GENETICS & INHERITANCE

INHERITED CHARACTERISTICS  Leaf shape in plants  Coat colour in guinea pigs  Tongue rolling ability  Hair & eye colour These are all examples of the many characteristics determined by genetic information passed on from generation to generation during sexual reproduction These are all examples of the many characteristics determined by genetic information passed on from generation to generation during sexual reproduction

INHERITED CHARACTERISTICS  The alleles inherited from parents may be the same or different.  A DOMINANT allele always shows in the appearance of an organism  A RECESSIVE allele only shows if the organism has inherited two of them *

Symbols  To represent dominant and recessive traits, we use the first letter of the allele, using a capital letter for the dominant and a small letter for the recessive.  Eg mouse colour, Brown is dominant over white coat colour.  Brown =B White = b

INHERITED CHARACTERISTICS If a pea plant inherited the allele for tallness from each of its parents then its genotype would be TT. tt – inherited allele for dwarfness Tt – inherited a dominant & recessive allele for height TT or Tttt BB or Bb bb

KEY WORDS  DOMINANT – always shows up in appearance of organism  RECESSIVE – only shows if organism has inherited two of the alleles  GENOTYPE – the set of genes an organism has, the alleles it carries for a characteristic (usually written as letters)  PHENOTYPE – the appearance of the organism (blue-eyed, tall)

GENOTYPE & PHENOTYPE  The phenotype of an organism depends on the genotype!  An organism which has inherited 2alleles the same is called HOMOZYGOUS (TT,tt)  An organism which has inherited 2 different alleles is HETEROZYGOUS (Tt)

PREDICTING PHENOTYPES FROM GENOTYPES  In humans the allele for tongue rolling is dominant (R) to the non-rolling (r)  In mice the allele for black coat is dominant (B) to white coat (b).

MONOHYBRID CROSS  A monohybrid cross is a breeding experiment which follows the inheritance of one characteristic.  Organisms which are true breeding pass on the same characteristic to their offspring over many generations. Parents X X Gametes F1 (first) generation Gametes Second generation YY yellow seeds YY Pollen Y Ovules Y Pollen Y Ovules Y All offspring genotype YY – yellow seeds

GENERATIONS OF A MONNHYBRID CROSS  How do we know yellow is the dominant seed colour? Parent generation X Pea plant YY yellow seeds Pea plant yy green seeds Pollen Y Pollen y F 1 generation All offspring pea plants Yy with yellow seeds

THE F 2 GENERATION  To find out what happened to the recessive characteristic Mendel crossed the F 1 generation  The second generation – offspring of the F 1 generation. F 1 Generation X Pea plant Pea plant Yy – yellow seeds Yy – yellow seeds Punnet square showing 2 nd generation F 2 Yy YYYYy yYyyy Female gametes Male gametes Providing a phenotypic ratio of 3:1

TEST CROSSES  A practical problem for breeders is to be sure that an organism is true breeding.  Looking at phenotype is not enough.  A test cross can be done to identify the genotype of an individual with a dominant characteristic by crossing it with a homozygous recessive individual.

CO-DOMINANCE  It is possible for two contrasting members of a pair of alleles to be in equal dominance.  This type of inheritance is called Co- dominance.  Offspring turn out to have a phenotype which is in between each parent.

CO-DOMINANCE  Both black and white coats breed true when such horses are crossed but all offspring in fact have both black and white hairs.  These are called grey roans. X

GENOTYPES IN CO- DOMINANCE  Neither alleles is recessive. Both are capital letters. Phenotype Genotype Phenotype Genotype Black coat BB Black coat BB Roan coat BW Roan coat BW White coat WW White coat WW Both black and white phenotypes are true breeding homozygous. The roan phenotype is heterozygous

OTHER POSSIBLE CROSSES!? Red coat and white coat are co-dominant and intermediate phenotype is light red called red roan. X RRWW RW

HOMOZYGOUS x HETEROZYGOUS X PHENOTYPES RED BULL ROAN COW GENOTYPES RR RW GAMETES ALL R R or RW Do a punnet square to work out the offspring off this cross. F 1 GENOTYPES RATIO = PHENOTYPES RATIO =

OTHER POSSIBLE CROSSES!?  Blood group is determined by three alleles A, B, and O. A and B are co-dominant to O the recessive allele. A person with blood group A may have the genotype AA or AO. A person with blood group A may have the genotype AA or AO. A person with blood group AB must have genotype AB as both alleles are co-dominant.A person with blood group AB must have genotype AB as both alleles are co-dominant.

EXAMPLES OF POLYGENIC INHERIATNCE IN HUMANS  Height, weight, skin colour, foot size etc. show a pattern of polygenic inheritance.  There are many different expressions of these so they must be controlled by more than 2 genes. Skin colour Height Maize – kernel colour

COMPARING POLYGENIC & SINGLE GENE INHERITANCE SINGLE GENE POLYGENIC NUMBER OF GENES ONE MORE THAN ONE NUMBER OF PHENOTYPES 2MANY TYPE OF VARIATION DISCONTINUOUSCONTINUOUS TYPE OF INHERITANCE

DROSOPHILA  Very variable so many phenotypes to investigate  Short life cycle – days  Large offspring produced quickly become sexually mature in short period of time (8-9hrs after hatching)  Grey body (G) or ebony (black) (g) MALE - ROUND ABDOMEN FEMALE - POINTED ABDOMEN

Steps in Monohybrid Cross with Drosophila 1) Parents are chose, one with a grey body and other with ebony body 2) Breed the parents 3) The F 1 phenotype is observed 4) The F 1 generation is self crossed 5) The F 2 generation is examined and phenotypes counted 6) The experiment is repeated