egg

Gene for eye color
(blue eyes)

Gene for eye color
(brown eyes)

Homologous pair
of chromosomes

• One pair of Homologous Chromosomes:

Alleles – different genes (possibilities) for the same trait –

ex: blue eyes or brown eyes

Dominant and Recessive Genes

• Gene that prevents the other gene from “showing” –

dominant

• Gene that does NOT “show” even though it is present –

recessive

• Symbol – Dominant gene – upper case letter – T

Recessive gene – lower case letter – t

Dominant

color

Recessive

color

Example: Straight thumb is dominant to hitchhiker thumb

T = straight thumb t = hitchhikers thumb

(Always use the same letter for the same alleles—

No S = straight, h = hitchhiker’s)

Straight thumb = TT
Straight thumb = Tt
Hitchhikers thumb = tt

* Must have 2 recessive alleles
for a recessive trait to “show”

• Both genes of a pair are the same –

homozygous or purebred

TT – homozygous dominant
tt – homozygous recessive

• One dominant and one recessive gene –

heterozygous or hybrid

Tt – heterozygous

BB – Black
Bb – Black w/

white gene

bb – White

Genotype and Phenotype

• Combination of genes an organism has (actual gene

makeup) – genotype
Ex: TT, Tt, tt

• Physical appearance resulting from gene make-up –

phenotype
Ex: hitchhiker’s thumb or straight thumb

White fur (b)

Punnett Square and Probability

• Used to predict the possible gene makeup of offspring –

Punnett Square

• Example: Black fur (B) is dominant to white fur (b) in mice

1. Cross a heterozygous male with a homozygous recessive female.

Black fur (B)

White fur (b)

Heterozygous

male

White fur (b)

Homozygous
recessive female

Bb

Bb

bb

bb

Write the ratios in the following orders:

Genotypic ratio
homozygous : heterozygous : homozygous

dominant recessive

Phenotypic ratio
dominant : recessive

b

b

b

B
Possible offspring – 2N

Male gametes - N

(One gene in

sperm)

Female gametes – N

(One gene in egg)

Male = Bb X Female = bb

Genotypic ratio = 2 Bb : 2 bb

50% Bb : 50% bb

Phenotypic ratio = 2 black : 2 white

50% black : 50% white

BB

Bb

Bb

bb

B

b

B

Genotypic ratio = 1 BB : 2 Bb : 1 bb

25% BB : 50% Bb : 25% bb

Phenotypic ratio = 3 black : 1 white

75% black : 25% white

Cross 2 hybrid mice and give the genotypic ratio and
phenotypic ratio.

Bb X Bb

b

BB

Bb

Bb

bb

B

b

B

b

Example: A man and woman, both with brown eyes (B)
marry and have a blue eyed (b) child. What are the
genotypes of the man, woman and child?

Bb X Bb

Man = Bb

Woman = Bb

XX

XX

XY

XY

X

Y

• What is the probability of a couple having a boy? Or a girl?

Chance of having female baby? 50%

male baby? 50%

Who determines the sex of the child? father

X

X

Multiple Choice

Who is the father of genetics?

• Gregor Mandela

• Gregor Mendel

• Isaac Newton

• Albert Einstein

Drag and Drop

Which of the following can be passed on from parents to offspring?

Drag these tiles and drop them in the correct blank above

hair color

earlobes

rolling of tongue

height

behavior

favorite food

skills

skin color

blood type

Multiple Choice

What determines traits of a person?

• The food he/she eat everyday

• The wealth of a family

• The genes of the parents

• The skill of the family doctor

Multiple Choice

Which of the following is used as representation of a gene?

• DNA

• RNA

• Chromosomes

• Alleles

Poll

What is the probability for a husband and wife that are both heterozygous for a trait normal skin and hair color to have an albino child?

• 25%

• 50%

• 75%

• 100%

Multiple Choice

What is the possible genotype of the parents having all normal skin and hair color offsprings?

• both are heterozygous

• one is homo dominant and homo recessive

• one is hetero and one is homo recessive

• both albino for their trait

Multiple Choice

Hitchhicker thumb is a dominant trait. What is the probability of having a child that is hitchhiker if one is heterozygous and another is homo recessive?

• 25%

• 50%

• 75%

• 100%

Multiple Choice

Whose genes contributes to the male gender in the offspring?

• Mother's gene

• Father's gene

• Father's DNA

• Mother's DNA

Incomplete dominance and Codominance

• When one allele is NOT completely dominant over

another (they blend) – incomplete dominance

Example: In carnations the color red (R) is incompletely
dominant over white (W). The hybrid color is
pink. Give the genotypic and phenotypic ratio from a
cross between 2 pink flowers.

RW X RW
RR

RW

RW WW

R

W

R

W

Genotypic = 1 RR : 2 RW : 1 WW
Phenotypic = 1 red : 2 pink : 1 white

• When both alleles are expressed – Codominance

Example: In certain chickens black feathers are
codominant with white feathers.

Heterozygous chickens have black and white speckled
feathers.

Multiple Choice

In rose, red (R) is incompletely dominant with white (W). How many pink flowers will appear if we crossed pollinate Red and White flower?

• 1

• 2

• 3

• 4

Multiple Choice

If black color (B) is codominant with white color (W) for dogs, what would be the probability for both heterozygous dogs to have a BW color dog offsprings?

• 25%

• 50%

• 75%

• 100%

Sex – linked Traits

• Genes for these traits are

located only on the X
chromosome (NOT on the Y
chromosome)

• X linked alleles always show

up in males whether
dominant or recessive
because males have only
one X chromosome

• Examples of recessive sex-linked disorders:

1. colorblindness – inability to distinguish between

certain colors

Color blindness is the inability to distinguish the differences between certain colors. The most
common type is red-green color blindness, where red and green are seen as the same color.

You should see 58
(upper left), 18
(upper right), E
(lower left) and 17
(lower right).

2. hemophilia – blood won’t clot

A

B

Example: What would be the possible blood types of
children born to a female with type AB blood and
a male with type O blood?

AB X OO

AO

BO

AO

BO

O

O

Children would be type A or B only

Multiple Choice

Who is always affected in sex-linked disorder?

• males

• females

• both

• neither the male nor the female

Multiple Choice

A boy who has blue eyes will have a genotype X^bY. Who transmitted the trait?

• The mother

• The father

• The grandfather

• The grandmother

Multiple Choice

Colorblindness is an example of sex-linked recessive, meaning the disease is carried by recessive gene. What would be the possibility for a daughter to have the colorblindness trait if the mother is colorblind and the father is normal?

• 0% chance

• 25% Chance

• 50% chance

• 100% chance

Multiple Choice

An AB blood type mother who is married to a type AO man cannot have an offspring with blood type __________.

• A

• B

• AB

• O