Chapter 4 – Sex Determination and Sex Linked Characteristics

Sex Determination

• In sexually reproducing organisms, there is usually two distinct sexes – males and females– Sex chromosomes – different between sexes– Autosomes – any non-sex chromosome

Sex Determination

• XX-XO sex determination– Grasshoppers– XX – female

• Homogametic – all gametes contain same type of sex chromosome

– XO – male (one less chromosome)• Heterogametic – 2 different types of gametes in regard to sex

chromosomes (X or none)

• XX-XY sex determination– All mammals; some plants, insects, reptiles– XX – female– XY – male

XX - XY• Pseudoautosomal

region– Small region of both

X and Y chromosomes carry same genes

– Allow for homologous pairing during meiosis

– Not considered sex-linked in terms of heredity

Sex Determination

• ZZ – ZW sex determination– Birds; some fish and amphibians– ZW – female

• Heterogametic• Female determines sex of offspring

– ZZ – male• homogametic

Sex Determination• Haplodiploidy

– Hymenoptera (ants, bees, wasps)

– Females are diploid; produce eggs by meiosis

– Males are haploid; produce sperm by mitosis

• Develop from unfertilized eggs (parthenogenesis)

– Brothers have 50% relatedness; sisters have 75% relatedness (50% from mother and 100% from father)

Sex Determination

• Genic sex determination– No difference in chromosomes between sexes– Sex is determined by genotype at one or more gene

loci

• Chromosomal sex determination also determined by genes – happen to be on sex chromosomes– SRY gene on Y chromosome determines maleness

Environmental Sex Determination• Credidula fornicata

– Larva is free-living– 1st to land is female, and

attracts more larva, which are male

– As more stack on, the covered males become female

– Sequential hermaphroditism • Can be both female and

male, but not at the same time

• Temperature– Turtles, crocodiles, alligators– Incubation temperature of

eggs determine sex of offspring

Sex determination in Drosophila melanogaster

• 8 chromosomes– 3 pairs of

autosomes; 1 pair of sex chromosomes

– Females are XX; males are XY

– Y alone doesn’t determine maleness

• Genic balance system

Genic balance system in Drosophila• Ratio of X chromosomes:haploid sets of autosomes (X:A)

• X chromosome = female producing effects; autosomes = male producing effects

• X:A ratio– Female = 1.0 (2X:2n)– Male = 0.5 (1x:2n)– 0.5 < X:A < 1.0 = intersex

• Mix of male and female characteristics

– 1.0 < X:A = metafemale• Increased developmental problems; rarely emerge from pupal case

– 0.5 > X:A = metemale• Weak and sterile

Sex Chromosomal Abnormalities

• Nondisjunction – abnormal segregation of chromosomes during meiosis I and II

• Turner syndrome– 45, XO– 1 in 3,000 female births– Clinical features

• Short stature, broad chest and wide – spaced nipples, low hairline on neck, webbed neck, sterile, usually normal intelligence

– Mosaics may be fertile

Sex Chromosomal Abnormalities

• Klinefelter syndrome– 47, XXY– 1 in 1,000 male births– Clinical features

• Above average height, reduced facial and pubic hair, breast development, underdeveloped testes – sterile, usually normal intelligence

– Additional sex chromosomes

• XXXY, XXYY• Lower IQ more common

– Increases severity as #s increase

Sex Chromosomal Abnormalities

• Poly X females– 47, XXX– 1 in 1,000 female births– Clinical features

• Tall and thin, most are fertile, increased risk of mental retardation– Increases with additional Xs (XXXX)

• Jacob syndrome– 47, XYY

• Originally thought to be associated with increased violence, deviant/antisocial behavior

– Original study showed higher % in mental and penal institutions – Not supported by other studies

• Fertile - #s in population not known since usually asymptomatic

Male Determining Gene in Humans• Early development

– Gonads are undifferentiated and have both male and female reproductive ducts

• SRY gene– Sex-determining region on Y

chromosome– Begins expression approx 6

weeks after fertilization– Develops neutral gonads into

testes• Testes produce:

– Testosterone – male characteristics

– Mullerian-inhibiting substance – degeneration of female reproductive ducts

SRY gene

• In the absence of SRY:– Gonads develop into ovaries and male ducts degenerate

• Androgen-insensitivity syndrome– Phenotype is female– Menstruation does not occur– No internal female reproductive organs; has testes in abdominal

cavity, which produces testosterone• Have testes due to SRY gene

– Karyotype is male – XY– Receptor for testosterone is defective – can’t affect appropriate cells

• Default to female

Eye color in Drosophila• D. melanogaster

– Wildtype = red eyes

• P– red-eyed female X

white-eyed male

• F1

– All red eyes– Looks like Mendelian

inheritance

Eye Color in Drosophila cont• F2

– 100% females have red eyes

– 50%males have red; 50% have white

• Males are hemizygous for X-linked genes

• Reciprocal cross at P generation– F1 = all females have red;

all males have white– F2 = 50% of both females

and males have red; 50% white

X-linked Colorblindness in humans

• X-linked recessive trait

• More common in males than females– Affected males can only pass mutant allele to

daughters (100%)– If heterozygous, females can pass mutant

allele to both daughters and sons (50%); if homozygous, female will pass mutant allele to ALL offspring

X linked disorders

• Symbols– X = X chromosome– Y or / = Y chromosome

• X-linked dominant disorders– More common in females

• Have two chances to receive mutant allele; males have only one chance

Dosage Compensation• Females have two copies of each X-linked gene while males

have one 1 copy, yet females do not have twice the gene product

• D. melanogaster– Male gene expression is doubled

• C. elegans– Each female X gene’s expression is halved

• Placental mammals– Only one X remains active; other is inactivated

Barr body• Darkly staining body at edge of

nucleus

• Mary Lyon (Lyon hypothesis)– Barr body is an inactive X

• Early in embryonic development, a random X is inactivated in each cell– Once marked for inactivation, all

descendant cells will have the same X inactivated

• Only 1 X is kept active

Xist (X inactivation specific transcript)

• Is only expressed on inactive X

• Produces RNA molecule that coats chromosome

• Some genes on “inactive” X are kept active (mechanism unknown)– Why “normal” females are different from Turner

syndrome females

Holandric inheritance• Y linked gene

• All males are affected within family; no affected females– Used to trace ancestry through paternal lines

• Y chromosome have few genes, 2/3 is heterochromatin

• No trait has been definitely associated with Y chromosome in humans