7a (genetics) Flashcards
what is a gene?
sequence of bases on a DNA molecule that codes for a protein which results in a characteristic
what is an allele?
what is the difference between alleles?
versions of a type of gene.
most plants and animals carry 2 of each (one from each parent)
order of bases in each allele is slightly different
what is a loci?
the fixed position on a gene at which an allele is found
what is a genotype?
an organisms genetic constitution eg someone could have the genotype BB or Bb for eye colour
what is the phenotype of an organism?
the expression of the genetic constitution and its interaction with the environment
what is the dominant allele?
what is the recessive allele?
dominant is always expressed in the phenotype even if theres only one copy
recessive alleles are characteristics which only appear in phenotype if two copies are present.
what are codominant alleles?
when both alleles are expressed in the phenotype because neither is recessive
eg horses can have white or coloured hair allele but neither is recessive so horses with a copy of each have a coat with both hairs
what is homozygous/heterozygous?
homozygous- organism carries 2 copies of same allele
heterozygous- organism carries 2 different alleles for a gene
how many alleles for each gene do diploid organisms have at each locus?
how many alleles for each gene do haploid organisms have at each locus?
2
1
what is a genetic diagram?
a diagram that can be used to predict the phenotype and genotype of offspring if two parents are crossed
what is monohybrid inheritance?
the inheritance of a characteristic controlled by a single gene. monohybrid crosses show the likelihood of the different alleles of that gene being inherited by offspring of certain parents
what is a phenotypic ratio?
ratio of diff phenotypes in the offspring
what are the F1 and F2 generations?
F1 is first set of offspring and F2 is second set
what is the typical phenotypic ratio for a monohybrid cross of 2 heterozygous parents?
3:1 of dominant:recessive
what is codominance?
eg?
both alleles are expressed in the phenotype as neither one is recessive
eg sickle-cell anemia
what is the expected phenotypic ratio for a monohybrid cross of 2 heterozygous parents involving codominant alleles?
1:2:1
what is a multiple allele cross?
still only involve 1 gene but the gene can have more than 2 alleles eg blood type
what is dihybrid inheritance?
what is a dihybrid cross?
inheritance of 2 characteristics which are controlled by different genes
dihybrid crosses show the likelihood of offspring inheriting certain combos of the 2 characteristics from particular parents
what is the phenotypic ratio for a dihybrid cross with 2 heterozygous parents?
9:3:3:1 (dominant both:dominant 1 recessive 2:recessive 1 dominant 2:recessive both)
why is the phenotypic ratio for dihybrid crosses and codominance not 9:3:3:1?
there are more than 4 poss phenotypes in the offspring
what are male sex chromosomes?
female?
XY
XX
what is sex linkage?
when an allele that codes for a characteristic is located on a sex chromosome
why are most genes on sex chromosomes on the X?
because the Y is smaller so carries fewer genes
why are males more likely than females to show recessive phenotypes for genes that are sex linked?
males only have 1 X so often only have 1 allele for sex linkage. so because they only have 1 copy, they express characteristic even if its recessive
examples of X-linked disorders
colour blindness and haemophilia
what is an autosome?
what is an autosomal gene?
a chromosome that isnt a sex chromosome
autosomal genes are genes located on autosomes
why are genes on the same autosome said to be linked?
what does it mean when 2 genes are more closely linked?
because they stay together during independent segregation in meiosis 1 and their alleles are passed on to offspring together
theyre closer together on an autosome so less likely to be split up by crossing over
what is a back cross/ test cross?
crossing an organism with a double recessive type to see if someone is homozygous dominant or heterozygous
what is epistasis?
the interaction of different genes where the gene at one locus masks or supresses the expression of another gene at a second locus
what is recessive epistasis?
what is the phenotypic ratio of crossing homozygous dominant w homozygous recessive?
2 recessive alleles at one gene locus will mask the expression of alleles at a 2nd gene locus
9:3:4 (dominant both: dominant epistatic, recessive other: recessive epistatic)
what is dominant epistasis?
what is the phenotypic ratio of crossing homozygous dominant w homozygous recessive?
a dominant allele at one gene locus will mask the expression of alleles at a 2nd gene locus
12:3:1 in the F2 gen
what is the chi² test?
statistical test to see if results of an experiment support a theory
need to make null hypothesis that states there is no sig diff between observed and expected results
Used when categorical data
how do you work out chi² value?
Χ^2 = Σ x (O-E)^2 / E
Χ2 is the chi-square test statistic
Σ is the sum of
O is the observed frequency
E is the expected frequency
what is the critical value? how do you work it out?
the value of X^2 that corresponds to a 0.05 level of probability that the difference between the observed and expected results is due to chance
1. work out degree of freedom (no of phenotypes-1)
2.find critical value corresponding to the degrees of freedom and probability level of 0.05 in the table
how do you compare the X² value to the critical value?
if X^2 is larger than or equal to critical value, then there is sig diff between observed and expected so reject null as difference is not due to chance. vice versa
