Summary of Mendel’s Principles1. The inheritance of traits is determined by
genes which are passed from parent to offspring.
2. When 2 or more alleles for a gene exist, some alleles may be dominant and other alleles may be
recessive. (_______________________)
3. Genes are segregated from each other when gametes are formed. (____________________)
4. The alleles for different genes usually segregate independently of one another. (__________________________________)
Exceptions to Mendel’s Principles
Not all genes show simple patterns of dominant and recessive alleles.
The majority of genes have more than 2 alleles
Traits are sometimes controlled by more than one gene.
Exceptions to Mendel’s Principles
Incomplete Dominance – one allele is not completely dominant over another. There is an appearance of a 3rd phenotype. Heterozygous phenotypes show up somewhere in between. Example snapdragon flowers
Red Flowers x White flowers Pink Flowers
RR rrRr
Exceptions to Mendel’s Principles
Codominance – both alleles contribute to the phenotype of the organism. The offspring have a phenotype that expresses both alleles equally.
Examples Chickens:
Black feathers x white feathers speckled
black and white
Exceptions to Mendel’s Principles
Multiple Alleles – genes that have more than 2 alleles for a specific trait
Examples: Rabbit fur color has 4 different alleles
C = full color
Cch = chinchilla
Ch = himalayan
c = albino – no color
Exceptions to Mendel’s Principles Multiple Alleles continue:
Examples: The alleles IA and IB are always expressed – they are codominant. Both IA and IB are dominant to i.
Genotypes Phenotypes
IA IA or IA i A
IB IB or IB i B
IA IB AB
i i O
Exceptions to Mendel’s Principles Polygenic Traits – traits controlled by 2 or
more genes. The genes can be on the same chromosome or on different chromosomes. In polygenic inheritance, all heterozygotes are intermediate in phenotype.
Examples: Eye color in fruit flies – 3 genes Skin color in humans – there are three or four
genes involved in the inheritance of skin color in humans
Linkage
Linkage: Some genes that are close on the same chromosomes are less likely to be separated during crossing-over.
i.e. they end up getting shuffled together most of the time.
Example: red hair and
freckles.
Sex Linked
Traits that can only be passed to males or females.
The gene can be found on the X or Y chromosome.
Example: color blindness Can be found on the X chromosome because
females have two, males only have one chance for a “good” color vision gene.