4.3 inheritance Flashcards
what is a gene?
a sequence of DNA on a chromosome normally coding for a specific polypeptide, which occupies a specific position or locus
genes normally exist as two or more alleles.
what is an allele?
a different form of the same gene, coding for a specific polypeptide
what is the genotype of an organism?
- the genetic make-up
- i.e the actual alleles it possesses
what is the phenotype of an organism?
- it represents the characteristics of an organism
- the expression of an organism’s genetic make up combined with its interaction with the environment
when both alleles are the same, the organism is said to be (heterozygous/homozygous) for that gene?
homozygous
- e.g RR or rr
when both alleles are different, the organism is said to be (heterozygous/homozygous) for that gene?
heterozygous
- Rr
dominant allele definition
alleles that are always expressed
recessive allele definition
alleles that are only expressed if 2 copies are present
what are a few rules to follow to present a genetic cross between two organisms?
- choose a single letter to represent each characteristic e.g R
- use upper case letters to represent dominant features (R), lower case for recessive (r), and state what they represent
- clearly a punnett square to calculate crossing
- state the phenotype and ratios of offspring. first generation is represented by F1, the second by F2
what does monohybrid inheritance involve?
- the inheritance of a single gene
why did Mendel conduct his experiment with peas?
- they were easy to grow
- they showed clear differences in phenotypes e.g tall and dwarf plants, those with purple of white flowers, yellow or green seeds
- they produced a large number of seeds making the results reliable
monohybrid inheritance example:
- peas with purple flowers x white flowers
- (parents): PP x pp
- (gametes): P x p
- F1: all purple Pp
- F1 x F1
- (parents) Pp x Pp
- (gametes) P, p x P, p
- 1 PP (purple), 2 Pp (purple), 1 pp (white)
when is a test cross/backcross performed? (in monohybrid inheritance)
to show if a dominant characteristic is determined by one or two dominant alleles (heterozygous or homozygous dominant)
- i.e PP or Pp
what does a test cross involve?
- crossing the organism with the unknown genotype with the homozygous recessive
- e.g cross either PP or Pp with pp
- if the F1 generation are all purple then the purple plant was pure-bred/homozygous, but if there were 1 purple plant to 1 white plant then the plant was not pure-bred i.e heterozygous
what does co-dominance mean?
- both alleles involved are dominant and therefore both are expressed equally
- e.g ABO blood groups: A and B are co-dominant
- IA IA = blood group A
- IB IB = blood group B
- IA IB = blood group AB
(when showing co-dominance, it is easier to use a letter to represent the gene e.g I and use subscripts to show the alleles as you have to use different letters)
whats the difference between co-dominance and incomplete dominance?
- co-dominance - both alleles are expressed equally so both characteristics are seen
- incomplete dominance - has an intermediate phenotype result
what is dihybrid inheritance?
the simultaneous inheritance of two unlinked genes (genes on different chromosomes)
- (the determination of a trait by the inheritance of two genes)
- mendel carried out experiments with pea plants involving two different characteristics at the same time
- e.g plants that produced yellow or green seeds AND wrinkled or round seeds
- mendel noticed that the colour of the seed was inherited independently from the seed texture (wrinkled or round)
dihybrid inheritance example:
- pure bred (homozygous) yellow wrinkled peas x green smooth peas
- key: Y yellow, y green, R round, r wrinkled
- yellow round seeds x green wrinkled
- (genotype): YYRR x yyrr
- F1 genotype: YyRr
- F1 phenotype: yellow round peas
- F1 generation were self pollinated
- (phenotype): yellow round x yellow round
- (genotype): YyRr x YyRr
- F2: phenotype ratio:
9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled
when does autosomal linkage occur?
- when two different genes are found on the same chromosome and therefore cannot segregate independently
- this applied to autosomes, chromosomes other than the sex chromosomes
- crossing over could occur if the two genes are not too close together, but this is a random event, so does not always occur on every chromosome pair
in chi squared tests, if your calculated value (is less than/exceeds) the critical value (at p=0.05), there would be a significant difference between the observed and expected results, so you can reject the null hypothesis, as any differences seen were not due to chance at p=0.05. e.g due to linkage
(this means that mendel’s laws DO NOT apply)
- exceeds
in chi squared tests, if your calculated value (is less than/exceeds) the critical value (at p=0.05), there is not a significant difference between the observed and expected results, so you must accept the null hypothesis, as any differences seen were due to chance at p=0.05. e.g random fertilisation
(this means that mendel’s laws DO apply)
less than the critical value
what does Σ mean?
sum of
what is the chi squared tests used for?
- to determine if the difference between the observed and expected numbers of offspring of different phenotypes is due to chance or a real effect