In unit IV, we looked at Modifications to Mendelian patterns.
These were phenotypic patterns. We saw that the way
genotypes were translated into phenotypes was much more complicated
than simple Mendelian dominance. In this unit,
we will see that there are even deviations from the basic principles
of segregation and independent assortment - the
basic principles of heredity, themselves.
A. Sex Determination
- Although sex is determined by particular chromosomes in humans, this is not the typical case in life forms on Earth. Before we look at the consequences of chromosomally determined inheritance, lets put it in perspective by looking at other ways that sex is determined.
1. Environmental
a. temperature (turtles, croc's) (warm eggs develop into daughters) Why? Reproductive success. Young females "invest" in daughters to guarantee their reproductive success (all daughters will mate...). Older females, having a secure reproductive investment of daughters already, can now try and "win the lottery" by taking a chance and producing male offspring. Most won't mate, but those that do will mate a lot (big reproductive payoff).
b. nutrients/size (large = females) Jack in the Pulpit Why? Reproductive success. It is cheap, energetically, to be a successful male; just the cost of sperm. It is more expensive, energetically, to be a female. Big gametes are just the beginning; then you have to nurture the embryo and invest in additional tissue, like fruit. So, if you are small, you can still be successful as a male. But, you can't be as successful if you are female and small. So, why be female when you are big? Well the first reason is a probabilistic argument.... if everyone else is male and you are the only female, then you will mate and all of the offspring in the population will be yours (HUGE relative reproductive success). A second reason is because you can adjust the QUALITY of your offspring. You can decide to invest in all of them, or can invest in fewer and give them each more energy. THis give them a better chance of survival, which increases the likelyhood that your genes (that are in these offspring), get passed on. So, if you are a female plant and you have received pollen from many males, you can abort the embryos that are doing poorly and only invest energy in embryos that are doing well. This increases your reproductive success. And, this doesn't happen only in plants. White-tailed deer have been shown to selectively abort male embryos in years where resources are in short supply. How can we understand this? Well, it is understandable in light of natural selection. Small male deer won't mate - they won't acquire a harem. Small does will mate. So, as a pregnant female with few resources should abort a male embryo that will be unlikely to mate in the futre, while she should bring a female offspring to term (as it WILL mate and thus contribute to her reproductive success.)
c. Hormones Some schooling fish have a dominant female that secretes a hormone that keeps the other fish male. When she dies, one of the other fish develops into a female and begins to produce that hormone, keeping the other fish as male. Why? reproductive success. By being the only female in the population, she garauntees that all the offspring will be hers... so she has the greatest reproductive success of any individual in the population (greatest selective advantage) and her genes for keeping other fish male will be passed to all the offspring.
This highlights an important point about selection: SELECTION (AND EVOLUTION) DO NOT NECESSARILY ACT "FOR THE GOOD OF THE SPECIES". Selection maximizes the immediate reproductive success among individuals. Curiously, what's good for an individual might be bad for the long-term success of the population as a whole. For instance, having only 1 female in the population REALLY inhibits the growth rate of the population. The population will be alot smaller than if you had many females all producing offspring. And, smaller populations are closer to extinction, so any random chance might bounce the population size to zero. But, selection FAVORS females that can inhibit the female development of their peers. (Like it favors male deer that can inhibit the mating of their male peers). So it happens, even if it is "bad" for the species in the long run. If an individual (male OR female) can usurp all of the mating (or a disproportionate share of the mating) in a population for themselves, then they will reproduce more than other members of the populations and these genes for usurping control will increase in frequency. So, in harem forming species, males that keep other males from mating have the most offspring, and the male offspring inherit a belligerent attitude that stimulates then to fight other males and try to keep them from mating. You see, it is not necessarily the TOTAL number of offspring that selection favors. It is the number of offspring produced REALTIVE TO other members of the population that selection favors. So, if a male can keep other males from mating, even if he only has 1 offsrping, he will still be doing better than the other males in the population and will be at a selective advantage (his genes will increase in frequency relative to theirs).
b. Lygaeus (another bug) - two sex chromosomes:
- In some species, the homogametic sex is male (WW) whereas the heterogametic sex(ZW) is female.... fowl.
- In humans and most mammals, the heterogametic sex is male and the homogametic sex if female. Have a "y", then develop into a male. Even if XXY. At least, usually. The sex determining region of the y (SRY gene) is only a part of the 'y' chromosome. And, sometimes it is cut off and attached to the X during male sperm production. So, this man produces "y" sperm that LACK the sry, and "X" sperm that have the sry. (This only occurs in the couple sperm where this mutation occurred; it would not occur in all this man's sperm. So, this is VERY rare. This sry "X" chromosome, when paired with the X of the mother, would produce a zygote that is XX but has the sry and would develop into a male child.
Obviously, Y-linked genes ONLY occur in males. However, these are few in number because the Y chromosome has very few genes (but it does have the all important "sry"...) However, for X-linked genes, the following patterns occurs. Dominant traits are more common in females, because they have 2 X chromosomes and therefore have twice the "chance" of receiving an X with a dominant allele on it. The obverse is also true: recessive traits are more frequently expressed in MALES. Since males only get one X, if they get a recessive allele on that X, it will necessarily be expressed (it's all they have). If a female receives a recessive gene on an X, they have the chance of still receiving a dominant gene on the other X, and therefore NOT express the recessive trait. So, deleterious recessives like hemophilia and red-green colorblindness are more common in males than females.
Study Questions
1. How does the particular type of sex determination in turtles,
by temperature, maximize reproductive success in
a harem-forming species?
2. Explain why females are often larger than males, even in species in which individuals change sex.
3. Consider this cross: AaXBY x AaXBXb
- How many types of gametes can the male make with respect to these loci? (answer: 4)
- How many phenotypes are possible in the progeny (Assume IA and complete dominance for both loci).
Answer: 2 at "A", 3 at "B" (B females, B males, and b males) = 6.
- What fraction of male offspring will express the Ab phenotype? 3/4 A x 1/2 b. (only 1/2 the SONS) = 3/8