iv. modifications to mendelian patterns

IV. Modifications to Mendelian Patterns

A. Intralocular Interactions

B. Interlocular Interactions:

C. Environmental Effects:

The environment can influence how an allele is expressed and the effect it has.


1. TEMPERATURE - Siamese cats and Himalayan rabbits – dark feet and

ears, where temps are slightly cooler. Their pigment enzymes function at cool temps.

- Arctic fox, hares – their pigment genes function at high temps and are responsible for a change in coat color in spring and fall, and a change back to white in fall and winter.


1. TEMPERATURE 2. TOXINS, ALLERGENS:

- people have genetically different sensitivities to different toxins. Certain genes are associated with higher rates of certain types of cancer, for example. However, they are not ‘deterministic’… their effects must be activated by some environmental variable.

PKU = phenylketonuria… genetic inability to convert phenylalanine to tyrosine. Phenylalanine can build up and is toxic to nerve cells. Single gene recessive disorder.

But if a homozygote recessive eats a diet low in phenylalanine, no negative consequences develop.

IV. Modifications to Mendelian Patterns

A. Intralocular Interactions

B. Interlocular Interactions:


D. The “Value” of an Allele:

1. There are obvious cases where genes are bad – lethal alleles

2. But there are also ‘conditional lethals’ that are only lethal under certain conditions – like temperature-sensitive lethals.

3. And for most genes, the relative value of one allele over another is determined by the relative effects of those genes in a particular environment.



Survivorship in U.S., sickle-cell anemia (incomplete dominance, one gene ‘bad’, two ‘worse’)

SS Ss ss



Survivorship in U.S., sickle-cell anemia Survivorship in tropical Africa (incomplete dominance, one gene ‘bad’, (one gene ‘good’, two ‘bad’)two ‘worse’)

SS Ss ss SS Ss ss




SS Ss ss SS Ss ss

Malaria is still the primary cause of death in tropical Africa (with AIDS). The malarial parasite can’t complete development in RBC’s with sickle cell hemoglobin… so one SC gene confers a resistance to malaria without the totally debilitating effects of sickle cell.




SS Ss ss SS Ss ss

As Darwin realized, selection will favor different organisms in different environments, causing populations to become genetically different over time.

V. Sex Determination and Sex Linkage

- Overview:

Mendel’s reciprocal crosses showed that the transmission of many traits was not influenced by the sex of the parent, nor the sex of the offspring. However, there are situations where this is NOT the case…


- Overview:

A. Some Questions About Sex…


- Overview:


1. Why sex?


- Overview:


1. Why sex?

- meiosis and sexual recombination during fertilization produces extraordinary variation which is adaptive in changing environments.


- Overview:


1. Why sex?


2. Why 2 sexes?


- Overview:


1. Why sex?


2. Why 2 sexes?

- There aren’t always 2 sexes…. In many species there are multiple “mating types” (fungi, for example).


- Overview:


1. Why sex?


2. Why 2 sexes?

- There aren’t always 2 sexes…. In many species there are multiple “mating types” (fungi, for example).

- Multiple sexes have an advantage: there are more potential mates available (with the only restriction being that organisms of the same mating type can’t mate).

2 sexes, equally represented: 50% chance of meeting opposite sex20 sexes, equal rep: 95% chance of meeting opposite sex

Advantageous for org’s with restricted mobility (fungi growing through soil).


- Overview:


1. Why sex?


2. Why 2 sexes?

- So, if multiple sexes is so great, why are most species 2-sexed?

2. Why 2 sexes?

- So, if multiple sexes is so great, why are most species 2-sexed? - It may have to do with ‘cytoplasmic wars’

2. Why 2 sexes?


When cells from different organisms contact one another, they can initiate a cellular ‘immune response’ – especially if they fuse. Proteins in the cytoplasm can be recognized as foreign and start a ‘cytoplasmic war’ between the cells.

2. Why 2 sexes?



A solution is for one cell to ‘unilaterally disarm’ and NOT donate cytoplasmic elements – just donate chromosomes.

2. Why 2 sexes?



A solution is for one cell to ‘unilaterally disarm’ and NOT donate cytoplasmic elements – just donate chromosomes.

With multiple sexes, how are these decisions made? Consider a simple hierarchy:


SEX 1 – NEVER DISARMS

SEX 2 – Disarms for 1, not for 3-5.

SEX 3 – Disarms for 1 and 2, not for 4-5.

SEX 4 – Disarms for 1-3, not for 5.

SEX 5 – ALWAYS DISARMS




SEX 3 – Disarms for 1 and 2, not for 4-5.

SEX 4 – Disarms for 1-3, not for 5.


Sexes 2-4 have to make a choice; and we should expect some frequency of errors (because nothing is perfect). So, matings involving sexes 2-4 will have a lower frequency of successful fertilization than those involving 1 and 2.




SEX 3 – Disarms for 1 and 2, not 4-5.

SEX 4 – Disarms for 1-3, not 5.



What do we call differential reproductive success?








What do we call differential reproductive success? Riiiight…..Selection









So what happens to the population?









So what happens to the population? Right…. The population becomes dominated by two sexes; one that never disarms and always donates the cytoplasm (female and egg), and one that always disarms and gives nothing but chromosomes (male, sperm).


- Overview:


B. Sex Determination


- Overview:



1. Environmental:

- Temperature dependent sex determination in crocodilians, turtles, and some lizards:

1. Environmental:


How? – May involve temperature dependent enzymes (like aromatase) that convert testosterone to estrogen. Change in activity with temperature, like the genes for coat color in himalayan rabbits and arctic fox (also reverse effects there).

1. Environmental:


How? – May involve temperature dependent enzymes (like aromatase) that convert testosterone to estrogen. Change in acxtivty with temperature, like the genes for coat color in himalayan rabbits and arctic fox (also reverse effects there).

Why? … when you see a non-random characteristic in organisms, what’s your hypothesis?

1. Environmental:


How? – May involve temperature dependent enzymes (like aromatase) that convert testosterone to estrogen. Change in acxtivty with temperature, like the genes for coat color in himalayan rabbits and arctic fox (also reverse effects there).

Why? … when you see a non-random characteristic in organisms, what’s your hypothesis? …. Riiiight….selection. So why might it be adaptive, in terms of reproductive success?

1. Environmental:


Why?

Crocs, turtles, and some lizards have a ‘polygynous’ mating system….

1. Environmental:


Why?

Crocs, turtles, and some lizards have a ‘polygynous’ mating system…. One big male holds a territory and acquires and mates with most of the females in an area.

1. Environmental:


Why?

Crocs, turtles, and some lizards have a ‘polygynous’ mating system…. One big male holds a territory and acquires and mates with most of the females in an area.

SO! Daughters will probably mate, but only the rare son, who can acquire a harem, will mate. Daughters are a safe reproductive investment; sons are riskier, but with a potentially bigger reproductive payoff.

Why? SO! A young female turtle digs a shallow nest – its warm – most her eggs develop as daughters.

MT FT

Why? SO! A young female turtle digs a shallow nest – its warm – most her eggs develop as daughters.

MT FT

This is adaptive, as most of her daughters will mate… she has gauranteed her reproductive success by making a safe investment early in life…

Why? SO! As she ages, she grows larger, and digs a deeper nest with a higher fraction of males

MT FT

This is ALSO adaptive. Her daughters are also reproducing her genes. In fact, cumulatively, several reproducing daughters would produced more of her genes than she would each year!

With her reproductive security assured, making males is low risk (if they don’t mate, no biggie), but it could pay off BIG (if they become a dominant male and mate ALOT.)

Why? SO! As she ages, she grows larger, and digs a deeper nest with a higher fraction of males

MT FT

So, in their mating system, temperature dependent sex determination may be adaptive.


- Overview:



1. Environmental:

- Temperature dependent:

2. Developmental:

Hermaphrodites have both sex organs, but all their cells are the same genetically. The key is in differential gene activation in different tissues; just like tissue specialization for other tissue types.


- Overview:



1. Environmental:2. Developmental:3. Chromosomal:

Sex correlates with a particular complement of chromosomes; suggesting that the genes that govern sexual development are all on this chromosome.


- Overview:



1. Environmental:2. Developmental:3. Chromosomal:

Sex correlates with a particular complement of chromosomes; suggesting that the genes that govern sexual development are all on this chromosome.

NOTE that this is NOT equivalent to ‘genetic’ sex determination. In all cases presented above, sex determination is influenced by many genes; just that in some organisms the action of those genes is affected by temperature, or proteins/chemicals produced elsewhere in the organism, and the genes are not all concentrated on one chromosome.

3. Chromosomal:

You are familiar with the ‘X – Y’ system, but there are several:

a. Protenor sex determination: Sexes differ in chromosome number

Order: Hemiptera “True Bugs”Family Alydidae – Broad-headed bugs

3. Chromosomal:


a. Protenor sex determination: Sexes differ in chromosome number b. Lygaeus sex determination: Sexes have different types of sex

chromosomes – heterogametic and homogametic sexes… (Fowl: female=ZW, Male=ZZ)

Order: Hemiptera Family: Lygaeidae “Chinch/Seed Bugs”

3. Chromosomal:



chromosomes – heterogametic and homogametic sexes… (Fowl: female=ZW, Male=ZZ)

- Sex Determination in Humans:


Lygaeus sex determination, whereby the presence of the Y determines maleness.

transcribed

Not transcribed



SRY – codes for a product called the ‘testis determining factor’ – triggers undifferentiated gonad to become testis.



Evidence:

Some XY individuals lack the SRY region, or have a mutation in it, and they are phenotypically female.

Some XX individuals have an sry that has been transposed, and they are phenotypically male.

Experimental insertion of sry-homologs in mice stimulates XX embryos to become male.

SRY – codes for a product called the ‘testis determining factor’ – triggers undifferentiated gonad to become testis.

3. Chromosomal:



chromosomes – heterogametic and homogametic sexes… (Fowl: female=ZW, Male=ZZ) c. Balanced sex determination (Drosophila): The ratio of autosomal sets to X

chromosomes determines the sex:

3. Chromosomal:



chromosomes – heterogametic and homogametic sexes… (Fowl: female=ZW, Male=ZZ) c. Balanced sex determination (Drosophila): The ratio of autosomal sets to X

chromosomes determines the sex:

Governed by several genes on autosomes that are activated differently, and their transcripts are spliced differently, depending on the ratio of X/autosomal sets…suggesting there is another x-linked gene that might work in a dosage dependent way.


- Overview:



C. Sex Linkage


- Overview:



C. Sex Linkage

Where sex is determined chromosomally, there are obviously going to be correlations between sex (a phenotypic trait determined by those sex chromosomes) and OTHER TRAITS governed by OTHER GENES on those SEX CHROMOSOMES. So SEX LINKAGE is an example of a broader phenomenon of ‘linkage’ – patterns of correlated inheritance between traits governed by genes on the same chromosome. In this case, the correlation is between sex and other traits governed by the sex chromosomes.

C. Sex Linkage

Sex linkage was first described by Thomas Hunt Morgan at Columbia in 1920’s.

Found a novel white-eyed male in his culture. Mated it with a red-eyed female, and all flies were red eyed, as expected. Then did the f1 x F1 cross, and got a 3:1 ratio, as expected. However, all white eyed flies were MALE.

C. Sex Linkage


Found a novel white-eyed male in his culture. Mated it with a red-eyed female, and all flies were red eyed, as expected. Then did the f1 x F1 cross, and got a 3:1 ratio, as expected. However, all white eyed flies were MALE.

By crossing the F1 females (heterozygotes) with white males, he produced some white females that he could use in a reciprocal cross; which revealed a different pattern, dependent on the sex of the white eyed fly.

C. Sex Linkage


C. Sex Linkage

Human x-linked traits are hemophilia and red-green colorblindness, among others. Genes on the y are also sex-linked (holandric), but those with phenotypic effects other than ‘maleness’ are rare.


- Overview:



C. Sex Linkage

D. Dosage Compensation


With chromosomal sex determination, the sexes have a different complement of sex chromosomes. Consider X-Y Lygaeus determination; males have 1 ‘X’ and females have 2.



One might expect that females would produce TWICE the amount protein products from x-linked genes.



One might expect that females would produce TWICE the amount protein products from x-linked genes.

But for many proteins (enzymes), correct concentration (dosage) is critical to function. So, we see different methods whereby this initial difference in dosage is corrected – or ‘compensated’ for….


In mammals – all but one X in each cell is “turned off” in females. So, in normal females, one X is active and one is inactivated. In 47, XXX individuals, 2 X’s are off in each cell. In 47, XXY males, one X is turned off.


In mammals – all but one X in each cell is “turned off” in females.So, in normal females, one X is active and one is inactivated. In 47, XXX individuals, 2 X’s are off in each cell. In 47, XXY males, one X is turned off. Happens at different points in development for different tissues… and inactivation is then inherited by daughter cells.

- The inactive X is seen as a condensed mass on the periphery of the nucleus – a Barr Body

- Heterozygous females can be a ‘mosaic’ – exhibiting one phenotype in some cells/tissues/body regions (governed by 1 X) and another phenotype in another region (governed by the other X). Calico and Tortoiseshell female

cats.

iv. modifications to mendelian patterns

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