Mendelian Genetics 1 Flashcards
Genotype
Definition
the underlying genetic makeup of an organism
Phenotype
Definition
the physical appearance of an organism
genotype may interact with the environment in order to produce the phenotype
Locus
Definition
a particular place in a genome, a particular physical region on the chromosome
Allele
Definition
variant of a gene
Diploid
Definition
an organism that has two copies of their genetic information, one from each parent
Haploid
Definition
cells that only contain one copy of the organisms genetic information
Homozygous
Definition
two copies of identical alleles at an individual locus
Heterozygous
Definition
two different alleles at the same locus
Dominant
Definition
the dominant phenotype is that which is the same regardless of whether or not the organism is homozygous or heterozygous for that allele at that locus
DOMINANCE IS RELATIVE
Recessive
Definition
in a diploid organism, two copies of a recessive allele are required at a particular locus for the recessive phenotype
Wildtype
Definition
the wildtype phenotype or wildtype allele is that which is most common in the population
Mutant
Definition
alleles or phenotypes that are different from the wildtype, mutant simply means different or changed
Mendel’s First Law of Inheritance
the principle of segregation states that the two members of a gene pair ssegregate from each other in the formation of gametes
Mendel’’s Second Law of Inheritance
the principle o findependent assortment states that members of different gene pairs are transmitted independently of one another during gametic production
What is selfing?
self fertilisation
What is a cross?
cross fertilisation
Self Fertilisation
to produce offspring using gametes from a single organism
Cross Fertilisation
producing offspring by fusing gametes derived from different organisms
True-Breeding or Pure-Breeding Strains
strains of organisms that when mated among themselves always produce identical progeny because they are homozygous for the phenotype under study
Pleiotropy
Definition
a genetic locus where consequences effect more than one phenotype
Mendel’s Experiments
- carried out experiments using true breeding strains of the garden pea which showed different phenotypes for seven different traits:
- flower colour
- seed colour
- seed shape
- pod colour
- pod shape
- stem height
- flower position
Mendel’s Experiment
Smooth/Wrinkled - Description
- Mendel noted that when a true breeding smooth and wrinkled plants were crossed all of the F1 were also smooth
- the smooth phenotype was dominant to the wrinkled phenotype
- when the F1 plants were selfed both the original parental phenotypes reappeared in the F2
- Mendell predicted that each plant carried two copies of a genetic determinant, a gene
Mendel’s Experiment
Smooth/Wrinkled - Genotypes
Parental Cross: SS x ss produced F1 : Ss F1 cross: Ss x Ss produced F2: SS, Ss, sS, ss 3:1 ratio of smooth:wrinkled
Important Model Organisms for Genetics
- fruit flies
- nematode worms
- plants - Arabidopsis thaliana
- fungi
Arabidopsis
- agamous
- recessive mutant allele
- causes infertility
Drosophila
Wildtype RR, Rr - red eye colour WW, Ww - normal wings Mutant rr - white eye colour ww - vestigial wings
Genetic Nomenclature
- gene names always written in italics
- dominant alleles always written first, Bb not bB
- dominant alleles always written in capitals
- recessive alleles always written in lowercase
- wildtypes often denoted by +
Yellow Mice
Genotypes and Phenotypes
- -an example of an organism where where the recessive allele is the wildtype
- wildtype mouse hair colour is agouti, y/y
- Y/y mice are heterozygous an have yellow hair
- Y/Y mice are never found because Yelllow is a lethal gene when homozygous for the Y allele, homozygous YY embryos abort early in devlopment
Yellow Mice
Pleiotrophy
- in mice, Yellow has pleiotrophic effects, lethality an coat colour
- coat colour is a dominant trait relative to wildtype coat colour, but lethality is recessive to wildtype for embryonic survival
What is the simplest way to represent a genetic cross?
a Punnett Square
Ss x Ss
3:1
Ss x ss
1:1
Ss x SS
1:0
Test Cross
a cross between an individual of unknown genotype and an individual that is true breeding for the recessive trait
- if the F1 are all dominant phenotype then the unknown genotype must have been AA
- if the F1 are not all the dominant phenotype then the unkown genotype must have been Aa
- any organism with the recessive ohenotype must be aa so you don’t need a test cross
Snapdragons -Flower Colour
Genetic Crosses
R - red r - white parent cross: RR x rr produces F1: Rr - pink F1 cross: Rr x Rr produces F2: RR, Rr, Rr, rr 2:1:1 pink:red:white -this is an example of codominance or incomplete dominance
Snapdragons - Flower Colour
Explanation
- the gene codes for an enzyme which manufactures the colour pigment
- RR produces twice as much pigment as Rr so appears red where as Rr appears pink
- -rr does not produce any pigment so the flower are white
Yellow Mouse
Genetic Cross
parental cross: Yy x Yy
F1 Genotypes: YY, Yy, yy
but YY is lethal so aa class of progeny is missing from the 3:1 ratio
the observed ration in the progeny is 2:1
Mendel’s Experiment
Seed Colour & Seed Shape
Separate Traits
-seeds are either yellow or green
-seeds are either smooth or wrinkled
-parental cross: YY x yy
produces F1: Yy
F1 cross: Yy x Yy
produces F2: YY, Yy, Yy, yy
expect 3:1 ratio of yellow:green
parental cross: SS x ss produces F1: Ss F1 cross: Ss x Ss produces F2: SS, Ss, Ss, ss expect 3:1 ratio of smooth:wrinkled
Mendel’s Experiment
Seed Colour & Seed Shape
Traits Together
parental cross: SSYY x ssyy
produces F1: SsYy
cross F1: SsYy x SsYy
produces F2: 9:3:3:1 ratio of SmoothYellow:SmoothGreen:WrinkledYellow:Wrinkled Green
-this ratio comprises a 3:1 ratio of smooth to wrinkled within which there is a further 3:1 ratio of yellow to green
Mendel’s Experiment
Seed Colour & Seed Shape
Branch Diagram
trait one: seed colour
trait two: seed shape
-for the first trait 3/4 of the progeny are smooth and 1/4 are wrinkled
-for the second trait 3/4 of the progeny are yellow and 1/4 are green
-smooth yellow:
3/4 x 3/4 = 9/16
-smooth green:
3/4 x 1/4 = 3/16
-wrinkled yellow:
1/4 x 3/4 = 3/16
-wrinkled green:
1/4 x 1/4 = 1/16
Gene Pairs, Phenotypic Class, Genotypic Class and Genotytpe
- if the number of gene pairs is n, then:
- -the number of phenotypic classes is 2^n
- -the number of genotypic classes is 3^n
- -the number of genotypes is 4^n
What is the ratio for two gene segregation?
9:3:3:1
Multiple Genes, Single Trait Example
- Comb development in chickens
- four phenotypes; rose, pea, walnut and single
rose x single
produces F1: all rose
F2: 3:1 rose:single
->rose is dominant to single, looks life a single-gene trait
pea x single
produces F1: all pea
F2: 3:1 pea to single
-> pea is dominant to single, also looks like a single-gene trait
rose x pea
produces F1: all walnut
F2: 9:3:3:1 ratio of walnut to rose to pea to single
-> two gene segregation
RRpp = rose rrPP = pea RrPp = walnut rrpp = single
Multiple Alleles
Example
- coat colour in rabbits
- different colour variations derive from different combinations of alleles at the coat locus C
- -agouti rabbits carry at least one copy of the wildtype allele C+ which is dominant to all other alleles
- -chinchilla rabbits carry at least one copy of the Cch allele, homozygotes are chinchilla but heterozygotes carrying one Ca or Ch are light grey chinchilla
- -himilayan rabbits carry at least one Ch allele, if heterozygotes, the other allele must be albino, Ca, for the himilayan phenotype
- albino rabbits are homozygous for the ‘most recessive’ allele, Ca
Multiple Alleles vs Multiple Genes
- if loci are allelic you don’t get wildtype phenotypes when you cross individuals homozygous for different RECESSIVE alleles
- this cross test is called a complementation test
- we say that complementation occurs if we obtain the wildtype from a cross between individuals homozygous for recessive alleles, e.g. with chicken comb shape
Trihybrid Cross
parental cross: AABBCCxaabbcc produces F1: AaBbCc F1 cross: AaBbCc x AaBbCc produces F2: 27:9:9:9:3:3:3:1 64 genotypes and 8 phenotypes
