PTC Taster Gene

Scientific Investigation

Types of Inheritance

Mystery of evolution - why some people like Brussels sprouts but others hate them.

Substances similar to PTC give a bitter taste to green vegetables such as Brussels sprouts, broccoli and cabbage

They have found that a gene in modern humans that makes some people dislike a bitter chemical called phenylthiocarbamide, or PTC,

But they are also present in some poisonous plants, so having a dislike for it makes evolutionary sense."The sense of bitter taste protects us from ingesting toxic substances," the report said.

a recessive variant of the TAS2R38 gene which made some of them unable to taste PTC - an inability they share with around one third of modern humans.

"This feature ... is a mystery of evolution," said the report."These (bitter) compounds can be toxic if ingested in large quantities and it is therefore difficult to understand the evolutionary existence of individuals who cannot detect them."

• Blame it on your Genes?

• Blame it on your Parents?

Bitter

Homozygous Dominant

Homozygous Recessive

Heterozygous Dominant

1. Complete dominance2. Sex Linked3. Codominance 4. Incomplete Dominance 5. Multiple Alleles In a nutshell!

Types of Inheritance

1. Complete Dominance

Dominant/Recessive

• One allele is dominant over the other (capable of masking the recessive allele)

PP = purple pp = white Pp = purple

Dominant/Recessive• In pea plants, purple flowers (P) are dominant

over white flowers (p) show the cross between two heterozygous plants. Monohybrid Cross

P

p

P p

pp

Pp

Pp

PP- PP (1); Pp (2); pp (1)- ratio 1:2:1

- purple (3); white (1)- ratio 3:1

GENOTYPES:

PHENOTYPES:

2. Sex Linked Inheritance

• Genes found only on

Your sex chromosomes,

Chromosome pair 23.

X – Linked traits• Colour Blindness * Haemophilia

Question: Drawing a monohybrid cross for a homozygous recessive female x dominant male for Haemophilia, determine the genotypes and phenotypes of all offspring.

3. Codominance

Codominance

• The heterozygous condition, both alleles are expressed equally

• Sickle Cell Anemia in Humans

CN CN = normal cells

CS CS = sickle cells

CNCS = some of each

Problem: Codominance• Show the cross between an individual with

sickle-cell anemia and another who is a carrier but not sick.

CN CS

CS

CS

CN CS

CN CS

CS CS

CS CS

- NS (2) SS (2)- ratio 1:1

- carrier (2); sick (2)- ratio 1:1

GENOTYPES:

PHENOTYPES:

4. Incomplete Dominance

Incomplete Dominance

Incomplete Dominance• A third (new) phenotype appears in the

heterozygous condition.

RR = red rr = white Rr = pink

Problem: Incomplete Dominance

• Show the cross between a pink and a white flower.

- Rr (2); rr (2)- ratio 1:1

- pink (2); white (2)- ratio 1:1

R r

r

r

rrRr

rrRr

GENOTYPES:

PHENOTYPES:

So is PTC taster gene Incomplete Dominance?

• Since all people have two copies of every gene, combinations of the bitter taste gene variants determine whether someone finds PTC intensely bitter, somewhat bitter, or without taste at all.

• To people who are tasters, the paper tastes very bitter, but to non-tasters, it just tastes like paper

• More precise analysis of the phenotype indicated that the "taster" gene is incompletely dominant to the "nontaster" gene, so that people with two copies of the taster gene have a lower threshold of sensitivity to PTC than people with only one copy.

• When there is a T allele present, this usually predicts the ability of PTC tasting

• The heterozygotes tend to be weak tasters compared to the homozygous dominant genotype.

• other studies have indicated that another gene may determine taste sensitivity.

• The frequency of PTC taster and non-taster allele vary in different human populations

t

t

T t

Tt tt

Tt tt

1. Name the 3 possible genotypes and phenotypes for the PTC gene?

Refer to the Monohybrid cross below.

2. What are the offspring genotypes?

3. What are their phenotypes?

4. Draw a Monohybrid cross for a Supertaster crossed with a non-taster?

Complete PTC Gene Scientific Investigation

Refer back of task sheet and remember to look at criteria sheet carefully•Title Pg.•Introduction (including secondary data and in-text referencing) and justified hypothesis•Materials•Method (third person past tense)•Results (see format from draft sheet)•Discussion – observation of results, refer back to introduction and hypothesis. Analyse data, find relationships and justify your conclusions.•Conclusion – Summary •Bibliography

Extra on Multiple alleles if time

Red blood cell compatibility table[60][61]

Recipint[1]Donor[1]

O− O+ A− A+ B− B+ AB− AB+

O− Y N N N N N N N

O+ Y Y N N N N N N

A− Y N Y N N N N N

A+ Y Y Y Y N N N N

B− Y N N N Y N N N

B+ Y Y N N Y Y N N

AB− Y N Y N Y N Y N

AB+ Y Y Y Y Y Y Y

• http://www.youtube.com/watch?v=NJHgZhbn87Q&feature=related

5. Multiple Alleles

• There are more than two alleles for a trait

• Blood type in humans

• Blood Types?– Type A, Type B, Type AB, Type O

• Blood Alleles?– A, B, O (in book – IA, IB, I)

Rules for Blood Type

• A and B are codominant– AA = Type A– BB = Type B– AB = Type AB

• A and B are dominant over O– AO = type A– BO = type B– OO = type O

Problem: Multiple Alleles• Show the cross between a mother who has type O

blood and a father who has type AB blood.

- AO (2) BO (2)- ratio 1:1

- type A (2); type B (2)- ratio 1:1

GENOTYPES:

PHENOTYPES:

O O

A

B

AO

BO

AO

BO

Problem: Multiple Alleles• Show the cross between a mother who is

heterozygous for type B blood and a father who is heterozygous for type A blood.

-AB (1); BO (1); AO (1); OO (1)- ratio 1:1:1:1

-type AB (1); type B (1) type A (1); type O (1)- ratio 1:1:1:1

GENOTYPES:

PHENOTYPES:

A O

B

O

AB

OO

BO

AO

Exercises

• Textbook pg. 77. q. 1 to 4, 6, 7, 9