12-4 Beyond Medelian Genetics

Alleles are the possible “options” for a trait.

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

Blue roan Red roan

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

Rabbit Fur Colors

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.