inheritance Flashcards
genotype vs phenotype
genotype = genetic makeup of an organism
phenotype = expression of genetic makeup and its interaction with the environment
alleles
different versions of the same gene
- order of bases slightly different
- code for different versions of the same characteristic
can be dominant, recessive or codominant
at the same locus they can be heterozygous (one of each) or homozygous (same)
dominant vs recessive alleles
dominant = appears in phenotype, even with only one copy
recessive = only appears in phenotype if two copies pf allele are present
locus
fixed position of a gene on a chromosome
alleles found at same location on each chromosome in a pair
what is a carrier?
a person carrying an allele which is not expressed in the phenotype
but can be passed on to offspring
epistasis
allele for one gene masks the expression of the alleles for another
sex linkage
inheritance of genes carried on the sex chromosomes
males XY
females XX
Y chromosome smaller and carries fewer genes
- so most genes on sex chromosomes carried on X
example of sex linked disease
colour blindness
haemophilia
why are males more likely to express recessive sex linked disorders?
only have one copy of X
so only 1 copy of allele for sex linked genes
so express characteristics even if recessive
so more like then females
who have second allele to mask it
autosomal linkage
inheritance of 2 or more genes carried on the dame chromosome
- autosomes (not sex chromosomes)
do not separate in meiosis, inherited together, not independently segregated
- assume no crossing over
codominance
2 dominant alleles that both contribute to the phenotype
- expresses both or a blend
example of codominance
sickle cell anemia
blood types
- A and B codominant
- O recessive to A and B (must be homozygous)
what are multiple alleles?
when there are more than 2 alleles of the same gene
eg blood groups (A,B and O)
expected ratios of mono, di and codominant crosses
f1 = from homozygous parents (AA aa)
= all heterozygous offspring (Aa)
f2= from all heterozygous parents (Aa)
monohybrid
3:1
dihybrid
9:3:3:1
codominant
1:2:1
why do you not always get the expected phenotypic ratio?
- linked genes (sex and autosomal)
- epistasis
- small sample size
- random fertilisation of gametes
why do you not always get expected phenotypic ratio?
epistasis - one allele masks the expression of the other
= more common/ over represented
sex linkage = one gender more likely than the other to express certain alleles in phenotype
autosomal linkage = certain combos of alleles more likely in phenotype as inherited together
how to find genotype of unknown expressing dominant phenotype
breed with recessive phenotype
- must have 2 recessive alleles to express it (therefore genotype known)
if produce no recessive offspring = homozygous dominant (recessive masked)
if at least one recessive = heterozygous (must have a recessive allele)
monohybrid
- no recessive offspring = homozygous dominant
- at least one recessive = heterozygous dominant
dyhybrid
- no recessive = homozygous for both genes
- at least one recessive of one gene = homozygous for one, heterozygous for another
- at least one of each recessive = heterozygous both both
why do some offspring have different combination of characteristics to parents?
crossing over in meiosis
breaks linkage of genes
= random combination of alleles of lateral and paternal characteristics
use of chi2 test
determine if there is a significant difference between expected and observed results
if difference between is significant
suggests another factor involved
eg linkage, epistasis etc
interpretation of chi2
degrees of freedom = n-1
chi2 > critical = significant difference
- null rejected, another factor affecting results
critical > chi2 = not significant
- null accepted, any differences due to chance
compared calculated to critical
use 0.05 level
heterozygous vs homozygous
heterozygous = 2 different alleles
homozygous = 2 copies of the same allele
mono vs dihybrid
inheritance of one gene
inheritance of 2 genes at the same time
