Genetics can be fun (Chs 14 and 15)

Principles of Biology II,M. MarshallShippensburg University Biology Dept.Fall 2011

Figure 14.0x Mendel

Figure 14.1 A genetic cross

Figure 14.2 Mendel tracked heritable characters for three generations

Figure 14.x1 Sweet pea flowers

Figure 14.3 Alleles, alternative versions of a gene

Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

Figure 14.x2 Round and wrinkled peas

Figure 14.4 Mendel’s law of segregation (Layer 2)

Figure 14.5 Genotype versus phenotype

Figure 14.6 A testcross

Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

Segregation of homologs at Metaphase One determines the allele combinations of

gametes when the genes for the traits of interest are on separate chromosomes

Figure 14.8 Segregation of alleles and fertilization as chance events

We discussed this topic extensively in lab. Look over the lab hand-out sheet to review this topic, including the product and sum rules.

Non-Mendelian trait type 1., Incomplete dominance in snapdragon color

Non-Mendelian traits are any whose inheritance pattern does not result in standard Mendelian ratios in the F1 and F2

generations.

Both Mendelian and non-Mendelian traits can and do exist within the same species.

Incomplete dominance in carnations

Non-Mendelian trait type 2., Multiple alleles for the ABO blood groups

Human blood type differences involve different glycoprotein (with some glycolipid) types on the red blood cell membrane.

Technically type “O” is NOT the complete absence of such material, but the lack of the specific galactose sugars that constitute the type A & B molecules. The O gene cannot code for the glycosyltransferase enzyme to add these sugars, while the A and B genes code for different enzyme specificities.

ABO blood types as detected by the use of anti - A and anti - B protein antisera.

People who have never been exposed to other blood types still may have anti-A or Anti-B antibodies in their blood due to the fact that similar sugars are found elsewhere in nature, on food, on bacteria, etc. and these are similar enough that most people naturally have cross-reacting antibodies.

A transfusion mismatch, if it occurs, is likely to be mild on the first exposure. Subsequent mistakes, however can be life threatening as the first is likely to greatly increase the antibody concentration (titer).

The RH factor (+ / -) is a different gene altogether and involves a surface protein

Non-Mendelian trait type 3., Epistasis, where one gene modifies the expression of another.

In this example:

B = Black, which is dominantb = brown, which is recessive

But neither color can be expressed at all in the absence of at least one C allele:

C = color conferred by B gene is expressedc = color conferred by B gene is not expressed

The individual gene alleles actually operate in a Mendelian fashion, but their interaction makes this difficult to see at first.

Non-Mendelian trait type 4., Polygenic inheritance of skin color

Many traits in nature are expressed as a result of several to many genes working together.

This can create difficulties when the trait involved is one that we desire to modify or manipulate. Resistance to disease organisms works this way in many plant species.

Multi-gene inheritance, a more detailed view

Again, the alleles of the individual genes “behave” in a Mendelian fashion, but the fact that they all affect the same phenotype makes this difficult to see.

Non-Mendelian trait type 5., Environmentally variable phenotype

Hydrangeas produce blue-violet flowers when the soil pH is acidic, as this allows Al to be taken up from the soil. Alkaline pH is obviously not acidic, and it prevents Al uptake and the flowers have a pink coloration

Anthocyanins are flavonoid pigments that change color with pH and the ions that they are complexed with, as shown by this in-vitro demo done with rose pigments..See: http://www.demochem.de/p26_anth-e.htm

Non-Mendelian trait type 6., Pleiotropic effects of the sickle-cell allele.

Homozygous individuals suffer from sickle cell RBC damage, but heterozygous individuals have a milder form of the disease, AND are more resistant to the malaria parasite which completes much of its life cycle within the human host within the red blood cell. So in regions where malaria is endemic the sickle cell trait confers an advantage.

Pedigree analysis

Large families provide excellent case studies of human genetics

Dr Nancy Wexlar pursued her quest to map the location of the gene for Huntington's chorea using pedigree analysis connected to DNA finger printing. Huntington’s disease has some similarities with muscular dystrophy in that it involves a deterioration of muscle control, in this case due to neurodegeneration. It is different, however, in that it is an autosomal dominant trait (most mutations are recessive) that usually is not detected until the person is in their 30s, in many cases after they have had children of their own. Wexler’s work involved analyzing a large effected population living in villages on Lake Maracaibo, Venezuala, and met with success in 1983. since then the gene has been located and characterized, although the disease is still not totally understood.

Figure 14.17 Testing a fetus for genetic disorders

Chromosomal structural effects. Some traits have their inheritance influenced by

Figure 15.1 The chomosomal basis of Mendel’s laws

Non-Mendelian trait type 7: Sex-linked inheritance

T.H. Morgan was a geneticist who pioneered the use of fruit flies as a genetic “model system” which could be easily cared for and would rapidly breed in the lab One of the first mutations thathe discovered was for eye color.

This is a perfectly good example of how a sex-linked trait is inherited, but it involves fruit flies (Drosophila) and Drosophila genetics has its own (complicated ) convention for labeling alleles.

So lets look at a more straight-forward example – human color blindness.

Morgan’s first mutant was in a sex-linked trait – eye color

T.H. Morgan in his lab at Columbia U., circa 1910.

Sex-linked inheritance, another “view:” Color Blindness

The XN allele confers normal color vision, the Xn allele confers abnormal color vision

XNXN and XNXn women have normal vision; XnXn women and XnY men do not.

An XN Y man would be normal

A “carrier”

Color blind

The transmission of sex-linked recessive traits – Your textbook’s version

Color vision test

- - means you’d see nothing in particular; no obvious number

Sex-linked inheritance patterns

Non-Mendelian trait type 8: Linkage - Evidence for linked genes in Drosophila

You should get a 1:1:1:1 ratio from a standard test cross, but in this case you do not.

How can you explain the relatively small number of “recombinant” phenotypes?

Drosophila testcross

Linkage: Linkage with Recombination due to crossing over can explain the seemingly “odd” ratio.

Recombinant numbers are small, as the chance of it occurring between these two loci is small. The closer together the loci are, the lower the recombinant numbers will be.

Linkage: Recombination due to crossing over

Crossing over at any given location is a rare event. The only crossovers that will be detected as recombinant progeny are those occurring between the two loci involved. The higher the recombinant numbers, the greater the distance between the loci . The number of recombinants seen are a composite (a sum total) of many different cross-overs that all occurred at some point between the two loci.

Linkage: Using recombination frequencies to construct a genetic map

If the cross-over frequency between locus b and vg is as shown, and that between b and cn and cn and vg (calculated as total recombinants / total progeny *100) are also as shown (as arrived at from data from the three crosses involved), then the relative positions of the loci MUST be as shown also. The % can be converted to “map units” which imply no real physical distance, but do accurately indicate relative positioning and relative spacing.

Linkage: A partial genetic map of a Drosophila chromosome

Long before DNA sequencing was possible, loci positions for hundreds of traits were worked out by painstakingly doing crosses involving linked genes. This was done for many of the model systems used by geneticists, fruit flies, certain fungi, corn, tomatoes, etc.

Aberrations from the expected results also indicated that certain DNA sequences could actually move their location over time. This was first discovered in corn. Today these “jumping genes” are known as mobile or transposable elements, similar to the PV 92 Alu sequence that we used in lab.

Figure 15.x1 Translocation

X inactivation and the tortoiseshell cat

Because female cells have two X chromosomes, one is inactivated through condensation and its genes are not available for transcription. As the locus for “tortoise shell” coat color is located on the X, and different skin cells differ in which X is inactivated, a mottled appearance can result if the cat is heterozygous.

The Calico cat also has white areas where neither XO nor XB are expressed.

Figure 15.11 Meiotic nondisjunction

Figure 15.14 Down syndrome

Alterations of chromosome structure occur during DNA replication.

Genomic imprinting

The alleles of certain traits can be silenced if they come from one sex or the other. In a given species only certain genes behave this way, but the behavior is fairly uniform with certain genes being silenced only if they come from the male or female on a case by case basis. So these traits behave almost as if they were an example of a sex-linked inactivation, but they are almost always found on autosomal (non-sex)chromosomes. So unlike standard mendelian traits that are inherited the same in reciprocal crosses (where male and female are switched) imprinted gene traits would not be. The imprinting effect is “erased” in the next go-round of gamete production.

Cytoplasmic inheritance in tomato leaves

The small amount of DNA in mitochondria and chloroplasts can contain genes that code for detectable traits; these are inherited through the maternal line only as the progeny organelles come from the egg.