Inheritance (lectures 12-14) Flashcards
what is continuous variation?
phenotypes that show a large range of variation
POLYGENIC - multiple interacting genes
environmental influences
what is the dominant allele?
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
what is the recessive allele?
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
what is mendels first law?
2 copies of each gene segregate independently
what is mendels second law?
the copies of each gene segregate independently of those of other genes
doesn’t always happen - linked genes are inherited together
what is a monohybrid cross?
cross of 2 true breeding lines differing in 1 trait
looks at 1 gene/characteristic
what is the ratio of genotypes in the F2 generation of a monohybrid cross?
1 : 2 : 1
as long as the genes are not linked
what is the ratio of phenotypes in the F2 generation of a monohybrid cross?
3 : 1
as long as the genes are not linked
what does the hardy-weinberg equilibrium state?
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
what does the hardy-weinberg equilibrium assume?
infinitely large population
random mating
no evolutionary forces
what are the equations for hardy-weinberg equilibrium?
total frequencies of alleles in the population
p + q = 1
total frequencies of genotypes in the next generation
p^2 + 2pq + q^2 = 1
when does the hardy-weinberg equilibrium not work?
gene flow
genetic drift
non-random mating
natural selection
what is gene flow?
movement of an individual to an isolated population
what is genetic drift?
small populations - chance or inbreeding leading to a loss of infrequent alleles
what is non-random mating?
preferences for certain characteristics
what is natural selection?
ability to produce more offspring due to differential mortality (survival) or fecundity (family size)
examples of human autosomal recessive conditions
albinism phenylketonuria tay-scchs cystic fibrosis sickle cell anaemia haemochromotosis
examples pf human autosomal dominant conditions
Huntingdon's disease achondroplastic dwarfism polydactyl hypercholesterolemia FASPS - familial advanced sleep phase syndrome
what is an autosomal recessive pedigree?
disease generally appears in the progeny of unaffected parents
horizontal pedigree pattern
affected progeny includes males and females
what is an autosomal dominant pedigree?
appear in each generation of a pedigree
vertical pedigree pattern
usually affect 50% of offspring
affects both males and females
how are the X and Y chromosomes different?
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
what is sex linkage?
as men only have 1X, any allele of a gene on X that causes a disease is phenotypically expressed in male offspring
examples of X linked recessive disorders in humans
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
examples of X linked dominant disorders in humans
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
what is codominance?
mixture of 2 phenotypes
traits show up equally in F1
what is incomplete dominance?
heterozygous individual looks like neither parents
new phenotype formed
F1 resembles neither parent
what is pleiotrophy?
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
what is epistasis?
where 1 mutation hides/modifies the phenotype of another
what is the epistatic gene?
the gene that modifies or masks another gene
what is the hypostatic gene?
the gene that is being modified or masked
what is penetrance?
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%
what is expressivity?
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
how is penetrance and expressivity in polydactyly?
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
what environmental effect has an affect on phenotypes?
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
what is complementation analysis?
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
what is a complementation group?
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
what is linkage?
when genes are close on the same chromosome they can be linked and are inherited together
how are non parental genotypes formed when there is gene linkage?
independent asssortment of genes on different chromosomes
breakage and rejoining of homologous chromosomes during meiosis - recombination
what is recombination frequency?
no. of recombinants / total progeny x 100
what is a centimorgan?
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
