22/05/2015

1

Biology 3201 – Genetics Unit #2: Mendelian

Genetics #2Mendelian Genetics (part 2) and Beyond

Incomplete Dominance

O Incomplete dominance: a situation where NEITHER of the two alleles for a trait are dominant O Ex. Snapdragon flowers (red, white, heterozygous

is pink)

O Ex. Four o'clock flowers (red, white, heterozygous is pink)

O We represent the alleles with the same capital letter with a ‘ to show the differenceO Red flowers = RR

O White flowers = R’R’

O Pink flowers = RR’

22/05/2015

2

Incomplete Dominance

O Ex. What would be the genotypic and

phenotypic ratios of a cross between a red

flower and white flower?

O Parents: Red = RR; White = R’R’

R R

R’ RR’ RR’

R’ RR’ RR’

Genotypic ratio: 100% RR’

Phenotypic ratio: all pink

Incomplete Dominance

O Ex. 2 What would be the expected ratios of a

cross between two pink flowers?

O Parents : RR’ x RR’

R R’

R RR RR’

R’ RR’ R’R’

Genotypic ratio: ¼ RR: ½ RR’: ¼ R’R’

Phenotypic ratio: ¼ red: ½ pink: ¼ white

22/05/2015

3

Co-dominance

O Co-dominance: a situation where both alleles for a

trait may be dominant. Both alleles are expressed in

a heterozygous individual.

O Ex.

O Feather colour in chickens (black and white

expressed at the same time is barred plummage)

O Roan horses (red and white hair expressed at the

same time

O Blood type (AB – type A and B blood expressed at

the same time

Co-dominance crosses

O In chickens, the gene for black feather

colour is co-dominant to the gene for white

fur colour. What are the expected genotypic

and phenotypic ratios of a cross between a

black rooster and a white hen?

O Parents: BB x WW

O Note: in co-dominant situations, each allele

is given a different capital letter.

22/05/2015

4

Co-dominance crosses

B B

W BW BW

W BW BW

Genotypic ratio: 100 % BW

Phenotypic Ratio: all barred plummage (black and white)

Multiple Alleles

O Some genetic traits are expressed by

multiple (many) alleles. Such a trait is blood

type in humans. The blood type in humans

is controlled by 3 alleles resulting in 4

distinct blood types:

O Type A

O Type B

O Type AB

O Type O

22/05/2015

5

Blood types Genotype Phenotype

IAIA or IAi Type A

IBIB or IBi Type B

IAIB Type AB (A and B are co-

dominant). Most common

blood type

ii O (recessive) Least common

blood type

Blood Types

22/05/2015

6

Mixing Blood

O Only certain types of blood can be mixed. If

you mix the wrong types of blood

agglutination (clumping) will occur. This will

cause the blood cells in the person being

affected to clump together and the person

would die from a blood clot in the brain,

heart, etc.

Red blood cell compatibility

Blood group AB individuals have both A and B antigens on the

surface of their RBCs, and their blood plasma does not contain

any antibodies against either A or B antigen. Therefore, an

individual with type AB blood can receive blood from any group

(with AB being preferable), but cannot donate blood to any

group other than AB. They are known as universal recipients.

Blood group A individuals have the A antigen on the surface of

their RBCs, and blood serum containing IgM antibodies against

the B antigen. Therefore, a group A individual can receive blood

only from individuals of groups A or O (with A being preferable),

and can donate blood to individuals with type A or AB.

22/05/2015

7

Red blood cell compatibility

O Blood group B individuals have the B antigen on the surface of

their RBCs, and blood serum containing IgM antibodies against

the A antigen. Therefore, a group B individual can receive blood

only from individuals of groups B or O (with B being preferable),

and can donate blood to individuals with type B or AB.

O Blood group O (or blood group zero in some countries)

individuals do not have either A or B antigens on the surface of

their RBCs, and their blood serum contains IgM anti-A and anti-

B antibodies against the A and B blood group antigens.

Therefore, a group O individual can receive blood only from a

group O individual, but can donate blood to individuals of any

ABO blood group (i.e., A, B, O or AB). (Universal donors)

Crosses involving Multiple Alleles

O Ex. A woman with blood type A marries a

man with blood type AB. What are the

possible blood types for the offspring?

O Recall: Type A has 2 genotypes; IAIA or IAi, so

two separate crosses need to be done

22/05/2015

8

Crosses involving Multiple Alleles

O Cross 1: IAIA x IAIB

IA IA

IA IAIA IAIA

IB IAIB IAIB

Genotypic ratio: ½ IAIA : ½ IAIB

Phenotypic ratio: ½ type A: ½ Type AB

Crosses involving Multiple Alleles

O Cross 2: IAi x IAIB

IA i

IA IAIA IAi

IB IAIB IBi

Genotypic ratio: ¼ IAIA : ¼ IAi : ¼ IAIB : ¼ IBi

Phenotypic ratio: ½ Type A: ¼ Type AB: ¼ Type B

22/05/2015

9

Chromosome Theory of Inheritance

O Proposed by Walter Sutton and Theodor Boveri

(1902)

O Studied chromosomes during the various phases of

meiosis

O Findings:

O Chromosomes occur in pairs

O Chromosome pairs segregate (separate) during

anaphase I (this backs up Mendel’s claims of the

Law of Segregation

O Chromosomes align themselves independently

along the equator (this backs up Mendel’s idea of

independent assortment

Chromosome Theory of Inheritance

O What the theory says

O Mendel’s factors or genes are carried on

chromosomes

O The segregation and independent

assortment of chromosomes during meiosis

is what accounts for inheritance

O Note: The work of Sutton and Boveri

confirmed Mendel’s ideas about genes

(factors) and how they are inherited

(segregation and independent assortment)

22/05/2015

10

Thomas Morgan and Gene Linkage

O Studied fruit flies (Drosophila melanogaster)

O Did crosses involving eye colour

O Results:

O Found that chromosomes in fruit flies are the

same except one pair

O He called the dissimilar pair sex chromosomes

because he believed they determined the sex of

the fly

O found that certain traits such as eye colour in fruit

flies are found on the X chromosome. This is

what he called “sex-linkage”. Today we called this

“sex-linked” traits or genes

Sex-linked traits

O Sex-linked trait: a trait that is carried on one of the

sex chromosomes (X or Y)

O Morgan also found that certain genes on the same

chromosome are called “linked genes”

O He said that linked genes get inherited together and

not separately as Mendel had proposed (They do not

obey Mendel’s Law of Independent Assortment). This

would account for some differences in ratios of

crosses.

22/05/2015

11

Law of Independent Assortment in Modern TermsO Morgan also found that genes on the same

chromosome that are separated by a great

distance will separate as a result of crossing

over

O If crossing over does not occur, genes that

are located on the same chromosome will

be inherited together while those on

separate chromosomes will sort

independently.

Sex linked traits in humans

O Sex-linked traits are traits that are carried on sex

chromosomes (X and Y)

O Most often the traits are carried on the X

chromosome (because of the size difference)

O Most of the traits are recessive and sometimes may

be lethal

O Sex-linked traits affect males more often than

female. Since they are recessive, females would

have to have 2 defective X chromosomes. Males only

have 1.

22/05/2015

12

Sex linked traits in humans

O Examples

O Red-green colour blindness

O Male pattern baldness

O Hemophilia

O Duchenne Muscular Dystrophy

Sex-linked crosses

O Ex. A woman with normal colour vision and

with no instances of colour-blindness in her

family history marries a man who is colour-

blind. What are the possible ratios for their

offspring?

O Parents: XNXN x XnY

O Note: for sex-linked crosses, X and Y

chromosomes must be involved to

distinguish males and female offspring

22/05/2015

13

Sex-linked crosses

XN XN

Xn XNXn XNXn

Y XNY XNY

Genotypic ratio: ½ XNXn : ½ XNY

Phenotypic ratio: ½ carrier female: ½ normal male

Note: Carriers for recessive sex-linked disorders do not have

the disorder, but do have the ability to pass it on to their

offspring. Only females can be unaffected carriers

Sex-linked crosses

O Ex. 2 A woman who is a carrier for male

pattern baldness marries a male who is not

bald. What are the possible genotypic and

phenotypic ratios of their offspring?

O Parents: XBXb x XBY

22/05/2015

14

Sex-linked crosses

XB Xb

XB XBXB XBXb

Y XBY Xb Y

Genotypic ratio: ¼ XBXB : ¼ XBXb : ¼ XBY: ¼ Xb Y

Phenotypic ratio: ¼ normal female: ¼ carrier female: ¼

normal male: ¼ bald male

Note: out of the females, ½ were normal and ½ carriers but

physically they would look the same.

Sex-linked traits: important points

O 1. sex-linked traits are recessive (small

letters)

O 2. sex-linked traits are carried on the X

chromosome, normally

O 3. if a person has a big allele and a small

allele, they are a carrier of the trait. Only

females can be carriers. Males cannot be

carriers (only one X chromosome).

22/05/2015

15

Polygenic Inheritance

O Polygenic inheritance: where traits are affected by

more than one gene.

O Because of this, you get a range of phenotypes

instead of just two or three (ex. Not just tall or short

but some in the middle).

O Continuous variation: variation among individuals in

a population where there is a gradient of phenotypes

O Ex. Length of an ear of corn

O Human skin colour

O Human height

Polygenic Inheritance

O How does this work?

O Two or more genes work at the same time for the

same trait. This leads to variations in the expression

of the trait.

O Ex. In corn, the shortest lengths occur when the

alleles for both genes (four alleles in total) are

homozygous recessive. The longest lengths occur

when all the alleles are homozygous dominant.

All other lengths occur by other allele

combinations.

O CUT OFF FOR GENETICS #1 TEST