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Sex Determination

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Sex determination in living organisms and sex linkageSex: Sexual Reproduction- Coming together of genetic material from (two) individuals to form progeny that combinegenes from all (both) parentsCan there be sex without gender?- Many animals have separate genders.- In some animal species the males and females are very similar.- Other animal species show sexual dimorphism.- Some plants are unisexual and have male and female plants. For example, only female hollies can produce berries.- Hermaphroditism: both sexes in the same organism- Monoecious: both male and female reproductive structures in the same organism- Dioecious: either male or female reproductive structures in one organism Sex Determination- A biological system that determines the development of a sexual characteristics in an organismTypes of chromosomesSex Chromosomes:A chromosome that men and women have different amounts of. (X and Y in humans.)Autosomal Chromosomes: All the other chromosomes BESIDES the sex chromosomes. Sex Chromosomes:The Role of Sex Chromosomes- The X chromosome contains genetic information essential for both sexes; at least one copy of an X is required.- The male-determining gene is located on the Y chromosome. A single Y, even in the presence of several X, still produces a male phenotype.- The absence of Y results in a female phenotype.1Chromosomal Sex-Determination SystemsXX-XO system: XX – femaleXO – male . e.g grasshoppersXX-XY system:XX – femaleXY – male e.g mammalsZZ-ZW system:ZZ – maleZW – female e.g Birds, snakes, butterflies, some amphibians, and fishesHaplodiploidy system:Haploid set – maleDiploid set – female e.g Bees, wasps, and antsMultiple sex chromosome type – In some mammals e.g barking dear, kangaroo rat, there are more than two sex chromosomes in one of the sexes. In barking dear, females have two pairs of autosomes and one pair of sex chromosomes (XX), whereas the males have three sex chromosomes (X, Y1, Y2) in addition to the autosomes.Genic Sex-Determining System- In mosquito, the sex determining potency of X and Y chromosomes is due to a single gene M. The X and Y chromosomes of mosquito are homologous. Homozygotes (MM) are females, while heterozygotes are males i.e m is a sex gene.- In Papaya (dioecious) the genes M1 and M2 determine the sex of individual plants.Cytoplasmic sex determination- In bacteria e.g E. coli, the type of sex is controlled by cytoplasmic factors, called sex factor. Female cells, F- lack the sex factor while male cells, F+ have the sex factor. - As such, the latter cells can transfer genetic material to the F- individuals.Environmental Sex Determination- Some fishes, reptiles (snakes, lizards, turtles, crocodiles and alligators-sex determined after fertilization- Choice of sex determined by Temperature at which embryonic development takes place. - High temp favour production of females (Turtles and lizards). Alligators high temp favour production of males Sex Linkage- The gene for the trait is located on either the X or Y chromosome.Sex-Linked Characteristics- X-linked characteristics- Y-linked characteristicsX-Linked Color Blindness in Humans- Human eye detects only three colors—red, green and blue- Affected woman passes the X-linked recessive trait to her sons but not to her daughters- Affected man passes the trait to his grandsons through his daughters but never to his sons- Pattern of inheritance exhibited by X-linked recessive characteristics is sometimes called crisscross inheritance2Colourblindness- Result from a defect in one of three genes found on X chromosomeRed–green color blindness is inherited as an X-linked recessive trait in humansPossible Inheritance of Colorblindness AlleleCarrieran individual that has two alleles, one damaged one and one regular3Y-Linked Characteristics- Exhibit a distinct pattern of inheritance- Present only in males- All male offspring of a male with a Y-linked trait inherit the trait- Relatively little genetic information on the Y chromosome- Recognizing Sex-linked Inheritance- Alleles on sex chromosomes are inherited in predictable patterns- Y-linked trait can be inherited only from the paternal grandfather (the father’s father), never from the maternal grandfather- X-linked characteristics also exhibit a distinctive pattern of inheritance4CYTOPLASMIC AND MATERNAL INHERITANCE - Most characters are controlled by nuclear genes although a few are determined by genes in organelles like chloroplasts and mitochondria. - Because of the locations of the organelles in the cytoplasm (outside the nucleus), the genethey carry are called plasmagenes or cytogenes. - Their pattern of imheritance is known as extranuclear/extrachromosomal or cytoplasmic inheritance.- This pattern of inheritance does not follow the Mendelian type, where reciprocal crosses give identical results in which the nuclear contribution of both make and female gametes is equal. E.g red flower and white flower reciprocal cross gives all red flowers in F1 of both crosses.- In the case of cytoplasmic inheritance, the female gamete contributes wholly to the cytoplasm of the zygote while the male gamete’s cytoplasm contribution is negligible. - Extranuclear genes are therefore exclusively or mainly from the female parent to the progeny. Thus, cytoplasmically oriented characters show maternal resemblance. - Cytoplasmic inheritance or maternal effect is suspected when reciprocal crosses show significant difference. Examples: Inheritance of leaf colour in the 4 O’clock plant (Mirabilis jalapa). - This plant has green, white or variegated leaves. - It was found that seeds borne on green leaved branches produce green leaved seedlings, while seeds from white leaved branches produced albino seedlings. - Seeds borne on branches with variegated leaves gave rise to plants with only green, variegated or white leaves. In all these cases, the pollen source was immaterial. - This showed that the inheritance of chloroplasts in this plant is strictly maternal.- Cytoplasmic: male sterility (cms) in plants. - Failure to produce functional pollen is controlled by cytoplasmic genes in maize, wheat, onion, sugar beet etc. - In some cases it has been controlled by an interaction of nuclear and cytoplasmic genes. In maize, male sterility is strictly controlled by cytoplasmic genes.Male sterile plant X Male fertile plant Male sterile plants- Repeated back crossing of male sterile plants with the male sterile offspring resulted in male


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