Exceptions to mendel's laws
- 1. Exceptions to Mendel’s Laws: Co-dominance Incomplete Dominance Multiple Alleles Lethal Genes Test Cross
- 2. B=brown b= blue Test Cross: designed to reveal the genotype of an organism that exhibits the dominant trait – Brown eyes might be BB or Bb… how can we tell?? cross the organism with an individual expressing the recessive trait….bb What will this tell us?
- 3. Johnny has brown eyes… what is his genotype? Complete the Punnett Squares:
- 4. The Results: -if all the offspring of the cross show the dominant trait (brown eyes), then the parent must be homozygous for the dominant allele -ifboth phenotypes appear among the offspring then the parent must have been heterozygous for the trait
- 5. Proving the genotype: Ifyou are asked to prove a genotype of an individual (with a dominant trait), use a testcross Cross the unknown genotype with an individual who is homozygous recessive
- 7. Co-dominance: Two alleles of a gene are clearly expressed in the phenotype (equally) – Ex. Roan colour in cattle – coat has both red and white hairs in it – AB blood type in humans - IA & IB alleles are present
- 8. Punnett Square – Roan Cattle •In certain cattle, the hair colour can be red- RR, white- R’R’, or roan- RR’ •Cross a red bull with a white cow •Cross an offsrping from the F1 generation with a white cow
- 9. Incomplete Dominance: Two alleles of a gene appear to be blended into the phenotype – Red snapdragons X white snapdragons = pink!
- 10. Multiple Alleles: Polygenic Inheritance Some genes have 3 or more alleles BUT you can only have two alleles for a trait – Ex. Skin color or height in humans – Human blood types ABO – There are 4 phenotypes A, B, AB & O produced from 3 alleles IA-dominant, IB – dominant, & i - recessive
- 12. Coat Colour in Rabbits Four phenotypes and four alleles Allelic series is C > cch > ch > c (which is most dominant) Coat Color Phenotype Genotype Full Color CC, Ccch, Cch, Cc Chinchilla cchcch, cchch, cchc Himalayan chch, chc Albino cc
- 13. Punnett Square – Blood Groups: Show a cross between a person who has heterozygous A type blood & a person who has homozygous B type blood. IAi x IBIB Genotypes: Phenotypes:
- 14. Lethal Genes: Somegenes are lethal when present in the homozygous condition (dominant or recessive) – Ex. In chickens, when an embryo contains two copies of the recessive gene known as creeper, the embryo dies inside the eggshell. Chicks that are heterozygous for the gene survive – In mice, yellow coat colour is dominant to grey Mice that have YY (pure yellow) coat colour do not survive
- 15. Example with Mouse Coat Colour Yellow is dominant to grey Cross hybrid yellow mice Yy x Yy Resulting offspring: 2 yellow to 1 grey One genotype is missing Somehow the YY genotype is lethal The 2:1 ratio is the typical ratio for a lethal gene.
- 16. Answers to samples: A •child’s phenotype is O, the genotype must be ii (recessive) •father (A) and mother (B) must both carry the recessive allele •Father’s genotype IAi and mother’s genotype IBi B C Chance of having an AB type baby is 25%
- 17. a) The offspring occur in three types, classified as long, round and oval (intermediate), suggesting incomplete dominance • Long (L) Round (L’) Oval (LL’) - heterozygous Test this idea…. LL’ x LL’ Gives a ratio of 1 long to 2 oval to 1 round radishes. b) (LL x LL), all the offspring will be long. c) (L’L’ x L’L’), all the offspring will be round.
- 18. A) Since ¼ of the eggs don’t hatch – lethal gene Crested (C) and non-crested (c) then both parents would have to be Cc ( eg. Cc x Cc) All the eggs with CC genotype would not hatch Viable offspring would hatch in a ratio of 2 crested to 1 non-crested B) Cc x cc Ratio: 1Cc to 1 cc 1 crested to 1 non
- 19. A) Bull’s genotype RR, Cow’s genotype R’R’ B) RR’ x R’R’ All of the offspring have RR’ genotype and will be roan Ratio of 2 roan to 2 white calves