Gene and Phenotype

What was thought was the reason for families having resemblance?

the blending theory which stated that traits from parents would get mixed like a fluid in the offspring resulting in a new trait that resembles parents

What was the prediction made off of the blending theory?

if it blended like fluid then the parents individual traits would be lost in the offspring and cannot be recovered

What was Mendel’s method?

• he took the pollen from the anther of one plant and transferred it to another plant

• he also removed the anther of the other plant to ensure it could not self-pollinate

• he was able to control the crosses between different purebred pea plants

How was the blending theory proven to be faulty?

• Mendel crossed a purple flower and a white flower → 100% of the flower were purple

• if there had been blending, we would have seen a paler purple however we did not

• however, something else could have influenced the colour so he then he did a self pollination (monohybrid cross) from the F1 generation → if blending theory is correct you should get no white flowers

What was the result of the monohybrid cross and what did it prove?

• the result was a 3:1 ratio of purple to white flowers meaning that the bleeding theory was not correct

• the white phenotype reappeared → identical to the parent suggesting the parent trait is not lost

• also suggests that the element responsible for flower colour works like particles, and can be separated and maintained in the next generation

What were the results of his experiments with the other 6 traits?

• same results as the colour

• in F1 generation only one parent trait was apparent

• in F2 generation the recessive (other parent trait) reappeared in 3:1 ratio

What were the conclusions drawn from the experiments?

• there is a dominant trait that hides the recessive trait

• the element producing the trait is transmitted to the next generation in a predictable pattern (3:1)

What does it mean by visible?

when you do a cross its the phenotype that you see (the idea of expression but not in terms of gene function → gene function is different)

What was Mendel’s final hypothesis?

• there are two copies of the element (associated with phenotype), but only one is visible (dominant and recessive)

• only one of the two elements is transmitted to the next generation and each of the two parental elements have an equal chance of being transmitted

• in a monohybrid cross since ¾ combinations contain at least one dominant trait you get 3:1 ratio of dominant to recessive trait

Why did Mendel’s experiment work?

the traits studied were affected by only one gene

• if you look at a persons height for example you won’t get the 3:1 ratio because its controlled by multiple genes

he also obtained a large number of progeny to confirm his hypothesis

Why is the ability to obtain a large number of progeny important?

• a low probability event can happen at any time during data collection process → if you only look at a small number of peas you might not see the less likely event

• helps to determine precise data

• can reduce variability between experiments

What is the element Mendel described?

A gene located on a chromosome that describe phenotype not molecular function

what did Mendel see at the molecular level?

• a cell carrying one gene (A/a) will produce 4 gametes → two with A and two with a → equal chance of transmitting A or a

Where does randomness come in?

At the molecular level during meiosis

• there is an equal chance of transmitting an A or a BUT when a progeny is produced, which 1 of the 4 used to produce the genotype of the next generation is RANDOM

What is a test cross?

the cross of an individual to a fully recessive individual (tester)

1. purple homozygous dominant X white homozygous recessive → gives 100% purple

2. purple heterozygous X white homozygous recessive → gives 50-50

What is a loss of function mutation?

• the loss of a function of a gene at the molecular level (gene not functioning)

• often recessive (phenotypic level)

for example in white flower plants there is a loss of gene required for pigment production → because wild-type (purple) is haplosufficient, white is recessive

What does haplosufficient mean?

whether you have one copy or two of the gene → one is enough to provide function

one copy of purple functions as if there is two copies (homozygous dominant and heterozygous)

What is an example of a haplosufficient gene?

cystic fibrosis - it affects the cells that produce mucus

• there is a mutation in the CFTR gene

• two carrier parents will have a ¼ chance for transmitting the disease

What is an allele?

a variation of a single gene

can be mutant alleles of a single gene