Genetics Basics

A gene can be described in two ways, by:

1. Genotype: genetic makeup; use letters to represent gene

▫(allele – different form of a gene; single letter)- Ex: BB, Bb, bb

2. Phenotype: physical trait; what you see- Ex Brown hair, blue eyes

Alleles can sometimes be described as Dominant or Recessive:

1. Dominant: this allele dominates the other

▫ use capital letters- Ex: Brown Hair (B) dominates blonde hair (b)

2. Recessive: this gene will only be expressed if 2 recessive alleles are present

▫ use lower case letters- Ex: blonde hair is recessive (bb)

A combinations of alleles can be Homozygous or Heterozygous:

1. Homozygous: same size letters (alleles) ▫ BB, bb

2. Heterozygous: different size letters (alleles)

▫ Bb Also called hybrid

Self-Quiz•What is an allele?

•What is the difference between genotype and phenotype?

•When will a recessive gene be expressed?

• Indicate whether the following are heterozygous or homozygous; Bb, BB, bb

•Why are there always at least 2 alleles for a gene?

A different form of a gene

Genotype represents genes, phenotype represents physical traits

When both alleles are recessive (lower case)

Bb-hetero, BB-homo dominant, bb homo recessive

You receive one allele for a gene from each of your parents

Punnett Square Basics

Dominate vs. Recessive Punnett Squares

• Remember: ▫ BB = homozygous dominant▫ Bb = heterozygous▫ bb = homozygous recessive

• Each parent donates 1 allele to their offspring

• How can we predict what alleles offspring will inherit?

Punnett Squares!Dad Mom

Offspring

bbBb

bb bbBb Bb

Ex 1: What is the predicted offspring of combining a parent that is Homozygous

recessive with one that is Heterozygous?

bbx Bb b b

B

b

2/4 = 50% Bb

2/4 = 50% bb

Bb

bbbb

Bb

Ex 2: A heterozygous brown bird is crossed with a black bird. Brown is

dominate to black. What is the predicted offspring of the baby birds

(genotypes and phenotypes)?Bbx

bbB bb

b

2/4 = 50% Bbbrown

2/4 = 50% bbblack

Bb

bbBb

bb

Self-Quiz• One cat carries heterozygous, long-haired traits (Ss), and its

mate carries homozygous short-haired traits (ss). Use a Punnett square to determine the probability of one of their offspring having long hair.

• In humans, brown eyes (B) are dominant over blue (b). 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. What is the man’s genotype? What are the genotypes of the children?

S s

s

s

Ss

Ss

ss

ss

B

b b

Bb Bb

?b ?b

Man’s genotype- BbBrown eyed children-Bb, BbBlue eyed children-bb b

Intermediate Traits Punnett Squares

Intermediate Traits•Sometimes, one trait does not dominate another.

Sometimes, both traits will be expressed. There are two types of intermediate traits

• Incomplete Dominance-a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. INBETWEEN trait 

• Codominance-cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.  COMBINATION trait

To complete a punnett square for an intermediate trait, use a combination of two different letters instead of upper vs. lower case (because there is no a need to show dominance)

Ex: a red flower and a white flower combine to make pink flowered offspring•R – red flower = RR (or CR CR)•W – white flower = WW (or CwCw)•RW – pink flower = (or CRCw)

A pink flower is crossed with a white flower. What are the genotypes and

phenotypes of the predicted offspring?

RWx WW RWW

W

2/4RW= pink = 50%

2/4 WW= white = 50%

RW

WWRW

WW

Roan Cow

•An example of codominance•Cattle can be red (RR = all red hairs),

white (WW = all white hairs), or roan (RW = red & white hairs together).  A good example of codominance.

•(the EOC loves to use this example, so learn it!)

Multiple Alleles

REVIEW• BASIC DOMINANCE-When the gene for one trait exists

as only two alleles & the alleles play according to Mendel's Law of Dominance, there are 3 possible genotypes (combination of alleles) & 2 possible phenotypes (the dominant one or the recessive one).

• INTERMEDIATE DOMINANCE-If there are only two alleles involved in determining the phenotype of a certain trait, but there are 3 possible phenotypes, then the inheritance of the trait illustrates either incomplete dominance or codominance.

• MULTIPLE ALLELES-If there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population.

Examples

• Basic-Hitchhiker’s thumb is a dominant trait. The possible genotypes are HH, Hh, hh. The possible phenotypes are Hitchhiker’s or no hitchhiker’s thumb.

• Intermediate-▫Codominance- The possible genotypes in cattle

are RR, RW, WW. The possible pheotypes are red, white or roan.

▫Incomplete-The possible genotypes in flowers are BB, BR, RR. The possible phenotypes are blue, blue & red or red.

Multiple Alleles•There are multiple alleles in a

population causing 4 or more phenotypes of a trait to exist

•KEEP IN MIND▫There may be multiple alleles within the

population, but individuals have only two of those alleles

Why?

…because individuals have only two biological parents.  We inherit half of our genes (alleles) from mommy, & the other half from daddy, so we end up with two alleles for every trait in our phenotype.

Example•Human Blood Type:

▫Blood type exists as four possible phenotypes: A, B, AB, & O.

▫There are 3 alleles for the gene that determines blood type: A, B, O

▫Alleles A & B are CODOMINANT What does that mean?

▫Allele O is recessive What is the genotype for O blood?

Combination. You “see” both traits. i.e. AB blood.

OO

Blood Typing Basics•Blood types are also represented as: IA, IB, io

or i- (this is how you will most likely see it expressed on your test. )

Phenotype GenotypeType O OO (io)

Type A AA, AO (IA IA, IB io)Type B BB, BO (IB IB, IB io)Type AB AB (IA IB)

Example•A husband is homozygous for blood group

B. He marries a woman with type O blood. What are the blood types of their predicted offspring? BO x OOB B

O

O

100% BO100% type B

BO BO

BO BO