In most cases when dealing with genes and alleles, the dominant/recessive rule always applies.

Dominant alleles mask the expression of a recessive allele.

Recessive alleles only express in the full homozygous state.

Ex. Hh dominant hh recessive

Ex) When Gregor Mendel crossed a purple-flowered pea plant with a white-flowered pea plant, the flowers in the next generation are all purple.

What is the dominant allele? Purple or White? ____________________

In many ways Gregor Mendel was quite lucky in discovering his genetic laws. He happened to use pea plants, which happened to have a number of easily observable traits that were determined by just two alleles.

For the traits he studied in his peas, one allele happened to be dominant for the trait & the other was a recessive form.

He also used a breed of mice that reflected the same patterns over and over, without variation.

If Mendel were given a black squirrel & a white squirrel & asked what their offspring would look like, he would've said that a certain percent would be black & the others would be white.

He would never have even considered that a white squirrel & a black squirrel could produce a GREY squirrel !

For Mendel, the phenotype of the offspring from parents with different phenotypes always resembled the phenotype of at least one of the parents.

There are 4 ways traits do not follow the dominant/recessive pattern shown before: ◦ INCOMPLETE DOMINANCE ◦ CO-DOMINANCE ◦ EPISTASIS (MULTIPLE ALLELES) ◦ LETHAL (TERMINATOR) GENES

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

The heterozygous does not show as dominant. It shows as a new trait.

Ex) When flowers are crossed (snapdragons, roses) their colour is a result of both parent colours mixing

Wavy Hair in Humans: When curly hair (dominant) is crossed with

straight hair (recessive), it creates a mix of both types. Wavy hair is the heterozygous.

There are over 30 different genes that combine to create your hair!

We can still use the Punnett Square to solve problems involving incomplete dominance.

The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other).

You will use 2 different letters in a monohybrid cross.

R = allele for red flowers W = allele for white flowers

RR = Red, WW = White and Rw = Pink

red x white ---> pink RR x WW ---> 100% RW

The trick is to recognize when you are dealing with a question involving incomplete dominance. There are two steps to this:

1) Notice that the offspring is showing a 3rd phenotype. The parents each have one, and the offspring are different from the parents.

2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.

1. A cross between a blue dragon & a white dragon produces offspring that are silver. The color of dragons is determined by just two alleles (monohybrid). a) What are the genotypes of the parent dragon in the original cross? b) What is/are the genotype(s) of the silver offspring? c) What would be the phenotypic ratios of offspring produced by two silver dragons?

2. The color of fruit for plant "X" is determined by two alleles (monohybrid). When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.

a) What are the genotypes of the parent orange-fruited plants?

This occurs when each of the alleles are equally expressed in a heterozygous offspring.

The "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.

With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.

Think… CO-OPERATION!

A very common example is a Roan cow.

This is when a red haired cow is crossed with a white haired cow to create a cow with both white and red hair.

The human blood type AB is a common example.

The red blood cells have the characteristics of both A and B blood.

Types A and B blood can actually have 2 different genotypes, depending on parent genetics.

Types O and AB only have 1 possible genotype.

When making a codominance punnet square, 2 letters must be used. 1 is the gene and then the other is the allele. The gene letter is a place holder for the allele. This just shows you that it’s codominance.

Example: FB or RT

We'll use "F" for the flower color allele. FR = allele for red flowers FW = allele for white flowers

red x white -------> red & white spotted flowers FRFR x FWFW ----> 100% FRFW

FR FR

FW FR FW FR FW FW FR FW FR FW

Blood Typing in Humans: A heterozygous type A blood is crossed with

a heterozygous type B blood. What is the resulting cross?

One parent is type A blood, while the other is type O blood. Show both possible crosses and outcomes.

1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

Epistasis is when the presence of one gene affects or eliminates the expression of another gene.

In this example, one gene controls whether the squash has colour or not and a separate gene controls what colour the squash will be if it is coloured. So the allele W=no colour will produce white sqaush. The allele w=colour, will allow the sqaush to have colour.

Therefore, no colour is dominant to colour.

If the recessive gene, w, is present then a separate gene controls whether the squash will be yellow or green.

The allele G=yellow and the allele g=green. Therefore yellow is dominant to green.

However if the dominant form of the color gene (the first gene) is present then it does not matter what allele is present for the second gene.

In mice and other mammals black coat is dominant to brown. However a second gene determines whether or not pigment will be deposited in the hair. D=deposit pigment, d=no pigment deposited. “D” will give the black or brown colour but a mouse with “dd” will be white (albino – no colour), regardless of the black or brown gene.

What happens if a black mouse that is

heterozygous for black and pigment genes mates with a mouse that is heterozygous for black and homozygous recessive for pigment genes? What phenotype ratio results?

Genes that cause an organism to die, only in the homozygous condition are called lethal or terminator alleles.

If the mutation is caused by a dominant lethal allele, the heterozygous offspring will live, while the homozygous dominant offspring will die in-utero or shortly after birth.

If the mutation is caused by a recessive lethal allele, the homozygote for the allele will have the lethal outcome.

Discovered first in mice. It was noticed that some mice lived when they had a yellow coat and some mice died shortly after birth if they had a yellow coat. All white coated mice lived.

All viable offspring are heterozygous.

Offspring with a homozygous dominant pairing will no develop in utero.

http://www.youtube.com/watch?v=Bn7eGRsYYGI

When completing lethal gene punnet squares, they are set up the same as normal monohybrids.

The key comes in looking at viable (living) offspring to determine if the gene is homozygous dominant linked or homozygous recessive linked.

Gerbils have a gene which can cause the gerbil to have white markings on his or her fur.

Gerbils who have no white markings are homozygous recessive for the solid color allele; gerbils which are marked with white patterns are heterozygous.

The homozygous white-marked genotype is lethal; these gerbils don't survive the development period, and are not among the babies delivered when a litter is born.

If two white spotted gerbils mate, figure out the prospects for their offspring.