Exceptions to Mendel’s Laws:

Co-dominance

Incomplete Dominance

Multiple Alleles

Lethal Genes

Test Cross

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?

B=brownb= blue

Complete the Punnett Squares:

Johnny has brown eyes… what is his genotype?

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

-if both phenotypes appear among the offspring then the parent must have been heterozygous for the trait

Proving the genotype:

If you 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

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

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

Incomplete Dominance:

Two alleles of a gene appear to be blended into the phenotype– Red snapdragons X white snapdragons = pink!

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

Coat Colour in Rabbits

Coat Color PhenotypeFull ColorChinchillaHimalayanAlbino

GenotypeCC, Ccch, Cch, Cccchcch, cchch, cchc chch, chc cc

Four phenotypes and four allelesAllelic series is C > cch > ch > c (which is most dominant)

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 IBIBGenotypes:

Phenotypes:

Lethal Genes:

Some genes 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

Example with Mouse Coat Colour

Yellow is dominant to grey Cross hybrid yellow mice Yy x Yy

Somehow the YY genotype is lethal The 2:1 ratio is the typical ratio for a lethal gene.

Resulting offspring:2 yellow to 1 grey

One genotype is missing

Answers to samples:

•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

A

C Chance of having an AB type baby is

25%

b) (LL x LL), all the offspring will be long.

c) (L’L’ x L’L’), all the offspring will be round.

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.

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 hatchViable offspring would hatch in a ratio

of 2 crested to 1 non-crested

A)

B) Cc x cc

Ratio: 1Cc to 1 cc1 crested to 1 non

A) Bull’s genotype RR, Cow’s genotype R’R’

All of the offspring have RR’ genotype and will be roan

B) RR’ x R’R’

Ratio of 2 roan to 2 white calves