Genetics After Mendel

Incomplete Dominance

• Can happen when neither gene is completely dominant over the other. (Tall pea plants are dominant over dwarf ones)

• Example: In snapdragons, red colour (RR) and white colour (rr) are equally dominant, and the result is a heterozygous offspring (Rr) that is the colour pink!

• At first you might think that the colours just blended like paint would. However, when scientists crossed the hetero-zygous (Rr) in the F1 generation with one another, there were some pure red and pure white flowers that resulted.

• Many human traits show incomplete dominance. Examples: hair, skin, eye colours

Co-Dominance

• This is where two alleles are expressed at the same time. Neither allele is dominant.

• Example: a homozygous red horse and a homozygous white horse make a “roan” horse (a blend of red and white). However, it is not a true blend, there will be full hairs on the horse that are red, and some that are white.

• In this case, when we represent the alleles with letters we would say: CWCW for a white horse and CRCR for a red horse. A roan horse would be CRCW. If we cross two roan horses we would get a 1:2:1 ratio.

What’s the Difference?

• Incomplete: where neither allele dominates and both have an influence: partial expression of both traits.

• Codominance: where both alleles are expressed fully.

Human Blood Typing

• Human blood types show co-dominance and simple dominace together, so in this case there are 3 different alleles that might be found at each locus on the homologous chromosomes. When there are more than two alleles possible for any given gene, we call it multiple allelism

• For humans, the blood types are: A (represented by IA) B (represented by IB) O (represented by i)*A and B are co-dominant over O

Blood Typing Genotypes

Genotype Phenotype

IAIA or IAi Type A

IBIB or IBi Type B

IAIB Type AB

ii Type O

Multifactorial Traits

• Is a term that is used to describe traits whose phenotypic expressions is controlled by numerous genes on different loci (plural of locus)

• A multifactorial trait is often influenced by both internal and external environments as well as other factors

• Example: a multifactorial trait is human’s height. Anywhere between 140cm and 200cm.

• A dominant/recessive “normal” trait for pea plants is height. Either tall or dwarf

• The multifactorial trait shows a continuous distribution

• The other trait, where only one pair of alleles is involved shows a discontinuous distribution

Environmental Effect

• The environment can actually determine some traits. For example, the trait for tallness may not be fully expressed if the individual is deprived of the proper nutrients during his or her growing stages

Linkage

• Humans have an estimated 27000 to 40000 genes on our 46 chromosomes.

• Any genes that appear on the same chromosome are said that gene linkage is in effect.

• When we have two genes on the same chromosome, they do not assort independently of one another, like Mendel said in his Law of Independent Assortment

Linkage Cont’d

• Thomas Morgan studied fruit flies and he crossed GgWw (grey-body, normal wing) with ggww (black body, small wing). So the expected ratio would be 1:1:1:1 for the phenotypes, or 25%, 25%, 25%, 25%. BUT!!! What he observed was quite different: 41.5%, 8.5%, 8.5%, 41.5%.

• He realized that somehow these genes (body colour and wing size) were linked (on the same chromosome) so that they could not assort independently of one another

Linkage continued…

• Notice that not all of the F1 generation fruit flies were like either parent. 17% (8.5 + 8.5) were a combination. These new flies, termed recombinants were the result of crossing over during prophase I of meiosis, therefore making the recombinants different than the original parents

Gene Mapping

• Gene maps were made of the fruit flies’ chromosomes. This is where the scientists have identified the actual location of genes on a specific chromosome

• Scientists have figured out that the further away two genes are on a chromosome, the greater chance of a crossing over, than those that are linked close together

• If two traits always seemed to appear together, it meant that the two were not only on the same chromosome, but were very close on that chromosome

Human Genome

• The Human Genome Project is where scientists completed a catalogue of our entire genetic makeup.

• Declared complete in April 2003.

• This will allow scientists to be able to find defective genes easily, and help with cures.