L5 - departure from mendelian ratios

Question 1

why may we get a deviation from mendelian ratios

Answer 1

1. there are different variations of gene dominance
2. sons X chromosomes can only be inherited from mother
3. sex linkage genes
4. sex influenced genes
5. mitochondrial and chloroplast DNA only inherited from maternal side

Question 2

what are the types of loss of function mutation

Answer 2

amorphic (null)

hypomorphic (leaky)

Question 3

describe amorphic (null) mutation

Answer 3

recessive

mutation where no protein produced at all / protein has no function

Question 4

describe hypomorphic(leaky) mutation

Answer 4

recessive

protein function reduced due to less protein made/ protein less active

Question 5

what are the types of gain of function mutation

Answer 5

hypermorphic
neomorphic
antimorphic (dominant -ve)
dominant lethal

Question 6

describe hypermorphic mutation

Answer 6

dominant

increase in protein activity due to more protein made/more active

Question 7

describe antimorphic (dominant -ve) mutation

Answer 7

disturbance in function that interferes with protein encoded by norm allele

Question 8

describe neomorphic mutation

Answer 8

dominant

alters function of protein completely (new function)

Question 9

describe incomplete/partial dominance

departure from mendelian ratios

Answer 9

results in blend of characteristics

eg a flower may be CC red and cc white but all Cc offspring are pink rather than red

Question 10

describe co dominance

departure from mendelian ratios

Answer 10

where presence of both dominant alleles are detected equally

eg blood group AB

Question 11

describe lethal alleles

departure from mendelian ratios

Answer 11

a combination of alleles that can be lethal in utero and so are never seen in a population

Question 12

give an example of lethal alleles

departure from mendelian ratios

Answer 12

in mice, homozygous alleles for the yellow coat colour is lethal .
yellow allele is dominant to wild type but recessive lethal

Question 13

how do lethal alleles affect humans?

Answer 13

7.5% of all conceptions are miscarried due to being homozygous for lethal genes

Question 14

define genetic load

Answer 14

collection of all lethal alleles in the pop. gene pool

Question 15

list the deviations from mendelian ratios due to reasons other that partial/incomplete dominance

Answer 15

1. X linkage
2. sex influenced
3. maternal inheritance

Question 16

describe X linkage

Answer 16

unique inheritance patterns because males inherit their X from mother

Question 17

who do X linked disorders affect most and why?

Answer 17

1. males, because they only have one X if the disease is recessive they will still be affected

Question 18

describe characteristics of inheritance of x linked disorders

Answer 18

1. all sons of affected/carrier mother will be affected
2. cant pass father to son
3. all daughters of affected father are carriers
4. males cant be carriers

Question 19

describe influence of sex on departure from mendelian ratios

Answer 19

genes found on autosomes but are influenced by sex

can be
sex limited
sex influenced

Question 20

what is sex limited?

give example of sex limited trait

Answer 20

(genes on autosomes) phenotype limited to one sex only

eg milk yield in cattle

Question 21

what is sex influenced?

give example of sex influenced trait

Answer 21

genes on autosomes
trait usually dependant on hormones

eg male baldness dominant in males recessive in females

Question 22

describe maternal inheritance on departure from mendelian ratios

Answer 22

mitochondria and chloroplasts inherited exclusively from maternal side

Question 23

describe characteristics of maternal inheritance (of a disease)

Answer 23

1. all offspring of affected mothers will be affected
2. offspring of affected father will be unaffected
3. affects M&F equally

matrilineal

Question 24

define matrilineal

Answer 24

feature of maternal inheritance (eg of mtDNA) that doesn’t follow autosomal or sex linked inheritance

Question 25

describe incomplete penetrance

as explanation of departure from mendelian ratios

Answer 25

when phenotype associated with genotype fails to appear

eg not all individuals with mutant allele will have mutant disorder (polydactyly)

Question 26

describe variable expressivity

as explanation of departure from mendelian ratios

Answer 26

phenotype of mutant genotype varies in severity due to environment / interaction from other genes

eg 4 people with same mutant genotype (wardenburg syndrome) may have different severity of a disorder

• hearing loss
• grey hair
• differently coloured eyes

Question 27

define pleiotropic

Answer 27

when an allele affects more than one characteristic

eg hearing loss, greying of hair

Question 28

what does a pleiotropic gene indicate?

Answer 28

the gene is required by several tissues/organs

Question 29

what controls penetrance and expressivity?

Answer 29

environment

genotype at other loci

Question 30

what causes an allele to be dominant or recessive?

Answer 30

the type of mutation

eg gain/loss of function