Inheritance (lectures 12-14)

Question 1

what is continuous variation?

Answer 1

phenotypes that show a large range of variation
POLYGENIC - multiple interacting genes
environmental influences

Question 2

what is the dominant allele?

Answer 2

allele that expresses itself at the expense of an alternate allele

the phenotype expressed in the F1 generation from the crossing of 2 pure lines

Question 3

what is the recessive allele?

Answer 3

allele whose expression is suppressed in the presence of a dominant allele

phenotype will disappear in F1 generation from the crossing of 2 pure lines but will reappear in the F2 generation

Question 4

what is mendels first law?

Answer 4

2 copies of each gene segregate independently

Question 5

what is mendels second law?

Answer 5

the copies of each gene segregate independently of those of other genes

doesn’t always happen - linked genes are inherited together

Question 6

what is a monohybrid cross?

Answer 6

cross of 2 true breeding lines differing in 1 trait

looks at 1 gene/characteristic

Question 7

what is the ratio of genotypes in the F2 generation of a monohybrid cross?

Answer 7

1 : 2 : 1

as long as the genes are not linked

Question 8

what is the ratio of phenotypes in the F2 generation of a monohybrid cross?

Answer 8

3 : 1

as long as the genes are not linked

Question 9

what does the hardy-weinberg equilibrium state?

Answer 9

gene frequencies and genotype ratios in a randomly breeding population remain constant from generation to generation thus populations retain genetic variability

alleles don’t disappear unless there is a selective pressure

Question 10

what does the hardy-weinberg equilibrium assume?

Answer 10

infinitely large population

random mating

no evolutionary forces

Question 11

what are the equations for hardy-weinberg equilibrium?

Answer 11

total frequencies of alleles in the population
p + q = 1
total frequencies of genotypes in the next generation
p^2 + 2pq + q^2 = 1

Question 12

when does the hardy-weinberg equilibrium not work?

Answer 12

gene flow
genetic drift
non-random mating
natural selection

Question 13

what is gene flow?

Answer 13

movement of an individual to an isolated population

Question 14

what is genetic drift?

Answer 14

small populations - chance or inbreeding leading to a loss of infrequent alleles

Question 15

what is non-random mating?

Answer 15

preferences for certain characteristics

Question 16

what is natural selection?

Answer 16

ability to produce more offspring due to differential mortality (survival) or fecundity (family size)

Question 17

examples of human autosomal recessive conditions

Answer 17

albinism 
phenylketonuria 
tay-scchs 
cystic fibrosis 
sickle cell anaemia 
haemochromotosis

Question 18

examples pf human autosomal dominant conditions

Answer 18

Huntingdon's disease 
achondroplastic dwarfism 
polydactyl
hypercholesterolemia 
FASPS - familial advanced sleep phase syndrome

Question 19

what is an autosomal recessive pedigree?

Answer 19

disease generally appears in the progeny of unaffected parents
horizontal pedigree pattern
affected progeny includes males and females

Question 20

what is an autosomal dominant pedigree?

Answer 20

appear in each generation of a pedigree
vertical pedigree pattern
usually affect 50% of offspring
affects both males and females

Question 21

how are the X and Y chromosomes different?

Answer 21

they show little homology
X has way more genes than Y
many genes on Y are involved in male sex development
many genes on X are not involved in sex development

Question 22

what is sex linkage?

Answer 22

as men only have 1X, any allele of a gene on X that causes a disease is phenotypically expressed in male offspring

Question 23

examples of X linked recessive disorders in humans

Answer 23

haemophilia - failure of blood to clot
fragile X syndrome
deuteranopia - red-green colour blindness

for serious X linked disorders all cases are in boys - always inherited from the mother who is heterozygous

Question 24

examples of X linked dominant disorders in humans

Answer 24

hypophosphatemia

affected males pass the condition to all their daughters but none of their sons
affected heterozygous females pass the condition to half their sons and daughters

Question 25

what is codominance?

Answer 25

mixture of 2 phenotypes

traits show up equally in F1

Question 26

what is incomplete dominance?

Answer 26

heterozygous individual looks like neither parents
new phenotype formed

F1 resembles neither parent

Question 27

what is pleiotrophy?

Answer 27

some genes affect more than 1 unrelated characteristics
they are called pleiotropic

eg. Manx phenotype in cats is due to a dominant allele of a gene that severely disrupts spinal development
all manx cats are heterozygous
it is recessive lethal - offspring with 2 copies don’t make it to birth

Question 28

what is epistasis?

Answer 28

where 1 mutation hides/modifies the phenotype of another

Question 29

what is the epistatic gene?

Answer 29

the gene that modifies or masks another gene

Question 30

what is the hypostatic gene?

Answer 30

the gene that is being modified or masked

Question 31

what is penetrance?

Answer 31

measures the proportion of the population with a particular genotype show the expected phenotype

can be complete or incomplete

if 15% of individuals with a particular mutation appear wild type, the penetrance is 85%

Question 32

what is expressivity?

Answer 32

refers to the range and symptoms that can occur in different people with the same genetic condition

depends on interaction with other genes as well as environmental influences

Question 33

how is penetrance and expressivity in polydactyly?

Answer 33

incomplete penetrance - due to a dominant mutation, but children with extra digits are born to parents with the normal number

variable expressivity - some individuals have 1 extra finger, other have extra digits on each hand and foot

Question 34

what environmental effect has an affect on phenotypes?

Answer 34

temperature

eg. siamese cats - homologous for a thermosenstive allele of a gene encoding an enzyme catalysing melanin production
protein doesn’t work at normal body temperature but works at slightly cooler temperatures

Question 35

what is complementation analysis?

Answer 35

test for determining whether two mutations associated with a specific phenotype represent two different forms of the same gene (alleles) or are variations of two different genes

can only be done with recessive alleles

Question 36

what is a complementation group?

Answer 36

a set of mutations mapping to the same chromosomal locus that fail to complement each other when crossed
they produce a mutant phenotype when crossed

Question 37

what is linkage?

Answer 37

when genes are close on the same chromosome they can be linked and are inherited together

Question 38

how are non parental genotypes formed when there is gene linkage?

Answer 38

independent asssortment of genes on different chromosomes

breakage and rejoining of homologous chromosomes during meiosis - recombination

Question 39

what is recombination frequency?

Answer 39

no. of recombinants / total progeny x 100

Question 40

what is a centimorgan?

Answer 40

2 loci are 1 cendtimorgan (cM) apart if recombination is observed

cendtimorgan is commonly called a map unit (mu)

in humans, 1 cendtimorgan is around 1 million bp