more gene interactions slide 2recombinationrecombination slide 3chromosome mappingchromosome mapping...
TRANSCRIPT
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…