chapter 4 – sex determination and sex linked characteristics
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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 - PowerPoint PPT PresentationTRANSCRIPT
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