-The Genetics of SEX DETERMINATION

Sex-determination system - a biological system that determines the development of sexual characteristics in an organism

• Most sexual organisms have two sexes.

• Occasionally there are hermaphrodites in place of one or both sexes. *sila ung may both male and female reproductive organs.

• There are also some species that are only one sex due to parthenogenesis, the act of a female reproducing without fertilization. *form of asexual reproduction by self-impregnation resulting in the production of a zygote from an unfertilized egg.

Sex Determination In Humans

When you were born, after the doctor and your parents determined that you were healthy, what is the next question your parents probably asked?

• “Is it a boy or a girl?”

True or False

1. The father determines the sexual category of the offspring. - TRUE – The male sperm has 2 types of chromosome– X and Y– the Y chromosome determines the sexual category of the child

Ex: King Henry VIII

3 Levels of Sexual Development

1. Chromosomal sex -Presence or absence of the Y chromosome

2. Gonadal sex (primary sex determination) - whether the gonads develop as testes or ovaries depends on the presence or absence of the SRY gene, usually found on the Y chromosome

3. phenotypic sex (secondary sex determination) - all of the internal and external structures develop along male or female lines depending on which hormones are secreted by the gonads.

• Phenotypic sex also has a couple of distinct systems: the internal ducts, and the external genitalia

• Two important times: pre-natal development and puberty

Chromosomal Sexual Determination

• In humans, there are 23 pairs of chromosomes.

• The 23rd pair of chromosomes is different.

• Twenty two pairs of chromosomes are the same in both males and females. They determine general body characters. They are called the autosomes.

• The twenty-third pair of chromosomes is different and determines sex. They are called the sex chromosomes.

There are two sorts of sex chromosomes:

• The large one is called the X chromosome; and the smaller one is called the Y chromosome.

In humans, females have two similar X chromosomes (XX). Males have two different sex chromosomes: X and Y chromosome (XY).

• Sex is determined by a pair of sex chromosomes.

Chromosomal Aberration - Any range resulting in the duplication, deletion, or rearrangement of the chromosome material.

Sex Chromosomal Aberration

1. **Klinefelter Syndrome (**XXY) - has an additional X chromosome at chromosome #23

Characteristics of Individuals:

• general male physical characteristics

• testes are small which do not normally produce sperm

• mostly mentally retarded

• arms are longer than average; breasts are developed, voice is high-pitched

• talkative; have delicate skin, low IQ

• usually will develop osteoporosis

1. **Turner Syndrome (**X0) - individuals have only 1 X chromosome

• occurs in 2 out of 2000 people the sex chromosomes during meiosis

• results from the nondisjunction of

Characteristics of Individuals:

• general female physical characteristics

• short stature (usually under 5ft)

• short, webbed neck

• broad, shield- like chest

• mentally retarded however some may excel

• secondary sex characteristics do not develop

• usually sterile

Poly X (Superfemale, Metafemale) - individuals have more than 2 X chromosomes (XXX)

• occurs in 1 out of 1, 200

Characteristics of Individuals:

• physically female, sterile and retarded

• perfectly normal, however, in some case, have underdeveloped sex characteristics

1. Jacob’s Syndrome (Metamale) individuals have more than 2 Y chromosomes (XYY)

• occurs in 1 in every 250 live male births

Characteristics of Individuals:

• above average height, low IQ

• physically normal but are slightly epileptic

1. Autosomal Chromosome Aberration

a. Aneuploidy (2n +/- chromosome) - addition or loss of a single chromosome

i. Monosomy - loss of 1 chromosome

1. Cri-du-chat syndrome (cry of the cat) - deletion of chromosome #5 lower arm

• anatomic malformation

• mentally retarded

• malformed larynx and glottis

ii. Trisomy – addition of 1 chromosome

1. Down Syndrome (Mongoloid) - addition of 1 chromosome on chromosome #21

• low IQ (70), short lifespan

• mentally retarded, rounded

• short, prone to respiratory illness

1. Edward Syndrome - addition of 1 chromosome on chromosome #16,#17 or #18

• elongated skull, short neck

• mentally retarded, malformed ears

• Sterile

1. Patau Syndrome - addition of 1 chromosome on chromosome #13,#14 or #15

• Polydactyl *baby is born with one or more extra fingers, cleft palate

• mentally retarded, malformed ears

• sterile

1. Structural Abnormalities

i. Deletion/ Deficiency - a condition where a portion of a chromosome is lost; breaks in the chromosome

1. Philadelphia Syndrome - deletion of an arm on chromosome #21

ii. Duplication - a condition where there is duplication of chromosomes

iii. Inversions - a segment of a chromosome is turned around 180 degrees

iv. Translocations - involves the movement/change of location of a segment of a chromosome to a new place in the genome

Other Chromosomal Abnormalities

1. Tay-Sachs Disease – multiple kinds of mutations in chromosome #15

2. 13 Q Deletion Syndrome – deletion of the lower arm of the chromosome #13

3. Aniridia-Wilms Tumor Syndrome – deletion of the upper arm of the chromosome #11; blindness, kidney tumor, mentally retarded and retarded growth

4. 18 Q Deletion Syndrome – deletion of the arms on chromosome #18; heart diseases, large eyes and ears but normal lifespan

5. Prader-Willi Syndrome - deletion of the arms in chromosome #15; respiratory problems, bird-like head, obese

Sex Determination in Animals

1. ZW Sex Chromosomes - found in birds, some reptiles, and some insects and other organisms.

• The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ).

1. UV Sex Chromosomes - In some Bryophyte and some algae species. females carry a U chromosome, whereas males carry a V chromosome

Mendelian Genetics: Pedigree Analysis

Pedigree Analysis - Used to study the inheritance of genes in humans. Useful when studying any population when progeny data from several generations is limited or any population with uncontrolled crosses.

• Useful when studying species with a long generation time.

Family Pedigree - Shows the history of a trait or illness in a family

• Allows geneticists to analyze human traits and/or diseases.

Human Pedigrees

• Symbols are used to represent different aspects of a pedigree such as the following

• Heterozygous who fail to express a recessive train, when known to be heterozygous with certainty, have only the left half of their square or circle shaded. (i.e. carrier)

Instructions for Analyzing a Pedigree Chart

1. Determine whether the trait is sex linked or autosomal.

• If the trait is sex linked, it will be expressed more frequently in males, whereas if it is autosomal it will appear in both sexes equally.

1. Determine if the trait is dominant or recessive

• If the trait is dominant, every child that expresses the trait will have a parent that expresses the trait; if it is recessive, both parents can appear normal as both parents may be heterozygous for the trait.

Sex Related Inheritance

Sex-Limited Traits

• Many traits, especially secondary sex characteristics, are only expressed in one sex.

• It affects a structure or function of the body that is present in only male or only females.

• A good example is lactation, which is limited to females. However, as any dairy farmer knows, daughter of “good bulls” give more milk

Sex-Influenced Traits

• Traits that are dominant in one sex but recessive in the other are said to be sex influenced.

• Horns in sheep, coat color in ayrshire cattle and pattern baldness in humans are all good examples.

• Traits governed by genes in the sex chromosomes. Another example is, color blindness in humans (X Chromosomes) and hairy ears in humans (Y Chromosomes).

Mode of Inheritance

• Y–linked inheritance

• X linked recessive inheritance

• X-linked dominant inheritance

• Autosomal dominant inheritance

• Autosomal recessive inheritance

• mtDNA-Linked inheritance

Y-Linked VS X-Linked Inheritances

• Genes carried to the Y chromosomes are said to be Y-Linked, and those on the X chromosomes are X-Linked

• Y-linked traits are rare because the Y chromosomes have few genes. These traits are passed from male to male only. Also known as holandric inheritance, is the determination of a phenotypic trait by an allele (or gene) on the Y chromosome. E.g. infertility

Y linked VS X-Linked inheritance

• Genes on the X chromosome have different patterns of expression in females and males.

• In females, X-linked traits are passed just like autosomal traits – that is 2 copies are required for a recessive allele, and one for a dominant allele

• In male, however, a single copy of an X-linked allele causes expression of the trait or illness. A man inherits an X-linked trait only from his mother.

• X-linked traits are always passed on the X chromosomes from mother to son or from either parent to daughter, BUT there can be NO direct male-to-male transmission of X-linked traits

X-Linked Recessive inheritance

• X - linked recessive trait is if the causative allele is present in two copies.

Criteria for an X-linked Recessive trait:

• Always expressed in the male

• Expressed in a female homozygote but very rarely in heterozygote.

• Passed from a heterozygote or homozygote mother to an affected son.

• Affected female has an affected father and a mother who is affected or a heterozygote.

Rules of Inheritance X-Linked Recessive

• More males than females are affected

• Affected sons are usually born to unaffected mothers, thus the trait skips generations

• Approximately, ½ of carrier mothers’ son are affected

• It is never passed from father to son

• All daughters of affected fathers are carriers

X-Linked Dominant Inheritance

• Dominant x-linked conditions and traits are rare and gene expression differs between the sexes.

• A female who inherits a dominant x-linked allele has the associated trait or illness, but a male who inherits the allele is usually severely affected because he has no other allele to mask its effect.

Criteria for an X-Linked Dominant Inheritance

• Expressed in female in one copy

• Much more severe effect in males

• High rates of miscarriages due to early lethality in males.

• Passed from male to all daughters but to no sons.

Rules of Inheritance X-Linked Dominant

• Both males and females are affected; ;often more females than males are affects

• Does not skip generations

• Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father

• Affected fathers will pass the trait on to all their daughters

• Affected mothers if heterozygous will pass the trait on to ½ of their sons and ½ of their daughters