Genetics (Recap)Let’s look at this again

What is a Gene?

GeneThe part of a chromosome that codes for a

specific traitHas two Alleles

◦One from mom and other from dad◦Can either be dominant or recessive

If Dominant (T) then the trait will be expressed If Recessive (t) then the trait can only be

expressed when there’s another recessive allele

How Are They Written?

A gene is homozygous (pure) when bothalleles are the same

◦Ex. TT (homozygous dominant) or tt (homozygous recessive)

A gene is heterozygous (hybrid) when bothalleles are different

◦Ex. Tt

Genotype◦Name of the gene: TT, Tt or tt

Phenotype ◦What the gene looks like or does…◦Ex. albinism genotype is aa but the phenotype is

lack of skin or hair color

What is a Monohybrid Cross?

A cross that involves only one gene:Example:A homozygous dominant tall plant iscrossed with a heterozygous tall plant. Givethe genotypic and phenotypic ratios for theoffspring:

TT x TtT T

T

t

T T T T

T t T t

Genotype Ratio:2TT and 2Tt

Phenotype Ratio:

4 Tall Plants

What Are Some Examples?

In silkworms a single gene determines the color of the cocoon. The yellow cocoon allele is dominant. It is indicated by “C”. The white allele is recessive, and indicated by “c”. Complete the following Punnett Square assuming a homozygous dominant male fertilizes a homozygous recessive female.

CC x ccC C

c

c

C c C c

C c C c

Genotype Ratio:4Cc

Phenotype Ratio:

4 yellow cocoons

What Are Some Examples?

The length of fur in guinea pigs is controlled by a single gene. The dominant allele “F” produces short fur. The recessive allele “f” produces long fur. Complete the following Punnett Square assuming a heterozygous male fertilizes a homozygous recessive female

Ff x ffF f

f

f

F f f f

F f f f

Genotype Ratio:2 Ff and 2 ff

Phenotype Ratio:

2 short haired 2 long haired

What is a Dihybrid Cross?

A Cross that involve TWO traits

In race horses, black hair (F) and a trotting gait (G) are dominant traits. Recessive traits are chestnut hair (f) and a pacing gait (g). The genes for hair color and gait are on two different chromosomes.Determine the possible offspring from a cross between two heterozygous black Trotters. FfGg x FfGgGamete Combinations:

FG Fg fG fgFG Fg fG fg

FFGG FFGg FfGG FfGg

FFGg FFgg FfGg Ffgg

FfGG FfGg ffGG ffGg

FfGg Ffgg ffGg ffgg

FG Fg fG fg

FG

Fg

fG

fg

Genotype Ratio: FFGG l, FFGg ll, FfGG ,ll FfGg llll,

FFgg l , Ffgg ll, ffGG l, ffGg ll,

ffgg l,

Remember:

FF or Ff: Black Hairff: Chestnut HairGG or Gg: Trotting Gaitgg: Pacing Gait

So: Black Hair Trotters: 9Chestnut Trotters: 3Black Hair Pacers: 3Chestnut Pacers: 1

What is Incomplete Dominance?

When traits mix like paint Example:

◦When Red snapdragons are crossed with white snapdragons the resulting offspring are pink.

RR x WW

R R

W

W

R W R W

R W R W

Genotype Ratio:4 RW

Phenotype Ratio:

4 Pink

What is Incomplete Dominance?

When traits mix like paint Example:

◦Cross a pink snapdragon with another pink snapdragon.

RW x RW

R W

R

W

R R R W

R W W W

Genotype Ratio:1 RR, 2 RW, 1 WW

Phenotype Ratio:

1 Red, 2 Pink, 1 White

What is Codominance?

When both forms of a trait are displayed at the same time

Example◦ABO blood Types◦Type A: IAIA, or IAi◦Type B: IBIB, or IBi◦Type AB: IAIB◦Type O: iiCross a homozygous type A with a type O

Genotype Ratio:4 IAi

Phenotype Ratio:4 Type A

IAiIAi

IAiIAi

IAIA X ii

IA IA

i

i

Cross a type AB with a type O Homozygous type A: IAIB Type O: ii Genotype Ratio:

2 IAi; 2 IBi

Phenotype Ratio:2 Type A2 Type B

Probabilities:

Type A: 2 out of 4Type B: 2 out of 4

IBiIAi

IBiIAi

IAIB X ii

IA IB

i

i