BY322 CH5

CH5 Genetics

sexual dimorphism

the physical differences between the males and females of a species

heteromorphic chromosomes

a pair with some homology but differences in size, shape, and/or staining properties , these are typically the sex chromsomes

Human sex chromosomes

females have XX, males have XY

autosomes

chromosomes that are the same in males and females

sex is determined randomly by the father

eggs always donate an X, sperm can donate an X or Y; thus determining the sex. Genes, not chromosomes cause sex differentiation.

hemizygous

with regards to sex chromosomes males are this. The Y chromosome lacks homology with most genes on the X chromosome; therefore, most alleles present on X chromosome of the male will be expressed, even if they are recessive.

Y

this chromosome has fewer genes than its counterpart, it has between 75 and 200 genes while its counterpart has 900-1400

y-centered sex determination

he SRY gene (Sex-determining Region of Y) on the Y chromosome is the master gene for sex determination; triggers formation of testes, which produce the male sex hormone (testosterone); without testosterone, ovaries develop and produce female sex hormones (estrogens).

x-linked inheritance patterns (x-lingage)

Sons receive X chromosomes from their mothers and Y chromosomes from their fathers (never an X from the father); daughters receive X chromosomes from mother and father. Males have only one X (plus one Y, which fails to mask genes on X due to lack of alleles); therefore, males can express a single recessive allele carried on the X chromosome. A heterozygous female has two X chromosomes and may not show symptoms. Thus, more males than females have X-linked recessive genetic disorders, and males inherit the disorders from their mothers.

Nettie Stevens

was an early American geneticist. In 1906, she discovered that male beetles produce two kinds of sperm, one with a large chromosome and one with a small chromosome. When the sperm with the large chromosome fertilized eggs, they produced female offspring, and when the sperm with the small chromosome fertilized eggs, they produced male offspring. This pattern was observed in other animals, including humans, and became known as the XY sex-determination system.

Thomas Hunt Morgan (1920)

Discovered X-linked inheritance in studies of a white-eyed mutation in Drosophila. Reciprocal crosses between white-eyed and red-eyed flies did not yield identical results. Inheritance pattern clearly related to sex of parent carrying mutant allele. Morgan concluded the gene is on the X chromosome

Hemophilia A

an x-linked recessive disorder; Bleeding caused by lack of blood-clotting protein

red-green color blindness

an x-linked recessive disorder; : Inability to distinguish certain colors caused by altered photoreceptors in the eyes

Duchenne muscular dystrophy

an x-linked recessive disorder; : Degeneration of muscles caused by lack of the structural protein dystrophin. A lethal X-linked recessive disorder. Onset prior to age 6; lethal around age 20. Occurs only in males. Females can only be heterozygous carriers who do not develop the disorder.

sex-limited inheritance

Patterns of gene expression may be affected by sex even when not on X chromosome; some gene expression depends on the hormone constitution of an individual. For example: feather plumage in chickens is controlled by an autosomal gene. Hen-feathering controlled by dominant allele (H) expressed in both sexes. Cock-feathering controlled by recessive allele (h) only expressed in hh males (hh females have hen-feathering phenotype). Pattern baldness in humans: autosomal genes are responsible for phenotypes. Allele B (baldness) is dominant in males but recessive in females; males with BB or Bb exhibit baldness; females with Bb are normal; BB females have baldness trait, but less pronounced

Herman Hinking (1891)

Nuclear structure in sperm cells of insects was discovered in 1891 by German Biologist, Hermann Henking: labelled X-body. For example, some insects: female somatic cells (diploid) have 14 chromosomes, including two X; male somatic cells have 13 chromosomes with one X chromosome. Such organisms have a XX/X0 chromosome system (males lack Y, thus X only). Moreover, fertilization by X-bearing sperm produced female offspring; fertilization by X-deficient sperm: male offspring. Thus, males produce unlike gametes (males are the heterogametic sex); their gametes determine the sex of progeny. Females produce like or uniform gametes (they are the homogametic sex). Note that in some organisms, females are heterogametic, and males are homogametic

XX/XY sex determination system

: found in humans and most other mammals, as well as some insects. Females have two X chromosomes (XX), whereas males have X and Y chromosomes.

XX/X0 sex determination system

found in grasshoppers, cockroaches, nematodes, some rats, and other organisms. Females have two X chromosomes (XX), whereas males have one X (X0).

ZW/ZZ sex determination system

found in birds, some reptiles, some insects, and other organisms. ZW is reversed compared to XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ).

ZZ/Z0 sex determination system

found in some moths. In moths, there is one sex chromosome (Z). Males have two Z chromosomes (ZZ), whereas females have one Z (Z0)

UV sex chromosomes

: in mosses and some algae, the haploid (gametophyte) stage of the life cycle includes male or female individuals that produce male and female gametes respectively. During meiosis (spore production), sex chromosomes known as U and V assort into spores that give rise to female gametophytes (with U chromosome) or male gametophytes (with V).

haploidiploidy

: found in insects such as ants and bees (Hymenoptera). Sex determination is controlled by a sex determiner (csd) locus; unfertilized eggs develop into haploid individuals which have a single, hemizygous copy of csd and are therefore males; fertilized eggs develop into diploid individuals which are generally females. Females can decide the sex of their offspring by holding received sperm and either releasing it into their oviduct or not; this allows them to create more workers, if needed in the colony

Klinefelter Syndrome

e (47, XXY): Individuals have more than one X chromosome: often XXY plus 44 autosomes; recognized as male. Tall with long arms and legs; large hands and feet; testes rudimentary, fail to produce sperm; slight breast enlargement and hips often rounded

Turner Syndrome

(45, X): 45 chromosomes with a single X. Female external genitalia and internal ducts but ovaries rudimentary. Short stature, skin flaps on back of neck, underdeveloped breasts, broad shield-like chest. Normal intelligence. Absence of Y: no masculinization occurs. Also symbolized as XO.

Triple X

Three X chromosomes along with normal set of autosomes (47, XXX). Results in female differentiation (1/1000 live births). Often perfectly normal; unaware of condition. Underdeveloped secondary sex characteristics, sterility, sometimes intellectual disabilities.

Tetra-X; Penta-X

tetra (48, XXXX); Penta (49, XXXXX) karyotypes have been reported

Karyotype 47, XYY Condition

(47, XYY) Presence of additional Y chromosome. Males; over 6 feet tall; may have subnormal intelligence; may have personality disorders. The above occur as a random errors in the production of gametes; generally, not heritable; due to nondisjunction during meiosis: failure of sex chromosomes to segregate during meiosis.

Sexual differentiation in humans

Y chromosome houses genetic information for maleness: gene provides “signal.” In early embryonic development, the human embryo is androgynous or hermaphroditic: gonadal phenotype is sexually indifferent (with bipotential gonads). By 5th week, gonadal ridges form either ovaries or testes. If XY: gonadal ridge develops into testes; in the absence of Y, the ridge forms ovarian tissue. Mullerian duct is a paired embryological structure that gives rise to female reproductive organs; these degenerate in males; Wolffian duct is a paired embryological structure that gives rise to male reproductive organs; these degenerate in females.

pseudoautosomal regions (PARs)

Present on both ends of Y chromosome; shares homology with regions on X. These synapse and recombine with X during meiosis; thus, provides pairing regions that are critical for the segregation of X and Y chromosomes during male gametogenesis.

Male-specific regions of Y (MSY)

95% of Y chromosome. Does not recombine with X. Some portions share homology with genes on X chromosome.

SRY: Sex-determination region Y

Critical gene controlling sexual development. Becomes active in XY embryos at 6–8 weeks of development. SRY gene encodes protein Testis-Determining Factor (TDF). Present in all mammals: triggers testes formation. Causes undifferentiated gonadal tissue of embryo to form testes. Deviations from normal sex determination:

XX male syndrome

males with two X and no Y (with translocated SRY gene on an X chromsome)

Swyer Syndrome

females with one X and on Y; most likely missing or mutated SRY gene

research on transgenic mice

produced from fertilized eggs injected with foreign DNA. Mice have Sry gene, comparable to human SRY. Mouse DNA with Sry injected into XX mice eggs: most offspring being males. TDF believed to be a transcription factor (a protein that regulates gene expression): behaves as master switch controlling genes involved in sexual differentiation.

Dosage compensation

Females have potential to produce twice as much product for X-linked genes: may create “genetic dosage” difference between genders. Possible explanation: Barr Bodies

Barr Bodies

(sex chromatin body): genetic mechanism in mammals. Inactive X chromosomes: highly condensed; lies against nuclear envelope of interphase cells. Arises randomly from one of the two X chromosomes. Questions: If one of two X chromosomes is inactivated, why is Turner syndrome (45, X0) not entirely normal? Why are females with triple or tetra Xs not normal? Why does an extra X in Klinefelter syndrome (47, XXY) result in a characteristic phenotype?

The Lyon Hypothesis

Inactivation of X chromosome is random; all descendant cells have same inactivation. Research on animals heterozygous for X-linked coat-color genes: heterozygote female mice show mottling coat color for coat-color genes on X chromosome. Calico and tortoiseshell cats show black and yellow-orange patches of fur color.

Mosaics

somatic cells display two different genetic cell lines with different kayotypes

ratio of x chromosomes to sets of autosomes can determine sex

Fruit Flies (Drosophila) have same sex chromosomes as humans: female XX and male XY. However, Y chromosome does not determine sex (Y chromosome has no role in sex determination).

Genetic Balance Theory

sex is determined by ratio of X chromosomes to haploid sets of autosomes (A): threshold of maleness is reached when X:A ratio is 1:2 (X:2 A). Presence of additional X (X X:2 A) alters balance and results in female differentiation

Nematodes (C. elegans)

have two sexual phenotypes: males (with one testes) have one X chromosome and no Y; hermaphrodites (have both testes and ovaries) have two X chromosomes and no Y. Eggs are fertilized by stored sperm (self-fertilization); majority of offspring are hermaphrodites; less than 1% are males.

TSD

Temperature-dependent sex determination. Environment, specifically temperature, has profound influence on sex determination. Some reptile species use ZZ/ZW or XX/XY in sex determination; in others TSD is the norm. Enzymes involved in steroids, androgens, and estrogen synthesis are affected (inhibited) by temperature

patterns of temperature-dependent sex determination in reptiles

Case I: Low temperatures yield 100% females; high temperatures yield 100% males. Case II: Exact opposite occurs. (In the news: Sea turtles: “hot chicks, cool dudes”) Case III: Low and high temperatures yield 100% females; intermediate temperatures yield various proportions of males

Aromatase

temperature effect on enzymes; converts androgens (male hormones such as testosterone) to estrogens (female hormones such as estradiol). Thermosensitive factors mediate transcription of gene. Sex-determining mechanisms involving estrogens are characteristic of nonmammalian vertebrates