Biology 30 Genetics Beyond Mendel
http://www.bozemanscience.com/science-videos/2012/5/6/chromosomal-genetics.html
Incomplete Dominance
· In some cases, an intermediate phenotype is shown
· Neither allele is dominant
· In snapdragons, flower color can be red, pink, or white. The heterozygous condition results in pink flowers (or an intermediate trait)
· A white snapdragon crossed with a red snapdragon produces all pink offspring
· Two pinks crossed together produce 1/4 white, 2/4 pink, and 1/4 red
· When dealing with incomplete dominance and codominance it does not matter what letter you use, as long as the heterozygous condition always denotes the intermediate trait. In the diagram R is used, but you could also use W or even P. Ww = pink, Pp = pink if these letters are used.
· Sickle cell disease is incompletely dominant in humans. AA x aa = Aa (sickle cell trait), where some blood cells will have abnormal shapes
Codominance
· Both alleles can be expressed
· For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.
· This phenotype might seem to support the blending theory. (The blending theory predicts pink F1 progeny.)
· The F2 progeny, however, demonstrate Mendelian genetics. When the F1 roan individuals self-fertilize, the F2 progeny have a phenotypic ratio of 1 red:2 roan:1 white.
· This mode of inheritance is called incomplete dominance.
· The phenotypic outcomes for cow color and incomplete dominance in general can be explained biochemically.
· One allele of the gene codes for an enzyme that functions in the production of the red color. The other allele codes for the gene to make white color. If both alleles are present, both are expressed, resulting in a cow that has some red and some white.
· Mendel's laws are not compromised here, he just happened to find in peas examples of complete dominance only.
Blood Types - Multiple Alleles and Codominance
· In humans, there are four blood types (phenotypes): A, B, AB, and O
· Blood type is controlled by three alleles. A, B, O
· O is recessive, two O alleles must be present for the person to have type O blood
· A and B are codominant. If a person receives an A allele and a B allele, their blood type is type AB
· Crosses involving blood type often use an I to denote the alleles - see chart.
· When doing blood type crosses, you will need to know whether at type A or B person is heterozygous or homozygous. Type O's are automatically OO and type AB is automatically AB. Crosses are performed the same as any other.
· The blood type determines what antibodies are located within the blood. Type A blood has type B antibodies. If type B blood is put into their bodies, their immune system reacts as if it were a foreign invader, the antibodies clump the blood - can cause death.
· Type AB blood has no antibodies, any blood can be donated to them - they are called the "universal acceptors"
· Type O blood has no surface markers on it, antibodies in the blood do not react to type O blood, they are called the "universal donors"
Many Genes Have Multiple Alleles
· A population might have more than two alleles for a given gene.
· In labrador retriever, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles.
· Even if more than two alleles exist in a population, any given individual can have no more than two of them: one from the mother and one from the father.
Labrador Retriever Genetics
Black is dominant to chocolate B or b
Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e
/ BBEE
BbEE
BBEe
BbEe
/ bbEE
bbEe
/ BBee
Bbee
bbee
Polygenic Traits
· Polygenes mediate quantitative inheritance
· Individual heritable characters are often found to be controlled by groups of several genes, called polygenes.
· Each allele intensifies or diminishes the phenotype.
· Variation is continuous or quantitative (adding up) - also called quantitative inheritance
· Seed Color in wheat - aabbcc, Aabbcc, AaBbcc, AaBbCc, AABbCc, AABBCC (light, intermediate colors, dark)
· In humans - hair color, height, skin color
Environment and Phenotype
· Temperature, water, food sources can have an affect on how a gene is expressed
· Siamese cats have a gene that codes for darker pigments - this gene is more active at low temperatures. Parts of the body that are colder will develop the darker pigmentation - ears, feet tail of the siamese cats
Chromosomal Inheritance
Chromosomal Theory of Inheritance: simply states that chromosomes are carriers of genetic information (Walter Sutton)
Every organism has sex chromosomes (that determine sex) and the rest of the chromosomes are called autosomes
Humans have 22 pairs of autosomes, 1 pair of sex chromosomes (see karyotype)
Fruit flies have 3 pairs of autosomes, 1 pair of sex chromosomes
Sex Linked Genes
· Some genes are located on the X chromosome. Females receive two alleles for these genes, but males only receive one.
· When doing a punnet square, use large X's and Y's to denote male and female, use superscript letters to designate the alleles
· If you are trying to determine if an allele is sex-linked, and which is dominant, a reciprocal cross is performed
White eyed male x red eyed female | Red eyed male x white eyed female
XrY x XRXR | XRY x XrXr
Human X-Linked Disorders
Colorblindness
· Red-green colorblindness makes it difficult for the person to discern the two colors (test yourself!)
· If the parent is a male, the genotype is automatically known. A colorblind male has to be b, since he only has one allele and colorblindness is recessive. A normal male must then be B
· Females can be heterozygous for the colorblindness trait - they are called carriers. A female can be BB - normal, Bb - carrier, or bb - colorblind
· The following shows a cross between a normal man and a woman who is a carrier.
Muscular Dystrophy
· characterized by the wasting away of muscles
· life expectancy: ~20 yrs
· females can be carriers for the disease, where they pass the disease to their sons only
Hemophilia
· also known as "bleeder's disease"
· blood does not clot properly
· disease was present in the royal family, starting with Queen Victoria
Fragile X Syndrome
· caused by triplet repeats in a gene on the X chromosome
· causes mental retardation
· named because the X chromosome had an odd appearance - the tip of the chromosome seemed to be attached only by a small thread
· the number of repeats of the gene determines the severity of the disease
Problems:1. What is the chance that a woman with hemophilia will have a child with hemophilia? (What sex would the child need to be?)
2. In a cross where a brown-haired female is crossed with a black-haired male, all the male offspring have brown hair and all the female offspring have black hair. What must be the genotypes of the parents? Which allele is dominant?
3. In drosophila (fruit fly), the allele for eye color is located on the X chromosome, where red eyes is dominant to white eyes. Show the cross of a heterozygous red-eyed female and a white eyed male.
4. A man who is colorblind marries a woman whose father is colorblind. What is the chance that they will have colorblind children?
Linkage Groups and Chromosome Maps
New alleles arise by mutation
· Different alleles exist because any gene is subject to mutation, or change, to a stable, heritable new form
· Alleles can randomly mutate to become a different allele depending on DNA sequence changes.
· Wild type is a term used for the most common allele in the population.
· Other alleles, often called mutant alleles, may produce a phenotype different from that of the wild-type allele.
· An alternate form of designating alleles. Alleles that are wild type are expressed with a +
· Ex. Red eye color (w+) is dominant to white eye color (w). The red eye is the wild type. Don't let this confuse you, its just a different way to express alleles.
Linkage Groups
So far, your studies and practice with crosses have involved alleles located on separate chromosomes, and crosses follow Mendel's Law of Independent Assortment
In actuality, many genes are located on the same chromosome, and they do not assort independently, instead, they are inherited together, they won't follow the normal rules of punnet squares, and the ratios obtained from crosses do not have the normal ratios. Consider the following chromosome map of the fruit fly: All the alleles are located on chromosome 2 of the fruit fly, and are inherited together.
When performing crosses with linkage groups, I find it best to draw a little picture of the chromosomes to show how they are inherited.
Example: A fly that is heterozygous for long wings (Ll) and heteroyzygous for long aristae (Aa) is crossed with another fly of the same type. AaLl x AaLl. In both cases the dominant allele is located on the same chromosome.
Before you set about making a 4x4 square, you need to consider the linkage groups. Sketch them!
The results of the cross would change considering the arrangment of alleles. Show the cross that would occur if the dominant alleles were on DIFFERENT chromosomes. (See answer)
The expected ratios are not always correct, because remember also that during meiosis, chromosomes overlap and exchange genes. In the case above, you may get an "odd" long wings, short aristae because the genes were exchange during meiosis. Confusing, huh?
How Chromosome Maps Are Determined
Speaking of crossing over, it is this frequency (the odds of the genes exchanging during meiosis) that determines how far apart the alleles are on a chromosome. Alleles that are farther apart, like the aristae allele and the brown eye color allele are more likely to exchange than one closer together, like the aristae allele and the long wing allele. In other words, alleles that are close together tend to stick together.
That being said, biologists use the percent of crossing over to determine the locus of alleles on a chromosome. The distance between alleles is measures in MAP UNITS, or MU. On the diagram above the long wing allele is 13MU from the aristae allele. The image above is a linkage map because it shows the distance between the alleles
The map unit = the percent of time the allele is known to crossover. Ex. the antenna allele and wing alleles will exchange 13 % of the time.
5 Practice Questions (assume no crossing over occurs)
1. A dumpy winged (ww) fruit fly with long aristae (AA) is crossed with a long winged (Ww) short aristae (aa). Show the cross and the phenotypic proportions.
2. A fruit fly with short legs (ll) and vestigial wings (ww) is crossed with one that is heterozygous for both traits. Assuming the dominant alleles are on separate chromosomes, show the cross and the expected phenotypic proportions.
3. A fruit fly with short legs (ll) and long aristae (Aa) is crossed with on that has long legs (Ll) and long aristae (AA). Show the cross and the expected phenotypic proportions.
4. Two fruit flies that are both heterozygous for the dumpy wing and short leg traits (WwLl) are crossed together. The resulting offspring are counted. 20 of the offspring have long wings and long legs, 8 of the offspring have dumpy wings and short legs. Show a chromosome map of the two parents, denoting which alleles are located on the chromosomes.
5. In fruit flies, red eyes is a dominant allele located on the X chromosome. The recessive condition results in white eyes. The tan body trait is also X-linked and is dominant to yellow bodies. A female who is heterozygous both traits with the dominant alleles located on the same chromosome is crossed with a white eyed, yellow bodied male. Show the cross and the phenotypic proportions (Don't forget these traits are X-linked!)
6. In two sweet pea strains, B = blue flowers, b = red flowers. L = long pollen grains, l = round pollen grains. In a cross between a heterozygous plant and a plant that has red flowers and round pollen, 44% of the offspring are blue, long; 44% are red, round; 6% blue, round; 6% are red, long. How many map units separate these two alleles?
Lethal Genes
· Some genes are lethal when both alleles are present. Lethality can occur before or after birth
· Huntington's disease in humans is caused by a lethal allele, death occurs later in life
· Other examples: Mouse coat color (yellow), Creeper legs in chickens, Manx Cats (no tails)
An example is the "creeper" allele in chickens, which causes the legs to be short and stunted.
· Creeper is a dominant gene, heteroyzous chickens display the creeper phenotype
· If two creeper chickens are crossed, one would expect to have (from mendelian genetics) 3/4 of the offspring to be creeper and 1/4 to be normal
· Instead the ratio obtained is 2/3 creeper and 1/3 normal.
· This occurs because homozygous creeper chickens die.
Manx Cats
Cats possess a gene for producing a tail. The tailless Manx phenotype in cats is produced by an allele that is lethal in its homozygous state. The allele interferes with normal spinal development, in heterozygous cats this results in lack of a tail.