Mendelian Inheritance Flashcards
reasons for working with peas
many varieties of characters (colour) and trait (purple)
short generation time
large number of offspring from each mating
easy to observe and record
could strictly control mating
true breeding
over many generations of self pollination plants produce only the same variety as the parent plant e.g. true breeding purple plant will give successive generations of purple flowered plants through self pollination
hybridisation
mating or crossing of two true breeding varieties
parents - P generation
F1 - first filial generation - offspring of hybrid mating
F2 - second filial generation - allowing F1 hybrids to self pollinate
P generation
parent generation in hybridisation process - mating of two true breeding varieties
YY x yy (hybrid)
F1 are their offspring
F1 and F2 generation
F1 - first filial generation / F2 second filial
F1 offspring of 2 true breeding hybridisation
F2 - offspring of 2 F1s
Mendels findings were as a result of him following through to the F2 stage
what enabled Mendel to discover the basic patterns of inheritance
If he had stopped with the F1 generation he would not have been able to work out the pattern of dominant and recessive traits as these only become evident with the F2 generation
what are Mendel’s two laws of hereditary
the law of segregation
the law of independent assortment
what was the main theory about inheritance before Mendel
the blending model which stated that genetic material contributed by two parents mixed and that over successive generations mating will led to a uniform population
when did Mendel do is work
Began in 1857
the law of segregation
alternative versions of genes account for variations in inherited characters
for each character an organism inherits two copies (two alleles) of a gene from each parent
if the two alleles at the locus differ, then one is the dominant allele (determines the organisms appearance)
the other recessive allele has no noticeable effect on the organisms appearance
the two alleles for a heritable character segregate during gamete formation and end up in different gametes
Give an example of the law of segregation for pea flower colour
cross pea flowers with purple and white flowers
F1 - all purple (the purple colour is dominant so all flowers have PP or Pp
F2 - proportion of 3:1 purple flowered to white flowered
The heritable factor for the recessive trait has not been lost but was masked by the presence of the factor for purple flowers
alleles
version of a gene e.g. purple flowers or white flowers
at a particular locus on the DNA
what is a gene
a sequence of nucleotides at specific locus along a chromosome
how can we have different alleles for a particular gene
genes are sequence of nucleotides
there can be variations in the nucleotide sequence
this variation can affect the function of the encoded protein the thus the inherited character of the organism
diploid
has two sets of chromosomes- one set inherited from each parent
a genetic locus is represented twice in a diploid cell once on each homologue of a specific pair
of chromosomes
somatic cell
all non sex cells
describe the arrangement of genes on chromosomes
each diploid cell has two sets of chromosomes - one set inherited from the mother and one from the father
so a gene is represented twice - once on the chromosome inherited from the mother and once on the chromosome inherited from the father
the variety of the gene - the allele - can be the same on both chromosomes (PP)
or they can be different (Pw)
what happens if the alleles for a trait differ
the dominant allele determines the appearance of the organism
the other allele is recessive and so not seen
homozygote
an organism that has a pair of identical alleles for a gene encoding a character
they are homozygous for that gene
homozygous
have two identical alleles for a gene e.g. PP or pp
heterozygote / heterozygous
an organism that has two different alleles for a gene
heterozygotes produce gametes with different alleles
e.g. Pp
phenotype
organism’s observable traits
this can be different to the genotype because some genes might be recessive and therefore not observable
genotype
genetic makeup
can be different from phenotype
e.g. PP and Pp plants have the same phenotype but different genotypes
test cross
breeding an organism of unknown genotype with a recessive homozygote to reveal the genotype of the organism
monohybrid cross
looking at only one genetic character e.g. colour of flowers
dihybrid cross
cross looking at two characters e.g. pea colour and shape
how many different combinations are possible from a dihybrid cross
16 (4 x 4)
Ratio 9:3:3:1
law of independent assortment
two or more genes sort independently
each pair of alleles segregates independently of any other pair of alleles during gamete formation
(some genes are inherited together if they are close to each other on the chromosome )
multiplication rule
to determine the probability of an event - multiply the probability of one event by the probability of the other event i.e. probability of purple flowers (1:2 or half) probability of white (1:2 , half) = half multiplied by half is one quarter so one quarter PP one quarter Pp One quarter Pp one quarter pp
addition rule
the probability that two or more mutually exclusive events will occur is calculated by adding their individual probabilities
e.g quarter of wrinkled peas + 1/4 of yellow =
complete dominance
when the alleles of the gene are different the dominant allele observed in all cases
the phenotypes of the heterozygote and the dominant homozygote are indistinguishable
incomplete dominance
when the alleles are different the phenotype is a blend (somewhere between the two parental varieties) e.g. snapdragons - when red snapdragons are crossed with white some of the offspring are pink (those with one dominant and one recessive)
ratios for phenotype and genotype
PP x pp
results in 3 purple and 1 white in appearance so the phenotype ratio is 3:1
however the genetic ratio is different
results in 1/4 PP 1/2 Pp and 1/4 pp so the ratio is 1:2:1
codominance
Codominance refers to a type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual. That is, instead of one trait being dominant over the other, both traits appear, such as in a plant or animal that has more than one pigment colour.
codominance example
red flower + white flower =
RR red flower
rr white flower
Rr - red petals and white petals
pleiotropy
most genes have multiple phenotypic effects
single gene affect a number of characters e.g. cystic fibrosis / sickle cell disease
Mendel - same gene that causes flower colour also causes the colour of the seed coating
epistasis
one gene affects the phenotype of another because the two genes interact
the phenotypic expression of a gene at one locus alters that of another gene at a second locus
e.g. labradors - colour gene is affected by the pigment depositing gene /if the dog is recessive at the pigment locus then it is yellow - no pigment regardless of the other gene
quantitative characters
characters like human skin colour or height that are graduations along a continuum
polygenic inheritance
two or more genes that work together to make a single phenotypic character e.g. height - 180 genes impact on height
skin pigmentation in humans is controlled by many separately inherited genes
pedigrees
collecting information about a families history for a particular trait / creating a family tree
use for calculating the probability of future children inheriting a trait / having a particular genotype or phenotype
carriers
heterozygotes (two forms of the gene) though phenotypically normal can transmit the recessive allele to their offspring
most people who have recessive disorders are born to parents who are carriers of the disorder but have normal phenotype
examples of diseases caused by recessive genes
passed on through carriers
cystic fibrosis (4% are carriers) sickle cell disease
examples of diseases caused by dominant alleles
achondroplasia (dwarfism)
people who do not have it are homozygous for the recessive allele
Most lethal dominant gene diseases are not passed on because the organism dies before reproducing
some like Huntington’s disease only appear after reproductive age
when does the segregation of alleles occur in meiosis
anaphase 1 - when alleles separate
who found proof in chromosomes that supported Mendel’s theory
Morgan (early 1900s) working with fruit fly - Drosophilia
why was the choice of Drosophila good
prolific breeders - many offspring and mate often
only 4 chromosomes
chromosomes can be seen with a light microscope
three autosomes and one pair of sex chromosomes
wild type
phenotype for a character most commonly observed
in natural populations
mutant phenotypes
alternatives to wild type
homologous chromosomes
two chromosomes in a pair carry genes controlling the same inherited characters
carry the same genes
somatic cells
all the cells in the body except gametes
sex chromosomes
chromosomes involved in sex determination
all other chromosomes are called autosomes
diploid cell
any cell with two chromosome sets
has a diploid number of chromosomes -2n
humans - diploid number is 46
drosophila - 8 (2 x 4)
haploid cell
cells tat have a single set of chromosomes
gametes are haploid cells
for humans the haploid number is 23
drosophila - 4
X - Y system for sex determination
Females have XX Males XY (Y smaller)
each egg receives one x
each sperm can have x or Y
Fathers pass X to all their daughters but none to their sons
sex linked gene
a gene located on either sex chromosome
chromosomes on the X chromosome are called X linked (1100 in humans)
chromosomes in the Y chromosome are Y linked
(78 in humans) - mainly related to function of testicular function
X chromosome genes for many characteristics
hemizygous
because females have XX they can be homozygous or heterozygous for a particular allele on a X chromosome
because males have only on X chromosome they only have one form of the allele - they are hemizygous (hemi - they only have half)
why do more males than females have X linked disorders
Females have two X chromosomes so can be XX Xx Xx and xx
if x is recessive have 1/4 chance of inheriting
If a male has only one X it can be X or x - so have 1/2 chance
if the male receives a recessive six linked gene from their mother they will express the trait
examples of sex linked traits
colour blindness
muscular dystrophy
haemophilia
baldness
Barr body
females have XX so twice as many of the genes located on the X chromosome as males
in females one X is inactivated during embryonic development
As a result males and females have the same effective dose (protein making ability) of most X linked genes
the inactive X forms the Barr body
why don’t females - XX- produce double the proteins for genes on the X chromosome as males (XY)
inactivation of one X chromosome during embryonic development
As a result both males and females have the same dosage of most X linked genes.
linked genes
genes located near each other on the same chromosome tend to be inherited together in genetic crosses
genes that are genetically linked
(different from Mendel’s law of independent assortment
parental types
offspring that have a phenotype that matches either of the phenotypes of the parents
e.g. round yellow pea X green wrinkly pea = round yellow and green wrinkly offspring which are parental types but also yellow wrinkly and green round which are recombinants
this is for genes that are not linked
recombinants
offspring that have new combinations of phenotypes
e.g. round yellow pea X green wrinkly pea = round yellow and green wrinkly offspring which are parental types but also yellow wrinkly and green round which are recombinants
nondisjunction
when members in a pair of homologous chromosomes do not move apart properly during meiosis 1 or sister chromatids fail to separate during meiosis II. One gamete receives two of the same type of chromosome and another gamete receives no copy
aneuploidy
a zygote with an abnormal number of chromosomes
monosomic - missing one chromosome
trisomic - having three of a chromosome
monosomy and trisomy are estimated to occur in 10-25% of human conceptions and are the main reason for pregnancy loss
monosomic
aneuploid with a missing chromosome
trisomic
aneuploid with an extra chromosome
down syndrome
trisomy21
polyploidy
have more than two complete chromosome sets triploidy - 3n tetraploidy - 4n Common in the plant kingdom some fish and amphibians are polyploid
monohybrid ratios
P- true breeding HH X hh
F1 - all express dominant allele 1 : 1 : 1 = Hh Hh Hh Hh
F2 - 3 : 1 HH Hh Hh hh
