PATTERNS OF HEREDITY

Take notes on the following slides on your foldable.

PATTERNS OF HEREDITY Based on what we have learned, what

offspring phenotypes would you expect from a cross between these two horses?

White Horse Red Horse

White or Red Offspring, Right?Not necessarily…

Strawberry Roan

PATTERNS OF HEREDITY How did this happen?

Most of the time, traits display more complex patterns of heredity than the simple dominant-recessive pattern

What is the simple dominant-recessive pattern known as?Mendelian Genetics

Let’s look at Mendelian Genetics one more time…and then let’s look at the other patterns of inheritance!

MENDELIAN GENETICS Again, Mendelian genetics shows a

pattern where a clear dominant trait masks a recessive trait.

Cleft chins, dimples, and freckles are all examples of traits that follow Mendelian patterns.

MENDELIAN GENETICSPunnett Square

Dimples (D) are dominant over no dimples (d)

Cross two parents that are heterozygous for dimples. Complete the ratios for genotype and phenotype.

d D

D

d

DD

Dd

Dd

dd

Genotype: 1:2:1

Phenotype: 3:1

INCOMPLETE DOMINANCE Occurs when

the heterozygous allele combination results in a blend of the dominant and recessive trait.

Remember the horses…

INCOMPLETE DOMINANCE

Straight, wavy and curly hair is one example of incomplete dominance.

Another example is carnation color. If the red and white alleles are both present, the resulting offspring will be pink (a blend of red and white).

INCOMPLETE DOMINANCEPunnett Square

Red flower color (R) is incompletely dominant over white flower color (r) in carnations.

Cross two parents that are heterozygous for flower color. Complete the ratios for genotype and phenotype.

R r

R

r

RRRr

Rrrr

Genotype: 1:2:1

Phenotype:1:2:1

CO-DOMINANCE

Occurs when both alleles are expressed when the heterozygous allele combination is present.

Not a Blend!! Human blood is an example of co-dominance.

A and B alleles are both dominant over the recessive allele, O. If both the A and B alleles are present, both will be expressed.

CO-DOMINANCEPunnett Square

Cross two parents that are co-dominant for blood type. Give the ratios for genotype and phenotype.

IA

IB

IA

IB

IAIA

IAIB

IAIB

IBIB

Genotype: 1:2:1

Phenotype:1:2:1

SEX-LINKED (X) TRAITS Occurs when a trait is passed on from

generation to generation on one of the sex-determining chromosomes (X, Y)

The dominant and recessive representative alleles are written as superscripts on the X and Y.

XRY XrY

Only females can be carriers of X-Linked traits. What is a carrier?

Why can’t males be carriers of X-Linked traits?

XRXR XRXr XrXr

SEX-LINKED TRAITSPunnett Square

Cross a female carrier for colorblindness with a normal male. List the possible phenotypes.

XB Xb

XB

y

XBXB

B – not affectedb - affected

XBy

XBXb

Xby

Possible Phenotypes: Normal FemaleCarrier FemaleNormal Male Colorblind Male

POLYGENIC TRAITS

What does Poly mean? Many

Genic looks like? Genes! Occurs when several genes influence a

trait

The genes may be on the same chromosome or on different chromosomes

So POLY GENIC means MANY GENES!!!

POLYGENIC TRAITS Eye color, height, weight, and hair and

skin color are all polygenic traits.

MULTIPLE ALLELES

Genes with three or more alleles are said to have multiple alleles.

Human blood has 3 alleles: A, B, and O.

How many possible genotypes for blood?

6

How many possible phenotypes?

4i i

MULTIPLE ALLELESPunnet Square

Cross a parent with an IAIO genotype with a parent that has the genotype IBIO for blood.

List the possible phenotypes.

IA IO

IB

IO

IAIB

IAIO

IBIO

IOIO

Possible Phenotypes: Type AB BloodType A BloodType B BloodType O Blood