Mendelism Flashcards
What were early models of inheritance, and what was their drawback?
Blending inheritance
Acquired characteristics were thought to be passed down to offspring
These models could not explain atavism, the reappearance of characteristics that skip generations.
What was Mendel’s breakthrough and what plant did he work on?
He thought of genes as particles rather than fluids.
He studied peas, which are hermaphrodites that can self-fertilise.
He worked with pure inbred lines that differed in seven characteristics.
What was Mendel’s first experiment and what can be interpreted from the results?
P1: Yellow x Green
F1: all yellow
This shows that genes are not blended but that one character is hidden. These plants were then self-fertilised.
F2: 3 yellow : 1 green
Genes are particles bearing information; this information can be concealed but it is still present. Each pure line has 2 copies of the particle for colour.
What is Mendel’s first law, also known as the Law of Segregation?
The two members of a gene pair segregate into separate gametes; half of the gametes carry one member of the pair and the other half carries the other member.
Offspring will acquire one of these genes from each parent.
What is the pattern for dominant inheritance?
Unaffected parents only have unaffected children.
Within families with one affected parent, about half the children are also affected.
Genotypes A/a and a/a can be clearly identified from the phenotype.
Every affected parent will be A/a (otherwise all their children would be affected) and every unaffected child will be a/a.
What is the pattern for recessive inheritance?
Affected individuals may have unaffected parents.
The phenotype can skip generations.
Around one child in four in an affected family will have the condition.
An affected child must be a/a and their parents must be both A/a
What were the results in the F1 and F2 plants when Mendel did a dihybrid cross?
P1: SSYY x wwgg
F1: All SwYg
F2: 9 smooth yellow : 3 smooth green : 3 wrinkled yellow : 1 wrinkled green
What was Mendel’s Second Law, the law of Independent Assortment?
During the formation of gametes, the segregation of alleles at one locus is independent of that of the segregation of alleles at another.
The pattern of inheritance of one characteristic is independent of another characteristic.
What are useful applications of Mendel’s Second Law?
Recombination means that no two individuals share the same set of alleles at freely recombining loci; ie. everyone’s DNA is unique to them.
This is useful in DNA fingerprinting, where culprits of crimes can be identified using small amounts of DNA.
When famine was common in India, tall but productive pure lines of rice were available, but were easily damaged by the weather. These were crossed with a short but low yield variant. The recombinants generated were short with a high yield.
How are recombinants potentially dangerous?
Bird and human flu can simultaneously infect pigs, where they can potentially recombine to form a new virulent version of the virus.
Different strains of HIV have recombined to become resistant to many antiviral drugs.
What is incomplete dominance?
The expression of the heterozygote is an intermediate between the different homozygotes.
MIrabilis japonica → AA = red, Aa = pink, aa = white
Tay-Sachs disease → AA = high hexosaminidase activity
Aa = intermeditate activty, aa = no activity
What is codominance?
A condition in which the phenotypic effects of a gene’s allele are fully and simultaneously expressed in the heterozygote.
Red cow x white cow = red cow with white spots
Sickle cell anaemia shows codominance because carriers have both type of red blood cells, but enough normal red blood cells in order to be able to live normally.
What is an example of a gene that is controlled by multiple (more than 2) alleles?
The ABO blood group system in humans. This shows both complete and co-dominance in the same system.
O is the universal recessive while A and B are codominant.
A, B and O are genes that code for antigens on the surface of red blood cells.
Type O has no antigens but anti-A and anti-B antibodies in the blood. Type A has A antigens and anti-B antibodies. Type B has B antigens and anti-A antibodies. Type AB has A and B antigens and no antibodies.
Type O can give to type A, B and AB, so it is known as the universal donor.
Type A and type B can both give to type AB.
Type AB can receive blood from any group but cannot donate it. It is therefore known as the universal receiver.
What is a lethal allele, and what is the result from crosses?
Lethal alleles are ones where the homozygote may be lethal.
Yellow coated mice:
Yy x yy = 1 Yy : 1 yy
Yy x Yy = 2 Yy : 1 yy
There is another class of genotype, YY, and while it is dominant for coat colour it is also simultaneously recessive for viability.
The same is true for Manx alleles and brachydactyly.
What is pleiotropy?
When one gene affects many characteristics.
Examples:
All blue-eyed white cats are deaf because they have no melanin, hence producing white coat colour. They also have no melanin in their ears, which is needed to be able to hear.
Sickle cell Hb involves one amino acid change but this produces many detrimental effects in the body eg. becoming anaemic, enlarged spleen, heart failure, bossed skulls etc. as well as malaria resistance in heterozygotes.
These are all side effects of the sickle cells clumping together and blocking blood vessels, not having enough working red blood cells, etc.
