MORE GENE INTERACTIONS

Slide 2 Recombination

Slide 3 Chromosome Mapping

Slide 4 Sex Linkage

Slide 5 Cats and Codominance

Slide 6 Other work…

Slide 7 Pleiotropy and Polygeny

Slide 8 Epistasis - Complementary

Slide 9 Epistasis - Supplementary

Slide 10 Collaboration

Even if genes are linked you still may find offspring where the linked genes have separated.

This is called Recombination. It obviously increases variation.

The further the two genes are from each other on the chromosome, the greater the chance that they will be separated during crossing over.

RECOMBINATION

Recombinants

A AA

B BB

a aa

b bb

Expected gamete

Expected gamete

A B

Ba

Ab

a

b

Lab manual page 109

CHROMOSOME MAPPING

The further apart the genes are the more likely they will be to cross over.

This allows us to be able to “map” the chromosome based on the frequency that the genes separate.

Crossover value (%) = No. of recombinants

No. of offspring 100

If we compare the crossover values for genes on the same chromosome we can get comparative distances, hence, map the chromosomes.

See the page below for examples.

Lab manual page 110-111

XY System

Humans, fruit flies have a pair of sex chromosomes, XY, for determination of one’s sex.

  Sperm determines sex.

XO & ZW SYSTEM & CHROMOSOME

# SYSTEMS

XO System• Grasshopper, crickets.• “O” stands for absence of chromosome.• Sperm determines sex.

ZW System• Birds.• Egg determines sex.

Sex determination by chromosome #

•Ants, bees lack sex chromosome.

•Sex determined by chromosome number (autosomes).

•Females develop from fertilized eggs (diploid).

•Males develop from unfertilised eggs (haploid), fatherless.

SEX LINKAGE

The disease is recessive. The gene X is normal, while Xc is colour blind. Y carries no information.

Note: Sex-linked genotypes tend to use Xsomething to indicate that it is carried on the X.

So… XX is a normal female

What other possibilities can you have?

Explain why it is less common for women to be colour blind.

Lab manual page 118-119

XcX = normal female XcXc = colour blind female

XY = normal male XcY = colour blind male

Some genes are carried on the X-chromosome (called sex-linked instead of autosomal). This means that males have only one allele for that gene while females have two.

A classic example is colour blindness. Have a look at this picture. DO NOT SAY WHAT YOU SEE.

CATSIn cats one aspect of coat colour is controlled by a sex-linked gene with alleles that are codominant.

These 2 females (XoXo) and (XbXb) are crossed with a male (XbY)…

Draw Punnett squares to find the offspring of each cross.

What is the XbXo offspring called?

How do we get male Tortoiseshell?

Klinefelters!

SOME OTHER WORK

A summary comparison of different types of inheritance mechanisms:

Autosomal DominantAutosomal RecessiveSex-linked DominantSex-linked Recessive

A way to show a number of generations of individuals and how they are affected by a specific trait is a pedigree chart.

Males

Females

Dead

Affected (but not dead – yet)

Normal

GENE-GENE INTERACTIONSPleiotropy: One gene ( one protein) controls many phenotypes

e.g. The Hawaiian happy spider (why’s it called that?)

Each leg of a pair is affected by the same gene they are the same length.

Leg pairCheesy grin!

Polygeny: Many genes control one phenotype (e.g. Human skin colour)

Lab manual page 130

TyrosinaseMelanin

a series of

enzymesThyroxine Tyrosine

Protein

Phenylalaninehydroxylase

Phenylalanine

essential amino acid

Trans-aminase

Hydroxyphenylpyruvicacid

Errors in Metabolism 1 The faulty metabolism of phenylalanine is associated with various

disorders, depending on which step in the metabolic pathway is affected:

Phenylpyruvicacid

Phenylketonuria

This in turn causes

Faulty enzyme results inbuildup of

Mental retardation, 'mousy’ body odor, light skin color, eczema, excessive muscular tension and activity.

Faulty enzyme causes

Albinism

Complete lack of the pigment melanin in body tissues, including the skin and hair

Faulty enzymes cause

Cretinism

Dwarfism, mental retardation, low levels of thyroid hormones, retarded sexual development, yellow skin color.

(PKU)

Epistasis (supplementary): One gene alters the outcome of the phenotype of another

Substance Product A Product B

Enzyme 1

Enzyme 2

If Enzyme 2 is bung we’ll only get product A.

If Enzyme 1 is bung we’ll get nothing, no matter what Enzyme 2 is doing.

What will the genotype ratio be for this cross?

Try him.

9:7

Mad! Think 9:3:3:1, but group the last 3 sets.

PPcc ppCC

PpCc

Parental

F1

PC

Pc

pC

pc

PC Pc pC pcF2

Complementary genes: Both need to be present for either to work.

Supplementary genes (epistasis): the second gene adds more to the first.

Coat color in Labrador retrievers is controlled by two genes (B and E). At least one dominant allele for both genes is required to produce a black dog (B_E_).

a. What are the genotypes of two black parental dogs that, when mated, produce black puppies, yellow puppies and brown puppies?

b. What proportions of black and yellow puppies do you expect from this cross?

c. This cross is an example of a what type of gene interaction?

Dogs homozygous for the recessive allele b that have at least one dominant E allele (bbE_) are brown and dogs homozygous recessive for E (ee) are always yellow.

Lab manual page 132/3

COLLABORATION

This is where 2 genes interact to make a product different to that which either could make independently.

The most common example is comb types in chickens.

Lab manual page 129

See more chickens…