Sex-Related Topics A grab bag of subjects, vaguely related to the typical eukaryotic condition of having 2 sexes
Sex Determination • Many groups use sex chromosomes to determine sex. Mammals have the X and Y chromosomes • XX = female, XY = male • All other chromosomes are called “autosomes” Thus, humans have 46 chromosomes, 44 autosomes plus 2 sex chromosomes.
X and Y Chromosomes • The X has many genes on it, just like the autosomes. Most of the genes on the X have nothing to do with sex. • The Y has very few genes on it. It consists of mostly inactive DNA. • One gene on the Y is very important: SRY. The SRY gene is the primary determinant of sex. • If SRY is present, testes develop in the early embryo. The testes secrete the hormone testosterone, which causes development as a male. • If SRY is absent (no Y chromosome), ovaries develop instead of testes, and the embryo develops into a female. • The X and Y chromosomes share a common region at their tips, the pseudoautosomal region. Crossing over in meiosis occurs in this region.
Sex Determination in Birds • Birds use a system of sex chromosomes very similar to mammals. The bird sex chromosomes are called Z and W. • Big difference from mammals: in birds, a ZZ individual is male, and a ZW individual is female. • We can define some terms: “homogametic” means having both sex chromosomes the same, like female (XX) mammals and male (ZZ) birds. “Heterogametic” means having different sex chromosomes, like male (XY) mammals and female (ZW) birds.
Sex Determination in Drosophila • Drosophila also have X and Y chromosomes, with XX female and XY male. • However, Drosophila don’t use the SRY gene to determine sex. Instead, they use the ratio of X’s to sets of autosomes. • 1 X plus 2 sets of autosomes is a normal diploid male. • 2 X’s plus 2 sets of autosomes is a diploid female. • The difference between sex determination mechanisms comes in the odd cases: --an XXY individual has a Y, so is a male mammal. However, 2 X’s plus 2 sets of autosomes makes it a female Drosophila. ---an XO individual (i.e. only 1 X, no other sex chromosomes, but otherwise diploid) is a female mammal (no Y) but a male Drosophila (1 X plus 2 sets of autosomes).
Other Mechanisms • Hymenopterans (wasps, bees, ants) are mostly female. Females are diploid, and males are haploid. Thus, a virgin female can lay unfertilized eggs that will hatch into males that can then fertilize her to produce more females. • Nematodes (roundworms) have a single sex chromosome, the X. An XX individual is female, but an XO (only 1 X) is a hermaphrodite, an individual with both male and female sex organs. No true males exist.
More Mechanisms • Some species have environmentally determined sex. Among reptiles, the temperature at which the eggs develop determines the sex. For example, in the turtles, eggs incubated at 30oC become female, while those incubated at lower temperatures become male. • Some species have both sexes on the same individual: this is very common among the angiosperms (flowering plants), where 90% of the species have hermaphroditic flowers, and many of the rest have separate male and female flowers on the same plant. A few plants (e.g. date palm and holly) have separate male and female plants.
Sex Linkage • Genes that are sex-linked are on the X chromosome. Genes on the Y are NOT sex-linked; they are called “holandric” instead. • Because males (mammals, that is) have only 1 X, any gene on the X in a male is expressed, whether dominant or recessive. In contrast, females have 2 X’s, so recessive traits are often covered up by the dominant normal (wild type) allele. In most cases, genetic diseases are recessive. Thus, most sex-linked genetic diseases are much more common in males than in females. • having only 1 copy of a gene is called “hemizygous”; sex-linked genes in male mammals are hemizygous. That is, it is not possible for these genes to be either homozygous or heterozygous, since those conditiosn imply having 2 copies of the gene.
Common Sex-Linked Traits • red/green colorblindness. The genes for the red and green receptors are on the X. The blue receptor is on an autosome. • hemophilia. Blood doesn’t clot. Two of the genes for proteins involved in clotting are on the X. • Duchenne muscular dystrophy. Muscles degenerate, leading to death before age 20 in most cases.
Sex-linked Inheritance Patterns • The father gives his X to his daughters only; sons get his Y instead. • Sons get their X from their mother. • “Reciprocal crosses” are crosses with the same phenotypes in the parents, but with reversed sexes. Reciprocal crosses usually give different results with sex-linked traits. • For example, a colorblind male x normal female gives all normal offspring. However, a normal male crossed with a colorblind female gives colorblind male children and normal female children. • Colorblind females can occur as a result of a cross between a colorblind male and a heterozygous (carrier) female.
Dosage Compensation • In mammals, males have 1 X while females have 2. Having only 1 copy of any other chromosome would be lethal. How can the X be present in 1 copy or 2 copies and produce normal offspring in either case? • Basic answer: only 1 X is active in each female cell.
Lyon Hypothesis • It has long been known that female cells contain “Barr bodies”, blobs of chromatin located on the inside of the nuclear membrane. Each female cell has 1 Barr body; male cells don’t have Barr bodies. • Mary Lyon proposed that Barr bodies are inactive X chromosomes, and that mammalian cells inactivate all but one of their X’s, converting the extras into Barr bodies. • Proof: XXY individuals are male, but have a Barr body; XO individuals are female but have no Barr bodies; XXX individuals are female with 2 Barr bodies in each cell.
Specifics of Inactivation • When the embryo has about 200 cells, each cell randomly inactivates one of its X’s, independently of the other cells. The inactive X stays inactive throughout the individual’s life, through many cell generations. • A common example: tortoiseshell cats have patches of black and orange fur. Almost all tortoiseshells are female. Heterozygous for the X-linked coat color gene, one allele black and the other allele orange. Only 1 allele is expressed in each cell, and patches on the fur result from cell division of the original embryonic cells that randomly chose an X to inactivate. • A similar human condition: anhidrotic ectodermal dysplasia: absence of sweat glands in the skin.
Sex-Influenced Traits • A sex-influenced trait is an autosomal trait that is dominant in one sex and recessive in the other. Good examples: male pattern baldness in humans and horns in sheep. • Pattern baldness is found in both sexes, but is rarer in females. Females usually get very thin hair all over, instead of the classic receding hairline and bald spot on top that men get. • Baldness is autosomal, but it is dominant in males and recessive in females. Thus, male heterozygotes are bald but female heterozygotes have normal hair.
Sex-Limited Trait • A sex-limited trait is expressed in one sex but not the other. This is usually due to anatomical or physiological limitations. • An example: ability to produce milk is sex-limited, because only females have breasts, the milk producing glands. • Similarly, susceptibility of prostate cancer is limited to men, because only males have a prostate gland.
Mitochondrial Genes • The mitochondria are organelles that produce most of the energy for eukaryotic cells. Aerobic metabolism--the Krebs cycle and the electron transport chain that produces ATP both occur in the mitochondria. • Mitochondria possess a small circle of DNA, like bacteria but unlike the linear eukaryotic chromosomes. They also have other characteristics similar to bacteria. • The “endosymbiont hypothesis” put forth by Lynn Margulis states that mitochondria (and chloroplasts in plants) are descended from free-living bacteria, which developed an intracellular symbiosis with primitive eukaryotic cells. • Over time, most of the bacterial genes have moved into the nucleus, but about 30 genes still remain in the mitochondrial genome. • Analysis of the DNA sequences of the remaining genes has allowed scientists to identify the bacterial groups that the mitochondria and chloroplasts came from.
More Mitochondrial Genes • Genes found in the mitochondria: --ribosomal RNA and transfer RNA. Mitochondrial ribosomes are of the prokaryotic type, not eukaryotic. --some electron transport chain proteins • The genetic code is slightly altered in mitochondria. For example, UGA is a stop codon in the nucleus, but is codes for Tryptophan in the mitochondria of humans and yeast. Also, AUA codes for Isoleucine in the nucleus, but it codes for Methionine in human mitochondria (but not yeast mitochondria).
Mitochondrial Inheritance Pattern • Mitochondria are inherited strictly from the mother. The father’s mitochondria are not passed to his offspring. • Thus, any mitochondrial trait found in the mother will be found in all of her children. • This fact has allowed tracing of mutations in mitochondrial DNA through the human species. The basic conclusions are that there is more genetic diversity on Africa than in the entire rest of the world (implying that our species evolved in Africa), and that the woman who was the common ancestor of all humans lived 100-200,000 years ago.
Heteroplasmy • Sometimes an individual has more than one kind of mitochondria. This is called heteroplasmy. Since mitochondria are divided randomly during cell division, different cells get different proportions of the two types. • If one mitochondrial type is mutant and the other is normal, severity of symptoms will vary in different tissues depending on the proportions of the two types.
Maternal Effect Genes • As we will discuss later, the egg cell in many animals is haploid for only a very brief time, long after it has been created. • During production of the egg, the mother puts many proteins and RNAs into the egg that are produced by diploid maternal cells. • Thus it is not surprising that some traits in an offspring are determined by its mother’s genotype, not the offspring’s genotype. • The maternal effect rule: “Mother’s genotype determines offspring’s phenotype.”
Shell Coiling in Lymnea • The dominant D allele causes coiling to the right, while the recessive d allele causes coiling to the left. Thus, all offspring of a dd mother will coil to the left, and all offspring of DD or Dd offspring will coil to the right. The father’s genotype and the offspring’s genotype has no effect on the offspring’s phenotype. • “Mother’s genotype determines offspring’s phenotype.”