Ch. 4: Sex Determination

Examples of environmental sex determination

(Crepidula fornicata [ slipper limpet], turtles, crocodiles, alligators, some birds) (ZZ-ZW bearded dragon lizards when eggs are incubated at high temps ZZ become phenotypic females)

Examples of genic sex determination

(some plants, fungi, protozoans, fish)

The role of sex chromosomes

1. X chromosome contains genetic info essential for both sexes; one X always required 2. male-determining gene located on Y chromosome. single copy of this chromosome usually produces a male phenotype 3. absence of Y chromosome usually results in female phenotype 4. Genes affecting fertility are located on the X and Y chromosomes. female usually needs at least two copies of X chromosome to be fertile 5. additional copies of X chromosome may upset normal development in both males and females

Mechanism of random inactivation

1. cell somehow assesses the number of X chromosomes present 2. one X chromosome is selected to become the active X chromosome and all others are silenced

Poly-X females

1/1000 female births; no distinctive features; tend to be tall and thin for Triple-X syndrome; severity of intellectual disability increases as the number of X chromosomes goes beyond

Evolution of the Y chromosome

A mutation of a gene on one chromosome causes maleness, mutations at other genes affect male characteristics, suppression of crossing over keeps genes for male traits linked to male-determining gene, over time lack of crossing over bw the X and Y chromosomes leads to degeneration of the Y

What does the chromosome theory of heredity state?

Genes are located on the chromosomes, which serve as vehicles for the segregation of genes in meiosis

How do genes in the pseudoautosomal regions of sex chromosomes behave?

Genes in these regions are homologous and exhibit autosomal patterns of inheritance rather than the sex-linked inheritance seen for most genes on X and Y chromosomes

How do chromosomal, genic, and environmental sex determination differ?

In chromosomal sex determination--males and females have chromosomes that are distinguishable. In genic sex determination, sex is determined by genes, but the chromosomes of males and females are indistinguishable. In environmental sex determination, sex is determined fully or in part by environmental effects

Z-linked characteristics

In organisms with ZZ-ZW sex determination, males are the homogametic sex (ZZ) and carry two sex-linked alleles; males may be homozygous or heterozygous; females are the heterogametic sex (ZW) and possess only a single Z-linked allele

How is dosage compensation achieved in placental mammals?

In placental mammals, dosage compensation is accomplished by the random inactivation of one X chromosome in the cells of females

How is sex determined in humans (XX-XY)

Sex determining region Y (SRY) gene on the Y chromosome determines maleness

How is sex determined in reptiles?

The sexual phenotype is affected by temperature during embryonic development (turtles, warmer temps = females but alligators cooler temps = females)

How is sex determined in fruit flies? (XX-XY)

X : A ratio (X=number of X chromosomes, A = number of haploid sets of autosomal chromosomes)

Where is the gene for the androgen receptor located?

X-chromosome (always inherited from mothers)

examples of X-linked characteristics

X-linked white eye in Drosophila, X-linked color blindness

Turner syndrome

XO; 1/3000 female births; underdeveloped secondary sex characteristics; women are short with low hairline, broad chest, folds of skin on the neck; sterile

XX-XO System (grasshoppers, insects)

XX = female, XO = male (O signifies the absence of a sex chromosome); sex of an organism is determined by which type of male gamete fertilizes the egg

XX-XY System (mammals, some plants, many insects, amphibians, reptiles)

XX=female, XY=male (male is heterogametic sex, female is homogametic sex)

Klinefelter syndrome

XXY, XXXY, XXXXY, XXYY; 1/1000 male births; men have small testes and reduced facial and pubic hair; often taller than normal, sterile

What molecule aids in X-inactivation?

Xist an RNA molecule aids in X-inactivation

ZZ-ZW System (birds, snakes, butterflies, some amphibians, some fishes)

ZZ = male, ZW = female female is heterogametic, male is homogametic; female always inherits W from her mother

Misconceptions of Sex-Linked Characteristics

a characteristic in which males and females differ is sex linked (many autosomal characteristics that differ bw the sexes can be due to the influence of sex hormone on expression of autosomal genes); a characteristic found more frequently in one sex is sex linked

How does the SRY gene determine maleness in human development?

all humans possess undifferentiated gonads and both male and female reproductive ducts; after 6 weeks after fertilization, gene on the Y chromosome becomes active and causes human embryo to develop as a male and in the absence of this gene, a human embryo develops as a female

sexual reproduction

alternates between haploid and diploid states

Mechanism of sex determination in fruit flies

although the sexual phenotype of a fruit fly is predicted by the X:A ratio, sex is actually determined by genes on the X chromosome

hermaphroditism

both sexes are present in the same organism

X-linked characteristics

characteristics determined by a gene or genes on the X chromosome; most sex-linked characteristics are X linked

Y-linked characteristics

characteristics determined by a gene or genes on the Y chromosome; traits which are only present in males and always inherited from the father (all male offspring of a male with a Y-linked trait will display the trait)

sex-linked characteristics

characteristics determined by genes located on the sex chromosomes

How can sex be determined among dioecious species?

chromosomally, genetically, or environmentally

Barr bodies

condensed, darkly staining bodies in the nuclei of cells that are inactive X chromosomes for placental mammals (regardless of how many X's only one is active)

How is dosage compensation achieved in fruit flies?

doubling the activity of the genes on the X chromosome of males but not of females

dosage compensation

equalization in males and females of the amount of protein produced by X-linked genes since having too few or too much protein can be detrimental during development

What is fundamental difference in males and females?

gamete size

heterogametic sex

gametes of this sex have different sex chromosomes

homogametic sex

gametes of this sex have the same sex chromosomes

Androgen-insensitivity sydrome

genotype = XY; androgen receptor is defective in females with androgen insensitivity syndrome so their cells are insensitive to testosterone; women with androgen-insensitivity syndrome have female external sexual characteristics and internalized teses

What does androgen-insensitivity syndrome illustrate about the influence of genes on a person's sex?

human sexual development is influenced by other genes in addition to SRY gene on Y chromosome; most people carry genes for both male and female characteristics; key to maleness and femaleness lies not in genes but in the control of their expression

Lyon Hypothesis

hypothesis proposes one X chromosome in each female cell becomes inactivated and suggests that the X chromosome that gets inactivated is random and varies from cell to cell

hemizygous

males cannot be either homozygous or heterozygous for X-linked loci

sex determination

mechanism by which sex is established

What process causes the genetic variation seen in offspring produced by sexual reproduction?

meiosis

Random X-inactivation

neighboring cells tend to have the same X chromosomes tend to have the same X chromosome inactivated, producing a patchy pattern (mosaic) for the expression of an X-linked characteristic in heterozygous

autosomes

nonsex chromosomes

YY

not viable (lethal--need at least one X)

SRY gene

on the y chromosome, a gene that triggers male development

monoecious

organisms that bear both male and female reproductive structures

sex

refers to sexual phenotype (most organisms only have 2) **sometimes an organisms has chromosomes or genes normally associated with one sex but an anatomy corresponding to the opposite sex

What determines sex in chromosomal sex-determining and genic sex determining systems?

sex is controlled by individual genes (difference is with chromosomal sex determination, the sex chromosomes also look different in males and females)

genic sex determination

sex is determined by genes at one or more loci; sex is genetically determined but there are no obvious differences in the chromosomes of males and females i.e. there are no sex chromosomes

How is sex determined for slipper limpets?

sex is determined environmentally by the limpet's position in the stack

environmental sex determination

sex is determined fully or in part by environmental factors; sometimes environmental factors can override chromosomal sex determination

pseudoautosomal region

small regions where sex chromosomes are homologous and allows them to pair (since S and Y chromosomes aren't homologous) *located at the tips of the X and Y chromosomes

dioecious

species in which the organism has either male or female reproductive structures (ex. humans)

sex chromosomes

the chromosomes which determine sex in organisms that differ between males and females

Example of X-inactivation

tortoiseshell and calico cats

sequential hermaphroditism

type of sexual development in which each individual animal can be both male and female, although not simultaneously

nondisjunction

when chromosomes fail to separate in anaphase I of meiosis