8.2 Transfer of genetic information Flashcards
Explain why crossing over between two different genes, located on a pair of homologous chromosomes, results in a maximum of 50% of gametes with the recombinant alleles. (3)
- crossing over occurs between chromatids (between the same
homologous chromosomes) (1) - therefore (only) half the chromosomes (produced by anaphase II)
will be recombinant chromosomes (so maximum of 50 %) (1) - crossing over does not always take place (so can be less than 50 %)
Define genotype
The genetic consitution of an organism
Define phenotype
The expression of an organism’s genetic constitution, combined with its interaction with the environment.
What is an allele?
Different forms of a particular gene, found at the same locus (position) on a chromosome.
A single gene could have many alleles.
What is meant by a dominant allele?
An allele whose characteristic will always appear in the phenotype, whether one or two are present.
What is meant by a recessive allele?
An allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be present.
What is meant by codominant alleles?
give example
Two dominant alleles that both contribute to the phenotype, either by showing a blend og both charcateristics or the characteristics appearing together.
eg: speckled chickens
recieve B (black) allele and W (white) allele and express BOTH so they are co-dominant alleles
Incomplete would be like snap dragons Rr heterozygous is pink (red/white)
What is meant by homozygous and heterozygous?
Homozygous= both alleles are dominant or both alleles are recessive.
Heterozygous = one allele is dominant, the other is recessive.
What is meant by multiple alleles?
eg
A gene with more than two alleles.
so that many different version of that trait exist within a population.
blood type
How can non-interacting, unlinked genes be inherited?
can be monohybrid or dihybrid.
- monohybrid = a phenotypic characteristic is controlled by a single gene.
- dihybrid = two phenotypic characteristics are controlled by two different genes present on two different chromosomes.
Give an examplease of a monohybrid cross
cross between tall pea plants and dwarf pea plants
give an example of a dihybrid cross
cross between pea plants with yellow round and green wrinkled seeds
What is the resultant ratio of a dihybrid cross? F1 gen and F2 gen (when you start with two RRYY x rryy)
F1 will be RrYy (all yellow and round)
F2 will be 9:3:3:1 (round and yellow: green and round: yellow and wrinkled: green and wrinkled)
What did mendels results with dihybrid crosses lead him to believe?
his theory of independent assortment: that any one pair of characteristics may combines with any one of another pair
(because the way chromosomes line up on the equator is independent of the position of others)
(we know now that characteristics Mendel chose were on different chromosomes and so behaved independently)
what is a recombinant?
individuals that show new combinations of characteristics
what is meant by autosomal linkage?
when genes are located on the same (non-sex) chromosome.
Give an example of automosomal linkage
drosphilia (fruit flies); colour and wing length are autosomally linked, meaning they are inherited in pairs.
Explain why automosomally linked genes are inherited in pairs, referring to meiosis.
Genes on the same chromosome, particularly those closest together, are unlikely to undergo recombination during meiosis. Therefore they are inherited as if they were the same gene.
How could you tell if genes are linked
a genetic cross between two heterozygous individuals will produce a ratio of phenotypes close to a 3:1 ratio whereas if they were unlinked it would be a 9:3:3:1 ratio (dihybrid unlinked inheritance)
Why do genetic crosses of linked genes not produce an exact 3:1 ratio
because a small number of recombinants will also be produced
in this sense recombinants occur because of crossing over
what affects if recombinants are produced as a result of crossing over?
- occurs because of crossing over of chromosomes in meiosis forming chiasmats
- formation of recombinants depends on whether crossing over occurs and where on the chromosome it takes place
- the futher apart the alleles are on the chromosome, then the more likely it is that they will be affected by crossing over and recombinants formed.
formula for cross over frequency
number of recombinants/total number of offspring x100
what would a small number of recombinants suggest (linked genes)
that the 2 genes are close together on the chromosome
What is meant by sex-inkage. Give an example
Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual e.g: haemophillia
