BEYOND MENDELIAN GENETICS

Conditions that Mendel didn’t explore:

Incomplete DominanceCo-dominanceMultiple AllelesPolygenic Traits

Lethal gene

KCI 2014

Limitations of Mendelian genetics

• Some genes are NOT independently assorted simply because they are too close to each other on a chromosome

• some alleles are incompletely dominant

• some genes are sex-linked

• A single trait can be controlled by many genes which can have multiple alleles

Incomplete dominance• A condition in which

neither allele for a gene completely conceals the presence of the other; it results in intermediate expression of trait

Flower color of snapdragons

3 phenotypes are observed in F2 in a 1:2(blended traits):1 ratio

Incomplete dominance• However, the alleles of flower color do

segregate according to Mendel’s law of independent assortment

• In human, traits showing incomplete dominance:

How to represent incomplete dominance?• Use superscripts rather than upper-case and

lower-case letters since neither allele is dominant over the other

• Eg. Snapdragon flower colour: both alleles affect the colour of flower C

• 2 alleles are therefore CR (red) and CW (white)

HbAHbA: normal biconcave disk

HbSHbS: sickle cell S shape

HbAHbS sickle cell trait (some normal and some sickle blood cells under prolonged low O2 condition) resistant to malaria giving the individuals a heterozygous advantage

Incomplete dom.

Sickle cell disease in humans.Trait concerned: Shape of blood cellsHb: letter chosen to represent hemoglobin

https://www.youtube.com/watch?v=R4-c3hUhhyc

If both parents have sickle cell trait (heterozygous), what is the chance of their offspring having sickle cell anemia?

P: HbAHbS x HbAHbS

F1: HbA HbS

HbA HbA HbA HbA HbS

HbS HbA HbS HbS HbS

Answer: 25% chance their offspring will have sickle cell anemia (HbSHbS)

Probability of offspring in F1

Genotype 25% HbA HbA

50% HbA HbS

25% HbS HbS

Phenotype 25% normal blood shape

50% with sickle cell trait

25% crescent blood shape (sickle cell anemia)

Codominance• Is the condition in which both alleles for a trait

are equally expressed in a heterozygous offspring; both alleles are dominant

E.g a. Shorthorn roan cowCattle can be red (HRHR = all red hairs), white (HWHW = all white hairs), or roan (HRHW = red & white hairs together)

b. Human AB blood type (two types of protein ("A" & "B") appear together on the surface of blood cells.)

Codominance

Correct the letter HR, not Hr as expressed in your textbook

What’s the difference between incomplete dominance and codominance?

• E.g. a. Incomplete dominance:- Both alleles are partially expressed- Offspring shows a third phenotype

that is something between the parents’ phenotypes (i.e. a blend)

b. Co-dominance:- Both alleles are fully expressed- Offspring shows a third phenotype

that is completely different from both the parents’ phenotype

Correct the letter r to R in this co-dominance cross

Multiple alleles• So far we’ve studied traits controlled by one gene with

two alleles• An individual has only 2 alleles for a gene but many

different alleles for a gene can exist within a population

• A gene with more than 2 alleles is said to have multiple alleles

Reminder

Human blood types: example of both codominance and dominance

• One single gene determine a person’s blood type: A, B or O

• This gene is designated as I and have 3 alleles that code for antigen proteins attached to the RBC’s membrane

Alleles

of the I

gene

What do they enable RBC’s membrane to have?

IA Polysaccharide A

IB Polysaccharide B

i No polysaccharide

IA IB i

IA IAIA IAIB IAi

IB IAIB IBIB IBi

i IAi IBi ii

Genotype Blood type

IAIA IAi

Type A

IBIB

IBiType B

IAIB Type AB

ii Type O

Possible alleles from a female

Pos

sibl

e al

lele

s fr

om a

mal

e

RECAP: http://wps.aw.com/bc_goodenough_boh_3/104/26722/6840887.cw/content/index.html

Multiple Alleles (Cont.) – Rabbit Coat

Predict the genotypic ratio and phenotypic ratio of a cross between a heterozygous Chinchilla (cchc) with a heterozygous dark grey (Cch).

P: cchc x Cch

F1: cch c

C Ccch Cc

ch cchch chc

Genotypic Ratios: 1 Ccch: 1Cc : 1 cchch: 1 chc

Phenotypic Ratios: 2 dark grey: 1 light grey: 1 Himalayan

One trait (i.e. rabbit coat color)is influenced by multiple alleles

Complete attached practice problems on incomplete-, co-

dominance and multiple alleles

Taking up Practice problem

Incomplete dominance practice problemSpongeBob loves growing flowers for his pal Sandy! Her favorite flowers, Poofkins, are found in red, blue, and purple.

1. Write the correct genotype for each color if R represents a red gene and B represents a blue gene.Red - Blue - Purple -

2. What would happen if SpongeBob crossed a Poofkin with red flowers with a Poofkin with blue flowers. (a) Give the genotypes and phenotypes for the offspring.(b) How many of the plants would have red flowers? _____%(c) How many of the plants would have purple flowers? _____ %(d) How many of the plants would have blue flowers? _____ %

3. What would happen if SpongeBob crossed two Poofkins with purple flowers? (a) Give the genotypes and phenotypes for the offspring.(b) How many of the plants would have red flowers? _____%(c) How many of the plants would have purple flowers? _____ %(d) How many of the plants would have blue flowers? _____ %

Codominance practice problem

1. In some chickens, the gene for feather color is controlled by codominance. The allele for black is B and the allele for white is W. The heterozygous phenotype is known as erminette.

• a. What is the genotype for black chickens? ____• b. What is the genotype for white chickens? ____• c. What is the genotype for erminette chickens? ____

2. If two erminette chickens were crossed, what is the probability that:a. They would have a black chick? ____%b. They would have a white chick? ____%

3. In a certain fish, blue scales and red scales are codominant. When a fish has the genotype B R, it has a patchwork of blue and red scales. What happens if

you breed this fish with a fish that only has Blue Scales.

 Step 1: Determine the genotypes of the parents

CB CR = CB CB = CR CR =

Multiple alleles practice problem1. In the 1950’s, a young woman sued film star/director Charlie Chaplin

for parental support of her illegitimate child. Charlie Chaplin’s blood type was already on record as type AB. The mother of the child had type A and her son had type O blood.

a. Complete a Punnett square for the possible cross of Charlie and the mother.

b. The judge ruled in favor of the mother and ordered Charlie Chaplin to pay child support costs of the child. Was the judge correct in his decision based on blood typing evidence? Explain why or why not.

*Use Punnett squares to support your answer.

Alien Genotyping:

2. In these aliens, horns are controlled by three allelles• One center horn (A) is codominant with two horns (B). If an

alien inherits both alleles (AB), then the alien has 3 horns. A recessive allele (O) results in an alien have no horns. Match the genotype to each of the pictures below.

Incomplete dom.

Show:a. Pink x Redb. Pink x Pinkc. White x White