NONMENDELIAN

GENETICS

INCOMPLETE DOMINANCE

DEFINITION: When neither allele is dominant over the other. When the two alleles are found together, they result in an intermediate trait.

LET’S TRY A PROBLEM:

In Mountain Boomers, the genes for length of tail exhibit “incomplete” dominance. Use a Punnett Square to predict the result of a cross between a homozygous long-tailed and a homozygous short-tailed Mountain Boomer. What do the offspring look like?

DETERMINE THE LETTERS YOU WILL USE TO REPRESENT THE TRAITS:

L = long-tail S = short tail LS = medium-tail

DETERMINE THE GENOTYPES OF THE PARENTS THAT ARE BEING CROSSED:

Long-tailed Parent = LL Short-tailed parent = SS

DETERMINE THE POSSIBLE GAMETES THAT EACH CAN PASS ON:

LL = GAMETE L SS = GAMETE S

DRAW A PUNNET SQUARE AND INSERT THE GAMETES ON EACH SIDE.

L

S

LS

ANSWER: OFFSPRING WILL ALL BE MEDIUM-TAILED

How many offspring will be medium-tailed if you cross two medium-tailed lizards?

MULTIPLE ALLELES

DEFINITION: When not just two but THREE or more alleles of the same gene code for a single trait.

EXAMPLE: ABO BLOOD TYPE

In humans, the A allele for blood type and the B allele for blood type are both dominant over the O allele for blood type. Neither A nor B blood types is dominant over the other, and when they mix you get AB blood type.

What are the four types of human blood?

Type A Type B Type AB Type O

What are the possible genotypes for human blood?

AA AO BB BO AB OO

PROBLEM: If a man with type A blood marries a woman with type B blood, can they have a child

with type O blood?What are the possible genotypes of the parents?

Man = AA or AO Woman = BB or BO

Draw Punnett Squares to represent the different Possibilities:

A A A A A O

B

B

B

B

B

O

AB AB

AB AB AB AB

AO AO

AB BO

AB BO

A O

B

O

AB BO

AO OO

ANSWER: ONLY IF BOTH PARENTS ARE HETEROZYGOUS FOR THEIR BLOOD TYPE CAN THEY

HAVE A CHILD WITH TYPE O BLOOD.

PROBLEM: CAN A MALE HETEROZYGOUS FOR BLOOD TYPE B, BE THE FATHER OF A CHILD WHO HAS TYPE

A BLOOD IF THE MOTHER IS TYPE O?

1) Determine the possible genotypes of the parents and child..

Dad = BO Mom = OO

Child = AA or AO 2) Place possible gametes in Punnett Square and multiply.

Dad = B or O; Mom = O

DAD

B O

M O

O

M

BO OO

ANSWER = NO, he can only have a type B or O

child.

Some genes alter the affects of other genesEPISTASIS occurs when the phenotype of one gene is

affected by another gene.

For example: The dominant coat color in mice is gray(B); the recessive black (b). However, another gene on a different chromosome allows for color. A = normal color pigment, a = no color. In order for a gray mouse to

be produced there must be both a dominant gray gene (B) and a dominant color gene (A). If a mouse is recessive for color pigment (aa), it will be albino.

Try this problem:If a mouse with the genotype AaBb (B = gray color, b = black color; A = color pigment, a= no color pigment) mates with a mouse with

the genotype aaBb. What are the phenotypes of their offspring?

1) Determine the cross. AaBb x aaBb

2) Determine the possible gametes for each parent.

Parent 1 =AB or Ab or aB or ab

Parent 2 = aB or ab

3) Place each parents gametes in the Punnett square and multiply. AB Ab aB ab

aB

ab

AaBB AaBb aaBB aaBb

AaBb Aabb aaBb aabb

4) Look at all the genotypes and determine the phenotypes.

All the A_B_ = ?

All the aa B_ = ?

All the A_ bb = ?

All the aabb = ?

Results :

_____gray; _____ black: _____ albino3/8 1/8 1/2

MULTIPLE GENES = POLYGENIC TRAITS

DEFINITION: When several genes work together to control the expression of a trait, causing the trait to

appear in a wide variety of forms.

Examples: Hair color, eye color, skin color, height

SEX DETERMINATION

In humans the 23rd pair of chromosomes = Sex chromosomes

These chromosomes determine if the child will be a male or female.

In MALES the 23rd pair are not HOMOLOGOUS = X and Y allele.

In FEMALES the 23rd pair are HOMOLOGOUS = X and X allele.

The FATHER determines the sex of the baby.

LET’S SEE HOW THIS WORKS:1. Each parent can pass on only

ONE of their alleles. Females only an X; Males an X or Y. Place them in the square and see the results.

X X

X

Y

XX XX

XY XY2. 50% chance of male or female

SEX LINKED GENES

Sex Linked Genes are genes located on the X or Y chromosome. There are very few traits on the Y chromosome, so most males get only ONE

allele for these traits because they get only 1 X. Females get two alleles, one on each X. THUS, males have a greater chance of getting

a disease that is found on the sex chromosomes.

EXAMPLE: Color Blindness

Color Blindness (red-green) is a recessive trait carried on the X chromosome. Only females can be carriers. Males either have the

disease or they don’t.

SAMPLE PROBLEM:

If a red-green colorblind man marries a woman with normal color vision whose father was colorblind, what will be the

expected phenotypic results of their children? 1) To begin the problem – DETERMINE the GENOTYPES of the P1

generation (parents). Because the trait is on the X chromosome we write it a little differently. To write the genotype we must include the X and Y allele and a superscript for the colorblindness allele above each X allele.

The Y allele does not carry the colorblindness allele. You include a 0 superscript above the Y.

Male Genotype = XcY0 Female = XCXc

2) Next determine the possible gametes each parent can pass on to their offspring.

Male gametes = Xc or Y0 Female = XC or Xc

3) Place the possible gametes for each parent in the Punnett Square and multiply to get the possible offspring from this cross.

Male

Xc Y0

Female

XC

Xc

XCXc XCY0

Carrier Normal male

XcXc XcY0

colorblind colorblind male

Female

4) Answer the question:

½ the males and ½ the females will be colorblind

½ the females will be carriers of the disease

ANOTHER SAMPLE PROBLEM

CALICO CATS

One pair of genes for coat color in cats is sex-linked. The gene B produces yellow coat, b produces black coat, and the heterozygous (carrier) Bb produces tortoise-shell coat

[CALICO]. What kind of offspring will result from the mating of a black male and a calico female?

Gametes = Male _________ Female _________XBY0 XBXb

XB Yo

XB

Xb

XBXB XBY0

XBXb XbY0

Offspring =

½ females calico, ½ black; ½

males black, ½ yellow

Can you get a calico male?

Linked Genes

When two or more genes are on the same chromosome, we say they are linked.

Linked genes are not inherited by independent assortment. They usually split together during meiosis (not randomly) because they

are on the same chromosome.

You can tell if two or more genes are linked by looking at the results of a particular cross.

• Sample Problem

If the genes G or g and W or w are on the same gene, what type of genotypic results would you get from the following cross:

GgWw x GgWw

In unlinked genes, the gametes an individual could pass on with these genotypes would be GW or Gw or gW or gw

With linked genes the possible gametes is different because they pull together during meiosis. The gametes would be: GW or gw.

Draw the Chromosomes Here: See white board.

Compare the two Punnett Squares – Linked and UnlinkedUNLINKED

GAMETES = GW OR Gw OR gW OR gwPunnett Square

GW Gw gW gw

GW

Gw

gW

gw

GGWW GGWw GgWW Ggww

GGWw GGww GgWw Ggww

GgWW GgWw ggWW ggWw

GgWw Ggww ggWw ggww

Classic phenotypic numbers =

9 dominant for both traits; 3 dominant for first, recessive for second; 3 recessive for first, dominant for second; and 1 recessive

for both traits

LINKED

GAMETES = GW or gw GW gw

GW

gw

GGWW GgWw

GgWw ggww

Classic phenotypic numbers = 3 dominant for both traits; 1 recessive for

both traits

If the genes on the same chromosome cross over with their homologous pair the number results may even appear more

unusual (not typical 3:1).

AS A GENERAL RULE: The CLOSER together two genes are the less likely they will crossover. The FURTHER APART two

genes are the more they are likely to cross over.

SEX INFLUENCED TRAITSDEFINITION: Certain traits that are dominant in

one sex and recessive in the other. Certain chemicals cause these differences.

EXAMPLE: BALDNESS

Baldness is dominant in males and recessive in females.

SAMPLE PROBLEM: If a man who is not bald mates with a female that is not bald but

whose father was bald, what % of their children will be bald?

Determine the genotypes:

Male Genotype: bb Female Genotype: Bb

Cross the parents: bb x Bb using a Punnett Square

Bb

bb

b

B

b

Determine the possible offspring:

How many boys will be bald?

How many girls will be bald?

Can women be bald?

Who determines if male offspring will be bald?

MUTATIONSDEFINITION: A mistake during DNA replication causing a change in

either an individual gene or the whole chromosome.

TYPES OF MUTATIONS:1) CHROMOSOME – PART OF MANY GENES OR AN

ENTIRE CHROMOSOME IS AFFECTED.

A) Deletion: piece of chromosome breaks off

Example: Cri du chat – Mental retardation, small head, unusual face features, cat like cry

B) Duplication: Segment of chromosome repeats itself.

Example: Fragile X syndrome – Mental retardation

C) Inversion: Piece breaks and rejoins in reverse order on the same chromosome

D) Translocation: When a piece breaks and rejoins to different chromosome

E) Nondisjunction: A chromosome pair fails to separate during meiosis; leads to polyploidy (extra numbers of chromosomes) or aneuploidy (too

few chromosomes)

Types of Diseases Caused by Nondisjunction

DOWN’S SYNDROME = Extra 21st Chromosome; 2n = 47

TURNER’S SYNDROME = XO

Female whose sex organs don’t develop, sterile, no mental retardation, short stature

SUPER MALE = XYY

Found in many criminals, fertile.

KLINEFELTER’S – XXY

Large breasts, high voice, sterile, retarded male

TRISOMY X =

XXX

NORMAL

2) GENE MUTATIONS = Change in the DNA at one gene site

A) POINT MUTATION – Change in just one base in a single gene. May be addition, deletion or substitution

of a base.

1)Frameshift = Type of point mutation - Only addition or deletion (more disastrous)

Example = Sickle Cell Anemia

Mutations can occur in either somatic or sex cells. Those that occur in somatic cells affect only the individual. Those that

occur in sex cells affect the offspring too.

CAUSES OF MUTATIONS:

1) Unknown 2) Age 3) Mutagens – viruses, radiation, air pollution, foods and their additives,

chemicals (thalidimide, DDT)