Beyond Mendel’s Laws…Incomplete Dominance

• There is still a dominant and a recessive trait, however the heterozygous individual will show an intermediate form of the two traits!

• ”Blending”• Another way of saying this is that the dominant allele is

not completely dominant, which is why we say it is incomplete.

• • Example: Flower colour in snap dragons•

• Red is dominant (R)• White is recessive (R’) – note for incomplete

dominance we do not write a lower case “r”…why do you think this is?

• In snap dragons, flower colour is determined by incomplete dominance. Red flowers (RR), white flowers are (R’R’), and pink flowers are (R, R’).

• If red and pink flowered plants are crossed, what are the chances of producing a red flower?

• Let RR be red• Let RR’ be pink• Let R’R’ be white

RR

RR’

RR

RR’ R R

RR XRR’

R R

R

R’

RR XRR’

R R

R RR

R’

RR XRR’

R R

R RR RR

R’

RR XRR’

R R

R RR RR

R’ RR’

RR XRR’

R R

R RR RR

R’ RR’ RR’

Results

• ¼ RR = 25% chance Red• 2/4 RR’• ¼ R’R’

Sample Incomplete Dominance problems

•

• If a pink flower is crossed with a white flower, what is the chance of producing a pink flower?

• If 2 pink flowers are crossed, what is the chance of producing a pink flower? What about for producing a white flower?

• If a pink flower is crossed with a white flower, what is the chance of producing a red flower?

Co-Dominance• There are even situations where the heterozygous individual

will show both forms of the two traits! • • In other words, both traits are dominant, or “codominant”• • Example: Feather colour in chickens• • Black feathers are dominant (B)• White feathers are also dominant (W) Note! We write both

dominant alleles as upper case• Barred feathers (BW) is the heterozygous expression

• In chickens, feather colour is determined by codominance. The allele for black feathers (B) is codominant to the allete for white feathers (W). The heterozygous chicken (BW) has some black and some white feathers – often referred to as “barred” feathers/plumage.

•

• If a “barred” chicken is mated with a black feathered rooster, what is the chance of producing barred chickens?

• Let BB be black• Let WW be white• Let BW be barred

• ½ BB• ½ BW, therefore we have a 50% chance of

producing barred chickens

Sample Codominance questions

• If a black chicken is crossed with a white rooster,

what is the chance of producing a white chicken?

• If a barred chicken is crossed with a barred rooster,

what is the chance of producing a black chicken?

Human Blood Types (Multiple Alleles)

• Every trait that we’ve looked at so far has involved only two possible alleles per trait: one dominant and one recessive. (eg. T for tall, t for short)

• • We’ve seen instances where there are two

dominant alleles (codominance) or where the recessive gene partially shows through in heterozygotes (incomplete dominance)…but it was always only two alleles.

• Some traits have more than two alleles! They still exist in pairs only (one from mom and one from dad) but human blood types have three alleles on the same gene (chromosome 9):

• • IA

• IB

• i•

• There are FOUR phenotypes and SIX possible genotype combinations:• • Type A Homozygous IA IA

• Heterozygous IA i• • Type B Homozygous IB IB

• Heterozygous IB i• • Type AB IA IB

• • Type O i i• • • There are two dominant alleles, IA and IB and one recessive allele, i.

• The presence of IA means that the red blood cells will have the A-type glycoprotein on their surfaces

• • IB means that the B-type glycoprotein will be on

the surface• • If someone has both IA and IB they will have both

glycoproteins (Type AB blood)• • i means that there will be no glycoprotein on the

surface and is called Type O (very important fact!)

Human Blood sample problems 1) A man who has homozygous type A blood marries a woman with

type AB blood. What blood types could their children have?• 2) A woman with type O blood marries a man with homozygous

type B blood. What genotypes and phenotypes could show up in the children?

• 3) What will the blood types be for children whose mother has

heterozygous B blood and father has type O?• A lawyer is defending a man with type B blood. A woman who is

known to have type A blood, has a child with type O blood and is claiming that this man in the father. Using a Punnett Square, show how you would determine whether or not he is the father!

• • The identification of two newborn babies were accidentally

mixed up in the hospital! You have been given the blood types of the babies and parents and been asked to sort out which baby belongs to which parent:

Baby 1: type O Mr. Brown: type A

Baby 2: type A Mrs. Brown: type AB

Mr. Smith: type B

Mrs. Smith: type B

Sex Linked Genetics

• • Human beings have 23 pairs of chromosomes, 22 of them are

common to both men and women and are referred to as the AUTOSOMAL CHROMOSOMES.

• • The 23rd pair of chromosomes is called the SEX CHROMOSOMES

because these determine what sex (male or female) the person will be. Each sex chromosome is called either X or Y and looks like this:

• • Woman XX• Man XY

• Many genes are carried on the X chromosome, while very few are on the Y (approximately 1% of the genes compared to X).

• Since so few genes are on the Y chromosome, we basically treat it as if there is no chromosome there at all. Men have only one copy of the X chromosome so this means that SEX-LINKED genes on the X chromosome are expressed in males whether they are recessive or dominant!

• Let b = colour blindedness• • Let B = normal colour vision• We write the alleles as

superscripts on top of the X to indicate that it is sex-linked.

Let’s look at an example: A woman is colour blind, which we know to be a sex-linked recessive trait (XbXb). She marries a man with normal vision (XBY). What are the chances of the children having normal vision?