Modifications Of Mendel Flashcards
Incomplete dominance
The expression of the heterozygous is an intermediate between the dominant homozygotes
E.g. Four o’clock plant, Mirabilis japonica is normally red but a white variety is known
All F1 is pink. Self fertilize these:
a a A Aa Aa
A Aa Aa
All F1 are pink - unlike wither parent. If we self fertilize F1 we get:
A a A AA Aa
a Aa aa
We get a ratio of 1red: 2pink: 1white:
Incomplete dominance (applications- Mendels peas)
- At the molecular level, Incomplete dominance is everywhere, even in Mendel’s peas. The
heterozygotes for look like intermediates between dominant homozygotes (smooth) and the
recessive homozygotes (wrinkled). This is due to the activity of an enzyme which builds up starch;
intermediates have half of the activity of the enzyme compared to the homozygote dominants
Incomplete dominace (application - Humans)
- In humans, children with Tay-Sachs disease (a homozygous recessive disease: the allele is common in the Ashkenazi Jewish population) have no hexosaminidase enzyme acitivity
- People with two dominant normal alleles have high activity and heterozygotes have an intermediate activity.
- enabled pre natal diagnosis
Codominance
- A condition in which the phenotypic effects of a gene’s allele are fully and simultaneously expressed in the heterozygote
Codominance (application on animals)
- Cross red and white cows, get a red cow with white spots
- In human sickle cell haemoglobin - heterozygotes produce two types of haemoglobin which is easily distinguished using basic technologies such as gel electrophoresis
Multiple alleles
- when three or more alleles can occupy a given locus
Multiple alleles (application in plants and drosophilia)
- pattern of clover leaves is determined by 5 alleles
- drosophilia; wild type has red eyes(++), also ww gives white eyes
- we now know that there are multiple alleles and different loci which can produce a wide variety of eye phenotypes.
Multiple alleles (application in humans)
- In humans, eye colour is also determined by multiple alleles and several loci although pigments involved are quite different
- ABO blood group system in humans :
- The loci codes for antigens which are found
on the surface of blood cells
- three alleles - A, B and O. O is recessice to
both A and B. A and B are codominant:
Blood Group Genotype
O O
A AO or AA
B BO or BB
AB AB
Blood groups were used as the first paternity tests:
B O A AB AO
O BO OO
i.e. an A parent and a B parent have children who are groups A, B, AB and O
Lethal alleles
Homozygotes may be lethal
Lethal alleles (application in mice)
- e.g. Yellow (light coat colour) mice. Cross yellow with wild type - get a ratio one yellow to one wild type. Suggests yellow is dominant and that yellow parent was a heterozygote
Y y y Yy yy
y Yy yy
Yy x yy —> 1 Yy : 1yy
When yellows are crossed with each other = confusing ratio- 2 yellow: 1 wild type
When dissecting pregnant mother - theres another class YY embryo but they die very early
Y allele is dominant for coat colour but recessive for viability.
Lethal alleles (application in cats and humans)
Manx cat, no tail
- if a cat inherits two copies of the Manx allele it is lethal.
- brachydactyly is the same
- Achondroplasia - form of dwarfism is similarly a dominant condition which is lethal in homozygotes
Pleiotropy
One gene affects many phenotypes
Pleiotropy (application in animals)
- e.g. All blue eyed cats are deaf due to the fault in the melanin pathway. Tabby cats are particularly aggressive, pleiotropy on coat colour and behaviour
- sickele cell haemoglobin involves change in a single base - changes one amino acid from glutamic acid to valine. This causes a change in the haemoglobin shape and changes its molecular properties. This changes the shape of the shape of the red blood cell and they sickle
Effects of sickle cells
Anemia, heart failure, paralysis, enlarged spleen, bossed skulls, brain damage, poor circulation - as well as malaria resistance in heterozygotes
Sex limitations
The expression of a phenotype is dependent on sex
Sex limitation application in humans
- due to sex differences in hormones which interact with gene loci
-e.g. In peacocks, both males and females have the genes which code for an elaborate tail, but they are only expressed in the males because only they produce testosterone
Sex limitation application in humans (characteristics)
- Secondary sexual traits in males such as beads and sex-limited male pattern baldness
only effect males because the traits are only expressed in the presence of testosterone. Many
mental disorders are more common and more sever in males, such as schizophrenia and ADHD. - Autism is a mental condition which develops during early childhood and is characterised by a difficulty in communication and relating to other people.
- These conditions are associated with many loci on different chromosomes that vary in their contribution to the phenotype.
- In the same way, BRCA1 (BRCA=BReast CAncer) mutations are associated with an increase in breast
cancer in women because of gene interaction with cell division genes and oestrogen
Gene interaction
Several genes affect one character
Gene interaction examples (A locus)
- Mouse Coat colours; many different inbred lines. At least 5 interacting loci
- A locus: determines the distribution of colour in hair shaft
- wild type mouse - “agouti”. Each hair has a dark shaft with a yellow band. A is wild type and dominant. Recessive allele a removes the yellow band, so that aa mice lacking a genetic change at any other colour locus are black.
Gene interaction examples (B locus)
B locus: codes for colour of hair
B codes for black, b for brown. BB and Bb mice are black; bb are brown. Consider this cross:
P AA BB (agouti) x aa bb (brown)
F1 all Aa Bb (agouti)
Cross these together (F1:F1):
Ratio Genotype Phenotype
9 A- B- Agouti
3 A- bb Cinnamon (brw y flecks)
3 aa B- Black
1 aa bb Brown
A-bb is a new phenotype (Cinnamon) formed by gene interaction
Gene interaction examples (C locus)
C locus: determiens presence of absence of pigment
CC or Cc animals have pigment, cc is albino. The albino allele is epistatic to the other loci; the cc genotype prevents the A or B loci from expressing their effects
P BB CC (black) x bb cc (albino)
F1 all Bb Cc (black)
Cross these together (F1 x F1):
Ratio Genotype Phenotype
9 B- C- Black
3 bb C- Brown
3 B- cc Albino
1 bb cc Alibino
Gene interaction (S locus)
S locus: controls the pigment distribution on the body
SS or Ss are not spotted, but ss is piebald - large patches of colour - say black and dilute black - on the same individual
Similar loci for coar colour are found in many mammals such as horses. The albino alleles in captive tiger populations are a problem in their conservation because they are highly inbred.
Gene interaction in humans
- The dominant white spotting loci in mice is homologous to KIT gene involved with cell division and associated with cancer.
- Secretor (Se) locus/phenotype similar to. Secretor phenotype involves interaction between ABO locus and Se locus. Those who are SeSe or Sese produce ABO blood group substances in their saliva and other bodily fluids such as semen. However, sese genotypes do not do this
Complementation
When you cross two strains of an organism with the same phenotype but different homozygous to produce a new phenotype
Foxglove normally purple petals but some populations have individuals with white ones
Within each population the white one is simple recessive
If we cross white plants from different populations all F1 plants are purple not white - there is a ratio of 9 purple to 7 white in the F2
There are two loci involved and homozygosity for a white allele at either of them will produce a white plant
There is a chain of producing purple which if broken at any point will stop the pigment from being producing purple and break at any point = no pigment
1 w1w1 W2W2 (white line 1) x W1W1 w2w2 (white line 2)
F1 All W1w1 W2w2 (purple) Self-fertilise
F2 9 W1- W2- (purple) 9
3 W1- w2w2 (white)
3 w1w1 W2- (white) 7
1 w1w1 w2w2 (white)
Ratio shows that white phenotype can be produced by mutations at two separate loci
Complementation tests
They show whether two alleles at a locus are involved or two or more different loci.
It was first done in fruit flies. Have a blue eyed fly turn up, which is recessive to white type. Make a homozygous line. In different line green eyed fly appears do the same t make a homozygous line
Cross blue homozygote with the green, if they are alleles at the same locus the F1 with be blue or green, depending on which is dominant, (or blue-green if dominance is intermediate).
If two loci are involved then F1 are heterozygous at both loci and have red eyes the loci complement
If we have many mutants we can sort them into ‘complementation groups’ within a group they do not complement between they do
Drosophila eye colour was the first biochemical pathway worked out using this method. Broadly
speaking there are two pathways: Pteridine pathway makes red colour pigment and the
ommochrome pathway makes brown colour pigments
Human deafness and Complementation
Many different loci affecting hearing - they affect all different parts of out ear, for example - malleus, incus, stapes of the middle ear, some nerve connections to the brain or the brain itself
Some mutations are dominant, and some are recessive. Some mutations have pleiotropic effects, termed syndromic deafness. Deaf individuals have additional health effects including problems with the heart, brain, eyes and joints. Wardenberg syndrome for example is associated with a mutation which can cause deafness, a white strip in the hair and mental abnormalities.
A break in the chain for any of them will lead to deafness
Hearing phenotype: Sound → A → B → C → D → E → F → Perception of sound.
Genotypes: First individual AA → BB → cc → DD → EE → FF – deaf
Second individual AA → BB → CC → DD → ee → FF – deaf
Offspring AA → BB → Cc → DD → Ee → FF – can hear
Complementation between C and E locus gives normal hearing as Cc and Ee heterozygotes have
functioning steps in auditory chain. As a result, two deaf people often have hearing children as each parent is homozygous for a recessive abnormality at a different locus and child is double heterozygote.
Also a link between deafness and the environment interaction. Women with ‘Derbyshire neck’ caused by iodine shortage often have children who are deaf and can be cured by adding iodine to salt. Some children with measles or rubella also become deaf. Being exposed to loud noises over long periods of time causes deafness. But here there is also a gene environment interaction; Individuals with darker skin are less likely to go deaf due to exposure to loud noises than individuals with whiter skin.
