Genetic Diagrams

Conventions

• Start by showing parental phenotypes then genotypes then gametes.
• A gene is represented by a single letter, upper case for dominant and lower cas for recessive
• Where a gene has more than two alleles, the gene has an upper case letter and the alleles are denoted in supercript.
• Where this is codominance the same convention applies

MONOHYBRID CROSSES

P1

Trait: Seed shaoe

Alleles: R- Round | r - Wrinkles

Cross: Round seeds x Wrinkles seeds

                  RR       x       rr

They would all be Rr

Genotype: Rr

Phenotype: Round

Genotypic ratio: all alike

phenotypc ratio: all alike

• homozygous dominant x homozygous recessive
• offring all heterozygous (hybrids)
• offspring called F1 generation
• genotypic and phenotypic ratio is all alike

F1 monohybrid cross

Traits: Seed shape

Allels: R- Round | r - Wrinkled

Cross:     Rr       x       Rr ( round seeds x round seeds)

Result:

RR                                   

Rr

Rr

rr

Gentotpes: RR, Rr, rr

Phenotype: Round and wrinkled

G ratio: 1:2:1 (RR:Rr:rr)

P.ratio: 3:1 (3 are rounded and 1 is wrinkles)

SEX LINKAGE

• a characteristic is sex linked if the gene that codes for it is found on one of the sex (X and Y) chromosomes

Convention

• when dealing with sex-linked characteristiccs in humans, the genotype of the male is XY (heterogametic) the female XX (homogametic)
• the sex linked allele is then denoted in supercript

Haemophilia A

• several factors are needed for blood to clot
• one of these is factor VIII - found on the X chromosome
• in it s recessive form the allele produces a non functional protein, blood takes longer to clot
• male only have on X chromosome
• if this has the recessive allele they will have haemophilia
• only having on allele for a characteristics is aka hemizygous

X^H= allele for normal factor VIII

X^h = allele for non functioning factor VIII

Cross between a carrier female and normal male
Results:

X^HX^H - normal female

X^Hy - normal male

X^HX^h - carrier female

X^hy - male with haemophilia

Duchenne muscular dystrophy (DMD)

• gene for muscle protein, dystrophin is on th X chromosome
• it is needed in muscular contration
• boys with te disease develop muscle weakness in early childhood and are usually wheelchair bound by age 10
• death often occurs due to complications of muscle degeneration (skeletal and heart uscle) by the early 20s

CODOMINANCE 

• this means thatthe homozygous dominant (AA) and the heterozygous dominant (Aa) both have the same phenotype = look the same
• however, in many cases when considering a pair of characters
• two alleles of a gene have an effect when they are present together
• when choosing symbols to represent co-dominant alleles it would be misleading to use a capital and lower case letters as this would suggest that the one with the cap was dominant
• instead, a capital letter is chosen to represent the gene and then two different superscript to represent the co dominant alleles
• a good example on incomplete dominance is found in snapdragons with respect to flower colour 

SICKLE CELL ANAEMIA

• the beta-strands of haemoglobin differ by 1 amino acid at position 6 (glutamic acid instead of valine)
• abnormal haemoglobin is de oxygenated and insoluble
• this deforms RBC making them inflexible and often sickle shaped
• they are unable to squeeze through capillaries
• after many oxygenated and de oxygenation cycles the RBC can be irreversibly damaged and sometimes destroyed
• when lodged in capillaries they can block the blood flow
• eventually organs esp heart lungs and kidneys become damage 

Sickle cell heterozygotes

• these individuals produce RBC with half of their haemoglobin normal and half sickled
• normal haemoglobin prevents sickling
• thus heterozygotes are symptomless carriers and at whole organism level this condition is considered to be recessive
• however, a molecular and cellular level because both alleles contribute to the phenotypes as observed in RBC it is co dominant

• Normal genotype: H^A H^A
• Sickle-cell gentoype: H^S H^S
• Symptomless heterozygous: H^A H^S

DIHYBRID INHERITANCE

• inheritance of two pairs of contrasting characters is known as dihybrid inheritance
• in pea plants:
• round seeds are dominant to wrinkled seeds
• yellow seeds are dominant to green seeds
• Y = allele for yellow seed coat
• y = allele for green seed coat
• R = allele for round seed shape
• r = allele for wrinkled seed shape
• when pure breeding plants with round, yellow seeds are crossed with plants that have wrinkled green seeds, all of the F1 generation had round yellow seeds
• These plants were allowed to self fertilise or were cross with each other and the resulting seeds were examined
• The expected ratio of 3 dominant to 1 recessive was found for both seed colour and for seed shape
• also, the inheritance of seed colour is independent of the shape of the seed and that nw combo of the characters appear in the F2 generations as well as the parental combination of the charcters

• Dihybrid inheritance is concerned with the inhereitences of the two genes at different loci
• genes that are on difference chromosomes (unlinked) are separated independently, and all combinations of two pairs of genes are possible
• a ratio of 9:3:3:1 is common when two factors are controlled y two independently sorted genes

EPISTASIS

• there are cases where different genes on different loci interact to affect one phenotype
• where one gene masks or suppresses the expression of another gene it is called epistasis
• they may either
• work antagonistically resulting in masking
• the homozygous presence of a recessive allele e.g. aa. may prevent the expression of another allele at a second locus.
• alleles at the st locus are epistatic to the alleles on the 2nd locus these are hypostatic
• work together in a complementary fashion
• recessive epistasis
• this can be seen in the inheritance of flower colour in salvia where two gene loci A/a and / on different chromosomes are involved
• a ration of 9:3:4 is observed
• this ration indicated recessive epsistasis
• the homozygous aa is epistatic to both alleles of gene B/ neither the allel or combiniion for purple  BB/b or the allele combination for pink b can be expressed i there s no dominant allele A present