Genetics - Lecture 6: Sex Determination and Sex Linkage

Sexual Reproduction

The formation of offspring that are genetically distinct from their parents

• In eukaryotes

  • sexual reproduction consists of two processes that lead to an alternation of haploid and diploid cells: meiosis produces haploid gametes (spores in plants), and fertilization produces diploid zygotes

Biological Sex

defined based on the size of the gametes produced: males produce small gametes (sperm or pollen), and females produce relatively large gametes (eggs or ovules)

• Male and female sexual phenotypes – genetic, anatomical, and physiological characteristics.

Sex Determination

The mechanism by which biological sex is established

• Hermaphroditism

• Dioecious

Hermaphroditism (Monoecious)

organisms that bear both male and female reproductive structures

Dioecious

organism has either male or female reproductive structures

Chromosomes Theory of Hereditary

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

• Definitive proof of this theory was provided by the discovery that the sex of certain insects is determined by the presence or absence of particular chromosomes

Nettie Stevens (1905)

First to show chromosomes determine sex and that the chromosomes of males and females were different.

•Females – two large (X) chromosomes

•Males – a single large chromosome (X) plus a smaller one (Y)

Sex-determining systems

• Genetic

  • XX-XO

  • XX-XY

  • ZZ-ZW

  • Genic

• SSEnvironmental

XX-XO Sex Determination

Ex: Grasshoppers

• In females (XX), the two X chromosomes pair during meiosis and separate → each egg receives one X chromosome.

• In males (XO), the single X chromosome separates during meiosis → half the sperm receive an X, half receive no sex chromosome.

• Males produce two types of gametes (X and no X) → heterogametic sex.

• Females produce only one type of gamete (X) → homogametic sex.

• In the XX–XO system, sex is determined by the type of sperm that fertilizes the egg:

  • X sperm + X egg → XX (female)

  • No-X sperm + X egg → XO (male)

XX-XY

Ex: Humans (most mammals), some plants, insects, and reptiles

Male - heterogametic sex

Female - homogametic sex.

XX-XY Variation

• 10 sex chromosomes

Ex: Duck-billed platypus

ZZ-ZW Sex Determination

Female - heterogametic

male - homogametic

• birds, reptiles, butterflies, amphibians, and fish

Sex determination

Sex is genetically determined, but there are no obvious differences in the chromosomes of males and females, so there are no sex chromosomes.

Genotypes at one or more loci determine the sex of an individual.

• Some plants, fungi, protozoans, and fishes

Comosomal sex-determiing systems

sex is actually determined by individual genes

Environmental sex determination

• Sequential hermaphroditism

• Temperature dependent

Sequential hermaphroditism

each individual animal can be both male and female, but not at same time

• Ex: Slipper Limpets - stack position determines sex

Temperature Dependant

Ex: turtles

• warm incubation temperatures = more females

• cool temperatures = males

• Reverse for alligators

Sex-determination in humans

XX-XY sex determination

• Males: XY, Females: XX

• Males: SRY Gene on Y chromosome

Atypical #’s of sex chromosomes:

• Turner Syndrome

• Klinefelter Syndrome

• Poly-X Females

• XYY Males

Phenotypes that result from atypical numbers of sex chromosomes show importance of Y chromosome in human sex determination

Turner Syndrome

• Single X chromosome (XO).

• 1 in 3000 female births

• Biologically female

Klinefelter Syndrome

• ≥ 1 Y chromosome, > 1 X chromosome

• 1 in 1000 male births

• Biologically Male

Poly-X females

• Triple X Syndrome: 3 X chromosomes

• 1 in 1000 female births

XYY Males

• Extra Y chromosome (XYY)

• 1 in 1000 male births

The role of sex chromosomes

1. X chromosome: contains genetic information essential for both sexes.

2. Male-determining gene is on the Y chromosome.

3. .Absence of Y chromosome usually = female phenotype.

4. Genes affecting fertility are located on the X and Y chromosomes.

• 2 copies of X required for female fertility.

5. Additional copies of X chromosome may affect development in males and females.

The male-determine gene in human

Sex-determining region Y (SRY) gene:

• Discovered in 1990

• Present on Y chromosome of other mammals

• Present in XX males

• Absent in XY females

• Presence of SRY gene on the Y chromosome causes a human to develop as a male

• In the absence of this gene, a human develops as a female

Sex determination of a fruit fly

Fruit fly (Drosophila melanogaster):

• XX/XY sex determination

  • Sex not determined by a specific gene on the Y chromosome

• 8 chromosomes

  • 3 pairs of autosomes, 1 pair sex chromosomes

• Females usually XX, and males usually XY

• Calvin Bridges – X:A ratio

  • Sexual phenotype predicted by the X:A ratio

• Sex determined by genes on the X chromosome

Sex-linked characteristic

Characteristic determined by a gene or genes on sex chromosomes.

• May be X-linked or Y-linked

• Inheritance differs from genes on autosomes

• Y chromosome contains little genetic information

• Most sex-linked characteristics are X linked.

X-linked white eyes in drosophila

Thomas Hunt Morgan (early 1990s) – first to describe patterns of sex-linked inheritance.

• Found white-eyed male among wild type reds

• Performed series of genetic crosses

Nondisjuction and the chromosome theory of inheritance

• Calvin Bridges - Chromosomes fail to separate in anaphase 1 (non-disjunction).

• Non-disjunction of 2Xs in XXY female flies.

• Linked the inheritance of a specific gene (the allele that determined white eyes) to the presence of a specific chromosome (X).

• Evidence that sex-linked genes are located on the X chromosome and confirmed the chromosome theory of inheritance.

X-Linked colour blindness in humans

Red-green color blindness

• Cone cell pigments (3)

• Blue: chromosome 7

• Red/Green: X-chromosome

• X-linked recessive trait

Z-linked characteristics

The same as in X-Y 

• Pattern of inheritance in males and females is reversed

• Males are homogametic sex (ZZ)

• Females are heterogametic sex (ZW)

Indian Blue Peafowl

• Cameo plumage (brown), results from a Z-linked allele (Zca) that is recessive to the wild-type blue allele (Z𝐶𝑎+).

Y-Linked characteristics

• Present only in males

• Y-linked traits exhibit a distinct pattern of inheritance

• Always inherited from father

• All male offspring of male with trait will inherit trait.

• Few genes (~150) on Y chromosome

  • Some influence male sexual development and fertility

  • Function of most poorly understood

  • Useful for determining genetic relationships

Dosage Compensation

mechanism to equalize the amount of protein produced by X-linked genes and autosomal genes

• Different numbers of X chromosomes in males and females – potential problem

  • The amount of protein produced by X-linked genes would differ in the two sexes

  • Females would produce twice as much

  • This difference could be highly detrimental