Beyond MendelBeyond Mendel

Beyond MendelBeyond Mendel Despite the importance Despite the importance

of Mendel’s work, it of Mendel’s work, it would be a mistake to would be a mistake to characterize the characterize the principles he discovered principles he discovered as “laws” because there as “laws” because there are important exceptions are important exceptions to most of them. to most of them. Not all genes show simple Not all genes show simple

patterns of dominant and patterns of dominant and recessive alleles.recessive alleles.

In most organisms, In most organisms, genetics is more genetics is more complicated, because the complicated, because the majority of genes have majority of genes have more than two alleles.more than two alleles.

In addition, many In addition, many important traits are important traits are controlled by more than controlled by more than one gene.one gene.

Incomplete DominanceIncomplete Dominance

A form of A form of intermediate intermediate inheritance in which inheritance in which heterozygous heterozygous alleles are both alleles are both expressed, resulting expressed, resulting in a combined in a combined phenotype.phenotype.

Incomplete DominanceIncomplete Dominance For example, in For example, in

cross-pollination cross-pollination experiments experiments between red and between red and white white snapdragon snapdragon plants the plants the resulting resulting offspring are offspring are pink.pink.

CodominanceCodominance A number of A number of

human traits are human traits are the result of 2 the result of 2 types of alleles types of alleles that are that are equally equally dominantdominant. .

Such traits are Such traits are said to be said to be codominantcodominant for for that trait.that trait.

CodominanceCodominance When an When an

individual is individual is heterozygous for heterozygous for such traits, the such traits, the resulting resulting phenotype or phenotype or expression of expression of these two traits these two traits is a is a blendingblending, , because both because both traits are traits are expressed expressed equally.equally.

CodominanceCodominance The alleles for curly hair and The alleles for curly hair and

straight hair are examples of alleles straight hair are examples of alleles for a trait that are codominant.for a trait that are codominant.

Individuals with curly hair are Individuals with curly hair are homozygous for curly hair alleles.homozygous for curly hair alleles.

Individuals with straight hair are Individuals with straight hair are homozygous for straight hair alleles. homozygous for straight hair alleles.

Individuals who are heterozygous, Individuals who are heterozygous, with one of each allele have wavy with one of each allele have wavy hair, which is a blend of the hair, which is a blend of the expressions of the curly and straight expressions of the curly and straight hair alleles.hair alleles.

Multiple AllelesMultiple Alleles Diploid organisms Diploid organisms

naturally have a maximum naturally have a maximum of 2 alleles for each gene of 2 alleles for each gene expressing a particular expressing a particular characteristic, one characteristic, one deriving from each parent. deriving from each parent.

In some cases, however, In some cases, however, more than two types of more than two types of allele can code for a allele can code for a particular characteristic, particular characteristic, as is the case of genetic as is the case of genetic coding for blood type in coding for blood type in humans or coat color in humans or coat color in rabbits.rabbits.

Multiple alleles result Multiple alleles result from different mutations from different mutations of the same gene.of the same gene.

Multiple AllelesMultiple Alleles

Coat color in rabbits Coat color in rabbits is determined by is determined by four alleles.four alleles.

Human ABO blood Human ABO blood types are types are determined by determined by alleles A, B, and O. alleles A, B, and O.

A and B are A and B are codominants which codominants which are both dominant are both dominant over O.over O.

Multiple AllelesMultiple Alleles

The only possible The only possible genotype for a type genotype for a type O person is ii. O person is ii.

Type A people have Type A people have either Ieither IAAIIAA or I or IAAi i genotypes. genotypes.

Type B people have Type B people have either Ieither IBBIIBB or I or IBBi i genotypes. genotypes.

Type AB have only Type AB have only the Ithe IAAIIBB (heterozygous) (heterozygous) genotype.genotype.

Multiple AllelesMultiple Alleles

The A and B alleles of The A and B alleles of gene I produce slightly gene I produce slightly different different glycoproteins glycoproteins (antigens)(antigens) that are on that are on the surface of each cell. the surface of each cell.

Homozygous A Homozygous A individuals have only individuals have only the A antigen, the A antigen, homozygous B homozygous B individuals have only individuals have only the B antigen, the B antigen, homozygous O homozygous O individuals produce individuals produce neither antigen, while a neither antigen, while a fourth phenotype (AB) fourth phenotype (AB) produces both A and B produces both A and B antigens.antigens.

Rh Blood type systemRh Blood type system When we are told our When we are told our

blood type, it is usually blood type, it is usually expressed as a letter expressed as a letter followed by either a followed by either a positive (+) or negative positive (+) or negative (-).  (-). 

This + or - refers to the This + or - refers to the Rh system of blood Rh system of blood testing and is usually testing and is usually quoted in combination quoted in combination with the ABO system with the ABO system described earlier in this described earlier in this lecture:lecture:

e.g. AB+ or O-e.g. AB+ or O-

Rh Blood type systemRh Blood type system

The Rh system is actually much The Rh system is actually much more complicated than the ABO more complicated than the ABO system because there are more system because there are more than 30 combinations possible than 30 combinations possible when inherited, however for when inherited, however for general usage, the Rh proteins general usage, the Rh proteins are grouped into two families:are grouped into two families:

either positive (+) or negative (-).either positive (+) or negative (-).

Rh Blood type systemRh Blood type system As with the ABO system, there is a As with the ABO system, there is a

dominant allele which happens to be dominant allele which happens to be the positive family.  the positive family. 

This means that the genetic pairs that This means that the genetic pairs that can exist in humans are as follows:can exist in humans are as follows:

GenotypeGenotype Blood typeBlood type ++++ Rh + Rh + +-+- Rh + Rh + ---- Rh - Rh -

Who can donate to who?Who can donate to who?

Polygenic TraitsPolygenic Traits

Traits controlled Traits controlled by two or more by two or more genes are called genes are called polygenic traitspolygenic traits..

Polygenic traitsPolygenic traits Usually polygenic Usually polygenic

traits are traits are distinguished by:distinguished by:

1.1. Two or more gene Two or more gene pairs contribute to pairs contribute to the phenotype. the phenotype.

2.2. Traits are usually Traits are usually quantified by quantified by measurement measurement rather than rather than counting. counting.

3.3. Phenotypic Phenotypic expression of expression of polygenic traits polygenic traits varies over a wide varies over a wide range. range.

Polygenic TraitsPolygenic Traits Human Human

polygenic traits polygenic traits include:include:

1.1. Height Height

2.2. Weight Weight

3.3. Eye Color Eye Color

4.4. Intelligence Intelligence

5.5. Skin Color Skin Color

6.6. Many forms of Many forms of behavior behavior