Gene Lecture 5
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Fundamental Genetics Lecture 5
Sex Determination and Sex Chromosomes John Donnie A. Ramos, Ph.D. Dept. of Biological Sciences College of Science University of Santo Tomas
Types of Sexually Reproducing Organisms Monoecious organisms produces two types of gametes in 1 organism (sperm cells and egg cells) Bisexual / hermaprhroditic Dioecious organisms Produces only 1 type of gamete in 1 organism (either sperm cells or egg cells) Unisexual / gonochoric Organism is either male or female Intersex - intermediate sexual differentiation (often sterile)
“Genetics is concerned with the differences of dioecious organisms and how is the mechanism of inheritance of these two types.”
1
Life Cycle of Clamydomonas Green alga Infrequent sexual reproduction Isogamous organisms Primitive means of sexual differentiation
Life Cycle of Zea mays
Monoecious plant Alternating gametophyte and sporophyte stages
2
Life Cycle of C. elegans Hermaphroditic nematode Two sexual phenotypes Male – with testes Hermaphrodites – testes and ovaries Genetic signal for maleness is determined by X chromosome anf autosomes No Y chromosome Hermaphrodites = XX Male = X Ratio of X chrom to autosomes determines sex of the individual 1.0 = hermaphrodites 0.5 = males
X and Y Chromosomes
XX-XO system
XX-XY system
Male is heterogametic sex while female is homogametic sex Birds, some reptiles, moths, butterflies, some fishes and amphibinas = ZZ/ZW system where ZZ=male while ZW=females (male is homogametic while female is heterogametic)
3
Sex Determination in Humans Spermatogenesis 44+XY
44+XX
Spermatogonium
Oogonium
44+XY
44+XX
Primary Spermatocyte
Primary Oocyte
22+X
22+Y
Secondary Spermatocyte
22+X 22+X 22+Y 22+Y Spermatozoa 1/4
Oogenesis
1/4
1/4
2/4 or ½ or 50%
1/4
22+X
22+X
Secondary Oocyte
1st polar body
22+X 22+X 22+X 22+X Ovum 100 %
2nd Polar Bodies
2/4 or ½ or 50%
Fertilization During fertilization, an egg (X-bearing) has an equal chance of being fertilized by an X-bearing sperm or a Y-bearing sperm. Sperm
Egg
Sperm
Egg
22+X
22+X
22+Y
22+X
50% or 0.5
100% or 1.0
50% or 0.5
100% or 1.0
Zygote
Zygote
44+XX
44+XY
0.5 or 50% (0.5)(1.0)= 0.5
0.5 or 50% (0.5)(1.0)= 0.5
50-50% of producing a male or female offpring
4
Sex Ratio in Humans Primary Sex Ratio (proportion of males and females conceived in a population) Secondary Sex Ratio (proportion of each sex that is born) Why birth rate of male is higher than female? (US survey on birth rate: Male=0.52 while females=0.48) Y bearing sperm is more motile (?) XY fetus has higher viability than XX fetus Why more females in than males in actual populations? (ranges from 1:2 to 1:5) Males have higher mortality rate than females
Implication of equal chances of producing male or female offspring: Can predict the number of males and females in a family through binomial distribution (m+f)n where n is the number of children.
Sex Chromosomes in Humans X chromosome Submetacentric Size between 7-8 groups Approximately 5.0-5.5 µm in length With satellites
Y chromosome Acroncentric Slightly longer than group G chromosomes 2 µm in length Has no satellites
5
Sex Differentiation in Humans Genetic Sex Males = XY while females = XX
Gonodal Sex (5th week) XY embryo will produce testes (medulla) while XX embryo will produce ovary (cortex) If the cell contains XY sex chromosomes, it will develop testes at 7th week it not ovary develops) Oogonia develops at 12th week, and by 25th week all primary oocytes have been produced and arrested at GO (remain dormant until puberty Prior to this stage, embryo is hermaphroditic
Genital Sex Secondary sex organs (external genitalia) and hormones
Somatic Sex Production of secondary sex characters (effect of hormones)
Nuclear Sex Males – Barr body negative Females – Barr body positive
Sociophysiological sex Influence of culture
Sex Chromosome Abnormalities Klinefelter Syndrome 2n=47 (XXY sex chromosomes) Occurs in 2 every 1000 male births Male genitalia and ducts Rudimentary testes (fail to produce sperm) = sterile Tall and long arms and legs Large hands and feet Slight enlargement of breast (gynecomastia) Rounded hips Below normal IQ Other forms: 48, XXXY; 48,XXYY; 49XXXXY; 49XXXYY
6
Sex Chromosome Abnormalities Turner Syndrome 2n=45 (X sex chromosome) Occurs in 1 every 2000 female births Most die in utero Female external genitalia Rudimentary ovary (sterile) Short stature, stocky, (usually under 5 ft) Webbing neck (Skin flaps on the back of neck) Flat chested (Underdeveloped breasts) Broad, shieldlike chest Low IQ
Sex Chromosome Abnormalities Metafemale, Poly X females 2n=47 (XXX) Genetically female Occurs in 1 every 1200 female births Some are perfectly normal Other cases, undeveloped secondary sexual characteristics leading sterility Mental retardation Rare cases: 48 (XXXX); 49 (XXXXX)
7
Sex Chromosome Abnormalities Jacobs Syndrome (Criminal Syndrome)
2n=47 (XYY) Phenotypically male First studied by Patricia Jacobs (1965) 9 out of 315 males in the Scottish maximum security prison Significantly above average in height (above 6 ft) Antisocial (nonviolent) behaviors, personality disorders High percentage have higher IQ (7/9)
Sex Chromosome Abnormalities Hermaphroditism
Individuals with both testicular and ovarian tissues Ambigous external genitalia Often musculinized Generally sterile (underdeveloped organs) Variable cases: (US study of 100 cases) 59 XX 21 XY 28 mosaics (XXY, XXX, etc.)
8
Other Causes of Sexual Abnormalities Attached X flies Observed in fruit flies only 2 X chromosomes are physically attached (inability to segregate during anaphase) Will result to metafemales
Polyploidy More than 2 sets of chromosomes
Transformer genes Autosomal genes that affect the sex of the individual Identified in Drosophila
Gynandromorphs Condition wherein ½ of the organism is male and the other half is female Male part contains XY chromosomes while female parts contain XX chromosomes
The Y Chromosome in Humans Contains Psuedoautosomal Regions (PAR) on both ends Share DNA sequence homology with X chromosome (making them pair during meiosis)
5 Nonrecombining region of the Y (NRY) – remainder of the chromosome Mainly Heterochromatic (devoid of structural genes) 7 genes without homology to other chromosomes Contains genes coding for Testes-Determining Factor (TFD)-proteins responsible for differentiatiation of gonodal tissue to become testes SRY gene (sex-determining region Y)-studied using transgenic mice Abnormal cases: Males without Y (XX) but contains SRY gene on one of the X Females with Y chromosomes but absence of SRY gene
9
Dosage Compensation for X Chromosomes in Females Females can produce twice gene products for genes located in the X chromosomes (could lead to abnormalities) Dosage compensation = mechanism employed to prevent overexpression Barr Bodies (Chromatin Bodies) Inactivated X chromosome on the periphery of the nucleus Follows N-1 rule where N is the number of X chromosomes
Dosage Compensation for X Chromosomes in Females If only 1 X chromosome is active, why are abnormalities observed? X chromosome does not occur in early development of embryo (before sex differentiation) Not all Barr bodies are inactivated The Lyon Hypothesis Inactivation of X chromosomes occur randomly in somatic cells Once inactivated, all daughter cells exhibit inactivated X chromosomes Controlled by a gene called X-inactivation Center (XIC)
10
Sex Determination In Drosophila Follows XY-XX system 2n=8 (6 autosomes and XY or XX sex chromosomes) First studies by Calvin Bridges (1916) Y chromosome does not determine sexes (unlike in humans) Ratio of X chromosome and the autosomes determines sex in Drosophila Based on initial findings that XXY flies are females while XO flies are males but are sterile. Studied triploid females (3n) = result of fertilization of diploid (2n) egg and haploid (n) sperm
Sex Determination In Drosophila Ratio of number of X chromosomes and number of sets of chromosomes present determines the sex Ratio of 1 = Female (fertile) Ratio above 1 = metafemale (sterile) Ratio between 0.5 to 1.0 = Intersex (invariably sterile, exhibit both male anf female morphology) Ratio of 0.5 = Male (fertile) Ratio below 0.5 = Metamale (sterile) Determined by Genetic Balance Theory Genes involved: Tra gene –transformer (autosomal gene) Sxl gene – sex lethal gene Dxl gene – double sex gene
11
Sex Determination In Reptiles Follows ZZ/ZW system where ZZ are males and ZW are females Sex determination is affected by temperature (incubation temperature of egg) Varies with different species Three observed patterns:
Low T=males High T = females
Low T=females High T = males
Low & high T = females Intermediate T = males
Tp = pivotal temperature; FT = female-determining temperature; MT= male-determining temperature Tp is narrow (1-5°C) Hormones are mainly affected (androgens and estrogens) Aromatase=converts androgens to estrogens
12
Sex Determination and Sex Chromosomes John Donnie A. Ramos, Ph.D. Dept. of Biological Sciences College of Science University of Santo Tomas
Types of Sexually Reproducing Organisms Monoecious organisms produces two types of gametes in 1 organism (sperm cells and egg cells) Bisexual / hermaprhroditic Dioecious organisms Produces only 1 type of gamete in 1 organism (either sperm cells or egg cells) Unisexual / gonochoric Organism is either male or female Intersex - intermediate sexual differentiation (often sterile)
“Genetics is concerned with the differences of dioecious organisms and how is the mechanism of inheritance of these two types.”
1
Life Cycle of Clamydomonas Green alga Infrequent sexual reproduction Isogamous organisms Primitive means of sexual differentiation
Life Cycle of Zea mays
Monoecious plant Alternating gametophyte and sporophyte stages
2
Life Cycle of C. elegans Hermaphroditic nematode Two sexual phenotypes Male – with testes Hermaphrodites – testes and ovaries Genetic signal for maleness is determined by X chromosome anf autosomes No Y chromosome Hermaphrodites = XX Male = X Ratio of X chrom to autosomes determines sex of the individual 1.0 = hermaphrodites 0.5 = males
X and Y Chromosomes
XX-XO system
XX-XY system
Male is heterogametic sex while female is homogametic sex Birds, some reptiles, moths, butterflies, some fishes and amphibinas = ZZ/ZW system where ZZ=male while ZW=females (male is homogametic while female is heterogametic)
3
Sex Determination in Humans Spermatogenesis 44+XY
44+XX
Spermatogonium
Oogonium
44+XY
44+XX
Primary Spermatocyte
Primary Oocyte
22+X
22+Y
Secondary Spermatocyte
22+X 22+X 22+Y 22+Y Spermatozoa 1/4
Oogenesis
1/4
1/4
2/4 or ½ or 50%
1/4
22+X
22+X
Secondary Oocyte
1st polar body
22+X 22+X 22+X 22+X Ovum 100 %
2nd Polar Bodies
2/4 or ½ or 50%
Fertilization During fertilization, an egg (X-bearing) has an equal chance of being fertilized by an X-bearing sperm or a Y-bearing sperm. Sperm
Egg
Sperm
Egg
22+X
22+X
22+Y
22+X
50% or 0.5
100% or 1.0
50% or 0.5
100% or 1.0
Zygote
Zygote
44+XX
44+XY
0.5 or 50% (0.5)(1.0)= 0.5
0.5 or 50% (0.5)(1.0)= 0.5
50-50% of producing a male or female offpring
4
Sex Ratio in Humans Primary Sex Ratio (proportion of males and females conceived in a population) Secondary Sex Ratio (proportion of each sex that is born) Why birth rate of male is higher than female? (US survey on birth rate: Male=0.52 while females=0.48) Y bearing sperm is more motile (?) XY fetus has higher viability than XX fetus Why more females in than males in actual populations? (ranges from 1:2 to 1:5) Males have higher mortality rate than females
Implication of equal chances of producing male or female offspring: Can predict the number of males and females in a family through binomial distribution (m+f)n where n is the number of children.
Sex Chromosomes in Humans X chromosome Submetacentric Size between 7-8 groups Approximately 5.0-5.5 µm in length With satellites
Y chromosome Acroncentric Slightly longer than group G chromosomes 2 µm in length Has no satellites
5
Sex Differentiation in Humans Genetic Sex Males = XY while females = XX
Gonodal Sex (5th week) XY embryo will produce testes (medulla) while XX embryo will produce ovary (cortex) If the cell contains XY sex chromosomes, it will develop testes at 7th week it not ovary develops) Oogonia develops at 12th week, and by 25th week all primary oocytes have been produced and arrested at GO (remain dormant until puberty Prior to this stage, embryo is hermaphroditic
Genital Sex Secondary sex organs (external genitalia) and hormones
Somatic Sex Production of secondary sex characters (effect of hormones)
Nuclear Sex Males – Barr body negative Females – Barr body positive
Sociophysiological sex Influence of culture
Sex Chromosome Abnormalities Klinefelter Syndrome 2n=47 (XXY sex chromosomes) Occurs in 2 every 1000 male births Male genitalia and ducts Rudimentary testes (fail to produce sperm) = sterile Tall and long arms and legs Large hands and feet Slight enlargement of breast (gynecomastia) Rounded hips Below normal IQ Other forms: 48, XXXY; 48,XXYY; 49XXXXY; 49XXXYY
6
Sex Chromosome Abnormalities Turner Syndrome 2n=45 (X sex chromosome) Occurs in 1 every 2000 female births Most die in utero Female external genitalia Rudimentary ovary (sterile) Short stature, stocky, (usually under 5 ft) Webbing neck (Skin flaps on the back of neck) Flat chested (Underdeveloped breasts) Broad, shieldlike chest Low IQ
Sex Chromosome Abnormalities Metafemale, Poly X females 2n=47 (XXX) Genetically female Occurs in 1 every 1200 female births Some are perfectly normal Other cases, undeveloped secondary sexual characteristics leading sterility Mental retardation Rare cases: 48 (XXXX); 49 (XXXXX)
7
Sex Chromosome Abnormalities Jacobs Syndrome (Criminal Syndrome)
2n=47 (XYY) Phenotypically male First studied by Patricia Jacobs (1965) 9 out of 315 males in the Scottish maximum security prison Significantly above average in height (above 6 ft) Antisocial (nonviolent) behaviors, personality disorders High percentage have higher IQ (7/9)
Sex Chromosome Abnormalities Hermaphroditism
Individuals with both testicular and ovarian tissues Ambigous external genitalia Often musculinized Generally sterile (underdeveloped organs) Variable cases: (US study of 100 cases) 59 XX 21 XY 28 mosaics (XXY, XXX, etc.)
8
Other Causes of Sexual Abnormalities Attached X flies Observed in fruit flies only 2 X chromosomes are physically attached (inability to segregate during anaphase) Will result to metafemales
Polyploidy More than 2 sets of chromosomes
Transformer genes Autosomal genes that affect the sex of the individual Identified in Drosophila
Gynandromorphs Condition wherein ½ of the organism is male and the other half is female Male part contains XY chromosomes while female parts contain XX chromosomes
The Y Chromosome in Humans Contains Psuedoautosomal Regions (PAR) on both ends Share DNA sequence homology with X chromosome (making them pair during meiosis)
5 Nonrecombining region of the Y (NRY) – remainder of the chromosome Mainly Heterochromatic (devoid of structural genes) 7 genes without homology to other chromosomes Contains genes coding for Testes-Determining Factor (TFD)-proteins responsible for differentiatiation of gonodal tissue to become testes SRY gene (sex-determining region Y)-studied using transgenic mice Abnormal cases: Males without Y (XX) but contains SRY gene on one of the X Females with Y chromosomes but absence of SRY gene
9
Dosage Compensation for X Chromosomes in Females Females can produce twice gene products for genes located in the X chromosomes (could lead to abnormalities) Dosage compensation = mechanism employed to prevent overexpression Barr Bodies (Chromatin Bodies) Inactivated X chromosome on the periphery of the nucleus Follows N-1 rule where N is the number of X chromosomes
Dosage Compensation for X Chromosomes in Females If only 1 X chromosome is active, why are abnormalities observed? X chromosome does not occur in early development of embryo (before sex differentiation) Not all Barr bodies are inactivated The Lyon Hypothesis Inactivation of X chromosomes occur randomly in somatic cells Once inactivated, all daughter cells exhibit inactivated X chromosomes Controlled by a gene called X-inactivation Center (XIC)
10
Sex Determination In Drosophila Follows XY-XX system 2n=8 (6 autosomes and XY or XX sex chromosomes) First studies by Calvin Bridges (1916) Y chromosome does not determine sexes (unlike in humans) Ratio of X chromosome and the autosomes determines sex in Drosophila Based on initial findings that XXY flies are females while XO flies are males but are sterile. Studied triploid females (3n) = result of fertilization of diploid (2n) egg and haploid (n) sperm
Sex Determination In Drosophila Ratio of number of X chromosomes and number of sets of chromosomes present determines the sex Ratio of 1 = Female (fertile) Ratio above 1 = metafemale (sterile) Ratio between 0.5 to 1.0 = Intersex (invariably sterile, exhibit both male anf female morphology) Ratio of 0.5 = Male (fertile) Ratio below 0.5 = Metamale (sterile) Determined by Genetic Balance Theory Genes involved: Tra gene –transformer (autosomal gene) Sxl gene – sex lethal gene Dxl gene – double sex gene
11
Sex Determination In Reptiles Follows ZZ/ZW system where ZZ are males and ZW are females Sex determination is affected by temperature (incubation temperature of egg) Varies with different species Three observed patterns:
Low T=males High T = females
Low T=females High T = males
Low & high T = females Intermediate T = males
Tp = pivotal temperature; FT = female-determining temperature; MT= male-determining temperature Tp is narrow (1-5°C) Hormones are mainly affected (androgens and estrogens) Aromatase=converts androgens to estrogens
12
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