inheritance

Question 1

genotype vs phenotype

Answer 1

genotype = genetic makeup of an organism

phenotype = expression of genetic makeup and its interaction with the environment

Question 2

alleles

Answer 2

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)

Question 3

dominant vs recessive alleles

Answer 3

dominant = appears in phenotype, even with only one copy

recessive = only appears in phenotype if two copies pf allele are present

Question 4

locus

Answer 4

fixed position of a gene on a chromosome

alleles found at same location on each chromosome in a pair

Question 5

what is a carrier?

Answer 5

a person carrying an allele which is not expressed in the phenotype
but can be passed on to offspring

Question 6

epistasis

Answer 6

allele for one gene masks the expression of the alleles for another

Question 7

sex linkage

Answer 7

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

Question 8

example of sex linked disease

Answer 8

colour blindness
haemophilia

Question 9

why are males more likely to express recessive sex linked disorders?

Answer 9

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

Question 10

autosomal linkage

Answer 10

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

Question 11

codominance

Answer 11

2 dominant alleles that both contribute to the phenotype
- expresses both or a blend

Question 12

example of codominance

Answer 12

sickle cell anemia

blood types
- A and B codominant
- O recessive to A and B (must be homozygous)

Question 13

what are multiple alleles?

Answer 13

when there are more than 2 alleles of the same gene
eg blood groups (A,B and O)

Question 14

expected ratios of mono, di and codominant crosses

Answer 14

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

Question 15

why do you not always get the expected phenotypic ratio?

Answer 15

• linked genes (sex and autosomal)
• epistasis
• small sample size
• random fertilisation of gametes

Question 16

why do you not always get expected phenotypic ratio?

Answer 16

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

Question 17

how to find genotype of unknown expressing dominant phenotype

Answer 17

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

Question 18

why do some offspring have different combination of characteristics to parents?

Answer 18

crossing over in meiosis

breaks linkage of genes
= random combination of alleles of lateral and paternal characteristics

Question 19

use of chi2 test

Answer 19

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

Question 20

interpretation of chi2

Answer 20

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

Question 21

heterozygous vs homozygous

Answer 21

heterozygous = 2 different alleles
homozygous = 2 copies of the same allele

Question 22

mono vs dihybrid

Answer 22

inheritance of one gene
inheritance of 2 genes at the same time